Following few facts about the DC series motor should be kept in mind while studying DC series motor speed control.
- The current flowing through the armature and field winding of a DC series motor is same.
- The flux produced by any coil is given by, flux (φ) = MMF/reluctance = NI/S
This expression shows that we can change the flux produced by a coil by changing either the number of turns (N) or by changing the coil current (I). These concepts are used here to explain the flux control methods.
Series Motor Speed Control by Flux Control
The flux control technique for controlling the speed of a DC series motor can be exercised in the following four ways:
- Field Diverter Method
- Armature diverter method
- Tapped field method
- Series-parallel connection of field
Field Diverter Method
The set up for field diverter method is shown in the figure. In this method, a variable resistor R is connected across the field winding. This resistor is known as the field diverter. In this series motor speed control method we provide a parallel path for the field current. By adjusting the value of R we can adjust the current flowing through the field winding.
As we reduce the resistance of diverter, the current will be diverted away from the field winding. Whereas if we increase the resistance of diverter, less current gets diverted through it, so more field current flows. This increases the flux and reduces the speed. By this method, we can control the speed only above the rated speed of the motor.
Armature Diverter Method
In this method, the variable resistor R (known as diverter) is connected across the armature winding as shown in the figure.
As we reduce the resistance of diverter, the armature current (Ia) will decrease. The motor draws more current to produce required torque. This increases the field current. Hence flux increases and speed decreases. Thus with a reduction in diverter resistance R, speed decreases.
This series motor speed control method is used for the constant torque loads. As large power is wasted in the diverter resistance, therefore this method is neither economical nor efficient.
Tapped Field Method
The set up for this method is shown in the figure. The change in flux is obtained by changing the number of turns of the field winding. A rotary switch is used to select the tapping.
By the use of a rotary switch, we can change the effective number of turns of the field winding. Since flux is proportional to MMF (NI), the flux will also change. The motor speed increases with the decrease in flux and vice versa. This method is useful for speed control above the rated speed.
Series and Parallel Connection of Field
In this DC series motor speed control method, the field winding is divided into two or more equal parts. These parts are connected either in series or parallel according to the requirement of the speed of the motor. Due to series parallel connections, the total MMF produced by field winding will change. This will change the flux φ which will change the motor speed.
With the help of this method; we can change the motor speed in steps only, we cannot change the motor speed smoothly.
Rheostatic Control (Armature Voltage Control)
In this method, a rheostat is connected in series with the armature. Due to insertion of the rheostat R in series with the motor, the armature current will flow through it and there is a voltage drop of IaR across it. Therefore, the voltage across armature winding will reduce and speed will also reduce because the speed is directly proportional to armature voltage.
As large power is wasted in the rheostat resistance (Ia2R), therefore this method is neither economical nor efficient.
Applied Voltage Control
As the speed is directly proportional to the applied voltage, we can get the variations in speed by changing the applied voltage. In this method; the maximum voltage that can be applied to the motor is rated voltage, the maximum speed will be the rated speed. Thus the only speed below the rated speed can be obtained. The variable voltage is usually obtained from an electronic circuit.