DC Machines

1
Chapter 15

• DC Machines

2
Objectives

• State Faradays Law and Lenzs Law
• Calculate the voltage generated by passing a wire
  through a magnetic field.
• Sketch a simple generator and describe how it
  operates.
• Describe a commutator and brush assembly and
  state how it works.

3
Objectives

• Find the force produced on a current-carrying
  wire in a magnetic field.
• State the differences between a shunt and
  compound dc generator and describe the
  performance characteristics of each.
• Sketch a simple dc motor and describe how it
  operates.
• State the differences among a shunt, series, and
  compound dc motor, and describe the performance
  characteristics and application examples of each.

4
15-1 Introduction
5
15-2 Magnetic Induction and the DC Generator

• Faradays Law e N dF / dt
• e the induced voltage in volts (V)
• N the number of series-connected turns of wire
  in turns (t)
• dF/dt rate of change in flux in Webers/second
  (Wb/s)
• e B L v
• B the flux density in teslas (T)
• L the length of the conductor that is in the
  magnetic field in meters (m)
• v the relative velocity between the wire and
  the flux, in meters/second (m/s)

6
Magnetic induction in a wire moving in a field.
7
Right-hand rule for magnetic induction.
8
Wire loop rotating in a magnetic field.
9
AC generator with slip rings and brushes.
10
DC generator with commutator and brushes.
11
DC generator output waveform.
12
DC generator with field control.
13
DC generator four-pole field.
14
DC generator rotor with two coils.
15
Coil and output waveforms for a two-winding rotor.
16
Rotor with several rotor coils and commutator
segments.
17
15-3 Shunt and Compound DC Generator

• Shunt Generator Model
• Compound Generator Model
• Efficiency

18
DC shunt generator model.
19
More precise dc shunt generator model.
20
Shunt dc generator with field rheostat.
21
Separately excited shunt dc generator.
22
Compound generator, (a) short shunt and (b) long
shunt.
23
Generator Efficiency

• Pin T nr / 7.04
• Pin the input power in watts (W)
• T the input shaft torque in foot-pounds
  (ft-lbs)
• nr the rotation speed of the shaft in
  revolutions per minute (rpm)
• ? Pout / Pin Vt It / (T nr / 7.04)
• ? the efficiency (dimensionless)
• Vt the generator terminal voltage in volts (V)
• It the generator output current in amperes (A)

24
Generator Losses

• Rotor Copper Loss
• This is the I2R loss in the rotor due to the
  resistance of the wire.
• This loss varies with the square of the rotor
  current.
• Rotor Core Loss
• Because the rotor core (the iron upon which the
  rotor windings are wound) is rotating inside a
  magnetic field, there will be eddy current and
  hysteresis losses in the rotor core.
• These losses vary with the field flux and the
  rotor speed.
• Field Copper Loss
• The I2R loss in the field windings due to the
  resistances of the wire.
• This loss varies with the square of the field
  current.

25
Generator Losses (continued)

• Brush Loss
• There is power loss in the brush-commutator
  interface.
• This loss is proportional to the rotor current
  and brush drop and is VbIa.
• Friction
• These are losses due to mechanical friction.
• They include the friction of the shaft bearings
  and the friction created by the commutator and
  brush assembly.
• Windage
• These are losses due to the wind resistance of
  the rotor.
• In most generators, cooling fins are attached to
  the rotor to circulate air through the generator,
  thus promoting cooling and allowing the generator
  to be operated at higher output currents.
• These cooling fins increase the windage loss.

26
15-4 Motor Action and the DC Motor

• F B L I
• F the resulting mechanical force in newtons (N)
• B the flux density in teslas (T)
• L the effective length of the wire (meters) in
  the field multiplied by the number of turns
• I the current in the conductor in amperes (A)
• Ia(start) (Vt Vb) / Ra
• Ia(start) the armature starting current in
  amperes (A)
• Vt the applied voltage in volts (V)
• Vb the brush drop in volts (V)
• Ra the armature resistance in ohms (O)
• Ia (Vt Vb Vcemf) / Ra
• Vcemf the induced counter emf in the armature
  windings in volts (V).

27
Force on a current-carrying wire in a magnetic
field.
28
Flux compression and resulting force.
29
Simple dc motor.
30
DC motor with electromagnetic field.
31
15-5 Shunt, Series, and Compound DC Motor

• Shunt Motor
• Series Motor
• Compound Motor
• Motor Efficiency

32
Shunt dc motor.
33
Series dc motor.
34
Compound dc motor.
35
Motor Efficiency

• ? Pout / Pin (T nr / 7.04) / (Vt It)
• ? the efficiency (dimensionless)
• Pout the output power in watts (W)
• Pin the input power in watts (W)
• T the shaft torque in foot pounds (ft-lb)
• nr the rotor speed in revolutions per minute
  (rpm)
• Vt the applied input voltage in volts (V)
• It the applied input current in amperes (A)
• For a separately excited motor
• ? (T nr / 7.04) / (Vt It Vf If)
• Vf the field voltage in volts (V)
• If the field current in amperes (A)

36
15-6 Dynamic Braking of DC Motors

• In dynamic braking the armature is connected to a
  resistive load after removing power, the energy
  stored in the rotor in the form of angular
  momentum will be transferred to the resistive
  load, rapidly decreasing the rotor speed.
• When plugging a motor, the motor is momentarily
  reconnected in such a way as to reverse the
  direction of rotation. This can cause excessive
  line currents and excessive torque on the rotor.