Electrical Machines

ETLS 747 – 01

Dr. Cal E. Hardie

05/18/2015

Practical view of AC Servo motor, AC

induction motor, and speed control in

industry

Ahmed Elhadeedy

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Abstract

One of the most known companies in the field of liquid packaging industry which manufactures those

machines is Tetra Pak. One of the main components in these machines are servo motors and AC motors.

Movement in a specific part of the machine is made using servo motors which it’s sensitive and it should

be precise movement because any lag or lead in this movement may lead to crash costly parts in these

machines.

We will discuss both AC and servo motors speed control introducing the classic way control and

showing the method that is used nowadays in the factories.

Some ways just use classic controls for high and low speed, and there is some devices like frequency

converter

When you have a new motor in a factory and you want to install it, there are very important

questions to be asked:

1- How this motor will be connected?

2- What is the most suitable practical way to start it?

3- What the easiest and the most convenient way to control it’s speed?

Introduction

Previously in the past few decades, speed control of the motors weren’t precise or helpful in the

automation. Nowadays, motors are mainly used in automation through speed control. In this term paper

we will discuss two types of speed control for two types of motor which is used widely in industry: AC

motors and servo motor. We will discuss briefly the construction of the two motor and we will take a

look on the system and the circuit of each motor in of the machine.

We will talk about a practical view of the Ac servo motors, AC induction motors, and speed controlling

in industry. We will discuss the construction of the servo motor and its whole control system. Common

starting methods and speed control of the AC induction motor will be discussed in this paper.

We will discuss the different ways of speed controlling like changing (voltage, number of poles, and

frequency), and which one is used in industry and why?

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Servo motor system overview

Figure 1: Servo system overview

Servo Motor

1- An AC servomotor is basically a two phase induction motor except for certain special

design features. A two phase servomotor differs in the following two ways from a normal

induction motor.

2- The stator has two windings.

3- The excitation voltage applied of two stator windings should have a phase difference of

90°.

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Figure 2: a) Induction motor b) Servo motor

The AC servomotor is basically a two phase induction motor with some special design features. The

stator consists of two windings (Power and control)

mounted in the stator, and there is a phase shift of 90° between the two windings. Each

One winding is called reference winding and the other is called a control winding. The exciting current

in the winding should have a phase displacement of 90°. The supply used to drive the motor is single

phase and so a phase advancing capacitor is connected to one of the phase to produce a phase difference

of 90°. The stator constructional features of AC servo motor are shown in figure 1.

Figure 4: Cup Rotor (Smaller Applications)

Figure 3: Cup rotor

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Figure 5: Squirrel cage servo motor rotor

Two types of the rotor of induction AC servo motor:

1- Hollow cup rotor (Smaller applications – low power)

2- Squirrel cage rotor (industrial applications)

- Aluminum bars which are shorted at the ends

- Similar design to squirrel cage induction motor

- Small diameter and long length to reduce moment of

inertia

- Aluminum conductors to reduce the motor’s weight

-

Principle of operation:

1- There is a phase difference between the voltage applied to the reference windings and the voltage

applied to control winding (90 degree)

2- The resulting magnetic field induces an e.m.f in the rotor, and the current in the rotor produces

it’s own flux

3- Rotor flux interacts with the rotating magnetic field and the rotor starts to rotate.

- AC amplifier: to make the phase shift

- Actual output : (sensors of positions)

- Potentiometer (error detector): to compare between the actual position and the desired position.

- Feedback inside the motor itself (angle encoder)

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Angle encoder: One of the unique things about the servo motors that

they do have an angle encoder which gives precise data

about the current position of the rotor

Figure 6: Angle Encoder

- How does angle encoder works? The angle encoder senses the angle position of the motor and

feeds this information back to the control unit. The angle encoder converts the angle into digital single

(for example: 0 and 1s).

This can be done using the code disk which it’s attached to the rotor shaft. Figure 7: code disk

Figure 8: Reading the code disk

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On one side of the code disc there are light-emitting diodes, and on the other side of the disc there are

light sensors (phototransistors) which works as follows

Phototransistor Output

Light received 1

No light received 0

So every position of the rotor and the attached disc will be transformed form an angle into a digital

output. For example (55º = 1101010010110) so the control unit can deal with this value because PLC

won’t understand the value 55 º.

Servo motors are used mostly in:

- Robotics

- Handling system

- Packaging technology

A Servo motor is known with high-torque unlike large industrial motors. Also known for its precise

speed and precise position control at high torque. This kind of motor has a high speed of response

because of low inertia.

Advantages:

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

2. They are capable of holding a static position.

3. They do not overheat.

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

5. They are able to accelerate and deaccelerate quickly.

6. High positioning accuracy

7. Compact design

8. low weight

9. less maintenance

10. computerized

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Why AC and not DC servo motor in industry (Bigger sizes)

1- DC are expensive (AC has simple construction)

2- Ac doesn’t have Commutation (Carbon brushes)

3- DC needs more maintenance

4- No maintenance needed for AC servo

5- less Friction losses in the AC servo motor

3-phase Induction AC motor advantages:

1- Simple construction

2- Cheap

3- Self-starting torque (that’s why it’s more

common and it’s widely used)

4- Low cost maintenance

Figure 9: Industrial AC motor system

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Motor construction:

The 3-phase motor circuit consists of:

1- Circuit breaker (disconnect) :

A circuit breaker is an automatically operated electrical switch

designed to protect an electrical circuit from damage especially

the wires. It works as a 3-phase power distributor and it

interrupts high amperes (short circuit)

Figure 10: Motor circuit and connection

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2- Contactor

A contactor is an electrically controlled switch used for switching a

power circuit, and it has more points to help in the control usage, like

activating a valve when the motor starts, and it doesn’t interrupt the

short circuit current.

3- Over load relay

It’s a switch is designed to open the circuit when the motor

starts to draw higher amperes, this relay is made to protect the

motor and the windings.

Basic 3-phase Induction motor connections

All induction motors has a small terminals box, which the terminals of the windings are.

It consists of 6 terminals, and we put the 3-phase power (3 wires) on 3 of them, and then

connect every two points as desired.

Figure 11: Motor terminals Star connection (left), and Delta connection (right)

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Figure 12: Motor plate

When to connect the induction motor as Wye or Delta?

Depends on the used voltage, and the rated voltage of the motor

Wye Delta

380V/1.7A 220V/3A

Low starting current High power motors,

the starting current will be 7:10 times the rated

current and it may cause the burn of the windings

For motors of power Less than 10kw For motors of power more than 10 kw

Lower volt

Higher volt Higher starting current

Common motor starting methods:

When the motor starts to run, it draws a high current (7 to 10) times of the rated motor current. This high

value of the current causes :

1- The temperature of the windings increases (windings may burn).

2- The voltage of the other devices may decrease.

3- Damage of cables, contactors, and protection devices.

Methods of starting :

1- Direct on-line starter :

Connecting the motor directly to a 3-phase power source. The starting current won’t be

reduced so it’s used with the motor of power of less than 5Kw

2- Stator resistance starter :

This method is done by adding variable series resistances to the stator. At the starting phase

the resistances value will be high, so the voltage and the current could be reduced, and after

the motor starts to run, those resistances could be reduced or removed.

Disadvantage: the copper losses increases due (I^2 * R)

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3- Wye-Delta starter

Used with the motors of 6-terminals, and usually runs on Delta connection, but starting the

motor with delta will be a problem because of the high current, so the motor starts on Wye

and then switches to Delta. (especially with high power motors)

Starting with Wye:

- The phase voltage reduces to : 1/√3 of the source voltage.

- The current is being reduced to: 1/3 the value of current in Delta connection. (about 67%

reduction)

After the motor reaches 70% or 80% of its rated speed, it switches to delta connection.

4- Autotransformer

This method is done by connecting the stator terminals to a 3-phase to a transformer to

reduce the voltage to get a suitable value of the starting current.

Disadvantage: expensive device.

Speed control comparison:

R2 is constant

s is small then sX2 is so small that it can be neglected.

T ∝ sE22

E2 is rotor induced emf

E2 ∝ V

Then T ∝ V2 and T ∝ 1/Ns

When the voltage decreases, the torque decreases, and speed decreases.

Disadvantages:

1) A large change in supply

voltage is required for

relatively small change in

speed.

2) If an autotransformer is

used to change the voltage,

it will be expensive and it

will take large space.

b) By changing the applied frequency

Synchronous speed of the rotating magnetic field of induction motor is given by

f = frequency of the supply

P = number of stator poles.

This method is widely used when we have a frequency inverter to convert the

voltage of 60 Hz to a voltage of controlled frequency.

1- We can get any

desired speed by

changing the

frequency even if

it’s more than

60Hz.

2- Shown in figure 14

that the motor

reaches the

maximum torque

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Waveforms with variable frequency

Figure 13: wavefroms of voltage of different frequencies

Figure 14: The relation between Toruq and Speed in when the frequency of stator

changes.

before the

synchronous speed.

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c) Changing the number of stator poles

From the equation above, if we changed the number of poles we can control

the speed

a stator is wound with two 3phase windings, one for 4 poles and other for 6

poles.

for supply frequency of 50 Hz

1- If 4 pole winding is connected, Ns = 120*50/4 = 1500 RPM

2- If 6 pole winding is connected, Ns = 120*50/6 = 1000 RPM

1- This method is not

practical way to use

because there is limits

on the speed.

2- Two or 3 speeds only.

Conclusion: This paper introduced two essential types of motors which are widely used in industry, and it

answers two main questions in this field. If you got a new motor in a factory what is the

most suitable way to connect it: Wye or Delta. The other question is based upon the

application of the motor which is what is the most convenient way to control the speed of the

motor?

Future: I guess the future of those two types of motors will improve, and this improvement will be

related to technology improvement. For example:

1- For AC induction motors which is made for industrial use, probably those motors will

have a built-in frequency inverter as a small box, which will reduce the price and space,

but still its related to the future of the electronic circuits and it’s size, which is going

smaller.

2- For AC servo motor, it may be more computerized; it may have a processor and an

embedded control unit to skip having a unit between the PLC modules and the motor

References:

1- Electrical Machines II, Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao, Indian

Institute of Technology Madras.

2- Electricity magazine , Servo motors technical paper number 64, Prof. Dr. Fathy abdelkader, Electrical

engineering and machine department, university of shiben Elkom.

3- Technical guide No. 4 (ABB guide to variable speed drives) 2011.

4- http://yourelectrichome.blogspot.com/2013/01/ac-servomotor.html

5- Google images