The Ward-Leonard Velocity Control Method is used to control the speed of a DC motor. It is a basic anchor control method. This control system consists of a direct current motor M1 and is powered by a direct current generator G. In this method, the speed of the DC motor (M1) is controlled by applying variable voltage across its armature.
The Ward-Leonard method of speed control is based on the fact that the speed of DC motors can be changed by varying the voltage applied to the armature. The schematic diagram of the Ward-Leonard method is shown in the figure.
Solid-State Control or Static Ward Leonard System:
Nowadays the Static Ward Leonard System is mostly used. In this system the rotating motor-generator set (MG) is replaced with a solid state converter to control the speed of the DC motor. Regulated rectifiers and choppers are used as converters.
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EXPLANATION: The Ward-Leonard method of speed control is achieved by varying the voltage applied to the armature.
Armature control for DC motors
One possibility is to implement armature resistance, where a variable resistor is placed in series with the circuitry of the armature. Once the resistance is increased, the current flow through the circuit is reduced and the armature voltage drop is less than the line voltage.
The flux control method is based on the principle that by varying the field flux ϕ, the speed of the DC shunt wound motor can be changed. 𝑁∝1φ In this method, a variable resistor (called the field resistor) is placed in series with the shunt field winding.
Back EMF is the system in the coil of an electric motor that opposes the current flowing through the coil when the armature rotates. As the speed varies, the winding properties can vary, resulting in back EMF variation.
The Hopkinson test is a method for testing the efficiency of DC machines. The Hopkinson test is known as a regenerative test or back-to-back test or heat-run test. This test requires two identical shunt machines that are mechanically coupled and also electrically connected in parallel.
Explanation: Plugging is the best braking method among all braking techniques. When plugged in, the value of the armature current is reversed and the mechanical energy is withdrawn. When clogged, a very high braking torque occurs.
There are three types of electric braking, all applicable to the common types of electric motors viz. Clogging (or counter-current braking), dynamic (or rheostatic) braking, and regenerative braking.
Explanation: In the Ward-Leonard speed control method, the speed can be reduced below the base value by reducing the armature voltage. Increasing the field current can reduce the speed, but this is not used in the Ward-Leonard method.
Therefore, the pole changing method is not used to control the speed of the slip ring induction motor.
Field failure relay, also known as field loss relay protects the shunt wound DC motor or main circuit under open field conditions or where field power is absent. The speed of the shunt wound DC motor increases when the field supply is weakened.
Explanation: The series-parallel control is superior to rheostatic control in terms of efficiency. However, it is limited to two speed levels. The method is usually used to control the speed of in-line traction motors.
Voltage regulation in transmission and distribution systems is typically achieved through the use of Stepped Transformers. This method adjusts the voltage on the line by changing the secondary EMF of the transformer by varying the number of secondary turns.
3.15A) means that the field flux is directly proportional to the armature current and therefore the torque is proportional to the square of the current. Reversing the direction of the applied voltage (and hence current) therefore leaves the direction of the torque unchanged.
Brake test concept:
It is a direct method to determine the efficiency of a DC motor. It consists in applying a brake to a water-cooled pulley mounted on the motor shaft, as shown in fig. The brake band is fixed using wooden blocks that grip the pulley.