Question

This question is about a d.c. compound wound machine acting as a motor A 500 W, 200 V compound motor has two field windings, one has a resistance of 0.03Ω and the other a resistance of 50 Ω.

i) Draw and label a circuit diagram. Which winding should be connected to the Z-ZZ terminal?

ii) If the armature resistance is 0.15 Ω, calculate the emf when it draws an armature current of 5A whilst running at 3000 rpm.

iii) When the load increases so that the motor draws a current of 10A, what is the new speed of the machine? (assume the field shunt current does not change – just the series field)

Answer 1

A compound motor with two field windings and specific resistances is discussed in this answer. It includes drawing a circuit diagram, calculating EMF, and determining the new speed based on changes in current.

Step-by-step explanation:

To answer this question, let's break it down into parts:

i) Circuit diagram

To draw the circuit diagram of the compound motor, you would have two field windings, one with a resistance of 0.03Ω and the other with a resistance of 50Ω. The field winding with a resistance of 0.03Ω should be connected to the Z-ZZ terminal.

ii) EMF calculation

To calculate the EMF when the motor draws an armature current of 5A at 3000 rpm, you need to know the armature resistance and the back EMF. Since the armature resistance is given as 0.15Ω and the motor is running at full speed, the back EMF is equal to the supply voltage, which is 200V in this case.

The formula to calculate the EMF is:

EMF = Supply voltage - (Armature current * Armature resistance)

Substituting the given values:

EMF = 200V - (5A * 0.15Ω)

iii) Speed calculation

To calculate the new speed of the machine when the current increases to 10A, we need to consider the relationship between armature current, speed, and back EMF. The back EMF remains constant, but the armature current affects the speed.

The formula to calculate the new speed is:

New Speed = (Back EMF - (Armature current * Armature resistance)) / Back EMF * Full Speed

Substituting the given values:

New Speed = (200V - (10A * 0.15Ω)) / 200V * 3000 rpm