This document discusses three main methods of controlling the speed of a DC motor: flux control, armature control, and voltage control. Flux control varies the motor's flux by adjusting the field current, allowing speeds above rated speed. However, it only works above rated speed and has a maximum speed limit. Armature control varies the motor's speed by adjusting the armature circuit resistance, permitting speeds below rated only. It wastes a large amount of power and has poor regulation. Voltage control involves adjusting the applied voltage using various methods like Ward-Leonard drives when the power source is AC or chopper control when it is DC.

1. ED&C – Speed
controlling methods of
DC Motor
BY
PK/AP/EEE/KEC

2. Speed control methods of DC Motor
• The speed of a DC motor is given by:
• Three main methods of
• controlling the speed of a D.C. motor
• By varying the flux per pole (f) - flux control method
• By varying the resistance in the armature circuit - armature control
method.
• By varying the applied voltage - voltage control method.2

3. Flux control method
• NαEb/Φ
• Φ α Ish
• If Ish decrease then
Φ also decrease
• N increase when
Φ decrease
• Speed above rated speed is possible
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4. Flux control method
Advantages
(i) An easy and convenient method
(ii) It is an inexpensive method since very little power is wasted in the shunt field rheostat
due to relatively small value of Ish.
(iii)The speed control exercised by this method is independent of load on the machine.
Disadvantages
(i) Only speeds higher than the normal speed can be obtained since the total field circuit
resistance cannot be reduced below Rsh—the shunt field winding resistance.
(ii) There is a limit to the maximum speed obtainable by this method.
The field of a shunt motor in operation should never be opened because its speed will
increase to an extremely high value.
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5. Armature control method
• NαEb/Φ
• Nα(V-IaRa-IaRc)/Φ
• If Ia reduce then
Eb also Reduce hence
Speed also reduced
• Below rated speed only
possible in this method
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6. Armature control method
Disadvantages
• (i) A large amount of power is wasted in the controller resistance since it
carries full armature current Ia.
• (ii) The speed varies widely with load since the speed depends upon the
voltage drop in the controller resistance and hence on the armature
current demanded by the load.
• (iii) The output and efficiency of the motor are reduced.
• (iv) poor speed regulation
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7. Voltage control method
When the supply is AC
1. Ward-Leonard system
2. Transformer with taps and an uncontrolled
rectifier bridges
3. Static ward leonard scheme or controlled
rectifier
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8. Voltage control method
When the supply is DC
1. Chopper Control
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9. Ward Leonard Drive
• when low rating and closed loop control is not desired
Control of generator filed is obtained by rheostats
• For high power application or closed loop control, the field is
supplied by a power amplifier
• Power amplifier Consists of
controlled rectifier
Chopper
Transistor amplifier
Magnetic amplifier
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10. Ward Leonard Drive
• For reversible drives
• Power amplifier capable of supplying
controlled field current in either direction
• In such case Power amplifier may be
Single phase or three phase fully
controlled dual converter
Four quadrant chopper
Four quadrant transistor amplifier
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11. Ward Leonard Drive
• When drives operates only in one direction
• power amplifier may be
Half controlled rectifier
Step down chopper
One quadrant transistor amplifier
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12. 12
Controlled
Rectifier
Uncontrolled
/Controlled
Rectifier
VM
Ia
ac motor
Three phase supply
Ward Leonard Drive
VG >Eb
Generator
DC Motor

13. 13
Controlled
Rectifier
Uncontrolled
/Controlled
Rectifier
VG < Eb
VM
Ia
Generator
Three phase supply
Ward Leonard Drive
Motor

14. Ward Leonard Drive
Applications:
• Rolling Mills
• Mine winders
• Paper mills
• Elevators
• Machines tools etc
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15. Ward Leonard Drive
Advantages:
• Inherent regenerative braking capability
• Employed for PF improvement etc
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16. Ward Leonard Drive
Disadvantages:
• High initial cost
• Low efficiency
• Requires frequent maintenance
• Produces more noise
• Large weight and size, needs large floors
area and foundation
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