Chapter 6 - Speed Control of a DC Motor.

Question 1

State Three methods of Speed control for DC Motors.

Answer 1

1. Flux Control with use of a Shunt Field Regulator (Variable Resistor) or a Series Field Divertor (Variable Resistor)
2. Armature Resistance Control with the use of a Variable Resistor increase resistance thus slowing current flow which in turn slows the motor down.
3. Terminal Voltage Control where supply voltage is controlled by a potentiometer.

Question 2

Explain how Field Control is achieved for a DC Shunt Motor?

Answer 2

• A Shunt Field Regulator (SFR) (a Variable Resistor) is placed in Series with the Field Windings adjustment of the SFR varys Field Current (If) which varys Flux (Φ)
• Increasing Resistance in the SFR decrease Field current, Decreasing Flux which in turn speeds up the motor.

Question 3

Explain how Field Control is achieved for a DC Series Motor?

Answer 3

• A Series Field Diverter (SFD) (a Variable Resistor) is place in a circuit in parallel with the field windings.
• This Allows two paths for Current to flow which mean some current will by-pass the field windings.
• Increasing Resistance in the SFD Increases Field Current (If), Which increases Flux thus Decreasing speed in the motor

Question 4

Give 2 Disadvantages of Field Control?

Answer 4

1. To Decrease Speed in a motor, flux must be increased Maxumum Flux is limited by saturation of the poles. this means that there is a minimum speed that can be obtained using flux contol before entering a deadband where we can make the motor go any slower.
2. From the torque equation. To increase speed Flux must be decreased this in turn decrease the torque in the motor.This mean less Torque at higer speeds.

Question 5

State the Advantage and Disadvantage of Armature resistance control.

Answer 5

Advantage:

• Speed control from Zero to Full Speed Can be obtained.

Disadvantage

• Large copper losses make this method of Speed Control inefficent (I²R = Copper losses)