Question

A 250- V separately- excited dc motor has an armature resistance of 2.5Ω. When driving a load at 600rpm with a constant torque, the armature takes 20A. The motor is controlled by a chopper circuit with a frequency of 400Hz, and an input voltage of 250V. What duty ratio is needed to run the motor at 400rpm, with the load torque maintained constant?

Answer 1

To run the motor at 400rpm with constant torque, a duty ratio of 80% is needed.

Step-by-step explanation:

To find the duty ratio needed to run the motor at 400rpm with constant torque, we need to determine the average armature voltage at this speed.

The back emf of the motor at 600rpm can be calculated using the formula:

Back emf = Supply Voltage - Armature Current x Armature Resistance

Given that the armature current is 20A and the armature resistance is 2.5Ω, the back emf at 600rpm is:

Back emf = 250V - 20A x 2.5Ω = 200V

The ratio of back emf and supply voltage is equal to the duty ratio, which can be calculated as:

Duty ratio = (Back emf / Supply Voltage) x 100%

Substituting the values, we have:

Duty ratio = (200V / 250V) x 100% = 80%