AC Servomotors

AC Servomotors

 Q. What is AC Servomotor?

 Q. What is meant by AC servo motor?

 Q. How does an AC servo motor work?

 Q. What are the applications of ac servo motor

• In automatic control system it becomes necessary to compare system parameters with some reference.
• Whatever is the error it is amplified and used to drive motors known as servomotors.
• These servomotors are coupled to shaft of the output.
• Thus signal error is converted into angular velocity to correct the error. Thus servomotors are not used to drive any load as usual motors.
• But they are used to correct position or velocity. Depending upon type of supply used servomotors are classified into :

1. A.C servomotor and
2. D.C servomotor

A.C. Servomotor :

Construction of AC Servomotors :

• Construction of ac servomotor is shown in Figure A.
• Basically it is an induction motor with two windings provided on the stator.
• These two windings are connected to two voltage sources which have 90° electrical phase difference.
• Due to this phase difference a rotating magnetic field is produced.
• One of the windings known as reference winding or main winding is connected to constant magnitude A.C. voltage.
• The other winding is connected to voltage obtained from servo amplifier.
• The rotor can be ordinary cage type rotor with aluminium bars embeded in slots and short circuited by end rings.
• For maximum flux linking air gap kept is minimum.
• In other type of rotor a drag cup is used due to which inertia of rotating system becomes low.
• This helps in reducing power consumption.
• The ac servomotors are used in the frequency range of 50 Hz to 400 Hz and from milliwatts power consumption to few hundred Watts.

Figure A

Speed control of AC Servomotors :

• The reference winding is connected to a constant AC voltage. But the control voltage from a serve amplifier is applied to the control winding.
• The speed of an AC servomotor can be changed by varying the AC voltage applied to the control winding.
• For a constant torque load, the speed of AC servo motor increases with increase in the voltage applied to the control winding

Closed Loop Speed Control of AC Servomotor :

Block diagram of AC Servomotors :

• Figure B shows the block diagram of a closed loop speed control system for an AC servomotor.

Figure B

Operation of AC Servomotors :

• The block diagram of an ac servo drive is as shown in Figure B. This circuit uses the speed feedback to keep the speed of the motor constant at the desired value.

1. Speed command :

• This potentiometer adjusts a voltage (generally between 0 and 10 V) which represents the set speed or the desired speed.
• 0 V represents minimum speed and 10 V represent the maximum desired speed.

2. Speed feedback :

• An encoder is used for producing the speed feedback signal. It is mounted on the shaft of the main motor.
• So its speed is same as that of the main motor. The encoder produces a dc voltage (0 - 10 V) which is proportional to the actual speed of the motor. This voltage is used as the speed feedback signal as shown in Figure B.
• The speed command is compared with the speed feedback signal by a comparator.
• Output of the comparator is "error in speed" which is applied to the speed controller.
• The controller produces a dc control voltage such that the error in speed is reduced to zero.
• The dc control voltage obtained at the output of controller is applied to an inverter.
• The firing pulses at the output of controller are applied to the SCRs connected in the inverter to produce a variable control voltage in order to operate the motor at the desired speed.