Speed control methods of motor

1. Power Electronics
161310109050 – Raval Arpit
161310109051 – Rohan Kaushik
161310109052 – Rohit Chavda

2. Basic Equation
Speed Control of Shunt Motors
 Flux control method
 Armature and Rheostatic control method
 Voltage control method :
• Ward Leonard system
Speed Control of Series Motors
 Flux control method :
• Field diverter
• Armature diverter
 Armature-resistance control

3. Flux control method
In this flux control method, speed of the motor is inversely proportional to the flux. Thus, by decreasing
flux and speed can be increased vice versa. To control the flux , he rheostat is added in series with the
field winding will increase the speed (N), because of this flux will decrease.

4. Armature Control Method
In the armature control method, the speed of the DC motor is directly proportional to the
back emf (Eb) and Eb = V- IaRa. When supply voltage (V) and armature resistance Ra are
kept constant, the Speed is directly proportional to armature current (Ia). If we add resistance
in series with the armature, the armature current (Ia) decreases and hence speed decreases.

5. Ward-Leonard System (Voltage Control)
This Ward–leonard system is used where very sensitive speed control of the motor is required. The arrangement
of this system is as required in the figure shown.
M2 is the motor, it controls the speed of the generator. M1 may be any AC motor or DC motor with constant
speed. G is the generator directly coupled to M1. In this method the output from the generator G is fed to the
armature of the motor M2 whose speed is to be controlled. The generator output voltage can be connected to
the motor M2 and it can be varied from zero to its maximum value, and hence the armature voltage of the
motor M2 is varied very smoothly. Hence very smooth speed control of motor can be obtained by this method.

6. Field diverter
A Rheostart is connected parallel to the series field as shown in fig. This variable
resistor is also called as a diverter, as desired value of the current can be diverted through
this resistor and hence current through field coil can be decreased.
Hence flux can be decreased to desired amount and speed(N) can be increased.

7. Armature diverter
Rheostat (Divider) is connected across the armature of the coil as shown in fig.
For a given constant load torque, if armature current is reduced, then flux must increase.
As armature torque Ta α ØIa.
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.

8. Armature-resistance control
In this method, an introducing resistance (R) is series with the armature of motor.
The voltage across the armature can be reduced. So the speed reduces in proportion with it.
It is seen that for a 4 pole motor, the speed of the motor can be obtained easily.

9. Rheostatic or Dynamic braking
Fig. shows dynamic braking of a shunt motor. The braking torque can be controlled by varying the
resistance R. If the value of R is decreased as the motor speed decreases, the braking torque may be
maintained at a high value. At a low value of speed, the braking torque becomes small and the final
stopping of the motor is due to friction.

10. Plugging
In this method, connections to the armature are reversed so that motor tends to rotate in the opposite
direction, thus providing the necessary braking effect. When the motor comes to rest, the supply must
be cut off otherwise the motor will start rotating in the opposite direction.

11. Regenerative braking
In one method, field winding is disconnected from the
supply and field current is increased by exciting it from
another source.
As a result, induced e.m.f. E exceeds the supply voltage V
and the machine feeds energy into the supply. Thus
braking torque is provided upto the speed at which
induced e.m.f. and supply voltage are equal as the machine
slows down
In a second method, the field excitation does not change but
the load causes the motor to run above the normal speed .
As a result, the induced e.m.f. E becomes greater than the
supply voltage V
The direction of armature current I, therefore, reverses but the
direction of shunt field current If remains unaltered.
Hence the torque is reversed and the speed falls until E
becomes less than V.

12. Stator Voltage Control with AC Voltage Controller
 Figure (a) shows the circuits of three phase half wave ac voltage controllers for star connected
stators.
In half wave ac 'voltage controller consists of 3 SCRs and 3 diodes Here one SCR and one diode in
antiparallel are connected between the line and motor in a phase.

13.  Figure (b) shows the circuits of three phase full wave ac voltage controllers for star connected stators.
 The full wave ac voltage controller consists of 6 SCRs Here two SCRs in antiparallel are connected
between die line and motor in a phase
 The main advantage of half wave controller is a 'saving the cost of system The disadvantage is that it
introduces more harmonics into the line current. The effective load voltage in three phase ac circuit
can be varied by varying the thyristor firing angles.

14. STATOR VOLTAGE CONTROL:
• The torque equation is given by:
From the torque equation of the induction machine given above, we
can see that the torque depends on the square of the applied
voltage. A reduction in the supply voltage will reduced the motor
torque and therefore the speed of the drive.
By controlling the firing angle of the thyristors connected in anti-
parallel in each phase, the rms value of stator voltage can be
regulated. As a consequence, motor torque and thus speed of the
drive is controlled.
Applications:
They are mainly used in low power applications such as fans, blowers
and centrifugal pumps, where the starting torque is low.
They are also used for starting high power induction motors to limit
the in-rush current.

15. Advantages and Disadvantages
Advantages of stator voltage control :
The control circuit is very simple
 More compact and less weight
 It’s response time is quick
 There is a considerable savings in energy and thus it is a economical method
Disadvantages :
The input power factor is very low.
 Voltage and current waveforms are highly distorted due to harmonics, which affects the efficiency
of the machine.
Performance is poor under running condition at low speed
Operating efficiency is low as resistance losses are high.
Maximum torque available from the motor decreases with decreases in stator voltage.
At low speeds, motor currents are excessive and special arrangements should be provided to limit
the excessive currents.