What is hopkinson's test? explain with circuit diagram.
Answers
Explanation:
Hopkinson’s Test is also known as Regenerative Test, Back to Back test and Heat Run Test. In Hopkinson Test, two identical shunt machines are required which are coupled both mechanically and electrically in parallel. One is acting as a motor and another one as a generator. The input to the motor is given by the supply mains.
The Circuit Diagram of the Hopkinson’s Test is shown in the figure below.
Hopkinson's-Test-figure-1Supply is given and with the help of a starter, the machine M starts and work as a motor. The switch S is kept open. The field current of M is adjusted with the help of rheostat field RM, which enables the motor to run at rated speed. Machine G acts as a generator. Since the generator is mechanically coupled to the motor, it runs at the rated speed of the motor.
The excitation of the generator G is so adjusted with the help of its field rheostat RG that the voltage across the armature of the generator is slightly higher than the supply voltage. In actual the terminal voltage of the generator is kept 1 or 2 volts higher than the supply voltage.
When the voltage of the generator is equal and of the same polarity as the of the busbar supply voltage, the main switch S is closed, and the generator is connected to the busbars. Thus, both the machines are now in parallel across the supply. Under this condition, when the machines are running parallel, the generator are said to float. This means that the generator is neither taking any current nor giving any current to the supply.
Now with the help of a field rheostat, any required load can be thrown on the machines by adjusting the excitation of the machines with the help of field rheostats.
Let,
V be the supply voltage
IL is the line current
Im is the input current to the motor
Ig is the input current to the generator
Iam is the motor armature current
Ishm is the motor shunt field current
Ishg is the generator shunt field current
Ra is the armature resistance of each machine
Rshm is the motor shunt field resistance
Rshg is the generator shunt field resistance
Eg is the generator induced voltage
Em is the motor induced voltage or back emf
