load test on scharge motor

7/13/2019 Load Test on Scharge Motor

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

ELECTRICAL MACHINES LABII

Load Test On Scharge Motor Page 1 of 4

LOAD TEST ON SCHARGE MOTOR

1. AIM:

(a) To study speed variation vs brush separation.(b) To perform load test on schrage motor at (i) subsynchronous speed and (ii) super synchronous

speed.

(c) To draw the following performance characteristics under the above conditions of operation

and compare them critically.

(i) Efficiency vs power output (ii) speed vs Torque. (iii) power factor vs Torque

2. APPARATUS:

S. No. Name Type Range Quantity1.

2.

3.

4.

Voltmeter

Ammeters

3 phase Variac

Lamp bank load

MI

MI

-

resistive

0-300/600V

0-20 A

400/0-400 V, 25 A

250 V, 3 Kw

1

5

1

2

3. THEORY:

The scharge motor is basically an invited three phase induction motor, in which 3-phase primarywinding is on the rotor, where as the 3 phase secondary winding is on the stator. The rotor of this machine

in addition to the main primary winding also carries a tertiary winding, housed in the same set of slots. The

tertiary winding is a closed commutator winding, the tappings of which are connected to the bars of the

commutator.The terminals of secondary winding on the stator are connected to set of brushes placed on the

commutator, so that any desired voltage from the tertiary winding can be injected into the circuit of the

secondary winding. As a result, the speed of the scharge motor changes, depending upon the magnitude anddirection of the injected emf into the secondary winding, thus achieving variable speed control with shunt

characteristics.

This motor has an additional advantage that input power factor can also be controlled along withcontrol speed. Commercially, these motors are normally built with speed range from about 50 percentbelow to about 50 percent above synchronous speed. Fig (2) shows the complete details of various

windings of scharge motor as per the above discussion.

Two brushes of each set connected to one phase of the secondary winding (i.e. a 1, a2) can be movedsimultaneously and in opposite direction by gearing arrangement provided with the motor. Fig (1) a, b, and

c illustrate three possible modes of action of this motor. In Fig (1)a, two brushes of a set (a 1, a2) are on the

same commutator bar, as such injected emf in the secondary is zero and the motor runs as an ordinaryinduction motor at nearby synchronous speed, with secondary winding short-circuited through the

commutator bar. Fig (1)b, corresponds to operation of the motor in synchronous region, where as Fig (1)c

clearly illustrates that the motor will run at super synchronous speeds.


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It may be observed that in all the above three diagram, the axis of the brush and the axis of secondarywinding remains the same. However, the phase angle of the injected voltage with respect to the secondary

induced voltage may be changed by shifting both the brushes a1, a2 together in one direction as clearly

shown in Fig (1)d. This causes a shift between the axis of the secondary winding and the brush-axis, which

provides a control on the input power factor.

Fig 2 : Speed and power factor control in scharge motor

The efficiency and the power factor of this motor are higher, when being operated at higher speed,

compared to its operation at lower speed.

The main disadvantage of this motor is its limited power handling capacity and the operating voltage.

Thu supply voltage is limited to 600 volts for reliable operation, because primary winding on the rotor is

fed through sliprings. Power handling capacity is limited to 25 KW per pole-pair, because of commutationproblems.

4. CIRCUIT DIAGRAM:

Fig 2 : Load test on 3-phase scharge motor


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5. PROCEDURE:

1. Connectthe circuit diagram of Fig 2.

2. Ensure that the brushes are set to minimum speed position, with the brush-axis in the neutral

Position. Moreover, there should not be any mechanical load on the motor shaft at the time of starting.

3. Switch-on the 3-phase ac supply and start the motor, using the direct-on line starter.

4. Record the brush separation, preferable in terms of electrical degrees and motor speed, after the speed of

the motor has become steady.

5. Reduce the brush separation and record the speed of the motor as well as the brush separation in

electrical degrees.

6. Repeat step no. 5 for various values of brush separation, till the brushes of one set are on the same

commutator bar i.e. short circuiting the secondary winding. It may be observed that under this condition,

the motor should run at a speed slightly lower than the synchronous speed.

7. Increase the brush separation in a direction opposite to that in step 4, 5 and 6, so that the scharge motor

runs in the region of super-synchronous speed. Now repeat step no. 5 for different brush separation.

8. Adjust the speed of the scharge motor to the desired value, at which load test is to be performed i.e.

subsynchronous speed, preferably 2/3 of the synchronous speed.

9. With the motor running on no load, record, current drawn, applied voltage, wattmeter readings and the

speed.

10. Increase the load torque gradually and record the readings of all the meters included in the circuit as

well as the speed for various settings of the load, till the current drawn is of rated value.11. Reduce the load on the motor gradually.

12. Adjust the speed of the motor in the super-synchronous region, preferably 4/3 of the synchronous

speed.

13. With the above brush setting, repeat step no. 9, 10 and 11.

14. Switch-off the ac supply to stop the motor.

6. TEST READINGS:

S.No.

Appliedvoltage

Inputcurrent

Wattmeter readings Spring balance readingsW1 W2 T1 T2 Speed


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11. RESULTS:

Load test on schrage motor at (i) subsynchronous speed and (ii) super synchronous speed is

performed.

12. PRECAUTIONS:

1. Loose connections should be avoided.2. Readings are taken without parallax error

13. VIVA QUESTIONS:

1.Why the primary winding of scharge motor is placed on the rotor?

2. What is the major function of the tertiary winding, provided in scharge motor?

3. Discuss various industrial applications of scharge motor.4. Show by suitable diagram, how speed control can be achieved in subsynchronous and super synchronous

and super synchronous regions.5. Discuss with suitable diagram, how power factor improvement is achieved in scharge motor.

6. Compare the efficiency of scharge motor at full load with the efficiency of 3-phase plain induction motorof same rating and at full load.

load test on scharge motor

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