aplicação e seleção de drives abb

Drives

Application and selection guidlelines for ac system drives: part 1 Kenneth D. Brink and Robert F. Van Lieshout

ABSTRACT This is a comprehensive examination of the applicatiom for ac system drives. It offiks guidelines for equipment choices depending on the f idmenta l requirements ofthe bad upon the drive. Part I of this article dbcusses ape& of induction motors and dc inverters as components of the drive pain. Part2 will investigate drive sizing method f i r ac paper mdcbine applications, line supply equipment, and imtalhtion as well a other consiakratiom unique to dc sys tm drives. KEywoRDs.. Alternating current, coating design, drives, engineering, evahation, jnbhing, immctions, paper industry, papermaking, selection, stanhrh, utilizdtion.

any papermaking, coating, and M finishing operations have used ac drives during the first decade of their commercial application. From these experiences many applications, techniques, and procedures have de- veloped. Preferred utilization and design criteria have evolved from specific operating systems. The widespread use of these successful criteria within the community of ac drive hardware suppliers, consultants, and users is leading to the establishment of numer- ous standards for ac system drives.

This paper serves to educate paper industry personnel in the application of ac system drives and to provide information for the proper selection of an ac system drive. It will include an examination of the methods used for sizing a drive for an ac paper machine application. Next there will be a re-

view of line supply equipment. Finally the paper will examine a variety of installation considerations as well as other considerations which are unique to ac system drives.

Part 1, which is offered here, provides an extensive investigation of induction motors and ac inverters. The appendix gives motor and inventor examples.

Induction motor component of drive t ra in

Historically, the separately excited de motor which has a proportional relationship between the armature current and the torque at constant flux density has found successful use in many paper machine drive applications. In conjunction with analog control, this proportional relationship has

Brink and Van Lieehout are both senior project engineers with ABB Drives, Inc., P.O. Box 372, Milwaukee, WI 53201.

made it relatively easy to use the de motors in applications requiring accurate torque control.

Torque in both ac and de motors is proportional to the magnetic flux density in the motor air gap. In de motors, a separate field controls this flu. The lack of a separate field in an ac induction motor would seem to pose a for- midable barrier to its use. Advances in digital control and better power switching systems now provide prac- tical techniques for controlling the torque in an ac motor.

In ac motors, as in de motors, only the input current controls the torque. The ac motor requires control of the field flu and m a t u r e magnets-mo- tive force (MMF). Generally these systems try to emulate the 90" angle between field components to copy closely the de motors. Unfortunately, these angles vary not only with load but also during transient changes. Nevertheless, it is possible to define the complex motor and control algo- rithm for today's digital control requirements relatively easily.

Vector sontrol Vector control is a method which allows control of both the phase and the amplitude of a current. The ac current has a reactive and a real component. The reactive component produces the air gap flux. The real component is proportional to the torque.

In vector control, the digital speed regulator output becomes the torque reference for a high-speed torque controller. This concept is identical to the speed and current regulator scheme which is used in all modern digital de

Vd. 76, No. 4 Tappi Journal 143

Drives

drive systems. If the application requires load control for a particular section, the motor can be driven by directly applying a torque reference with the speed controller providing a speed limitation in case the load drops off.

Vector control is suitable for applications demanding good dynamic characteristics and accurate speed control, The use of vector control gives induction motor control characteristics that are equivalent to and often exceed those of a digital de drive. The static speed measurement accuracy is typically 0.01% of the nominal speed setpoint. It is easy to achieve a 10-20 ms torque step response through the torque controller.

Vector control prevents the motor excitation from weakening when the load increases instantaneously and the total current is needed in the rotor to increase the torque. Such performance is superior to a de control system in which transient torque changes produce armature distortions with result- ing loss of air gap flux.

Commercial development of ac vector control algorithms has proceeded much farther than dcmotor algorithms. A later section of this paper presents a comparison of vector control with the conventionally applied scalar control.

slip Synchronous speed of an ac induction motor is the rotational speed of the air gap flux field with respect to the stator. It is directly proportional to .the rated frequency and inversely proportional to the number of poles. The number of poles is always an even number. This relationship is shown in Eq. 1:

Ns = 120 (FP) (1)

where

Ns = synchronous speed, rpm

F = rated frequency

P = number of poles

Base speed is the speed at full excitation, rated load, rated frequency, and ratedvoltage. Base speed is just below the synchronous speed of the motor.

The difference between synchronous speed and base speed is slip. Slip is an important characteristic that will be mentioned often in this paper. In modern motors with low losses, high efficiency, and small air gaps the magnitude of the slip is very small. Before the advent of variable frequency ac drives, synchronous speeds of 3600, 1800,900, and 720 rpm at 60 Hz with 2, 4, 6, and 8 poles were typical. Such speeds are gradually disappearingwith the appearance of large motors which are intended specifically for variable frequency drives at a rated frequency other than 60 Hz for optimum design. This rated frequency is the frequency at which rated voltage is reached.

Most of the present control systems allow the ac motor to perform simi- larly to the de motor. Figure 1 shows the familiar torque and power vs. speed curves. Normally paper machine and winder drives require a constant torque over the speed range of the motor. An exception is the unwind, or braking generator, on the winder where the motor runs in constant torque during acceleration and deceleration and in constant horsepower range during normal builddown. Motors for this application run well above the normal base and synchronous speed. Other authors have discussed the performance of ac motors in a winder application (1).

The ac motor is a highly inductive load for the inverter. The impedance increases directly with increasing frequency. To maintain a constant flux, it is necessary to increase the voltage in direct proportion to the frequency. In constant V per Hz mode, the motor can operate in a constant torque application similar to a de motor below its base speed. If, however, the frequency is increased while maintaining constant voltage, both the air gap flux and the torque decrease inversely proportional to the frequency. This is similar to the weak field mode in a de motor and is useful in constant hp applications.

Figure 1 also shows the breakdown torque. This is the maximum torque which a motor can produce. It is also the torque which corresponds to the

knee in Fig. 2. It is necessary to run a motor so that the breakaway or transient torque never exceeds the breakdown torque. Above base speed, the motor torque decreases inversely proportional to the frequency or speed, while breakdown torque decreases inversely proportional to the square of the frequency. It is important to note that the two torque curves do inter- sect. Motors should not be run near or beyond this point.

The connected load will determine the required torque. This in turn dictates the motor speed and slip. A smaller torque will result in lower slip relative to the rated slip. Other param- eters that are dependent on slip are power factor, efficiency, and input impedance. The sigmficance of slip and its relationships can be found by reducing the motor to its equivalent circuit (2).

Figure 2 is a plot of the torque and current for the induction motor against speed for a fixed frequency. Motors usually run in the linear portion of the torque curve where the torque is approximately proportional to slip. The curves in Fig. 2 move to the right or left as the frequency varies. Motors powered from inverters never experience the high starting m e n t or torque associated with the knee of this curve. This increases the life expectancy of a motor because it is never subjected to large transient forces and currents.

Notice how the current does not pass through zero as the motor goes from motoring to generating, This important and advantageous characteristic is discussed elsewhere (I). When the rotor of the induction motor is at synchronous speed, the machine draws a minimum current. As the slip increases in either direction, the current increases rapidly but smoothly without any discontinuity. This allows a very smooth transition between motoring and generating. There is no current zero o r dead band for this transition- only a phase shift.

144 April 1993 Tappi Journal

1. Power and torque curves 2. Induction motor torque and current at constant supply voltage and freauencv

n 2 2 400-

3

E 8 200

Breakdown toque

M~~~~ Power - - - - - - - - - - - - - - - - - - _ _ _ _ . - speed

50 hp Motor 50 hp Load

0 ’ I 432

MOTOR SPEED, rpm

100 hp Motor 50 hp Load

I I

SYNCHRONOUS SPEED, %

Current wave form for two motors with the same torque loading

+I + TIME

+ I

Harmonics In all switched power sources, it is necessary to consider the effect of harmonics. The output wave will consist of a fundamental and a series of sine waves at multiples of the fundamental frequency. These harmonics contribute to the root mean square (RMS) value of the current and voltage drawn by the motor. The harmonics in the current contribute to motor heating. Current standard motors will produce rated torque with a rated current at rated frequency and rated voltage from a pure sine wave power source. De- pending on the inverter, the same current may only produce 80% of rated torque. To produce 100% torque a motor would have tQ draw more than the motor rated current from the inverter.

Any increase in current results in increased motor heating. The harmonic content of the motor stator current produces a distorted wave form that is shown in Fig. 3 for typical motors.

The fundamental current and voltr age produce the torque at shaft speed. Even numbered harmonics do not exist in the air gap flux wave for a three- phase motor. The third harmonic and all multiples of it do not exist in the line-to-line voltage of a three-phase motor. Therefore the fifth, seventh, eleventh, etc., harmonics can contribute to the distortion of the field flux. Torque pulsations due to the harmonics manifest themselves as a high frequency ripple superimposed upon the load torque. One can use equivalent motor circuits and a phasor diagram to determine the magnitudes of the har-

monic torques (2). Using high switching frequencies for the pulse width modulation (PWM), inverters will reduce the effects of these harmonics. As a result, these harmonic or pulsating torques are insignificant compared to the fundamental and can usually be ignored.

It is important to note that, with conventional instruments such as digital and analog meters, it is not easy to measure accurately output currents and voltages on inverters due to the harmonics and the fixed de supply voltr age.

Totally enclosed fan-cooled (TEFC) energy-efficient motors may be able to run without derating over a narrow speed range of2:l. The deratingfactor would depend on the type of inverter, motor frame size, and speed range. Motor cooling, class of insulation, temperature rise, duty cycle, and ambient conditions all affect the temperature of the motor and are topics of later discussion.

Insulation and temperature rise Motors running from an inverter supply will encounter high peak voltages and steep voltage rises. Peak voltages and rates of rise will depend on the type of inverter. New motors with the latest insulation materials can usually handle these voltages. Some older motors, however, may encounter difficulty with insulation breakdown.

Vol. 76, No. 4 Tappi Journal 145 /

4. Tvoical thermal curve

FREQUENCY. Hz

Motors normally operate either with a NEMA insulation class B rise of 90% or a class F rise of 115°C above a 40°C ambient temperature. Since class B rise is less than class F rise, a motor with a class B rise will be physically larger than a motor with a class F rise by as much as 12%. Large motors above 300-400 hp are custom designed and the effects of the extra heating are considered in the design. Standard, off-the-shelf, motors must be derated to account for the effects of any extra heating. In addition, they must not exceed the rated temperature rise.

Motor manufacturers generally test their motors with different inverters to verify the performance and temperature rise.

It is important to remember that specifying a class B temperature rise will increase the size of the motor and the cost.

Service €actor It is possible to specify motors with either a 1.0 or 1.15 service factor. Mo- tors with the 1.15 service factor will be 15% larger than motors with the 1.0 service factor. Specifying a 1.15 service factor motor will increase the size and the cost.

A motor with both a class B temperature rise and a 1.15 service factor could be approximately 3040% larger

146 April 1993 'Tappi Journal

5. Minimum natural frequency

JlJm 100 10

.01 I I 1 10 100

FIRST NATURAL FREQUENCY OF TORSIONAL VIBRATION, Hz

than a motor VI.^.^ a 1.0 service factor and a class F rise. This increase is a more important factor with totally enclosed motors than with separately ventilated motors.

Thermal capacity It is necessary to consider the thermal rating of an ac motor the same as for a de motor. Because de motors are designed to run at variable speeds with a silicon controlled rectifer (SCR) power source and with forced air cooling, the thermal rating of the motor is less important.

Today most ac motors used in the paper industry are totally enclosed. These require less maintenance. To- tal enclosed motors are larger than open separate vent machines result- ing in a higher cost. Ambient temperature and the local environment affect the cooling of a t~tally enclosed motor and are important considerations for motor selection. Normally the motors are TEFC. In some applications with an extended speed range, a blower-cooled air-over (TEAO) motor will be used. Usually the outer skin temperature of the totally enclosed motor is hotter than the separately ventilated motor.

Figure 4 shows the typical thermal capacity of an ac motor. The actual limits will depend on the inverter and the motor frame. When selecting a

motor, the continuous torque over the speed range must be under the curve. It is also necessary to consider the duty cycle and overloads when selecting a motor.

Due to their size, larger motors may take as long as 4-6 hours to reach normal temperature from a cold start. This is a definite advantage for short- term overloads. Unfortunately, these motors will take a long time to cool down. In fact, the skin temperature will rise on a fan-cooled motor for a short period after the motor stops. In applications where the motors are not running continuously, a fan-cooled motor does not cool itself when stopped. This is a consideration when calculating the duty cycle. A blower- cooled motor will be cooled during the off period provided the blower contin- ues to run.

Thermal protection In larger, more critical motors, resistance temperature detectors (RTD) monitor the motor temperature. Nor- mally an alarm will alert an operator that the normal operating temperature has been exceeded and corrective action is required. Should the temperature increase even higher, the control can stop the drive. A bimetallic temperature contact can be used as an alarm or a drive-tripping mechanism on smaller motors.

Overload relays are ineffective because the temperature characteristics do not match the thermal capacity of the motors. For example, a motor running at a low speed with near rated current will overheat and never cause the relay heater to trip.

Mechanical kctors All vector-controlled drives require a digital tachometer. Paper machine drives usually have digital tachometers with 1024 pulses per revolution. Winder drives with slower speed requirements of 2-5 rpm may require 2048 pulses per revolution. It is important to make sure that these higher counts do not exceed the input frequency limit to the drive when running at top speed. Lower pulse per revolution tachometers are useful for distributed control system monitoring but not for the drive control.

Tachometer mounting and alignment is critical for all digital drives whether they are ac or de. It is possible to use shaft-mounted tachometers, but the rigidity and the alignment of the small stub shaft must be very good.

Insulated bearings may be required on large motors where the shaft voltages may exceed 0.3-0.8 V. The motor frame length and power rating will determine the need for these bearings. The bearing on the nondrive end is usually the one which is insulated. In this case the tachometer needs insulation to prevent a path for electrical current through the tachometer bearing. AC motors require no ground brush.

It is possible to spec@ V-ring or labyrinth-type seals, special coatings, and other special hardware in a similar manner to de motors. These op- tions do increase the cost of the motor, but they are necessary in some applications.

Special applications Running ac motors in parallel from a single source is often desirable. Such motors must be mechanically coupled

so that they can share the load. In this case, only one motor requires a tachometer. Other authors have docu- mented the performance and application of ac motors in parallel (3).

Because of the power limits for inverter switching devices, larger drives require a dual-fed stator. One can design motors with two separate wind- ings in parallel which can be fed from two inverters. An alternate method would utilize an inductor separating the two inverters with the single stator winding fed from the midpoint of the inductor.

Maximum motor loading Normally the maximum motor torque should not exceed 70% of the breakdown torque of the motor to maintain control. (See dryer motor example in appendix.)

Speed reducer and drive shaft Reducer and drive shaft requirements do not change dramatically when using ac drive vs. de drive. Today there is a tendency to reduce motor speeds to lower the ambient noise level and to increase bearing life.

The speed reducers must meet the necessary mechanical and thermal power requirements. Reducers must have service factors which will handle the peak loads as well as meet the endurance limits of the mechanical components. Reducers used in unusual applications such as a braking generator on awinder requiring awide speed range need special consideration. In such an application, it is important that the thermal power rating is not exceeded at the top speed of the motor. In addition, the mechanical power rating must not be exceeded at the base speed of the motor.

Normally one selects the gear ratio so the motor will run just above its rated base speed of about 8%. Brak- ing generators are an exception. It is also possible to choose gear ratios to allow the motor to run faster than base speed so the motor will not reach its thermal limit at low speed. (See ex-

ample in appendix.) Thermal limits of the reducer are a consideration in reducer selection.

Torsional and stiflFness effects Speed reducers, shaft couplings, and jack shafts all affect the torsional rigidity of the system. Torsional deflec- tions may be acceptable to cushion large torque transients in some applications. On a high-performance drive, however, they add unwanted flexibil- ity to the control loop. This decreased stiffness adversely affects the stability and the performance of the closed loop system (4).

The natural frequency of the mechanical system, f,, limits the response time of the drive system. By using basic regulator theory to analyze a system, it is possible to develop the curves shown in Fig. 5. Section inertia (J) divided by motor inertia (J,) provides a curve at 100 for use with dryer sections and another curve at 10 for use with wet-end low-inertia drives. From the figure, one can determine the first natural frequency for a desired drive response. For example, the first natural frequency must be greater than 5.25 Hz for a drive response of 1 s in a dryer section.

With the optimized control systems available today, there are multiple control loops regulating the complete paper machine. The ac drive system must have a predictable response characteristic to be a part of the complete control system.

AC inverter component of drive system

The paper up to this point has covered the most difficult part of designing an ac drive system, i.e., the proper definition of the application load requirements for a paper machine, coater, winder, etc., as well as the selection of the motor. The next step in the pro- cess is finding the proper ac inverter to provide an economical and efficient technique for controlling the speed or torque of a squirrel cage induction

Vol. 76, No. 4 Tappi Journal 147

Drives - _

A

Cross firing at low frequencies helps shape the voltage wave form 7. Giant transistor schematic I I

I ‘ I I UCN = 1.35 x mains voltage

motor. The selection of the proper inverter rating to feed the motor is fairly simple and straightforward. Simply stated, the motor data which was dictated by the demands of the application basically define the inverter size for a particular application.

AC drive systems equipped with vector control technology have successfully provided performance that was once thought achievable onlywith digital de drives. Such performance makes the system seem completely transparent to the operator, regard- less of the application. Most users judge the capabilities of a modern digital de drive by measuring the ease of controlling the steady-state speed accuracies of each section to 0.01% of the nominal setpoint while delivering constant torque over the range of maximum gear-in operating speed to zero.

Successful ac drive systems provide the same accuracies as de systems with a speed and torque regulator scheme similar to those successfully used in the control of de motor torque. The primary difference between the modern ac drive and its de counter- part is the way it controls the vari- ables that affect torque in an ac machine compared to a de machine. The look and feel of the drive to maintenance personnel are no more com- plicated than newer digital de systems provided there is a basic understand- ing of digitally controlled thyristor- type drives.

To motor phase conductor

The success of the ac drive system is due to the use of PWM technology and vector control as applied with modern, high-speed microprocessors.

Inverter theory

The ac inverter used for the control of a squirrel cage induction motor uses four main elements:

Line converter unit

DC intermediate circuit

Control unit

Inverter unit.

The line converter unit and de intermediate circuit will be covered in greater detail later in this paper.

The ac inverter used for sectional control in paper applications uses a PWM control scheme. The PWM inverter takes a fixed de voltage produced by the line converter unit and de intermediate circuit to produce a balanced, three-phase voltage at the desired frequency on the terminals of the induction motor. Maintaining constant torque requires proper V per Hz for appropriate excitation of the motor. This is equivalent to the fixed field in a de machine. One adjusts the RMS value of the required basic voltage for the motor by varying the widths of the individual pulses fed from the main de voltage. As the number of pulses increases, the actual RMS value of the output wave more closely represents the desired voltage to the motor.

8. Gate turn off thyristor with free wheeling

6 To motor

phase conductor

The very nature of the PWM drive lends itself to multiple motor applications because they are fed from a common fixed voltage source. Other methods of ac inverter control, such as variable voltage inverters (WI), tend to vary the intermediate circuit voltage together with the inverter output frequency. This usually results in poor speed control with sluggish response, because the large capacitor required in the intermediate circuit must be charged or discharged to change the motor terminal voltage. The PWM inverter, on the other hand, is a very accurate speed controller which operates over a large speed range, because the voltage control is a result of the inverter and not the inverter source. Another definite advantage of the PWM drive is its ability to vary the firing of the semiconductor to corre- s p n d with variations of the intame- diate circuit voltage level due to ac line fluctuations.


The control unit associatedwith the PWM inverter provides a combination of the proper regulator and su- pervisory and modulator control schemes necessary for successful operation of the inverters. The unit is completely digital and offers the following advantages:

0 Definition of numeric setpoints Repeatability of setpoints and pa- rameters

Accuracy

Utilization of standardized hardware and application specific soft- ware

Reduced number of components

Serial data communication.

The proper regulator control scheme for paper industry sectional applications and the way that a particular drive gathers information for the su- pervisory control will vary between various drive vendors. Although each vendor provides similar functionality, such information is proprietary.

The success of the PWM drive results from the control philosophy used in the modulator. The modulator gov- erns the inverter semiconductor device to produce the correct voltage pulses for the motor speed or torque control. The output voltage wave form of the PWM inverter regulates the flux produced in the motor to corre- spond closely to the flm produced by a sinusoidal voltage applied to the motor terminals. If the time interval used in the approximation of the proper voltage is shortened (increasing the switching frequency), the wave form of the flux will definitely improve. An infinite switching frequency, if it were attainable, would produce a flux that corresponds directly to a sinusoidal voltage.

The modulator that is provided in a PWM inverter has an extremely high control rate. This allows the modulator program to manipulate the output pulses to the semiconductors in a manner that ensures that the phaseto- phase voltage pulse area follows a

sinusoidal distribution within a given voltage half-cycle. When operating at low frequencies, the inverter employs cross firing of the semiconductors providing both positive and negative voltage pulses. This produces a more even voltage sine wave at the motor as shown in Fig. 6. As the frequency increases, the cross firing ceases. The pulse duration increases to the point where the inverter produces three pulses per half cycle at nearly 60 Hz.

One always maximizes the number of pulses to eliminate torque pulsations. A high number of pulses provide a much wider frequency and voltage control range. Zero frequency and voltage are possible. This condition allows better control at lower speeds, so the direction of rotation can easily be reversed at zero frequency.

The PWM inverter produces some losses associatedwith the switching of the power semiconductor devices. These losses are directly proportional to the switching frequency. Motor losses, on the other hand, are inversely proportional to the switching frequency of the drive. The appropriate number of switches per second, or number of pulses per half cycle at the operating frequency, must be optimized by the modulator control to maintain low harmonic currents and low torque pulsations in the motor without overheating the semiconductor device used. Typically, the power transistors that are used can handle switching frequencies greater than 1 kHz, while the silicon controlled recti- fiers operate at switching frequencies of less than one kHz. For proper speed control of ac motors with PWM control, the switching frequencies should normally be above 300 Hz.

All the information listed above indi- cates that the PWM inverter provides superior control of the ac motor throughout an infinite speed range. In- creasing the number of pulses at low operating motor frequencies is one method of reducing current pulsations when the motor is operating at low speeds. One negative effect of the PWM drive, however, is that additional motor heating can result from the voltage

curve produced from a pulse-type voltage controller on a squirrel cage induction motor. The PWM inverter causes those losses because the motor currents associated with the voltage controller are not completely sinusoidal.

Proper steps taken in the modulator program can optimize performance without excessive losses associated with either the motor or the semiconductor device. The PWM inverter uti- lizing this modulation scheme is far superior to other types of inverters for minimizing these currents.

Power devices

The modulation scheme specified above will control both stand alone and system-type inverters used to drive paper machine applications. The power semiconductors that are controlled by the modulator are either giant transistors (GTR) or gate turn- off thyristors (GTO). The use of one type of device vs. the other is strictly dependent upon the power rating of the individual inverter unit. A power rating of approximately 1000 kVA at 660 V presently limits the industrial application range of either GTR or GTO.

Figure 7 provides a schematic rep- resentation of a pair of GTR units feed- ing one phase of an induction motor. The benefits of the GTR compared to GTO are as follows: 0 Snubber capacitors are not neces-

sary to limit the voltage rise while turning off the transistor.

* The inverter output can withstand a short circuit without the need of additional components in the main circuit. A GTR is easier to manufacture than a GTO, although the GTR requires 2-3 times more silicon sur- face area than a GTO at the same electrical values.

The GTO has the characteristics of both a thyristor and a power transistor. Applying a negative current pulse to the gate will turn off the GTO. The peak value of the current pulse is ap-


Drives

proximately 25% of the anode current quency to compensate for the varia- Inverter SDecifiations and features The inverter described above must

established by the inverter desibmers and application engineers. These fieations

value. The pulse duration is typically

Figure 8 provides a schematic rep- resentation of GTO units used to feed one phase of an induction motor. The benefits of the GTO compared to GTR

tion in speed due to motor slip.

lier, is necessary in an ac motor if the torque must be controlled accurately and a high level of dynamic accuracy is required. In a de machine, a combina-

10-20 p. Vector as described ear- operate under specfications

the following items:

are related to the high current and tion of the motor field flux and the f 10% mains voltage variation - -

voltage ranges on which they can be applied.

armature current control the amount of torque that the motor delivers.

3% mains frequency

Scaler vs. vector control Thevarious types of control techniques that are available for the PWM drive include scaler, scaler with tachometer feedback, and vector. In scaler control, the speed of the motor is set by adjusting the supply frequency. The variation between the actual operating speed and the synchronous speed that corresponds to the supply frequency is dependent upon the motor design characteristics or slip. The slip causes the motor speed to settle to a value at which the driven machine will have the power it requires to handle the load.

Scaler control normally includes motor internal loss (IR) compensation and slip compensation. IR compensation supplies a voltage boost at low frequencies when there is a substan- tial voltage drop produced by the stator resistance of the motor. This voltage boost ensures achievement of proper magnetization for sufficient excitation of the motor. The IR compensation will also provide a high level of starting torque for the motor.

Slip compensation is useful when a constant speed is required irrespec- tive of changes in the load torque. The compensation effect can be adjusted as required by the slip of the driven motor.

Scaler control with tachometer feedback is useful for applications where a steady-state speed accuracy of f 0.10% is necessary and the level of dynamic response is not critical. One sets the speed of the motor by adjusting the supply frequency. The major enhancement that this control provides over standard scalar is an outer speed loop that modifies the reference fre-

Torque in an ac machine is composed of the same elements. There is, how-

Ambient temperature range of 0- 40°C

ever, only one resultant current that an ac motor receives. That current is composed of the fundamental component and the harmonic component. The harmonic component is largely produced by the switching of the inverter which does not produce any useful torque for the machine. The harmonics essentially result in heat losses. The fundamental component of current is the only factor that produces usable torque in the machine. This current is the result of the rotor field flux (magnetizing vector) and the stator flux (torque producingvector). Vector control regulates the magnitude and angle of the resultant vector by ma- nipulating the voltage and frequency to the motor. The amount of V per Hz controls the magnetization of the motor. The frequency establishes the sta- t o r rotating flux field velocity determining both the basic speed and torque of the motor. This control is implemented through the PWM inverter. The PWM inverter offers high efficiency, low motor harmonic torques, very wide frequency range, and a simple regulator.

One can measure the amount of actual torque that the motor is produc- ing by monitoring the phase currents and their relationship to the reference voltage. The slip of the machine is a computed value that compares the velocity of the rotating flux field in the rotor (through the 1024 pulse per revolution tachometer) to the rotating flux field in the stator (produced by the output frequency of the inverter). The combination of the two elements is a key factor for extremely accurate torque control.

95% relative humidity (nonconden-

Altitude less than 3300 ft above sea level.

The PWM inverter contains the following features:

Frequency stability at f 0.01% of the maximum frequency

sing)

Inherent regenerative capability

Electronic reversing with zero deadband

Adjustable field weakening point from 30-200 Hz

Active torque limiting adjustable between 30-100% of the nominal torque Efficiency at rated load and rated frequency better than 98% at the inverter Fundamental power factor on the input over the entire speed range near unity

Power limit setting capability

The actual frequency range of the inverter operates from 0-200 Hz. Most paper machine applications do not exceed 115% of the motor synchronous frequency. The main reason for this limitation, as stated earlier in this paper, is that the breakdown torque level of the induction motor decreases with the square of the speed. Thus the drive could lose control of the motor, if the level of load torque was too close to the motor breakdown torque.

The inverter unit itself provides inherent regenerative capabilities. Note, however, that either a regenerative ac


1. TvDical inverter sizes

Inverter ac current size, kVA output, A

460 volt unit 30 38 50 63 75 94 115 145 180 226 290 364 460 578 1300 1634 (Dual inverter)

575 volt unit 140 141 21 0 21 1 340 342 540 543 870 875 1550 1556 (Dual inverter)

to de line supply is necessary to supply power back to the ac line, or the inverter that will be regenerating must be fed from a common de bus with other inverters supplying motoring loads. Each inverter can provide re- generation back to its de supply without the need of additional bridge hardware. The frequency established by the modulator in combination with the driven load determine whether the motor is acting as a motor or an induction generator. The bridge that is used to establish the output voltage and frequency for the motor can transfer the power in either direction.

Parallel inverters If the power requirements of a particular application require more than approximately 800 hp at 460 V or 1000 hp at 575 V, a parallel set of inverters are necessary to feed the motor. Con- trol of the parallel connected inverters utilizes amaster-slave relationship. The master inverter has the same control card as the other individual inverters alongwith a parallel connection card that provides the synchroniza- tion of semiconductor control in the slave inverter. The output of the parallel inverter scheme requires either an output current imbalance limiting

reactor or a motor with a dual winding brought out to the terminal box. The latter is the preferred solution because it negates the need for an additional piece of hardware. Furthermore, it uses the simple dual stator connection scheme which was described earlier under the motor section.

Inverter rating The inverters are rated and intended to operate at either 460 or 575 V. The kVA ratings are achieved at the rated voltage. Experience shows that most modern drive system inverters for large hp sections carry a 575-V rating. This voltage normally provides a much more attractive alternative because of the decreased current for the same installed hp. Consequently, it requires smaller cable sizes than the equivalent 460 V unit. Table I shows typical inverter sizes which are commercially available.

Paper machine inverter sizing The size of power equipment chosen for paper machine and winder drive sections should be sufficient to handle the normal running loads and the peak load requirements with a comfortable safety factor or margin.

Most ac drive systems that utilize PWM techniques are not capable of providing short-term overloads because of the nature of the power semiconductor device that is used. The ac drive produces a fairly high peak current compared to its de drive counter- part. The semiconductor in an ac inverter is limited in the amount of peak current that it can handle before the device will fail. It is for this reason that the ac drives are sized without any momentary overload capability. Although the semiconductor device may be able to handle the fundamental current demanded by an overload condition, the peak currents created during commutation may exceed the acceptable level of current recom- mended for the safe operation of the device. A detection scheme is provided with hardware in each drive to cause a

fault if the peak currents exceed 230% of the nominal device rating.

Motor selection is very critical when avoiding the peak current problems defined above. The selection of an oversized motor for a particular application is not advisable because this will lead to an oversized inverter. This is because the PWM inverter is a self- commutating device. The largest value of peak current is the value that must be used to size the inverter. The peak current is the largest instantaneous output current determined from the sum of the fundamental and harmonic current components. The harmonic currents produced by the PWM drive with a larger, over-sized motor are greater than those of a smaller motor. This is directly related to the motor impedance. The larger motor will have a lower impedance and allow more harmonic currents to flow. Conse- quently, even when loading a larger motor with the same level of power as a small one, the current peaks will be higher as shown in Fig. 3. The net result is that a larger inverter will be needed for an oversized motor compared to a smaller motor at the same output power.

Application of standards The following portion of the paper will compare usual industry standards with the methods used for drive sizing in ac paper machine applications. Common practice states that the drive system should be capable of 150% of the nominal motor rating for 60 s on most sections and 200% of the nominal motor rating for 60 s on high inertia sections (5). These values were selected on the basis of standard or optionally available de motor overload ratings. This standard method of calculating normal running load (NRL) and recom- mended drive capacity (RDC) iswidely accepted and undisputed.

Acomplication does arise, however, with the "standard" overload requirements imposed on the power conver- sion equipment. It is important to emphasize here that it is not necessary to use substantially oversized

Vol. 76, No. 4 Tappi Journal 15 1

Drives

equipment to cover amomentary overload situation that may never exist on

The calculation of HPPeC follows from Eq. 3:

pertaining to the overload requirement. Generally the same argument

the machine.

Wet-end drives Assuming that the worst case overload which a wet-end section will experience under normal or abnormal conditions is the RDC horsepower, the 150% overload required for 60 s is questionable. This additional power is not required during acceleration or deceleration because the section is not even subject to the NRL loads without the presence of the sheet. After the sheet has passed through the section, the reference change to a new operating speed setpoint is so slow that the acceleration power is almost insignificant.

The standard RDC to NRL application margin ranges from 1.15 to 1.25 (5). We think that no additional overload is necessary as long as the section power converter has sufficient capacity to cover the RDC requirements. The inverters of ac systems with which we are familiar are typically selected with a minimum continuous overload capability of approximately 1.25 above the RDC. This margin is more than sufficient to handle any additional or abnormal loading that the drive may experience on this section of the machine.

Dry-end drives One handles dry-end drives differently from wet-end drives because they operate on high inertia sections. This requires special considerations during acceleration and deceleration. The calculated RDC power requirements for a dryer section result from the section NRL loads plus the additional power required to accelerate (or de- celerate, if the deceleration rate is sig- nificantly greater) the inertia in the time specified by the customer. The inertia hp equation defines the peak value of the additional power as shown in Eq. 2:

(2) HPacc = (HP,,, x Wta

HP- = (J x N'Y3.23 x lo6 (3)

where

t, = total time to accelerate or deceler-

J = roll inertia, lb-ft2

N = roll speed at gear-in speed, rpm

For most modern paper machines the acceleration time which is based on 22 ft,"in/s will typically be longer than 130 s. Consequently, the normally specified 200% thermal overload capability is only really available on a short-time basis. This is normally specified as 60 s. The actual thermal overload capability drops to 147% because of overload time being extended beyond 60 s.

The decrease in thermal capacity is based on Eq. 4:

ate to speed, s

T,, = [(I2,, x 60)/ts] O 5 = 15.49 x (t,)-0.5(4)

where 12,, = short-term overload torque re-

quirement (200% = 2PU)

The amount specified in the inertia hp calculation dictates the actual overload requirement for high inertia sections that must be satisfied by the drive. The overload is the acceleration or deceleration time specified in that inertia hp calculation.

On calender sections the NRL and RDC hp requirements follow from industry standards. Since most calender stacks are smaller than 8 rolls, the 200% for 60 s overload specified is not typically required. Every customer must speclfy their particular operation to determine ifthere is any chance that the calender will be required to pull higher tensions than normally specified, if the sheet should break at the reel. This condition can result in a nominal NRL and RDC hp that is slightly greater than standard recom- mendations.

After gathering all the above information and specifylng the power de- mand, it is necessary to raise a question

used for the wet end drive sections relating to lower than normal running loads during acceleration or deceleration applies also for most calenders. The main reasons are the fairly low inertia and the absence of the sheet (and tension hp) when starting and stopping the section. After the sheet passes through the section, the acceleration or deceleration to a new operating speed setpoint is so slow that the inertia hp is insignificant compared to the normal running loads. The only remaining consideration that is critical when selecting the proper overload for the drive is the additional power that may be required to provide sufficient control of the tension regulator to respond to a step load change. We have typically provided the inverters for calender sections with a minimum continuous overload capability of approximately 1.25 above the RDC. This margin is more than sufficient to handle any additional or abnormal loading that the drive may experience on this section of the machine.

One circumstance that could possi- bly increase the power requirements for a calender stack drive to the 150% short-term overload would be to jog a plugged stack. In most instances the customer opts to ignore this requirement and size the drive on the worst case running conditions. This avoids unnecessary over-sizing and reduces the drive cost. If jogging the plugged stackis animportant issue, the ac drive can be sized to deliver a larger continuous overload.

On reel sections the NRL and RDC hp requirements are selected from normal standards. The 150% or 200% for 60 s specified (5) is not normally necessary because the usual running loads include tension hp. They are typically large enough to handle the inertia power necessary to accelerate 2/3 of a full roll after a sheet break in a reason- able amount of time. If the calcula- tions determine that the inertia hp is substantially high compared to the nor-


mal running loads due to a lightweight sheet tension, the size of the motor and inverter may need to be increased to include the acceleration requirements specified by the customer.

Again we believe no additional overload is necessary as long as the section power converter has sufficient capacity to cover the RDC requirements. We provide inverters that possess a continuous overload capability approximately 1.25 times over the RDC on all ac systems. This should handle any additional or abnormal loading that the drive may experience.

Winder inverter sizing AC inverter equipment for winder applications does not differ from the paper machine guidelines presented earlier. After selecting proper hp requirements and motor sizes, the inverter capacity is based solely on the current ratings dictated by the motor

designers. Motor selection considers all overloads associated with acceleration or an emergency stop. The inverter must then cover the motor requirements.

Inverter sizing summary

The ac drive system inverters for paper applications cannot presently sus- tain short-term overloads because of the peak currents. The old de drive systems were able to use their inherent capability to contend with this oc- casional overload without adverse effects. Since modern paper machines run faster and faster, a question re- mains concerning the validity of specifying 200% for 60 s, especially on high inertia sections. Most of the ac systems installed today operate on the assumptions stated above. The power unit size covers the RDC hp requirements with a typical 1.25 continuous service factor. Dl

Literature cited

1. Eikenbeny, M. L. and Brink, K. D., TAPPI 1990 Engineering Conference Proceedings, TAPPI PRESS, Atlanta, p. 45.

2. Cochran, P. L., Polyphase Induction Mo- tors, Dekker, New York, 1989, ch. 5 & 6.

3. Kessler, N. andVan Lieshout, R. F.,TAPPI 1990 Engineering Conference Proceedings, TAPPI PRESS, Atlanta, p. 59.

4. Bentley, J. TAPPI 1991 Paper Machine Drive Short Course, TAPPI PRESS, At- lanta, p. 71.

5. Derrick, R. P., TAPPI 1978 Engineering Conference Proceedings, TAPPI PRESS, Atlanta, p. 381.

The authors are sincerely grateful to the many companies and individuals especially those noted in the literature citations whose efforts have con- tributed to the successful use of ac system drives throughout the paper industry.

Received for review July 1,1992.

Accepted Sep. 25,1992.

Presented at the TAPPI 1992 Engineering Con- ference.



he next smaller size

continuous overload se


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