1. Flux Control Method
In this method, the magnetic flux due to the field windings is varied in order to vary the speed of the motor.
As the magnetic flux depends on the current flowing through the field winding, it can be varied by varying the current through the field winding. This can be achieved by using a variable resistor in a series with the field winding resistor.
Initially, when the variable resistor is kept at its minimum position, the rated current flows through the field winding due to a rated supply voltage, and as a result, the speed is kept normal. When the resistance is increased gradually, the current through the field winding decreases. This in turn decreases the flux produced. Thus, the speed of the motor increases beyond its normal value.
2. Armature Control Method
With this method, the speed of the DC motor can be controlled by controlling the armature resistance to control the voltage drop across the armature. This method also uses a variable resistor in series with the armature.
When the variable resistor reaches its minimum value, the armature resistance is at normal one, and therefore, the armature voltage drops. When the resistance value is gradually increased, the voltage across the armature decreases. This in turn leads to decrease in the speed of the motor.
This method achieves the speed of the motor below its normal range.
3. Voltage Control Method
Both the above mentioned methods cannot provide speed control in the desirable range. Moreover, the flux control method can affect commutation, whereas the armature control method involves huge power loss due to its usage of resistor in series with the armature. Therefore, a different method is often desirable ?C the one that controls the supply voltage to control the motor speed.
In such a method, the field winding receives a fixed voltage, and the armature gets a variable voltage.
One such technique of voltage control method involves the use of a switch gear mechanism to provide a variable voltage to the armature, and the other one uses an AC motor driven Generator to provide variable voltage to the armature (the Ward-Leonard System).
Apart from these two techniques, the most widely used technique is the use of pulse width modulation to achieve speed control of a DC motor. PWM involves application of varying width pulses to the motor driver to control the voltage applied to the motor. This method proves to be very efficient as the power loss is kept at minimum, and it doesn’t involve the use of any complex equipment.
The above block diagram represents a simple electric motor speed controller. As depicted in the above block diagram, a microcontroller is used to feed PWM signals to the motor driver. The motor driver is L293D IC which consists of H-bridge circuits to drive the motor.
PWM is achieved by varying the pulses applied to the enable pin of the motor driver IC to control the applied voltage of the motor. The variation of pulses is done by the microcontroller, with the input signal from the push buttons. Here, two push buttons are provided, each for decreasing and increasing the duty cycle of pulses.