Ward Leonard method of speed control
When we talk about speed control of Dc motor we said that there is Ward Leonard method of speed control which based on varying the applied voltage to the armature, in this system we have a Dc motor (M) powered by a Dc generator (G) and we use a motor-generator set ( which consists of either Ac or Dc motor) to have the variable voltage we need, and that makes us think in the principle of Ward Leonard method, let’s see.
The principle of Ward Leonard method:
Firstly, we use a driving or prime motor which can be:
- A Dc shunt motor.
- A 3 phase induction motor.
- Synchronous motor.
- A diesel engine or any other constant speed drove.
And as shown in the figure the Ward Leonard system is made up of a constant speed driven motor and powers a Dc generator, and the output of the generator is fed to a Dc motor, when we vary the generator’s field current its output voltage will change so the speed of the controlled motor can vary smoothly zero to full speed.
As the generator shunt field current is the main base of control we require a control equipment for small current values, and we use a potentiometer or rheostat in the generator field circuit to vary the output voltage from zero to the full value and also in either direction, so the speed and direction of the controlled motor are determined by the generator output because the controlled motor has a constant excitation.
And we may use additional switching to reverse the controlled motor depending upon the compounding arrangements between the generator and the motor.
Advantages of Ward Leonard system:
We have many advantages of Ward Leonard method of speed control as:
- The availability of a very large range of speed variation.
- It’s very easy to reverse the direction of rotation by reversing the generator field current.
- The motor can run with a uniform acceleration.
- There is an inherent braking capacity.
- We have a good speed regulation.
- And the high overall efficiency.
Disadvantages of Ward Leonard system:
Unfortunately, we can find drawbacks in all systems and in Ward Leonard system we would find:
- We use two additional machines (motor-generator set ) of the same rating of the main Dc motor so it’s higher initial cost.
- It has a large size and weight.
- This system requires more floor area and by the way costly foundation.
- There are higher losses so it has a lower efficiency.
- It also produces more noise.
- It needs frequent maintenance.
Application of Ward Leonard system:
We usually use Ward Leonard method of speed control in:
- Colliery winders.
- Electric excavators.
- Mine hoists.
- Steel rolling mills.
- Paper machines.
- Diesel-locomotives, and so on where we require a very wide and sensitive speed control of Dc motor in both directions of rotation.
Static Ward Leonard system:
Static Ward Leonard system or solid state control is the most used nowadays, as we replace the motor-generator set with a solid state converter to control the speed of the Dc motor, and we use controlled rectifiers and choppers as a converter.
If we use an Ac supply we use the controlled rectifiers to convert fixed Ac supply voltage to a variable Ac supply voltage.
And if we use a Dc supply we use choppers to convert fixed Dc voltage to variable Dc voltage.