Speed Control of DC motor
DC motors consist of rotor-mounted winding (armature) and stationary winding (field poles). By changing the armature voltage or the field current will change the rotor speed.
Speed control means change of a speed to a value required for performing the specific work process. This adjustment should not be taken to include the natural change in speed which occurs due to change in the load on the drive shaft.
We know that the emf equation of DC motor is given as,
P = no. of poles,
Ø = flux/pole,
N = speed in rpm,
Z = no. of armature conductors,
A = parallel paths (Lap winding A=P) (Wave winding A=2)
Speed is directly proportional to armature voltage and inversely proportional to the magnetic flux produced by the poles
Speed Control of D.C. Shunt Motors
There are so many methods for controlling the speed of a DC shunt motor.
field Control Method
field rheostat control method is most reliable, economic and independent of load on the motor. that speed of motor is inversely proportional to flux. Thus by decreasing flux speed can be increased and vice versa.
This method is only applicable when we want speed which is higher than the normal speed of the motor. In this method, an increase in controlling resistance reduces the field current with a consequent reduction in flux and an increase in speed.Wider speed ranges tend to produce instability and poor commutation.
Armature Control Method
when we want to obtain low speed to control the low speed mechanical drive, we use armature rheostat control method. In this method, the speed at full load can be reduced to any desired value depending on the amount of resistance.
when supply voltage and armature resistance Ra are kept constant, speed is directly proportional to armature current Ia. Thus if add resistance in series with armature, Ia decreases and hence speed decreases.
- The output and efficiency of the motor are reduced.
- This method results in poor speed regulation.
Voltage Control Method
Multiple Voltage Control: the shunt field is connected to a fixed exciting voltage and the armature is supplied with different voltages. So the Voltage across armature is changed with the help of a suitable switch-gear devises. Armature speed is approximately proportional to the voltage across the armature. and speed control is possible.
This system is used , where very sensitive speed control of motor is required (e.g electric excavators, elevators etc.)
This method involves using a motor –generator (M-G) set.
- M2 is the motor whose speed control is required.
- M1 (prime mover) may be any AC motor or DC motor with constant speed.
- G is the generator directly coupled to prime mover.
- In this method the output from the generator is fed to the armature of the motor M2 whose speed is to be controlled. The output voltage of the generator can be varied from zero to its maximum value, and hence the armature voltage of the motor M2 is varied very smoothly. Hence very smooth speed control of motor can be obtained by this method. This method is best suited for steel rolling mills, paper machines, elevators, mine hoists, etc.
- good speed control over whole range in both directions.
- The speed of the motor can be adjusted through a wide range without resistance losses which results in high efficiency.
- This method is used for the speed control of large motors when a DC supply is not available.
Speed Control of D.C. Series Motors
variable resistance is directly connected in series with the supply. This reduces the voltage available across the armature and hence the speed reduces. By changing the value of variable resistance any speed below the normal speed can be obtained. This is the most common method employed to control the speed of DC series motors.
Flux Control Method
A Rheostat is connected parallel to the series field winding. This variable resistor is called as diverter, as desired amount of current can be diverted through this rheostat and hence current through field coil can be decreased. Hence flux can be decreased to desired amount and speed can be increased and it is vise-versa.
Rheostat is connected across the armature winding. When Armature current is reduced, then flux must increase. hence flux Ø will increase and subsequently speed of the motor will decrease.
Tapped Field Control
In this method, the flux is reduced by decreasing the number of turns of the series field winding as shown in above Fig. The switch S can short circuit any part of the field winding, thus decreasing the flux and raising the speed. With full turns of the field winding, the motor runs at normal speed and as the field turns are cut out, speeds higher than normal speed are achieved.
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