Mostly we want to control the speed of a DC motor on demand. This intentional change of drive speed is known as speed control of dc motor.
Speed control of a Direct current motor is either done manually by the operator or by means of an automatic control device. This is different from speed regulation – where the speed is trying to be maintained against the natural change in speed due to a change in the load on the shaft.
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SPEED CONTROL OF DC MOTOR
The relationship given below gives the speed control of dc motor,
Therefore, the speed of the 3 types of DC motors – shunt, series, and compound – can be controlled by changing the quantities on the right-hand side of the equation above.
Hence the speed can be varied by changing
- Resistance control or Rheostat control.
- Field flux φ Variation. This method is called field flux control.
- Applied voltage variation. Another name of this method is armature voltage control.
1.ARMATURE RESISTANCE CONTROL
Fig: (a) Speed control of a d.c. Shunt motor by armature resistance control.
(b) Speed control of a D.C. Series motor by armature resistance control.
In this system, a variable series resistor Re is put in the armature circuit. The figure (a), connection for a shunt motor above shows the process. In this case, the field is directly connected across the supply and therefore the flux Ф is not affected by the variation of Re.
Figure (b), D.C. series motor shows the method of connection of external resistance Re in the armature circuit. In this term, the current and hence the flux φ is affected by the variation of the armature circuit resistance.
The voltage drop in Re minimizes the voltage applied to the armature, and therefore the speed is reduced.
This method has the following drawbacks:
- In the external resistance Re a huge amount of power is wasted.
- Control is limited to give speed below normal and an increase of speed cannot is obtained by this method.
- For a conferred value of Re, the speed reduction is not constant but varies with the motor load.
This method is only used for small motors.
2.FIELD FLUX CONTROLφ
Since the field current produces the flux, and if we control the field current then the speed can be controlled. In the shunt motor, speed can be controlled by conjunctive a variable resistor Rc in series with the shunt field winding. In the diagram below resistor, variable resistor Rc is called the shunt field regulator.
Fig:(a) Speed control of a D.C. shunt motor by variation of field flux.
(b) The diverter in parallel with the series of D.C. Motor.
, Gives the Shunt Field Current
Any of the one systems can vary the field current of the series motor:
- A variable resistance Rd is connected in parallel with the series field winding. The resistor connected in parallel is called the diverter. A portion of the main current is diverted through Rd.
- The second method uses a tapped field control.
Here the ampere-turns are varied by varying the number of field turns. This arrangement is used in electric traction.
Figure: Tapped series field on D.C. motor
The advantages of field control are as follows:
- This is an easy and convenient method.
- The power loss in the shunt field is small because the shunt field current Ish is very small.
3.ARMATURE VOLTAGE CONTROL
We can control the speed of the D.C. motors by varying the armature of the applied voltage. The Ward-Leonard system of speed control works on these rules of armature voltage control. In this system, M is the main dc motor whose speed is to be controlled, and G is a separately excited dc generator. The generator G is driven by a three-phase driving motor which may be an induction motor or asynchronous motor. The combination of ac driving motor and the dc generator is also called the motor-generator (M-G) set.
Figure: Ward-Leonard drive
ADVANTAGES OF WARD-LEONARD DRIVES:
- This drive has a soft speed control of dc motor over a wide range in both directions.
- It has an inherent regenerative braking capacity.
- By using an overexcited synchronous motor as the drive for the dc generator, the lagging reactive volt-amperes of the plant are compensated. Therefore the overall power factor of the plant improves.
D.C MOTOR SPEED CONTROL THEORY:
To find the speed control of dc motor we start with the equation for the DC motor’s EMF. We know that the Electromagnetic force equation of DC motor is equal to:
Hence rearranging the equation:
- N = 60A E / PZØ
With k = PZ/60A, then:
- N = E / kØ
Hence with E = V – IaRa, we derive the speed of the DC motor (N):