EEP 305 ELECTRIC DRIVES LABORATORY

Experiment 3 Braking Mode of Operation of 3-phase

Induction Motor: DC dynamic braking

Low voltage plugging Capacitor braking

Diode braking

GROUP 3 Anupam Sharma 2009EE20443 Apoorv Chaturvedi 2009EE20444 Bhavesh Mishra 2009EE20446 Dhruv Mishra 2009EE20447 Gagan Gaurav 2009EE20448

18 January, 2012

Aim: Braking Mode of Operation of 3-phase Induction Motor:

a. DC dynamic braking b. Low voltage plugging c. Capacitor braking d. Diode braking

Apparatus:

Fluke Multimeter Ammeter (0 – 20 A) Tachometer Stop watch Capacitor banks Diodes Two way three phase switch 3-phase Induction Motor

Machine Ratings:

AC side: o Manufacturer: MAWDSLEY’s Ltd., Dursley, England o Continuous Rating o 5 bhp o 50 cycles o 1440 rpm o 440 Stator volts o 8.2 A Stator o 16.5 A Rotor o - Y

Circuit Diagrams:

DC Dynamic Braking:

Low Voltage Plugging:

Capacitive Braking:

Diode Braking:

Procedure:

1. Perform Stator Resistance Test and calculate the Stator Resistance of the machine. a. Measure resistance between the three phases of the stator. b. Take their average.

2. Perform Retardation Test to get the moment of inertia of the machine. a. Run the motor at rated speed. b. Cut off the supply and note the time the motor takes to stop.

3. Perform No load test to calculate Rc and Xm of the machine. a. Run the motor at rated speed. b. Note the power, current and voltage.

4. Perform Blocked Rotor Test and calculate Re and Xe of the machine. a. Block the rotor of the machine. b. Slowly increase the voltage applied to the machine. c. Note the values of voltage, current and power.

5. DC Dynamic Braking: a. Run the motor at the rated speed. b. Switch off the a.c. supply and apply DC voltage to the motor. c. Repeat the above for 2-lead and 3-lead connections.

6. Low Voltage Plugging: a. Run the motor at rated speed. b. Interchange the connection of two phases and apply a low voltage. c. Repeat for two sets of voltages.

7. Capacitive Braking: a. Run the motor at rated speed. b. Switch off the supply and connected three phase D connected capacitor

(uncharged) across the machine. c. Repeat for two sets of capacitor banks.

8. Diode Braking: a. Run the motor at rated speed. b. Put diodes in the supply side of two phases. c. Repeat for two sets of voltages.

Experimental Observations:

No Load Test:

Speed = 1480 rpm

S. no. W (Watts) Inl Vnl

1. 375 2.6 A 402.8 V

Stator Resistance Test:

Multimeter readings of resistance:

R – Y : 5.4 Y – B : 5.4 R – B : 5.4

Retardation Test:

Time taken by the motor to stop, t = 16 s

Power, PL = 375 W

Blocked Rotor Test:

Speed = 1480 rpm

S. no. Vblock Iblock W (Watts)

1. 111 V 8.06 A 0.52 kW

DC Dynamic Braking:

S. no. V Connection Starting speed Time (seconds)

1 25 2 lead 1480 rpm 4.5

2 25 3 lead 1480 rpm 3.0

3 51 2 lead 1480 rpm 2.0

4 51 3 lead 1480 rpm 1.0

Low Voltage Plugging:

Starting Speed = 1480 rpm

S. no. V Time (in seconds)

1. 30 V 13.51

2. 50 V 10.50

Capacitor Braking:

Capacitor C1: 7.5 kVAR, 440 V, 9.9 A, 9.5 kg, D connected

Capacitor C2: 5 kVAR, 440 V, 6.6 A, 7 kg, D connected

s. no. Capacitor used Braking Time (in seconds)

1. C1 8

2. C2 12.4

Diode Braking:

S. no. Voltage Braking Time (s)

1. 30 V 12

2. 50 V 8

Calculations:

PNL = 375 W

Prot = 375 – 3x(2.6/3)2x5.4

= 338.5W

RNL = 375/{3x(2.6/3)2}

= 55.47

ZNL = 402.8/(2.6/3)

= 268.33

XNL = 262.53

X1 + Xm = 262.53

RBL = PBL/3I2 = 520/8.062 = 8

R2’ = 8 – 5.4 = 2.6

ZBL = 111/(8.06/3) = 23.85

XBL = 22.47 = X1 + X2’

X1 = X2’ = 11.235

Xm = 251.295

d/ dt = 9.68

J = 375/(154.9x9.68)

= 0.2498 kg/m2

Graphs:

Running Retardation Test

Starting Plugging @ 30 V

Plugging @ 50 V Diode braking @ 30 V

Diode Braking @ 50 V Capacitor (5kVAR) braking

Capacitor (7.5 kVAR) braking DC dynamic braking (2 lead) @ 30 V

DC dynamic braking (2 lead) @ 50 V DC dynamic braking (3 lead) @ 30 V

DC Dynamic Braking (3 lead) @ 50 V

Simulation and Results:

Plugging:

Inference:

The electrical methods of braking are better than the mechanical methods of braking. The reasons are as follows:

1. Controllability of speed and braking torque.

2. Wear and Tear is less as compared to the mechanical braking.

3. Mechanical braking methods occupy larger space while electrical braking may not use any extra space (e.g. Plugging requires only phase reversal).

4. The efficiency of electrical braking is better than mechanical efficiency.

5. Electrical Braking causes less pollution than mechanical braking methods.

Conclusion:

Methods for producing internal braking can be divided into two groups: counter torque (plugging) and generating (dynamic and regenerative braking). In the counter torque group, torque is developed to rotate the rotor in the opposite direction to that which existed before braking. In the generating group, torque is developed from the rotor speed. Dynamic torque is developed by motor speeds that are less than the synchronous speed, and energy is dissipated within the motor or in a connected load. Developed torque is zero at zero rotor speed. For large motors, forced ventilation may be required during braking.

Comparison of ac dynamic braking- 2 lead and 3 lead:

In 2 lead connection, at high speeds (or at low values of slip), the impedance of positive sequence component part becomes very high. As positive and negative sequence components of current have to be equal, net breaking torque is small, and therefore, breaking is not very effective.

3 lead connection: in contrast to 2 lead connection, here magnitude of positive and negative sequence components of voltage are equal and not that of current. Positive and negative sequence parts of the circuit are independent, and therefore, there is no restriction imposed on negative sequence component of current by positive sequence part of equivalent circuit. Thus higher braking torques are obtained (compared to 2 lead case) at high speeds.

References:

Electrical Machinery by Fitzgerald Kingsley Umans Electrical Drives by GK Dubey MATLAB Simulink references