Dc motor are indispensable for many adjustable speed drives as Speed control can be achieved in dc motor.The expression for for speed of a dc motor is given by:
The above expression reveals that the speed can be controlled by adjusting any one of the three factors appearing on the right hand side of the expression:
- applied voltage to the armature terminals,V
- external resistance in the armature circuit,R
- flux per pole,Φ
Speed control methods are broadly classified as :
- Armature control methods
- Field control methods
Speed Control of DC shunt motor
1.Field control methods
Weakening of field causes increase in the speed of a motor and strengthening of field reduces the speed.Speed adjustment can be obtained by:
- Field-rheostat control involving the variation of flux by means of a field rheostat.
- Reluctance control involving the variation of the reluctance of the magnetic circuit of the motor.
- Field-voltage control involving the variation of voltage applied to the field circuit keeping the voltage applied to the armature terminals constant.
Field Rheostat control:
This method of speed control is very simple,convenient and most economical and used in modern electric drives.In this method the speed variation is achieved by means of a variable resistance inserted in series with the shunt field.An increase in controlling resistance reduces the field current with a consequent reduction in flux and an increase in speed.controlling resistance is made up of slide-wire type resistor to provide continuously variable speed cover the range. This method is independent of load on the motor.
Fig:Field Rheostat control For a DC shunt motor
The value of controlling resistance to provide a desired increase in speed can be determined form the magnetisation characteristics of the machine.
Let the initial values of back emf,flux and speed be Eb1,Φ1 and N1 respectively and let the back emf and flux be Eb2 and Φ2 respectively at new speed N2.
since N ∝ Eb/Φ
Eb1 and Eb2 can be calculated from the values of supply voltage,armature resistance and armature current Ia1 and Ia2. Therefore flux Φ2 can be determined by above equation.Form the magnetization curve of the machine the value of filed current corresponding to the value of Φ2 can be obtained .And from the value of field current ,the controlling resistance can be determined.
Limitations and Drawbacks
- Flux only can be reduced.
- creeping speeds cannot be obtained.
- High speed are only obtained by reducing torque.
- At the higher speeds the field is very weak and causes the armature current to increase which causes the overheating of armature,poor commutation and instability.To avoid instability caused by the relatively powerful effect of armature reaction on high speed dc shunt motor are often provided with a relatively weak series field winding.
fig:Speed-Torque characteristics of DC shunt motor with Field control
2.Armature control method
speed adjustment of dc shunt motors by armature control may be obtained by:
- Armature resistance control involving the variation of voltage applied to the armature terminals by means of a variable resistance connected in series with the armature.
- Shunted armature control involving the variation of voltage applied to the armature terminals by means of a combination of a variable resistance shunting the armature and a variable resistance in series with the shunted armature.
- Armature terminal voltage control involving variation of voltage impressed upon the armature circuit.
Armature Resistance control:
This method consists of a variable resistance connected in series with the armature.The voltage across the armature drops as the current passes through the series resistance and the remaining voltage applied to the armature is lower than the line voltage.Thus the speed is reduced in direct proportion to this voltage drop at the armature terminals.Field current will remain unaffected as the shunt field is directly connected across the supply mains.
fig:Armature Resistance control method for DC shunt motor
fig:speed-torque characteristics of DC shunt motor with Armature control
For a constant torque load,the armature current remains the same so input to the motor remains the same but the output decreases in proportion to speed.
Let Eb1 be the back emf at speed N1 with armature current Ia1 and no extra resistance in the armature circuit.When an extra resistance R is inserted in the armature circuit,the back emf be Eb2 speed N2 and armature current Ia2 and flux remaining the same.
From the above equation(i) we can determined the value of the resistance to be inserted in series with the armature.
Considering no load speed,
If the voltage drop in armature at no load is neglected then we have,
For the given resistance (Ra+R) the speed is a linear function of armature current Ia.
The motor speed will be Zero i.e the motor will be stalled when Ia=V/(R+Ra).This is the maximum current and is known as stalling current.
This method of speed control is employed where the speeds lower than rated one are required for a short period only.such as in printing machines,cranes and hoists where the motor is frequently started and stopped.It is also employed where the load drops off rapidly with decrease in speed as in fans and blowers.