Question

A PMDC motor driven by 10 V DC source draws 1 A current for a given load and spins at 4000 RPM. The motor was slowed down by using a friction pad. When the motor was completely stopped, the current rises to 2 A. What was the electromagnetic torque produced when the motor was spinning at 4000 RPM?

Answer 1

The student is asking for the electromagnetic torque produced by a PMDC motor at 4000 RPM, which requires additional information beyond what's provided to calculate. The question pertains to understanding the relationship between current, voltage, and mechanical power in the context of a PMDC motor's operation.

Step-by-step explanation:

The student wants to know the electromagnetic torque produced by a PMDC motor when spinning at 4000 RPM. To find the torque when the motor is running under load, we use the fact that the power input to the motor is equal to the product of the voltage and the current when running with the load. Under this condition:

Power Input = Voltage × Current
= 10 V × 1 A
= 10 W

When the motor is stopped by friction and drawing 2 A, the power input is:

Power Input = Voltage × Current
= 10 V × 2 A
= 20 W

This power is now converted entirely into heat, since the motor is not spinning. The difference in current between the loaded condition and the stalled condition implies that the back emf is reducing the effective voltage, and therefore the effective power when the motor is running.

Note: More information is needed to calculate the electromagnetic torque, such as the motor’s resistance or back emf, therefore an exact numerical answer cannot be provided with the information given in the question.