Electricity 1: Motors

What are the two main parts of a DC motor?

Field assembly and armature assembly

What type of energy conversion does a DC motor perform?

converts DC electrical energy into mechanical energy

Name three aircraft systems that commonly use DC motors.

Starters, autopilots, landing gear motors

What happens when a current-carrying wire is placed in a magnetic field?

a force acts upon the conductor

The force on the wire is always at right angles to what two things?

to the current and magnetic field

How does reversing the current affect the magnetic field around a conductor?

it reverses the direction of the magnetic filed around the conductor

What determines the direction of the force acting on a current-carrying conductor?

the direction is determined by the direction of current and magnetic polarity

Define torque.

the tendency for a force to produce rotations

What three factors determine the amount of torque produced by a coil?

the strength of the magnetic field, number of turns in the coil, and the coil’s position in the field

Using the Right-Hand Motor Rule, what do the thumb, index finger, and middle finger represent?

Thumb = direction of motion

Index = magnetic field

Middle = current

Why will a simple coil connected to a battery stop turning once aligned with the magnetic field?

Because once aligned with the magnetic field, torque becomes zero

What is the purpose of the commutator?

To reverse current in the armature each half-turn and maintian continous rotation

Why do practical motors use many coils instead of one?

To eliminate torque dead spots and ensure smooth, continuous torque

What are the major components of the armature assembly?

Laminated iron core, coils/windings, commutator, mounted on a shaft

Why is the armature core laminated?

To reduce eddy current losses and heating

What is the purpose of the brushes and how are they kept in contact with the commutator?

To conduct current to the commutator; springs keep them in contact

What is located in the end frame of a DC motor?

the drive-end bearing and mounting structure

How are the field windings connected in a series motor?

Connected in series

Why do series motors have high starting torque?

because the same high starting current flows through both fields and the armature, strengthening both fields. 

What happens to a series motor if operated with no load?

It can overspeed to destruction

Name at least two aircraft systems that use series motors.

Starters, landning gear motors, flap motors, cowl flap motors

How are field windings connected in a shunt motor?

In parallel with the armature

Why is shunt motor speed nearly constant?

Field current is constant which creates a constant magnetic field creating nearly constant speed

Why do shunt motors have lower starting torque than series motors?

The field has high resistance meaning a lower current and weaker field, meaning there is less torque needed to start

What two windings does a compound motor use?

a shunt winding in parallel and a series winding connected in series

What is the difference between cumulative and differential compound motors?

Cumulative: aides the shunt

Differential:series opposes the shunt

Which type of compound motor is rarely used in aircraft and why?

Differential because of the unstable speed characteristics

What causes counter EMF to be generated in a motor?

the rotation of the armature windings in the magnetic field

How does counter EMF affect the armature current at high speed?

It increases EMF, which decreases armature current

What happens to armature current when load increases?

The speed drops, causing counter EMF to drop and the armature current to increase.

Why is it harder to stall a series motor compared to a shunt motor?

because the increased current strengthens both field and armature windings

Explain the difference between intermittent duty and continuous duty motors.

Intermittent: runs for short periods up to 2 minutes, has to cool between uses. 

Continuous duty: can run for long periods of time at rated power

How can motor direction be reversed in general?

Reversing the current fow in either the field windings or the armature

What is a split field motor?

A motor with two field windings wound in opposite directions for reversing polarity

What type of switch is used in the switch-method of motor reversal?

Double pole, double throw (DPDT)

How does increasing field current affect motor speed?

More field current → stronger field → slower motor.

In a shunt motor, how does adding resistance to the field circuit affect speed?

Adding resistance reduces field current → weaker field → higher speed.

How can a series motor be made to run above normal speed?

By bypassing part of the field current with a rheostat in parallell with the series field

What are the two major categories of energy losses in DC motors?

Electrical losses and Mechanical losses

What are copper losses and why are they called I²R losses?

Copper losses are I²R losses—heat produced by current through resistance.

What causes eddy current losses?

Losses caused by the rotating iron core cutting through a magnetic field

What are mechanical losses in a motor?

Friction in bearings, brushes, and air resistance

How is power output of a motor expressed?

in horsepwoer

How is power input calculated?

volts x amps

What is the relationship between horsepower and watts?

1 hp = 746 watts

How is motor efficiency defined?

Efficiency = power output ÷ power input.

Name at least four items that should be inspected during DC motor maintenance.

Check wiring, terminals, fuses, brushes, commutator, overheating, cleanliness, mounting, etc.

What condition on the commutator requires attention?

Pitting, burning, roughness, scoring, corrosion, or high mica

When should a DC motor be lubricated?

Only when the manufacturer calls for it

What should be done if insulation on wiring is cracked or burned?

Replace the wiring