Speed control of dc motor

1. Speed control of DC motor
Presented by
Mrs. M.P.Sasirekha
Unit 3

2. DC Motor Speed Control Theory
• To derive the speed of a DC motor, we start
with the equation for the DC motor’s EMF
(Electromagnetic Force). We know that the
EMF equation of DC motor is equal to:
• Hence rearranging the equation:
• N = 60A E / PZØ

3. DC Motor Speed Control Theory
• With k = PZ/60A, then:
• N = E / kØ
• Hence with E = V – IaRa, we derive the speed
of the DC motor (N):

4. Speed of a DC motor
• Back emf Eb of a DC motor is nothing but the
induced emf in armature conductors due to
rotation of the armature in magnetic field.
• Thus, the magnitude of Eb can be given by EMF
equation of a DC generator.
Eb = PØNZ/60A
(where, P = no. of poles, Ø = flux/pole, N = speed
in rpm, Z = no. of armature conductors,
• A = parallel paths)
Eb can also be given as,
Eb = V- IaRa

5. Speed of a DC motor
• thus, from the above equations
N = E
b
60A/PØZ
but, for a DC motor A, P and Z are constants
Therefore, N ∝ K E
b/Ø (where, K=constant)
This shows the speed of a dc motor is directly
proportional to the back emf and inversely
proportional to the flux per pole.

6. Speed control methods of DC motor
Speed control of Shunt motor
• Flux control method
• Armature control method
• Voltage Control Method

7. 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.

8. Flux control method
• 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, Ish
2R loss is small.

9. Flux control method
• 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.

10. 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 Ra are kept
constant, then the speed is
directly proportional to
armature current Ia.

11. Armature control method
• 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.

12. 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.

13. 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.

14. Advantages of Armature Controlled
DC Shunt Motor
• Very fine speed control over whole range in
both directions
• Uniform acceleration is obtained
• Good speed regulation
• It has regenerative braking capacity

15. Disadvantages of Armature
Controlled DC Shunt Motor
• Costly arrangement is needed, floor space
required is more
• Low efficiency at light loads
• Drive produced more noise.

16. Field Rheostat Controlled DC Shunt
Motor
• In this method, speed variation is
accomplished by means of a variable
resistance inserted in series with the shunt
field.
• An increase in controlling resistances reduces
the field current with a reduction in flux and
an increase in speed.
• This method of speed control is independent
of load on the motor.

17. Field Rheostat Controlled DC Shunt
Motor
• Power wasted in controlling resistance is very
less as field current is a small value.
• This method of speed control is also used in
DC compound motor.

18. Disadvantages of Field Rheostat
Controlled DC Shunt Motor
• Creeping speeds cannot be obtained.
• Top speeds only obtained at reduced torque.
• The speed is maximum at minimum value of
flux, which is governed by the demagnetizing
effect of armature reaction on the field.

19. Speed Control Of DC Series Motors
• The speed control of d.c. series motors can be
obtained by two methods
• Flux control method
• Armature-Resistance control method.

20. Speed Control Of DC Series Motors

21. Flux control method
• In this method, the flux produced by the
series dc motor is varied. The variation of flux
can be achieved in the following ways:
• Field Diverters
• Armature Diverter
• Tapped Field Control

22. Field diverters
• In this method, a variable resistance (called
field diverter) is connected in parallel with
series field winding.
• A part of the line current passes through this
diverter and thus weakens the field.
Since N ∝ 1/ϕ , speed also varies with field
flux.

23. Field diverters
The lowest speed
obtained by this
method is the normal
speed of motor when
the current through
diverter is zero, ie,
diverter open
circuited.

24. Armature diverter
• In order to obtain speeds below the normal
speed, a variable resistance (called armature
diverter) is connected in parallel with the
armature of dc series motor.
The diverter reduces the armature current. As
a result flux get increased. So the speed
decreases since N ∝ 1/ϕ.

25. Tapped field control

26. Tapped field control
• In this method, the flux is reduced (and hence
speed is increased) by decreasing the number of
turns of the series field winding.
• The switch S can short circuit any part of the field
winding, thus decreasing the flux and raising the
speed.
• With full turns of the field winding, the motor
runs at normal speed and as the field turns
are cut out, speeds higher than normal speed are
achieved.

27. Armature-resistance Control

28. Armature-resistance Control
• In this method, a variable resistance is directly
connected in series with the supply.
• This reduces the voltage available across the
armature and hence the speed falls.
• By changing the value of variable resistance,
any speed below the normal speed can be
obtained.
• This is the most common method employed to
control the speed of d.c. series motors.

29. Thank you