The speed of DC motor is directly proportional to the back EMF (Eb) and inversely proportional to the flux (ф).
Ra = Resistance of armature circuit
V = Applied voltage
Ф = Flux per pole
Hence, from the above equation, we can say that the speed of the DC motor depends on Flux per pole, Armature resistance, and Applied Voltage.
According to controlled quantity, methods for Speed Control of DC Shunt Motor is classified as;
- Flux control method
- Armature control method
- Voltage control method
In this article, we will explain detail each method of speed control for DC shunt motor.
Flux Control Method (Field Control Method) of DC Shunt Motor
In the flux control method, a variable resistance is connected in series with the shunt field winding. By controlling the field resistance, we can control the field current.
The field current is directly proportional to the flux. And the flux is inversely proportional to the speed.
Hence, by controlling the field current, the speed of the DC shunt motor is controlled.
The connection diagram of the flux control method is shown in the figure below.
The rated field current is supplied when the rheostat is set at point A (minimum value resistance), and the motor rotates at rated speed.
As the resistance increases, the shunt field current decreases, and speed increases.
When the rheostat is set at point B (maximum value resistance), the speed reaches the highest speed. The graph of speed vs field current is shown in the figure below.
The advantages of the field control method are listed below.
- This method is easy, convenient, and more efficient.
- It is possible to change the speed smoothly.
- The shunt field current is very small. Hence, the power loss that occurs in rheostat is slight. And we can use a small size of a rheostat.
- It is possible to control speed above rated speed.
The disadvantages of the field control method are as listed below.
- We can reduce the flux below its rated value. Hence, it is possible to adjust the speeds which are only above the rated speed.
- There is a problem of commutation above the rated speed. This makes the system unstable.
- Speed below rated speed (creeping speed) cannot be achieved.
Armature Control Method of DC Shunt Motor
In this method, a variable resistance is connected across the armature winding. The connection diagram of this method is shown in the figure below.
In this method, we control the armature resistance, and hence we keep flux per pole constant.
A variable resistance is added to the armature resistance. So, total resistance across armature is;
Now, we assume that the input voltage is constant. And as the variable resistance increases, the armature voltage drop.
This reduces the voltage across the armature and results in decreased speed.
Disadvantages of Armature Control Method
A large amount of power loss occurs in variable resistance. Because it carries full load armature current. Therefore, this method is not suitable for a long period.
The voltage across the armature can be reduced but cannot increase. So, the speed of a shunt motor can be controlled only below the rated speed.
Due to the above drawbacks, this control method is not used for long-period applications. But it is used in printing machines, hoists, and cranes.
Voltage Control Method of DC Shunt Motor
In this method of speed control, the armature is supplied by a variable voltage source. Here, we will consider the flux as constant.
Hence, if we neglect the armature voltage drop, the speed of the shunt motor is directly proportional to the applied voltage. And armature voltage drop is very small. So, we can neglect it.
Ward Leonard’s method of speed control works in this principle.
An adjustable electronic rectifier is used to get variable voltage. This method gives an extensive speed control range with good efficiency and voltage regulation.
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