Motors Slides..PDF

8/10/2019 Motors Slides..PDF

1/28

D.C. MOTOR


2/28

- An Electric motor is a machine which converts electric energy into

mechanical energy.

Motor Principle

- Its action is based on the principle that when a current-carrying

conductor is placed in a magnetic field, it experiences a mechanical force

whose direction is given by FlemingsLeft-hand Rule.

- Constructionally, there is no basic difference between a d.c. generatorand a d.c. motor. In fact, the same d.c. machine can be used

interchangeably as a generator or as a motor.

- When its field magnets are excited and its armature conductors are

supplied with current from the supply mains, they experience a force

tending to rotate the armature.

- Armature conductors under N-pole are assumed to carry current

downwards (crosses) and those under S-poles, to carry current upwards

(dots). By applying FlemingsLeft-hand Rule, the direction of the force on

each conductor can be found. It is shown by small arrows placed above

each conductor.


3/28

- when the motor armature rotates, the conductors also rotate and hence cut the flux. In

accordance with the laws of electromagnetic induction, e.m.f. is induced in them whose

direction, as found by FlemingsRight hand Rule, is in opposition to the applied voltage

Significance of the Back e.m.f.

- Because of its opposing direction, it is referred to as counter e.m.f. or back e.m.f..Eb


4/28


5/28

The voltage V applied across the motor armature has to:

Voltage Equation of a Motor

*This is known as voltage equation of a motor.

Now, multiplying both sides by Ia, we get:

= +

=Electrical input to the armature

=Electrical equivalent of mechanical power developed in the armature

=Cu loss in the armature

Hence, out of the armature input, some is

wasted in loss and the rest is

converted into mechanical power within

the armature.


6/28

The gross mechanical power developed by a motor is:

Condition for Maximum Power

=

Differentiating both sides with respect to Ia and equating the result to zero, we get

= 2

Since

= + And

=

2

= 2 Therefore,

Thus gross mechanical power developed by a motor is maximum when back e.m.f. is

equal to half the applied voltage.

This condition is, however, not realized in practice, because in that case current would bemuch beyond the normal current of the motor. Moreover, half the input would be wasted in

the form of heat and taking other losses (mechanical and magnetic) into consideration, the

motor efficiency will be well below 50 percent.


7/28

The various stages of energy transformation in a motor and also the various losses occurring

in it are shown in the flow diagram below:

Power Stages in DC motors

From above, it is seen that A B = copper losses and B C = iron and friction losses.


8/28

A.C. MOTOR


9/28

- With the almost universal adoption of a.c. system of distribution of electric energy for light and

power, the field of application of a.c. motors has widened considerably during recent years.

Classification of A.C. Motors

- As a result, motor manufactures have tried, over the last few decades, to perfect various

types of a.c. motors suitable for all classes of industrial drives and for both single and three-

phase a.c. supply.

- This has given rise to bewildering multiplicity of types whose proper classification often offers

considerable difficulty. Different a.c. motors may, however, be classified and divided intovarious groups from the following different points of view :

With regard to current type:

1) single phase

2) three phase

With regard to :their speed

1) constant speed

2) variable speed

3) adjustable speed

With regard to their structural

features:

1) open

2) enclosed

3) semi-enclosed

4) ventilated

5) pipe-ventilated

6) riverted frame eye


10/28

Induction Motor: principle of operation

- As a general rule, conversion of electrical power into mechanical power takes place in the

rotating part of an electric motor.

- In d.c. motors, the electric power is conducted directly to the armature (i.e. rotating part)

through brushes and commutator

- Hence, in this sense, a d.c. motor can be called a conduction motor. However, in a.c. motors,

the rotor does not receive electric power by conduction but by induction in exactly the same

way as the secondary of a 2-winding transformer receives its power from the primary.

- That is why such motors are known as induction motors. In fact, an induction motor can be

treated as a rotating transformer i.e. one in which primary winding is stationary but the

secondary is free to rotate


11/28

Induction motor: Construction

- An induction motor consists essentially of two main parts : (a) stator, and (b) rotor.

(a) Stator

- It is made up of a number of stampings, which are slotted to receive the windings.

- It is wound for a definite number of poles*, the exact number of poles being determined by

the requirements of speed. Greater the number of poles, lesser the speed and vice versa.

- The stator windings, when supplied with current, produce a magnetic flux, which is of

constant magnitude but revolves (or rotates) at synchronous speed (given by Ns = 120

f/P). This revolving magnetic flux induces an e.m.f. in the rotor by mutual induction.


12/28

Construction continues

(b) Rotor

(i) Squirrel-cage rotor : Motors employing this type of rotor are known as squirrel-cage

induction motors.

(ii) Phase-wound or wound rotor : Motors employing this type of rotor are variously known as

phase-woundmotors or woundmotors or as slip-ringmotors.

Almost 90 per cent of induction motors are squirrel-cage type,

because this type of rotor has the simplest and most rugged

construction imaginable and is almost indestructible.


13/28

Power Stages in an Induction Motor

- Stator iron loss (consisting of eddy and hysteresis losses) depends on the supply

frequency and the flux density in the iron core. It is practically constant.

- The iron loss of the rotor is, however, negligible because frequency of rotor currents under

normal running conditions is always small. Total rotor Cu loss = 3

- Different stages of power development in an induction motor are as under :


14/28

SPECIAL MACHINES


15/28

Introduction

- This section provides a brief introduction to electrical machines which have special

applications. It includes machines whose stator coils are energized by electronically

switched currents, such as stepper motors. There is also a brief description of

servomotors. These motors are designed and built primarily for use in feedback control

systems.


16/28

Stepper Motors

- These motors are also called stepping motors or step motors. The name stepper is used

because this motor rotates through a fixed angular step in response to each input current

pulse received by its controller.

- In recent years, there has been widespread demand of stepping motors because of the

explosive growth of the computer industry. Their popularity is due to the fact that they can

be controlled directly by computers, microprocessors and programmable controllers.

- As we know, industrial motors are used to convert electric energy into mechanical energy

but they cannot be used for precision positioning of an object or precision control of speed

without using closed-loop feedback.

- Stepping motors are ideally suited for situations where either precise positioning or precise

speed control or both are required in automation systems.


17/28

Step Angle

- The angle through which the motor shaft rotates for each command pulse is called the step

angle

- Smaller the step angle, greater the number of steps per revolution and higher the

resolution or accuracy of positioning obtained.

- The step angles can be as small as 0.72 or as large as 90. But the most common step

sizes are 1.8, 2.5, 7.5 and 15.

- The value of step angle can be expressed either in terms of the rotor and stator poles

(teeth) Nr and Ns respectively or in terms of the number of stator phases (m) and the

number of rotor teeth.


18/28

Step Angle

- Resolution is given by the number of steps needed to complete one revolution of the rotor

shaft. Higher the resolution, greater the accuracy of positioning of objects by the motor

- A stepping motor has the extraordinary ability to operate at very high stepping rates (up to

20,000 steps per second in some motors) and yet to remain fully in synchronism with the

command pulses.- When the pulse rate is high, the shaft rotation seems continuous. Operation at high speeds

is called slewing. When in the slewing range, the motor generally emits an audible whine

having a fundamental frequency equal to the stepping rate. Iffis the stepping frequency

(or pulse rate) in pulses per second (pps) and is the step angle, then motor shaft speed

is given by

- Stepping motors are designed to operate for long periods with the rotor held in a fixed

position and with rated current flowing in the stator windings. It means that stalling is no

problem for such motors whereas for most of the other motors, stalling results in the

collapse of back emf (Eb) and a very high current which can lead to a quick burn-out.


19/28


20/28

Types of Stepper Motors

(1) Variable Reluctance Stepper Motor

(2) Permanent Magnet Stepper Motor

(3) Hybrid Stepper Motor

It has wound stator poles but the rotor poles are made of a ferromagnetic

material. It can be of the single stack type or multi-stack type which gives

smaller step angles. Direction of motor rotation is independent of the polarity

of the stator current. It is called variable reluctance motor because the

reluctance of the magnetic circuit formed by the rotor and stator teeth varies

with the angular position of the rotor.

It also has wound stator poles but its rotor poles are permanently

magnetized. It has a cylindrical rotor. Its direction of rotation depends on the

polarity of the stator current.

It has wound stator poles and permanently-magnetized rotor poles. It is best

suited when small step angles of 1.8, 2.5 etc. are required.


21/28

Servomotors

They are also called control motors and have high-torque capabilities.

Unlike large industrial motors, they are not used for continuous energy

conversion but only for precise speed and precise position control at high

torques.

Their basic principle of operation is the same as that of other electromagnetic

motors. However, their construction, design and mode of operation aredifferent.

Their power ratings vary from a fraction of a watt up to a few 100 W.

Due to their low-inertia, they have high speed of response. That is why they

are smaller in diameter but longer in length.

They generally operate at vary low speeds or sometimes zero speed.

They find wide applications in radar, tracking and guidance systems, process

controllers, computers and machine tools. Both dc and a.c. (2-phase and 3-

phase) servomotors are used at present.


22/28

Servomotorscontinues

Servomotors differ in application capabilities from large industrial motors in

the following respects :

1. They produce high torque at all speeds including zero speed.

2. They are capable of holding a static (i.e. no motion) position.

3. They do not overheat at standstill or lower speeds.

4. Due to low-inertia, they are able to reverse directions quickly.

5. They are able to accelerate and decelerate quickly.

6. They are able to return to a given position time after time without any drift.


23/28

Servomotorscontinues

These motors look like the usual electric motors.

Their main difference from industrial motors is that

more electric wires come out of them for power as wellas for control. The servomotor wires go to a controller

and not to the electrical line through contactors.

Usually, a tachometer (speed indicating device) is

mechanically connected to the motor shaft.

Sometimes, blower or fans may also be attached for

motor cooling at low speeds.


24/28

Problem 1: A hybrid VR stepping motor has 8 main poles which have been

castleated to have 5 teeth each. If rotor has 50 teeth, calculate the stepping angle.

Problem 2:A stepper motor has a step angle of 2.5. Determine (a) resolution (b)

number of steps required for the shaft to make 25 revolutions and (c) shaft speed,

if the stepping frequency is 3600 pps.

Solved Problems


25/28

Problem 3: A separately excited D.C. generator has armature circuit resistance of 0.1

ohm and the total brush-drop is 2 V. When running at 1000 r.p.m., it delivers acurrent of 100 A at 250 V to a load of constant resistance. If the generator speed

drop to 700 r.p.m., with field-current unaltered, find the current delivered to load.


26/28

Problem 4: A 440-V, shunt motor has armature resistance of 0.8 and field

resistance of 200 . Determine the back e.m.f. when giving an output of 7.46 kW at85 percent efficiency.


27/28

QUESTIONS AND ANSWERS ON SPECIAL

MACHINESQ.1. Do stepper motors have internal or external fans ?

Ans. No. Because the heat generated in the stator winding is conducted through

the stator iron to the case which is cooled by natural conduction, convection and

radiation.

Q.2. Why do hybrid stepping motors have many phases sometime more than six

?

Ans. In order to obtain smaller step angles.

Q.3. Any disadvantage(s) of having more phases?

Ans. Minor ones are: more leads have to be brought out from the motor, more

interconnections are required to the drive circuit and more switching devices are

needed.


28/28

Q.4. What is the main attraction of a multi-stack VR stepper motor ?

Ans. It is well-suited to high stepping rates.

Q.5. You are given a VR motor and a hybrid stepper motor which look exactly similar.

How would you tell which is which ?

Ans. Spin the rotor after short-circuiting the stator winding. If there is no mechanical

resistance to rotation, it is a VR motor and if there is resistance, then it is a hybrid

motor.

Q.6. How do you explain it ?

Ans. Since VR motor has magnetically neutral rotor, it will not induce any e.m.f. in the

shortcircuited winding i.e. the machine will not act as a generator and hence

experience no drag on its rotation. However, the rotor of a hybrid motor has magnetic

poles, hence it will act as a generator and so experience a drag.

QUESTIONS..conts

Documents

Motors Slides..PDF