How a Brake Test is Carried Out an D.C. Machine
Q. Explain with neat figure how a brake test is carried out an d.c.Machine
- This is the direct method of testing the machine.
- The machine is gradually loaded from no load to full load.
- The pulley is fitted on the shaft of the motor.
- The leather band brake, the tension of which is adjusted on the pulley, is supported around the pulley as shown in the following sketch.
- The output of the machine is converted into heat due to the friction between the band brake and pulley.
- The output is wasted and therefore only small machines up to 5 kW can be tested by this method.
Advantages and disadvantages of brake test
Advantages of D.C. Machine Break Test
- It is a direct method of testing.
- Temperature rise of the machine can be noted.
- Commutation difficulties can be observed.
- Results so found are accurate.
- Any type of motor shunt , series or compound can be tested by this method for series meter the precaution is taken that it should not run on no-load.
Disadvantages of D.C. Machine Break Test
- Suitable only for small machines upto 5 kW and not suitable for higher capacity machine due to the limitation of brake function, burning out of leather band etc.
- Pulley should be water cooled.
- Energy is wasted during testing.
- Requires more time for testing.
- Replacement of burn out belt may be required.
Procedure of D.C. Machine Break Test
- Connect the appropriate ranges of the meters with proper polarities as shown in Figure.
- See that the belt is loose initially for shunt or compound motor, but tight in case of series motor.
- Keep rheostats in minimum position and switch on the supply and start the motor with the help of starter.
- Adjust the speed of the motor to its rated value with rheostats Rf and take down the readings of meters for no load.
- With the help of hand wheel adjust the load on the brake pulley to the desired load conditions (such as 20 %, 40 % … 100 %).
- Keep the speed constant throughout the experiment as far as possible.
- Note down the meter readings and spring balance readings.
- See that there is sufficient water in the hollow pulley for cooling purpose.
- After taking the readings up to full load condition, gradually reduce the load and reduce the speed and stop the motor.
- Care should be taken to take the readings within a short time otherwise there is a possibility of burning out the belt due to heat produced by friction.
- Hollow pulley having water inside, helps cooling.