DC Motor
Torque and speed of a DC Motor
• It is proved that , the torque of a motor is a
function of flux and armature current, but it is
independent of speed.
• In fact, the speed depends on torque but
torque does not depends on speed.
• Ta proportional to (fi) (Ia)
• It is seen from the above that increase in flux
would decrease the speed and increases the
armature torque.
Torque and speed of a DC Motor
• If torque increases, motor speed must
increase rather than decrease.
• Suppose that the flux of a motor is decreased
by decreasing the field current, then the
following sequence of events takes place:
• Back Emf (Eb) drops instantly.
• Due to decrease in back emf(Eb), armature
current(Ia) is increased, since Ia = (V-Eb)/Ra
Torque and speed of a DC Motor
• Moreover, a small reduction in flux produces a
large increase in armature current.
• Hence the equation Ta PROPORTIONAL (fi)Ia
• A small decrease in flux is more balanced by a
large increase in Ia.
• With the result that, there is a net increase in
Ta.
• This increase in Ta produces an increase in
Motor speed.
Torque and speed of a DC Motor
• It is seen that, with the applied voltage (V)
held constant, motor speed varies inversely as
the flux.
• However, it is possible to increase flux, and at
the same time increase the speed, provided
armature current (Ia) will be constant, which is
done in DC ServoMotor.
Motor Characteristics
• The characteristic curves of a motor are those
curves, which shows the relationship between
the following quantities:
• Torque and Armature Current (Ta/Ia)
Characteristic: It is also called as Electrical
Characteristic.
• Speed and Armature Current(N/Ia)
Characteristic
Motor Characteristics
• Speed and Torque (N/Ta) Characteristic: It is
also called as Mechanical Characteristic.
• While discussing motor characteristics, the
following 2 relations are very important
• Ta Proportional to (FI) (Ia)
• N Proportional to (Eb/ FI)
Characteristics of Series Motors
• Torque and Armature Current (Ta/Ia)
Characteristic:
• We know that Ta proportional to Ia. In this
case, as field windings carry armature current
(Ia), then (FI) proportional to (Ia), upto the
point of magnetic saturation.
• Hence before saturation
• Ta proportional to FI Ia and
• Ta proportional to square(Ia)
Characteristics of Series Motors
• Torque and Armature Current (Ta/Ia)
Characteristic:
• At light loads, Ia and FI is small. But as Ia
increases, Ta increases as the square of
current (Ia).
• Hence Ta/Ia curve is parabola. After
saturation, FI is almost independent of Ia,
hence Ta Proportional to Ia, so the
characteristic or curve becomes a straight line.
Characteristics of Series Motors
• Torque and Armature Current (Ta/Ia)
Characteristic:
• The shaft torque (Tsh) is less than the armature
Torque (Ta) because of constant losses.
• So we conclude that on heavy loads, a series
motor produces a torque proportional to the
square of armature current.
• Hence in cases, where huge starting torque is
required for accelerating heavy masses like
electric trains, series motors are used.
Characteristics of Series Motors
• Speed and Armature current (N/Ia)
Characteristic:
• Variations of speed can be deduced from the
relation: N proportional to (Eb/FI)
• Change in Eb is small, for various load currents
and hence may be neglected for the time
being.
• With increased Ia, FI also increases. Hence
speed varies inversely as armature current.
Characteristics of Series Motors
• Speed and Armature current (N/Ia)
Characteristic:
• When load is heavy, Ia is large. Hence, speed is
low.
• When load current (I) and armature current Ia
falls to small value, speed becomes dangerously
high.
• Hence a Series motor should never be started
with out some mechanical load(not belt driven)
on it.
Characteristics of Series Motors
• Speed and Armature current (N/Ia)
Characteristic:
• Other wise, Series motor may develop
excessive speed and get damaged, due to
heavy centrifugal forces produced in it.
• It should be noted that, series motor is a
variable speed motor.
Characteristics of Series Motors
•
•
•
•
Speed and Torque (N/Ta) characteristic:
It is found that,
when speed is high, then torque is low and
When speed is low, then torque is high.
Characteristics of Shunt Motor
• Torque and Armature current (Ta/Ia)
Characterisitic:
• Assuming FI to be practically constant, then we
find that Ta proportional to Ia.
• Hence the electrical characteristic (Ta/Ia) is
practically a straight line through the origin.
• Since a heavy starting load will need a heavy
starting current. Shunt motor should never be
started on (heavy) load.
Characteristics of Shunt Motor
• Speed and Armature current (N/Ia)
Characteristic:
• If FI is assumed constant, then N proportional
to Eb.
• As Eb is also practically Constant, Speed is also
constant for most purposes.
• But strictly speaking, both Eb and FI decreases
with increasing load.
Characteristics of Shunt Motor
• Speed and Armature current (N/Ia)
Characteristic:
• However, Eb decreases slightly more than FI,
so that on the whole, there is some decrease
in speed.
• The drop varies from 5 to 15 % of full load
speed , which depends on saturation,
armature reaction and brush position.
Characteristics of Shunt Motor
• Speed and Armature current (N/Ia)
Characteristic:
• But, for all practical purposes, shunt motor is
taken as a constant speed motor.
• Because there is no appreciable change in the
speed of a shunt motor from No load to Full
load.
• It may be connected to loads which are totally
and suddenly thrown off, with out any fear.
Characteristics of Shunt Motor
• Speed and Armature current (N/Ia)
Characteristic:
• Due to the constancy of their speed, shunt
motors are suitable for driving shafts, machine
tools, lathes, wood-working machines and for
all other purposes, where an approximately
constant speed is required.
Characteristics of Shunt Motor
• Speed and torque (N/Ta) Characteristic:
• This curve can be deduced from
• Torque and armature current (Ta/Ia)
characteristic AND
• Speed and armature current (N/Ia)
Characteristic.
Compound Motors
• These motors have both series and shunt
windings.
• If series field helps the shunt field ie, series
flux is in the same direction, then the motor is
said to be cummulatively compounded.
• If series field opposes the shunt field, then the
motor is said to be differentially compounded.
• The characteristics of compound motors lie in
between those of series and shunt motors.
Cumulative-Compound Motors
• These machines are used, where series
characteristics are required and where, the
load is likely to be removed totally, as in the
case of coal cutting machines.
• Due to shunt windings, speed will not become
excessively high.
• But due to series windings, it will be able to
take heavy loads.
Cumulative-Compound Motors
• These motors have greatest application with
loads that require high starting torques.
• These motors are used to drive metalstamping machines, reciprocating pumps,
compressors and so on.
Differential Compound Motors
• In these motors, since series field opposes the
shunt field, the flux is decreased as load is
applied to the motor.
• This results in the motor speed remaining
almost constant or even increasing, with
increase in load.
• Due to this reason, there is a decrease in the
rate at which the motor torque increases with
load.
Differential Compound Motors
• These motors are not in common use.
• But these motors can be designed to give an
accurately constant speed under all conditions.
• These motors are rarely used for experiments
and research work.
• One of the biggest drawback of these motors is
that due to weakening of flux with increase in
load, the speed becomes unstable and motor
runs away, unless designed properly.
Comparison of Shunt, Series and
Compound motors
• Shunt Motor:
• Characteristics:
• Speed of a shunt motor is sufficiently
constant.
• For the same current input, its starting torque
is not as high as that of series motor.
• When the speed has to be maintained
approximately constant from NO Load to Full
Load.
Comparison of Shunt, Series and
Compound motors
• Shunt Motor:
• Characteristics:
• When it is required to drive the load at various
speeds, the shunt regulator enables the
required speed control, to be obtained easily
and economically.
Comparison of Shunt, Series and
Compound motors
•
•
•
•
•
•
Applications of Shunt Motor:
For driving constant speed line shafting lathes.
Centrifugal pumps
Machine tools
Blowers and fans
Reciprocating pumps
Comparison of Shunt, Series and
Compound motors
• Series Motor Characteristics:
• These motors have a relatively huge starting
torques.
• These motors have good accelerating torque.
• Have low speed at high loads and dangerously
high speed at low loads.
Comparison of Shunt, Series and
Compound motors
• Series motor applications:
• These motors are used
1. When a large starting torque is required for
driving cranes etc.
2. When the motor can be directly coupled to a
load such as a fan, whose torque increases
with speed.
Comparison of Shunt, Series and
Compound motors
• Series motor applications:
• These motors are used
3. If constant speed is essential, then, in fact the
decrease of speed with increase of load, has the
advantage that the power absorbed by the
motor, does not increase as rapidly as the
torque.
4. For driving centrifugal pumps.
Comparison of Shunt, Series and
Compound motors
• Compound motor Characteristics:
• These motors have
1. Variable speed
2. Adjustable varying speed (Regulator)
3. High starting torque
Comparison of Shunt, Series and
Compound motors
• Compound Motor Applications:
• These motors are used:
1. For intermittent high torque loads.
2. For Elevators
3. For Conveyors
4. For rolling mills: Ice Machines, Printing
presses, Air compressors
Losses and Efficiency of Motors
• The losses taking place in motors are the same as
in generators. They are
• Copper Losses
• Magnetic Losses
• Mechanical Losses
• The condition for maximum power developed by
the motor is IaRa = V/2 = Eb
• The condition for maximum efficiency is that
copper losses are equal to constant losses.
Power Stages of Motor
• The various stages of Energy transformation of
motor are as follows:
• PinPcuPaPifPout
• Where Pin is Motor input, Pin=VI watt
• Pcu is copper loss
• Pa is Driving power in armature, Pa=EbIa watt
• Pif is iron and friction losses
• Pout is Motor output in watt