Monday, October 17, 2011



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

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



  1. why dont we connect mechanical load to ac servomotor?

    1. we can..A servo could be considered as a small device that has a shaft for the purpose of output. The shaft’s angular moment could be regulated by the sending of a signal (coded) to the servo. The angular position of the output device, which is the shaft, would be maintained until the servo keeps receiving signals. By changing the signal sent, the position of the shaft could also be regulated...

      untill and unless the load is constant....


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