To study speed control of D.C. shunt motor by field current control method.

Experiment No.: 09

Aim of the Experiment:

To study speed control of D.C Shunt motor by field current control method.

Apparatus Required:

Sl. No.NameSpecificationRemarks
1D.C Shunt Motor2 BHP, 1440 rpm, 110V, 18.1A1 nos.
2Ammeter(0-10)A, PMMC1 nos.
3Ammeter(0-5)A, PMMC2 nos.
4Voltmeter(0-150)V, PMMC1 nos.
5TachometerDigital type1 nos.
6Connecting WirePVC Insulated Copperas per required.
7MultimeterDigital type2 nos.
8Rheostat(0-80)Ω2 nos.
Table:9.1

Circuit Diagram:

Theory:

D.C motor is made to have smooth and effective control of speed over a wide range. The shunt motor runs at a speed defined by the expressions-

\[E_b=\dfrac{(Z*N*ɸ*P)}{(60*A)}\] \[E_b= V – I_aR_a\] \[Where,\; N = \dfrac{E_b}{Kɸ}\] \[K = \dfrac{PZ}{60A},\; N\; is\; the\; speed, \] \[V \;is\; the\; applied\; Voltage,\] \[I_a \;is\; the \;armature\; current,\;\] \[R_a \;is\; the\; armature\; resistance\] \[ and \; ɸ \;is\; the\; field\; flux.\;\]

Methods for speed control of D.C Shunt Motor are

  1. Applied Voltage Control Method.
  2. Armature Current Control Method.
  3. Field Current Control Method.

Field Current Control Method:

Speed control by adjusting the air gap flux is achieved by means of adjusting the field current i.e by adding an external resistance in the field circuit.

The disadvantage of this method is that at low field flux the armature current will be high for the same load.

This method is used to run the motor above its rated speed only.

Procedures:

  1. Connect the apparatus as per circuit diagram.
  2. Connect the external rheostat in armature circuit and field circuit of D.C Motor.
  3. Keep at maximum value of resistance in armature circuit and minimum value of resistance in the field circuit of the D.C motor.
  4. Gradually increase the voltage till the motor attains the rated voltage.
  5. Increase the external resistance in the field circuit and record the readings as per observation table.

Precaution:

  1. Check the polarity of the Supply voltage, all instruments as well as D.C motor terminals.
  2. Don’t forgot to increase the external resistance in the armature circuit at the time of starting of D.C motor.

Observation Table:

Sl. NoSupply Voltage (VS) in volts.Field Voltage (VF) in volts.Armature Current (Ia) in Amps.Field Current (If) in Amps.External Resistance (R2) in Ohm.Speen (N) in RPM
1
2
3
4
5
6
Table: 9.2

Plot the graph between field current and speed.

Conclusion:

To be written by student.

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