Monday, February 22, 2016
Brake Test On DC Shunt Motor
In this article we are going to discuss about brake test on d.c. motor.Brake test is also a method of finding efficiency of dc motors.We took dc shunt motor as running machine.Brake test also called as direct loading test of testing the motor. Because loading will be applied directly on 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
Procedure of Brake Test On DC Shunt Motor
1. By adjusting the handle of the pulley take 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, 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 belt the test is called brake test.
5. The speed can be measured by 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 pulley in meters
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 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 are 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. 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 the belt friction lot of heat will be generated and hence there is large dissipation of energy.
2. Cooling arrangement is necessary to minimize the heat. Mostly in our laboratories we use water as cooling liquid.
3. Convenient only for small rated machines due to limitations regarding heat dissipation arrangements.
4.Power developed gets wasted hence brake test method is little expensive.
5. The efficiency observed is on lower side.