Machines

Question 1

What is the base speed of a DC machine?

Answer 1

The speed in RPM at which it delivers rated torque at rated armature voltage and field current. It is also equivalent to the rated voltage divided by a machine’s back-emf constant.

Question 2

What are the three key components of a DC machine, and what are their functions?

Answer 2

Stator: houses motor - copper windings (field windings) or permanent magnets prevent magnetic field

Rotor: Rotating element - field generated by either armature windings or permanent magnets

Commutator: Electrical connection between armature and external circuitry

Question 3

What issues are associated with the use of commutators?

Answer 3

• Expense
• Physical bulk
• Maintenance-intensive
• Sparking - fire and EMI hazard
• Mechanically limit machine to max. speed of 4000rpm

Question 4

What is the equation for the torque generated by a motor?

Answer 4

T = k I_a Φ

Φ = Magnetic field strength [Wb]

I_a = Armature current

k = Torque constant

Question 5

What is the equation for back-emf?

Answer 5

e_b = k ω Φ

Φ = Magnetic field strength [Wb]

ω = Speed [rads^-1]

k = Back-emf constant

Question 6

What are the advantages and disadvantages of using permanent magnets instead of field windings?

Answer 6

+ No I²R losses

• Non-adjustable field
• Expensive

Question 7

What are the benefits and shortcomings of using field windings in contrast to permanent magnets?

Answer 7

+ Adjustable field

+ Generate higher flux-density fields

• I²R losses

Question 8

Draw separately-excited, series, and shunt arrangement for a motor

Answer 8

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Question 9

Draw a graph of torque against speed, indicating full-field and weakened-field operation, constant-power and constant-torque regions, and labelling rated torque and base speed

Answer 9

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Question 10

What is power in terms of torque and angular velocity?

Answer 10

P = τω

Question 11

Why is it necessary to reduce field strength in order to attain higher motor speeds?

Answer 11

To ensure that the back-emf across the armature does not exceed the supply voltage

Question 12

Draw a buck converter in use for both field windings and an armature

Answer 12

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Question 13

What is the duty factor of the freewheeling diode in a buck converter driving a DC machine?

Answer 13

1 - δ, where δ is the duty factor of the controlling IGBT