Starting and Reversing speed of a DC shunt motor

OBJECTIVES:

  • To become familiar with the starting and reversing the speed of a D.C. shunt motor.
  • To become familiar with the starting of a D.C. shunt motor using 3-point starter.

CIRCUIT DIAGRAM:

THEORY:

                   When a D.C. motor is connected to its rated supply, it draws high current which is more than rated armature current. This current may cause damage to the armature winding. The motor voltage equation is given by-

V=Eb+IaRa

Where,  Eb= Back e.m.f= \( \frac{PφZN}{60A} \) ,  Ia= Armature Current,  Ra= Armature Resistance

Ia=(V-Eb)/Ra

So,  Eb = KØN

As, Eb α N, and at starting N=0, So Eb = 0,

At starting the armature current equation becomes

Ia=V/Ra

Generally the value of the armature resistance is very low for that huge current will flow through armature winding. To reduce the heavy inrush of such current a suitable rheostat is to be connected in series with the armature winding. Resistance needs to be gradually decreased with the increase of speed.

Torque equation is given by,

T= K Ia Ø   [Where, Ø = Flux per pole, Ia= Armature Current,  K= Constant]

Torque is related to Iand  Ø.. So, by changing the polarity of either Ior  Ø the direction of torque can be changed.

EQUIPMENTS/INSTRUMENTS REQUIRED:

Name of apparatus Quantity Type Range Maker’s Name
Voltmeter

 

Ammeter

 

Resistors

 

Techo Meter

 

3-point starter

 

DEVICE UNDER TEST:

  1. DC motor

 PROCEDURE:

  • The circuit is connected as shown in the circuit diagram.
  • The DC supply is switched on with minimum resistance in the field circuit and the starting handle is moved to first position slowly.
  • As soon as the arm makes contact with first stud of the 3 point starter, the motor stars running.
  • The armature current increases rapidly and settles to a steady value.
  • The armature current, armature voltage and field current at the instant of starting are noted.
  • Gradually all resistances are cut by further moving the arm to the right slowly.
  • When the arm reaches the run position, all the resistances are bypassed and starting arm would be attached with electromagnet.
  • The armature current, armature voltage and field current and speed are noted when values settle to a steady value.
  • The direction of rotation of motor is observed at different connection mentioned in table-2.
  • The supply is switched off and starting arm comes to off position automatically.

EXPERIMENTAL DATA:

TABLE-1

Obs. No. Operation mode Field Current

(If) A

Armature Current (Ia) A Speed (N)

rpm

1. At the start      
2. Final      

 TABLE-2

Particulars of observation Speed (N), rpm Direction of rotation
Connection as in circuit diagram    
Reverse field winding only

 

   
Reverse armature winding only    
Reverse both armature and field winding    

 PRECAUTION:

  • Voltage across armature should not be applied without applying voltage to the field winding of a D.C. motor.
  • All connections are to be tightly connected.

 CONCLUSION:

 

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