CHARACTERISTICS OF AC SERVO MOTOR

8. CHARACTERISTICS OF AC SERVO MOTOR

AIM: To study the characteristics of AC servo motor.

Front panel details:

1. Power : main ON/OFF switch to the unit with built in indicator.

2. RPM : tachometer to display to RPM

3. Ammeter : Ammeter to measure the DC motor armature current

4. Servomotor

ON/OFF : AC supply ON/OFF switch to the servo motor.

5. Load ON/OFF: ON/OFF switch to load the motor.

6. R : Potentiometer to vary the load 500 ohms/25watts.

7. Vdc : 12V unregulated DC supply to DC motor

8. Eb Terminals to measure the back EMF

9. Control winding: Control winding terminals of AC servo motor

10 Reference winding: Reference winding of AC servo motor.

11. Control voltage: Auto transformer to vary the AC supply to control

winding.

TABLE TO PLOT SPEED Vs BACK EMF:

Sl no ------- Speed(rpm)------- Backemf(volts)

. . .

. . .

. . .

PROCEDURE:

1. Study all the controls on the front panel.

2. Initially keep load control switch at OFF position, indicating the armature

circuit of dc machine is not connected to auxiliary dc supply – 12V keep

servo motor supply switch also at off position.

3. Ensure that load potentiometer and control voltage auto transformer at

minimum position.

4. Now switch on mains supply to the unit and also AC servo motor supply

switch vary the control voltage transformer. You can observe that the AC

servo motor will stars rotating and the speed will be indicated by the

tachometer in the front panel.

5. With load switch at OFF position switch ON AC servomotor and keep the

speed in the minimum position. You can observe that the AC servomotor

starts moving with speed being indicated by the tachometer and set the

speed for maximum speed. Now switch on the load switch and start

loading AC servo motor by varying the laod potentiometer slowly, Note

down the corresponding values of Ia and speed and enter these readings in

the table. And also note down the control panel.

S.No ----Ia(mA) ----N(rpm)---- P(watts)----Torque(Gm-cm)

. . . . .

. . . . .

. . . . .

. . . . .

Repeat for Vc=3/4th of 230V

Draw the graph of torque Vs speed. ....K2=DT/DN

Determine the motor constant K1:

1. Apply rated voltage 230V to control winding.

2. Apply load on the motor gradually till the motor wil run in the RPM.

3. Note down I and calculate torque

4. Decrease the load on the motor slightly the motor will run at certain rpm.

5. Reduce Vc slightly till the speed the motor comes to – N rpm

6. Repeat different loads.

7. Repeat for N2 rpm.

Draw the graph of Torque V/S VC : K1= DT/DVc

Ia measured by ammeter which is connected in series with the power supply &

variable resistance (load control). This method does not take in to the account

the no load torque developed by the ac servo motor. To measure torque

developed at no load (i.e. torque just required to rotate rotor of ac servomotor,

rotor of dc motor) the ac servomotor is switched off. Now the dc machine run

as the help of dc power supply, speed will be controlled by variable resistance

again we have to effect the measurement of Ia for a given speed. From the

product of Eb (back emf developed by the motor) and the armature current

taken, we can find the mechanical power developed at the shaft. Again we

must use the formula.

P= 2pi*NT/60

Torque = P*1.019*10^4*60/2pi*N

For various speeds, we can note down the no load torque required to be

developed by the motor. This torque is negligible & may not be taken in to

account for normal testing.

Torque calculation for a sample data:

Ia =0.17A

Speed N= 850 rpm

For speed 850rpm- Eb = 0.96V

Therefore power P = Eb*Ia = 0.96*0.17 = 0.1632 watts.

T= P*1.019 * 10^4*60/2piN

T = 0.1632*1.019*10^4 *60/2*3.142*850

T = 18.68 Gm-cm

GRAPHS: Respective graphs are drawn separately.

RESULT: