Speeed Control

Gomez, Christine S
BSECE
 Speed Control of Shunt Motor

1. Flux Control Method
It is already explained above that the speed of a dc motor is inversely

proportional to the flux per pole. Thus by decreasing the flux, speed can be increased
and vice versa. To control the flux, a rheostat is added in series with the field winding,
as shown in the circuit diagram. Adding more resistance in series with the field winding
will increase the speed as it decreases the flux. In shunt motors, as field current is
relatively very small, Ish2R loss is small. Therefore, this method is quite efficient. Though
speed can be increased above the rated value by reducing flux with this method, it puts
a limit to maximum speed as weakening of field flux beyond a limit will adversely affect
the commutation.
2. Armature Control Method
Speed of a dc motor is directly proportional to the back emf Eb and Eb = V -

IaRa. That means, when supply voltage V and the armature resistance R a are kept
constant, then the speed is directly proportional to armature current Ia. Thus, if we add
resistance in series with the armature, Ia decreases and, hence, the speed also
decreases. Greater the resistance in series with the armature, greater the decrease in
speed.
3. Voltage Control Method
a) Multiple voltage control:
In this method, the shunt field is connected to a fixed exciting voltage and armature is
supplied with different voltages. Voltage across armature is changed with the help of
suitable switchgear. The speed is approximately proportional to the voltage across the
armature.
b) Ward-Leonard System:
This system is used where very sensitive speed control of motor is required
(e.g electric excavators, elevators etc.). The arrangement of this system is as shown in
the figure at right. M2 is the motor to which speed control is required. M1 may be any AC
motor or DC motor with constant speed. G is a generator directly coupled to M1. In this
method, the output from generator G is fed to the armature of the motor M 2 whose
speed is to be controlled. The output voltage of generator G can be varied from zero to
its maximum value by means of its field regulator and, hence, the armature voltage of
the motor M2 is varied very smoothly.
Hence, very smooth speed control of the dc motor can be obtained by this
method.
 Speed Control Of Series Motor
1. Flux Control Method
 Field diverter: A variable resistance is connected parallel to the series field as

shown in fig (a). This variable resistor is called as a diverter, as the desired amount
of current can be diverted through this resistor and, hence, current through field
coil can be decreased. Thus, flux can be decreased to the desired amount and
speed can be increased.
 Armature diverter: Diverter is connected across the armature as shown in fig (b).
For a given constant load torque, if armature current is reduced then the flux must
increase, as Ta ∝ Ø Ia. This will result in an increase in current taken from the
supply and hence flux Ø will increase and subsequently speed of the motor will
decrease.
 Tapped field control: As shown in fig (c) field coil is tapped dividing number of
turns. Thus we can select different value of Ø by selecting different number of
turns.
 Paralleling field coils: In this method, several speeds can be obtained by
regrouping coils as shown in fig (d).
2. Variable Resistance In Series With Armature

By introducing resistance in series with the armature, voltage across the
armature can be reduced. And, hence, speed reduces in proportion with it.
3. Series-Parallel Control
This system is widely used in electric traction, where two or more mechanically
coupled series motors are employed. For low speeds, the motors are connected in
series, and for higher speeds, the motors are connected in parallel.
When in series, the motors have the same current passing through them,
although voltage across each motor is divided. When in parallel, the voltage across
each motor is same although the current gets divided.
 Speed control of separately excited DC motor

Speed of this type of dc shunt motor is controlled by the following methods:
I. Field control methods: Weakening of field causes increase in speed of the motor
while strengthening the field causes decreases the speed. Speed adjustment of this
type of motor is achieved from the following methods:
II. Field rheostat control: – Here a variable resistance is connected in series with the
field coil. Thus the speed is controlled by means of flux variation. Reluctance control
involving variation of reluctance of magnetic circuit of motor. Field voltage control by
varying the voltage at field circuit while keeping armature terminal voltage constant.
III. Armature control methods: Speed adjustment of separately excited DC motor by

armature control may be obtained by any one of the following methods :
i. Armature resistance control: – Here, the speed is controlled by varying the

source voltage to armature. Generally, a variable resistance is provided with
the armature to vary the armature resistance.
ii. ii. Armature terminal voltage control involving variation of variation of voltage
in armature circuit.

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Gomez, Christine S

 Speed Control of Shunt Motor

It is already explained above that the speed of a dc motor is inversely

Speed of a dc motor is directly proportional to the back emf Eb and Eb = V -

 Speed Control Of Series Motor

1. Flux Control Method

 Field diverter: A variable resistance is connected parallel to the series field as

2. Variable Resistance In Series With Armature

 Speed control of separately excited DC motor

III. Armature control methods: Speed adjustment of separately excited DC motor by

i. Armature resistance control: – Here, the speed is controlled by varying the

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