DC motors

Question 1

Explain how back EMF is produced and it’s effect on current in a DC motor

Answer 1

Back EMF is produced when the current carrying conductors cut the lines of flux produced by the stator. Back EMF opposes the current driving the armature.

Question 2

Sketch the torque/speed characteristic for a separately excited DC shunt motor and using the formula for speed explain it’s shape.

Answer 2

There is an approximately 10% drop in speed as current increases. This is because N is proportional to Ia.

Question 3

Describe briefly 3 methods of speed control for a DC motor

Answer 3

• Flux control, Increase flux to decrease speed
• Armature resistance control, increase Ra to drop speed (N)
• Voltage control, Increase voltage to increase speed

Question 4

What are 3 requirements that must be incorporated into a DC motor starter?

Answer 4

• Limit starting current
• Overcurrent protection
• It must switch off is there is a supply failure

Question 5

Explain with the use of a suitable formula why the starting current in a DC motor is higher than the running current

Answer 5

Ia = (V-E)/Ra
When speed is at 0 the back EMF is 0 volts so there is no back EMF to counter current.

Question 6

Explain with the aid of a simple graph why it is dangerous to operate a series DC motor on no load conditions, state the most common method used to overcome this problem.

Answer 6

Include speed and Ia graph for a series motor

It is dangerous because DC motors encounter an issue known as runaway under no load, this is where the speed will climb rapidly as Ia decreases. To counteract this a fan can be fitted to act as a load/brake.

Question 7

Draw a graph that will show the typical starting current of a DC motor with and without a face plate starter.

Answer 7

Graph should include Ia and time

Question 8

Explain with the aid of a diagram how torque is produced in a DC motor. Identify the 3 main parameters which affect the amount of torque.

Answer 8

Answer should include basic diagram showing:
- main flux
- Conductors
- Direction of force
- T = K Ia Φ

Question 9

Why is the flux in a DC shunt motor normally unaffected by the speed or load of the motor?

Answer 9

Should include a speed vs Ia (T) graph for a shunt motor.

The field coil is connected directly across the supply voltage and is therefore independent of changes in armature current or load.

Question 10

Draw a circuit diagram which shows the arrangement used for armature resistance speed control. State one advantage and one disadvantage

Answer 10

Advantage - Full range of speed control

Disadvantage - Low efficiency