DC/AC Motors

Question 1

Armature Runaway

Answer 1

Current will decrease and because current in stator coil decreases torque will decrease
Everything decreases

Question 2

Rotor Seizure

Answer 2

Everything increases

Question 3

Purpose of a electric motor

Answer 3

Convert electrical energy into mechanical energy

Question 4

DC Motor

Answer 4

Conductive wire armature / rotor rotating around poles of a permanent magnet

Connected to a DC supply source battery

Iron core and more armature coils used to increase output

Question 5

Starter Generator switching between motor to generator

Answer 5

DC motor supplied with current via field changeover relay - high torque no runaway

Engine runs to self sustaining speed start motor circuit isolated from power supply

Contact connect to the shunt field winding to voltage regulator

Question 6

Voltage and current on torque

Answer 6

Higher voltage or higher current = higher torque

Question 7

If speed goes up what happens to torque

Answer 7

Torque will decrease

Question 8

Relationship between torque and RPM

Answer 8

Inverse relationship

If RPM goes up torque goes down
If RPM goes down torque goes up

Question 9

Back EMF is

Answer 9

Production of EMF produces a opposing force in opposite direction

Opposes the voltage.current creating it therefore the rotation of the motor

Proportional to the motors speed

Question 10

Role of back emf in motor

Answer 10

Prevents the motor from going faster and faster

If no back emf current will go up and up

Question 11

Rotor seizure

Answer 11

Motor cannot turn therefore current builds up and catches fire as no back emf to slow it down

Excessive current in armature/torque increase to try and get going faster

Question 12

DC Motor series wound

Answer 12

Low RPM high current
Low RPM high torque

Question 13

Rotor Runaway

Answer 13

Motor accelerates to destruction
Load completely removed

Current and torque decrease to try and slow down

Series wound

Question 14

Left Hand rule for motors

Answer 14

First finger field
Second finger current
Thumb motion

Question 15

Types Of DC Motors

Answer 15

Series Motor
- when off loaded rpm is high
- must be started on load
- ideal for starter motor due to high torque at low rpm

Shunt Motor
- field winding in parallel
- fairly constant rpm across loads
- fuel pump/booster pumps etc

Compound
- connected in series/parallel
- expensive to not used often
- constant output with changes in load

Question 16

AC Motor

Answer 16

A rotating magnetic field is set up within stator field coils

Question 17

Dc Motor Series Wound

Answer 17

Excellent for high starting loads as they have high low speed torque

Low RPM/High Current
Low RPM/High Torque

Question 18

AC Induction Motor

Answer 18

Rotates at a specific speed which is slightly less than synchronous speed - asynchronous - difference is known as slip speed

Self starting motors with no brushes or slip rings

Question 19

Torque equation

Answer 19

Voltage x Current
————————
RPM

(Power)
————
RPM