International Islamic University,Islamabad

Electrical Machines LAB

EXPERIMENT # 9:

Demonstrate the Speed Torque Characteristics of Single Phase Induction Motor (Capacitor Start Type)

Name of Student: …………………………………..

Roll No.: ……………………………………………

Date of Experiment: ………………………………..

Report submitted on: ………………………………..

Marks obtained: ……………………………………

Remarks: ……………………………………………

Instructor’s Signature: ……………………………...

EXPERIMENT # 9:

Demonstrate the Speed Torque Characteristics of Single Phase Induction Motor (Capacitor Start Type)

Theory

Nature of magnetic field in the air gap due to single-phase stator windings is purely of pulsating type. As per classic theory such a field produces ZERO starting torque. Thus single phase induction motor does not start on its own. However, if the rotor is given an initial rotation in any direction, the resultant average torque increases in that direction and motor starts rotating and gives almost similar performance as that of 3-phase induction motor. In case of Capacitor start type motor, introduction of capacitor is high resistance auxiliary winding generates the rotating magnetic field at the time of starting and makes the motor self-starting.

As shown above along with main winding, the stator also carries auxiliary winding which is lightly resistive and connects to external series capacitor. These two windings are displaced in

space by 900with respect to each other. The rotor as shown is of squirrel cage type. As per

theory, the rotating magnetic field is created in space if two alternating fluxes are present which some phase difference between them. In this case, main winding is highly inductive, as such the current through it lags voltage by large angle. Whereas in case of auxiliary winding because of series capacitor, the current leads voltage. The fluxes produced by these currents accordingly

have phase difference between them and as a result rotating magnetic field is created due to which rotor starts rotating.

The auxiliary winding cannot carry load current continuously. A centrifugal switch is provided in series with this winding which operates at about 80% of full speed and as shown disconnects the auxiliary winding. The starting torque is proportional to angle of split between two currents. Generally about 400% of full load torque can be made available as starting torque by selecting proper value of capacitor. The direction of rotation can be easily changed by reversing the terminals of either of the windings. The motor finds applications in fans, blowers, washing machines, compressors, refrigerators where the motor has to start against load.

Connection Diagram

Please see the figure on the next page.

Procedure

1. Couple and align the single phase AC integrated motor with DC generator.2. Note here that 230V/50Hz is passed through EMT16 (4A MCB) for protection and applied to

auxiliary winding, further through EMT20F for metering, through start capacitor of 32uF and again through start push button provided on EMT16. The same supply is also applied to main winding on stator.

3. Another connection of 230V AC is given to dimmer to rectifier to generator DC which will excite the DC generator.

4. DC generator armature is connected to Voltmeter and ammeter to measure the load voltage and current.

5. Make the connections as per wiring diagram.6. The AC motor will run in capacitor start mode as soon as the start push button on EMT16 is

pressed.7. Observe that initially the EMT20F will show very high current till the motor picks up speed.

(Spend minimum time between putting ON MCB and pressing start PB on EMT8 to avoid damage to meter).

8. After the motor is started, load the DC generator which will in turn load the AC motor.9. Now take down the observation as mentioned in the observation table.

Observations

S. No

Input Voltage Input Current

DC Generator Voltage

DC Generator Current

Speed Torque