Another method of testing the d.c. motor is brake test method. This is a direct method of testing the motor. In this method, the motor is put on the direct load by means of a belt and pulley arrangement. Bu adjusting the tension of belt, the load is adjusted to give the various values of currents. The load is finally adjusted to get full load current. The power developed gets wasted against the friction between belt and shaft. Due to the braking action of belt the test is called brake test.
The Fig. 1(a) shows the experimental setup for performing brake test on a d.c. shunt motor while the Fig. 1(b) shows the belt and pulley arrangement mounted on the shaft of the motor.
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| Fig. 1 Brake test |
The tension in the belt can be adjusted using the handle. The tension in kg can be obtained from the spring balance readings.
Let R = Radius of pulley inmeter
N = Speed in r.p.m.
W1 = Spring balance reading on tight side in kg
W2 = Spring balance reading on slack side in kg
So net pull on the belt due to friction at the pulley is the difference between the two spring balance readings.
As radius R and speed N are known, the shaft torque developed can be obtained as,
Hence the output power can be obtained as,
Now let, V = Voltage applied in volts
I = Total line current drawn in amps.
Thus if the readings are taken on full load condition then the efficiency can be obtained as,
Adjusting the load step by step till full load, number of readings can be obtained. The speed can be measured by tachometer. Thus all the motor characteristics can be plotted.
1.1 Advantages
The advantages of brake test,
1. Actual efficiency of the motor under working conditions can be found out.
2. The method is simple and easy to perform.
3. Can be performed on any type of d.c. motor.
1.2 Disadvantages
The disadvantages of brake test,
1. Due to friction, heat generated and hence there is large dissipation of energy.
2. Some type of cooling arrangement is necessary
3. Convenient only for small machines due to limitations regarding heat dissipation arrangements.
4. The power developed gets wasted hence method is expensive.
5. The efficiency observed is on lower side.
Example :
In a brake test conducted on a d.c. shunt motor the full load readings are observed as,
Tension on tight side = 9.1 kg
Tension on slack side = 0.8 kg
Total current = 10 A
Supply voltage = 100 V
Speed = 1320 r.p.m.
The radius of the pulley is 7.5 cm.
Calculate its full load efficiency.
Solution :
W1 = 9.1 kg, W2 = 0.8 kg, I = 10 A, V = 110 V, R = 7.5 cm
Tsh = (W1 - W2) 9.81 x R = (9.1 - 0.8) x 9.81 x 0.075
= 6.1067
Pout = Tsh x ω = Tsh x (2πN/60)
= (6.1067 x 2π x 1320/ 60) = 844.133 W
Pin = VI = 110 x 10 = 110 W
%η = (/) x 100 = (844.133/1100 ) x 100
=76.74 %
Armature Voltage Control Method or Rheostatic Control
Three Point Starter
Brushless D.C. Motor
3. Convenient only for small machines due to limitations regarding heat dissipation arrangements.
4. The power developed gets wasted hence method is expensive.
5. The efficiency observed is on lower side.
Example :
In a brake test conducted on a d.c. shunt motor the full load readings are observed as,
Tension on tight side = 9.1 kg
Tension on slack side = 0.8 kg
Total current = 10 A
Supply voltage = 100 V
Speed = 1320 r.p.m.
The radius of the pulley is 7.5 cm.
Calculate its full load efficiency.
Solution :
W1 = 9.1 kg, W2 = 0.8 kg, I = 10 A, V = 110 V, R = 7.5 cm
Tsh = (W1 - W2) 9.81 x R = (9.1 - 0.8) x 9.81 x 0.075
= 6.1067
Pout = Tsh x ω = Tsh x (2πN/60)
= (6.1067 x 2π x 1320/ 60) = 844.133 W
Pin = VI = 110 x 10 = 110 W
%η = (/) x 100 = (844.133/1100 ) x 100
=76.74 %
Related Articles :
D.C. Shunt Motor Armature Voltage Control Method or Rheostatic Control
Three Point Starter
Brushless D.C. Motor
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Why this much preferable to dc shunt motor only
ReplyDeleteIs it possible to perfom on the generators?
ReplyDeleteR only on motors?
what is the reason?