DC Motors
KL3073
Direct Current (DC) Machines Fundamentals
 Dc motors are dc machines used as motors
 the same physical machine can operate as either a
motor or a generator-it is simply a question of the
direction of the power flow through it.
 Dc motors are compared by their speed
regulations.
Types of dc motors
 The separately excited dc
motor
 The shunt dc motor.
 The permanent-magnet dc
motor.
 The series dc motor.
 The compounded dc motor
The Equivalent Circuit of a DC Motor
 its equivalent circuit is exactly the same as that of a
generator except for the direction of current flow.
Figure: (a) The equivalent circuit of a dc motor. (b) A simplified equivalent circuit
The Equivalent Circuit of a DC Motor
 The internal generated voltage in this machine is
given by the equation.
 induced torque developed by the machine is given
by
 These two equations, the KVL equation of the
armature circuit and magnetization curve, are
necessary to analyze the behavior and
performance of a dc motor
Separately excited and shunt dc motors
 A separately excited dc motor is a motor whose
field circuit is supplied from a separate constantvoltage power supply
 a shunt dc motor is a motor whose field circuit gets
its power directly across the armature terminals of
the motor
 When the supply voltage to a motor is assumed
constant, there is no practical difference in
behaviour between these two machines
The Separately excited and shunt dc motors
(a) The equivalent circuit of a separately
excited de motor.
(b) The equivalent circuit of a shunt dc
motor.
 The KVL equation for the armature circuit is:
The Terminal Characteristic of a Shunt DC
Motor
 The terminal characteristic of a motor is a plot of
its output torque versus speed.
 How does a shunt dc motor respond to a load?
•When load ↑
•τ load > τind so ω↓
•EA ↓ = KФω↓
•IA ↑= (VT – EA ↓)/RA
•τind ↑ = KФIA
↑)
•τ load = τind
Induced Torque in the Rotating Loop
 Output characteristic of a shunt dc motor
VT
RA



2 ind
K ( K )
(a) Torque-speed characteristic of a shunt
or separately excited dc motor with
compensating windings to eliminate
armature reaction.
(b) Torque-speed
characteristic of the motor
with armature reaction
present.
Speed Control of Shunt DC Motors
 In general, there are 3 methods to control the speed of dc
shunt motor:
 Adjusting the field resistance RF (and thus the field flux)
 Adjusting the terminal voltage applied to the armature.
Less common method:
 Inserting a resistor in series with the armature circuit
Speed Control of Shunt DC Motors
Changing the Field Resistance
Speed Control of Shunt DC Motors
Changing the Field Resistance
The effect of field resistance speed control
on a shunt motor's torque speed
characteristic
Speed Control of Shunt DC Motors
Changing the Armature Voltage
The effect of armature voltage speed
control on a shunt motor's torque-speed
characteristic.
DC Machine Construction
 Inserting a Resistor in Series with the Armature
Circuit.
VT
RA



2 ind
K ( K )
The effect of armature resistance
speed control on a shunt motor's
torque-speed characteristic.
Safe Ranges of Operation for the 2 common
methods
Field Resistance Control
 If a motor is operating at its rated terminal voltage,
power and field current, then it will be running at
rated speed, also known as base speed.
 Field resistance control can control the speed of the
motor for speeds above base speed but not for speeds
below base speed.
 To achieve a speed slower than base speed by field
circuit control would require excessive field current,
possibly burning up the field windings.
Safe Ranges of Operation for the 2 common
methods
Armature Voltage Control
 If a motor is operating at its rated terminal voltage,
power and field current, then it will be running at
rated speed, also known as base speed.
 Armature voltage control can control the speed of the
motor for speeds below base speed but not for speeds
above base speed.
 To achieve a speed faster than base speed by armature
voltage control would require excessive armature
voltage, possibly damaging the armature circuit.
The Effect of an Open Field Circuit
 What would happen if the field circuit were
actually opened while the motor is running?
 The flux in the machine will drop, and EA will
drop as well. This would cause a really large
increase in the armature current, and the
resulting induced torque would be quite a bit
higher than the load torque of the motor.
Therefore, the motor’s speed starts to rise and
just keeps going up.
 This condition is called runaway.
THE PERMANENT-MAGNET DC MOTOR
 A permanent magnet dc motor (PMDC) is a dc
motor whose poles are made of permanent
magnets.
Advantage:
•Since the motors do not require an external field circuit.
•No field circuit copper losses.
•Smaller than corresponding shunt dc motors.
Disadvantages:
•cannot produce as high flux density >lower induced torque
•PMDC motors run the risk of demagnetization.
•not possible to control the speed of the PMDC motor by varying
the field current or flux.
THE SERIES DC MOTOR
 A series dc motor is a dc motor whose field
windings consist of a relatively few turns
connected in series with the armature circuit.
 The Kirchhoff's voltage law equation for this
motor is
The equivalent circuit of a series dc motor.
Induced Torque in a Series DC Motor
 the flux is directly proportional to the armature
current, at least until saturation is reached.
 the load on the motor increases, its flux increases
too.
 so the speed deceases
VT
RA



2 ind
K ( K )
 The induced torque is τind =KφIA
 φ =c IA
 τind =Kc(IA)2
The Terminal Characteristic of a Series DC
Motor
 Using equation φ= cIA the torque-speed characteristic
curve for the series motor can be derived as

VT
Kc
1
 ind

R A  RS
Kc
 ideal torque-speed characteristic is
Speed Control of Series DC Motors
 Unlike with the shunt dc motor, there is only
one efficient way to change the speed of a series
dc motor.
 That method is to change the terminal voltage of
the motor. If terminal voltage is increased, the
speed will increase for any given torque.
THE COMPOUNDED DC MOTOR
 A compounded dc motor is a motor with both a
shunt and a series field. Such a motor is shown in
Figure below.
The equivalent circuit of compounded dc motors: (a) long-shunt connection; (b)
short-shunt connection.
THE COMPOUNDED DC MOTOR
 The KVL for a compounded dc motor is:
 VT = EA + IA (RA + RS)
 and the currents are:
 IA = IL - IF
 IF = VT/RF

 The net mmf and the effective shunt field current are:
 Fnet = FF ± FSE - FAR
 IF* = IF ± (NSE/NF) IA – FAR/NF
The Torque-Speed Characteristic of a
Cumulatively Compounded DC Motor (CC)
 There is a component of flux which is constant and another
component which is proportional to its armature current
(and thus to its load).
 CC motor has a higher starting torque than a shunt motor
(whose flux is constant) but a lower starting torque than a
series motor (whose entire flux is proportional to armature
current).
 The CC motor combines the best features of both the shunt
and series motors. Like a series motor, it has extra torque
for starting; like a shunt, it does not overspeed at no load.
The Torque-Speed Characteristic of a
Cumulatively Compounded DC Motor (CC)
 A comparison of the torque-speed characteristics of each of
these types of machines is shown below:
(a) The torque-speed characteristic of a
cumulatively compounded dc motor
compared to series and shunt motors with
the same full-load rating.
(b) The torque-speed characteristic of a
cumulatively compounded dc motor
compared to a shunt motor with the same
no-load speed.