Brake Test

Brake Test

AU: Dec.-07

• 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. By 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. 5.6.1 (a) shows the experiemental setup for performing brake test on a d.c. shunt motor while the Fig. 5.6.1 (b) shows the belt and pulley arrangement mounted on the shaft of the motor.

• 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 in metre

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.

Net pull = W₁- W₂ kg = 9.81 (W₁-W₂) N

• As radius R and speed N are known, the shaft torque developed can be obtained as,

T_sh = Net pull x R = 9.81 (W₁-W₂) R N-m

Hence the output power can be obtained as,

T_sh = Net pull x R = 9.81 (W₁ - W₂) R N-m

Now let, V = Voltage applied in volts

I = Total line current drawn in amps.

Then P_in = VI W

• Thus if the readings are taken on full load condition then the efficiency can be obtained as,

% η = P_out / P_in × 100

• 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. 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.

2. Disadvantages

The disadvantages of brake test,

1. Due to friction, heat is 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.bit of

5. The efficiency observed is on lower side.

Ex. 5.6.1 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 = 110 V, Speed = 1320 r.p.m., The radius of the pulley is 7.5 Cm. Calcualte its full load efficiency.

Sol. :

Review Questions

1. Explain how to obtain efficiency of a d.c. machine by conducting brake test. State its advantages and disadvantages.

2. What precaution is necessary to be observed while performing brake test on a d.c. series motor? AU: Dec.-07, Marks 3