Electricity 1: DC Generators

What four factors determine the amount of voltage induced in a conductor?

1. Strength of the magnetic field

2. Angle at which the conductor cuts the field

3. Speed of movement

4. Length of the conductor in the field

What is the main function of a generator?

To convert mechanical energy into electrical energy using magnetic induction

Why does an elementary generator naturally produce AC?

Because the rotating armature loop continually changes position relative to the magnetic field, causing the induced voltage to reverse direction each half rotation.

What component converts AC to DC on a generator?

The commutator

How is a commutator different from slip rings?

Slip rings provide continuous AC output, while a commutator is split into segments and reverse the coil connection to maintain DC output

What is the “neutral plane”

The position where the armature conductors are parallel to the lines of magnetic flux and no voltage is induced

What is the effect of th adding more armature loops/coils?

Increases average voltage and reduces ripple creating smoother DC output

How can the voltage of a generator be increased without speeding it up?

Increase the number of turns of wire in the armature windings

Why do practical generators use electromagnets instead of permanent magnets? 

Electromagnets provide stronger fields and allow field strength to be controlled

What are the three major assemblies of a DC generator?

• Field frame (yoke)

• Rotating armature

• Brush assembly

Why is lamination used in the armature core?

To reduce eddy currents and minimize energy loss

What is residual magnetism?

The small amount of magnetism remaining in the iron field cores after current is removed

What happens if residual magnetism is lost?

The generator will produce little or no output unitl the field is flashed

What is the purpose of “flashing the field”?

To restore residual magnetism using an external DC source

What is the “coming-in speed”?

The RPM at which the generator begins producing usable voltage

Why are series-wound generators not used in aircraft?

They have poor voltage regulation—voltage rises and falls dramatically with load.

What is the main characteristic of a shunt-wound generator?

Field is connected in parallel with the load and contains many turns of small wire

What type of generator combines the advantages of both series and shunt windings?

A compound-wound generator

What is the difference between cumulative and differential compounding?

• Cumulative: series field aids shunt field (increases field strength)

• Differential: series field opposes shunt field (weakens field strength)

What is armature reaction?

Distortion of the main magnetic field caused by current flwoing inthe armature windings

What problems does armature reaction cause?

• Brush sparking

• Commutator pitting

• Lower generator output

Name two methods used to correct armature reaction.

1. Interpoles

2. Compensating Windings

What is the purpose of interpoles?

To cancel the armature's magnetic field and maintain a stable neutral plane during load changes.

What are interpoles?

What are compensating windings?

What voltage ratings are common for aircraft DC generators?

14 volts and 28 volts

Why must the generator rotation direction match what is specified on the data plate?

Wrong rotation direction reverses polarity of the output.

What are the main brush issues to look for during inspection?

• Wear and minimum length

• Proper spring tension

• Sparking

• High mica or dirty commutator

• Carbon dust contamination

Why must you never use emery cloth on a commutator?

It contains conductive materials that can cause arcing

What are the two most common types of voltage regulators?

1. Carbon pile regulator

2. 3 unit electro-mechanical regulator

What are the three “units” in a three-unit regulator?

1. voltage regulator

2. current limiter

3. reverse-current relay

What does the reverse-current relay prevent?

Battery current from flowing backward into the generator.

What types of aircraft typically use a generator control unit/GCU?

Turbine engine aircraft

List at least three functions of a GCU.

• Voltage regulation

• Overvoltage protection

• Reverse-current sensing

• Over-excitation protection

• Parallel generator operation

• Differential voltage sensing