DC Motors

Question 1

Left hand rule

Answer 1

To do with motors

Thumb = direction of motion
First finger = direction of field
Second finger = direction of current

Question 2

Back EMF

Answer 2

When the conductor of the motor moves in the magnetic field - voltage is generated in the conductor - direction acts in opposite to the supply voltage direction

Question 3

Series wound motors and where used

Answer 3

Have a high starting torque

Used in a/c starter motors and actuators

Take load off = will speed up

Question 4

Shunt wound motor and where used

Answer 4

Low starting torque

If you speed it up - back EMF will slow it down - remains constant speed

Used where constant speed is req - windscreen wipers, fuel pumps and rotary inverters

Question 5

Actuators

Answer 5

High speed reversible series-wound motors

Output normally converted to driving torque via a step down box

Motor actuators - self contained units - combine electrical and mechanical devices - capable of linear thrust over a short distance or a low speed turning effort - all reversible

Question 6

2 types of actuators and where they are most likely found

Answer 6

Rotary - rotary movement and are mainly used to rotate valves in air conditioning, hydraulic and fuel systems

Linear - driven directly from a reduction gearbox via a lead screw that extends/retracts a ram or plunger when rotated e.g trim tabs

Question 7

Purpose of friction clutches

Answer 7

Incorporated into transmission system to protect against the effects of mechanical over-loading

Question 8

Electromagnetic break

Answer 8

Stops them rotating when off