Electrical Drives & Control

1. Electrical Drives & Control
Topic : 1) Ward Leonard Method
2) Characteristics of Series Motor
Presenter : Pavithran S/O Selvam
Lecturer : Mr. Pratap Nair

2. Ward Leonard System
• It is combined armature and field control.
• This control system is consisting of a dc motor and powered
by a dc generator.
• Speed of the dc motor is controlled by applying variable
voltage across its armature.
• This variable voltage is obtained using a motor-generator set
which consists of a motor(either ac or dc motor) directly
coupled with the generator.
• It is a very widely used method of speed control of DC motor.

3. • Speed of M1 controlled by G.
• Shunt field of M1 is connected across the dc supply lines.
• G is driven by the motor M2 & its speed is constant.
• Output voltage of G is fed to M1 then it starts to rotate.
• Controlling of output voltage of G the speed of M1 also can be controlled.
• A field regulator is used to control field excitation which is connected across the
generator with dc supply lines.
• Direction of rotation of M1 can be reversed by excitation current of generator
with the help of reversing switch.

4. • At Nb, the motor armature is fed at rated voltage and its field current is
adjusted to the max. value , i.e the field is excited at rated voltage.
• Reducing the armature voltage provides a constant- torque speed control
where the speed can reduced below the base value, while the motor has full
torque capability (as If = max and Ia can have rated value.)
• For obtaining speed above Nb, the field is gradually weakened.
• The motor torque therefore reduces as its speed increases which
corresponds to constant kW drive.

5. Ward Leonard System
• Advantages of Ward Leonard System :
• The absence of an external resistance improves the efficiency
at all speeds .
• No special starting gear is required. As the generator induced
voltage is gradually raised from zero, the motor starts up
smoothly.
• Speed reversal is smoothly carried out.

6. Ward Leonard System
• Disadvantages of Ward Leonard System :
1. Very costly because 2 extra machines which are motor –
generator set are required.
2. Overall efficiency of the system is not sufficient especially it is
lightly loaded.

7. Ward Leonard System
Application Of Ward Leonard System :
• Colliery Winders
• Cranes
• Electric Excavators
• Mine hoists
• Elevators
• Steel rolling mills
• Paper Machines

8. Series Motor
Torque Vs Armature
• Field winding is connected series
with armature.
• Before magnetic saturation of the field, flux Φ is directly
proportional to Ia.
• Therefore, before magnetic saturation Ta α Ia2.
• After magnetic saturation of the field winding, flux ф is
independent of armature current Ia.
• Therefore, the torque varies proportional to Ia only, Tα Ia.
• After magnetic saturation , Ta-Ia curve becomes straight line.

9. Series Motor
Speed Vs Armature Current (N-Ia)
• Flux is directly proportional to Ia, speed is also inversely
proportional to Ia.
• Armature current becomes very small the speed becomes
dangerously high. ( it should never start without mechanical
load.)
• In Heavy loads, armature current Ia is large and speed is low
which results in decreased back emf Eb.
• Due to decreased Eb, more armature current is allowed.

10. Thank You….