Brake Test On DC Shunt Motor

In this article, we are going to discuss brake test on d.c. motor.Brake test is also a method of finding the efficiency of dc motors.We took dc shunt motor as running machine. Brake test also called for direct loading test of testing the motor. Because loading will be applied directly to the shaft of the motor by means of a belt and pulley arrangement.

How to conduct brake test on dc shunt motors?

Test Requirements:
1. DC shunt motor
2. Water-cooled pulley 
3. Spring balance

The procedure of Brake Test On DC Shunt Motor

1. By adjusting the handle of the pulley takes different readings of the spring balance.

2. The tension in the belt can be adjusted using the handle. The tension in kg can be obtained from the spring balance readings.

3. Adjusting the load step by step till full load, the number of readings can be obtained.By increasing the load is slowly, adjust to get rated load current.

4. The power developed gets wasted against the friction between belt and shaft. Due to the braking action of the belt the test is called brake test.

5. The speed can be measured by a tachometer. Thus all the motor characteristics can be plotted.

You can refer figure for the experimental setup for performing brake test on a d.c. shunt motor, belt & pulley arrangement mounted on the shaft of the motor.

Calculation Of  Brake Test On DC Shunt Motor

Let           R (or) r= Radius of the pulley in meters

               N = Speed in r.p.m.

               W1 = Spring balance reading on the tight side in kg

               W2 = Spring balance reading on the 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 on the rope = (W – S) kg  = (W – S) X 9.81 newtons……(1)

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

Tsh = Net pull X R=(W – S) X 9.81 X R   …..(2)

Hence the output power can be obtained as, Say speed of the pulley is N r.p.m., then, The above equation shows the output power of dc shunt motor in brake test.

  Now let, V = Voltage applied in volts

               I = Total line current drawn in amps.

As we know V,I am input parameters of dc motors in brake test.then,

Pin=V.I Watts  …..(3)

We have output and input.Then why late go and find the efficiency of dc shunt motor.

Efficiency (η)=Output/Input [No units]

From equation (2) & (3)

Advantages Of Brake Test On DC Shunt Motor

1. The actual efficiency of the motor under working conditions can be found out.

2. Brake test is simple and easy to perform.

3. It is not only for dc shunt motor, also can be performed on any type of d.c. motor.

Disadvantages Of Brake Test On DC Shunt Motor

1. In brake test due to the belt friction lot of heat will be generated and hence there is the large dissipation of energy.

2. The cooling arrangement is necessary to minimize the heat. Mostly in our laboratories, we use water as the cooling liquid.

3. Convenient only for small rated machines due to limitations regarding heat dissipation arrangements.

4.The power developed gets wasted hence brake test method is little expensive.

5. The efficiency observed is on the lower side.