There are two main Speed Control Methods of DC Motors:
- DC Shunt Motors
- DC Series Motors
And the rest of the DC motors are the hybrid ones i.e. manufactured from both the DC series and shunt motors. Now, due to the wide usage of these motors; their speed control is of main concern. The following are the details that describe how to control the speed of DC motors.
Speed Control Methods of DC Shunt Motors:
How can you control the speed of DC shunt motors? There are two common methods and one method which is used rarely because of its limitations.
- Field resistance RF adjustment
- Terminal voltage adjustment applied to the armature
- Inserting a resistor in series with the armature circuitry
Field Resistance Adjustment: To understand what happens when we change the field resistance, suppose that the field resistance increases gradually and observe the outcome of the motor. If this happens, then the field current decreases as it is clear from the following relation.
IF = VT/RF
Due to the decrease in the field current, the flux in the motor also decreases. Due to the decrease in the flux, the instantaneous decrease in the integrated voltage happens which is obvious from the following relation.
EA = KΦw
And hence, the increase in the armature current occurs.
IA = VT – EA/RA
The induced torque in the motor is given by
Tind = KΦIA
Since the flux in this motor decreases but the armature current increases, but which factor will decide that the induced torque will increase or decrease. The answer to this question is that the armature current will be higher in the result of decreasing the flux. Since the induced torque gets increased than the required load torque, the speed of the motor increases.
Infield resistance control, the lower the field current in a shunt (or separately excited) dc motor, the faster it turns; and the higher the field current, the
slower it turns. Since an increase in field current causes a decrease in speed, there is always a minimum achievable speed by field circuit control. This minimum
speed occurs when the motor’s field circuit has the maximum permissible current
flowing through it. If a motor is operating at its rated terminal voltage, power, and field current, then it will be running at rated speed, also known as base speed. Field resistance control can control the speed of the motor for speeds above base speed but not for speeds below the base speed. To achieve a speed slower than base speed by field circuit control would require excessive field current, possibly burning up the field windings.
Terminal voltage adjustment applied to the Armature: The second method of speed control of DC shunt motors is to control the terminal voltage at the armature without any changes in the voltage applied to the field. This must be kept in mind that the motor should be of separately excited one.
If the terminal voltage VT is increased, the armature current in the motor must rise which is given by
IA = VT – EA/RA
As armature current increases, the induced torque in the motor increases which is given by
Tind = KΦIA
Making the induced torque greater than the required load torque, making the speed of the motor increased. But as the speed of the motor increase, the integrated voltage EA, causing the armature current to decrease. This decrease in the armature current IA decreases the induced torque causing the induced torque equal to the required load torque at a higher rotational speed of the motor.
In armature voltage control, the lower the armature voltage on a separately
excited dc motor, the slower it turns; and the higher the armature voltage, the faster it turns. Since an increase in armature voltage causes an increase in speed,
there is always a maximum achievable speed by armature voltage control. This
maximum speed occurs when the motor’s armature voltage reaches its maximum permissible level. If the motor is operating at its rated voltage, field current, and power, it will be turning at base speed. Armature voltage control can control the speed of the motor for speeds below base speed but not for speeds above base speed. To achieve speed faster than base speed by armature voltage control would require excessive armature voltage, possibly damaging the armature circuit.
Insertion of the resistor in series with the armature circuitry: If a
the resistor is inserted in series with the armature circuit, the effect is to drastically increase the slope of the motor’s torque-speed characteristic, making it operate more slowly if loaded. Insertion of a resistor is a very wasteful method of speed control since the losses in the inserted resistor are very large. For this reason, it is rarely used. It will be found only in applications in which the motor spends almost all its time operating at full speed or in applications too inexpensive to justify a better form of speed control.
Speed Control Method of DC Series Motors:
Unlike with the DC Shunt motors, there is only one efficient method to control the DC series motors which are the controlling of the terminal voltages at the armature.
If the terminal voltage is increased, the first term in the following equation increases, increasing the speed of the motor.
W = [VT/(√KC√Tind)] – [(RA + RS)/KC]
There is also another method of inserting the series resistor with the armature circuit of the DC series motors but it is too much loss method and can cause damage to the motors.