Speed Control In DC Machine
One of the most important advantages of the dc machines which out ruled the ac machines in industry is the ability of the speed control of dc machine is much superior and easy compared to ac machines.
Speed of the dc machine can be controlled by:
- By applying the voltage to the terminals of the dc machine
- By introducing external resistance in the armature of the machine
- By varying the flux per pole (φ) of the machine
The first two methods involve the adjustments to be done on the armature side of the dc machine whereas the third method requires adjustment in the field side of the dc machine. Therefore speed control of the dc machine are classified as armature control method and field control method. In armature control method the speed of the dc machine decreases as the applying voltage will be reduced on the other hand in field control method weakens the flux per pole of the dc machine and speeds above the rated speeds are attained
Armature Control method of Speed Control:
In armature control method voltage applied to the armature terminals is varied with out changing the field current.
Speed of the dc shunt motor by armature control can be obtained by:
- By connecting an variable external resistance in series to the armature circuit so that voltage applied to the dc machine can be varied
- By connecting an variable resistance in shunt to the armature circuit and other variable resistance in series to the armature shunt circuit.
- By providing the varying voltage to the armature terminals. The voltage provided should be separate from the voltage that provide to the field circuit
i) Armature Resistance Control Method:
In armature resistance control method a variable resistance is connected in series to the armature circuit. When the resistance of the rheostat is increased, current flowing through the circuit reduces and the voltage drop at the armature is less compared to line voltage. Thus speed of the machine reduces in proportional to the applied voltage.
This method is employed only in applications where speed variation is required for shorter duration and not continuous. This is because of the larger resistance required and high power loss during the operation at the rheostat. Ex: printing machines, cranes, hoists
ii) Shunt Resistance Control Method:
Armature resistance method is simple and easy but have disadvantages as the speed not only depends on the resistance value but also on the load current (variation in the load during the drive operation results in the variation in the speed of the machine). Therefore a divertor (shunt resistance) is employed along with the series resistance.
In this method the change in the armature current (because of the load torque) does not affect the change in the voltage across the terminals hence the speed.
iii). Armature Voltage Control:
In this method of speed control a variable source of voltage is provided to supply the power to the armature circuit. Voltage to the field circuit should be different from the variable voltage source provided to the armature.
By this method disadvantages such as poor voltage regulation and low efficiency in case of voltage resistance method is avoided, however this method is costlier compared to the other. The adjustable voltage source can be from a adjustable voltage generator or from a adjustable rectifier
Field Control Method of speed control:
In DC machines weakening of the field of the dc machine increases the speed of the dc machine and strengthening of the field will cause to reduction in the speed of the machine. Hence by weakening the field of the machine speed above the rated speed can be attained.
Speeds above the rated speed can be attained by:
- Providing a variable resistance in series to the field circuit so that variation in the resistance value results in increase or decrease in the field of the machine resulting in the speed control.
- By variation in the reluctance of the magnetic circuit of the motor.
- By varying the applied voltage to the field circuit of the dc motor by keeping the voltage supplied to the armature circuit constant
i) Field Resistance Control:
In this method variable resistance is employed in series to the field circuit. As in this method when resistance value increases only field weakens. By weakening the field speed above the normal speed can be attained
ii) Reluctance Control Method:
This method requires motor to be constructed with special mechanical features such that reluctance of the magnetic circuit can be changed. This is more expensive and rarely employed
iii) Filed Voltage Control:
This method requires variable voltage source which is different from the voltage applied to the armature circuit.