Page 328 - Electronic Mechanic - TP - Volume - 2

ELECTRONICS MECHANIC - CITS

EXERCISE 220: Construct the circuit diagram for speed
control of DC shunt motor (phase control

method or ward Leonard method)

Objectives

At the end of this exercise you shall be able to:
• to demonstrate the speed control of DC shunt motor by using ward Leonard method
• to draw the speed torque characteristics of DC shunt motor.

Requirements

Tools/ Equipments/ Instruments
• Combination Plier 250mm - 1 No. • Rheostat 220s / 2A - 1 No.
• Screwdriver Set 150mm - 1 No. • Ammeter (0 - 2)A - 2 Nos.
• Wire Stripper - 1 No. • Voltmeter (0- 300)V - 1 No.
• Fuse - 5 Nos.
Materials/ Components
• 1 HP three phase induction motor - 1 No.
• 1 HP three phase AC induction motor - 1 No. • 1 HP DC shunt motor - 1 No.
• 1 HP DC shunt generator - 1 No. • 1 HP DC shunt generator - 1 No.
• 1 HP DC shunt motor - 1 No. • Ward leonard speed control
• Three phase 3 pole MCB - 1 No. system study trainer
• Star - delta starter - 1 No. • Ammeter (0 - 10)A MC - 1 No.
• DC MCB - 2 Nos. • Voltmeter (0 - 300)V MC - 1 No.

Procedure

This system is used where an unusually wide and very sensitive speed control is required as for colliery winders,
electric excavators, elevators and the main drives in steel mills and blooming and paper mills. M1 is the main
motor whose speed control is required. The field of this motor is permanently connected across the dc supply
lines. By applying a variable voltage across its armature, any desired speed can be obtained. This variable
voltage is supplied by a motor-generator set which consists of either a dc or an ac motor M2 directly coupled to
generator G. The motor M2 runs at an approximately constant speed. The output voltage of G is directly fed to
the main motor M1. The voltage of the generator can be varied from zero up to its maximum value by means of
its field regulator. By reversing the direction
of the field current of G by means of the reversing switch RS, generated voltage can be reversed and hence the
direction of rotation of M1. It should be remembered that motor generator set always runs in the same direction.
We know that, in DC Shunt motor, the back emf is given by
ϕZNP
Eb = V – IaRa =
60A
So Speed
N = (V-IaRa) / ZϕP x 60A (1)
N α 1/ ϕ and N α 1/ Ra (N is speed in RPM) Where ϕ is flux per pole & Ra is armature resistance.

• Connect the circuit as shown in circuit diagram.
• At base speed motor armature is fed at rated voltage.
• Field current is adjusted to the maximum value.
• For obtaining speeds below base speed armature voltage is reduced.
• For obtaining speeds above base speed field is gradually weakened.

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CITS : E & H - Electronics Mechanic - Exercise 218&219