Electrical Revolution

Introduction

The speed of DC motor is related to following equation

N a E_b / F

N = K ( V – I_aR_a ) / F

Therefore the speed of the DC motor can be controlled by varying

Speed control of DC Shunt Motor

( 1 ) Flux control method

The speed variation is obtained by inserting variable resistance is in series with the field circuit.
If the supply voltage is kept constant, back emf E_b is also constant.

N a 1 / F

It means that an increase in field resistance reduces the field current consequently reduction of field current and increase in speed.
As the field current is very small the field copper loss is very small. Therefore this method is very efficient and economical.
The maximum speed can be obtained by minimum value of flux which affects the effect of armature reaction.
At the same time increase in armature current causes over heating of armature, poor commutation and instability.
Therefore there is some limitation to obtain high speed.
The maximum to minimum speed can be obtained in the ratio of 6: 1 in the inter polar machine whereas it will be ratio of 2 : 1 in the non inter polar machine.
This method of speed control is applicable to achieve speed above normal or rated speed.

( 2 ) Armature resistance control

A variable resistor is inserted in series with the armature winding in this method.
As the supply voltage is kept constant, voltage across armature is equal to supply voltage minus voltage drop across rheostat.
As a variable resistor increases, the voltage drop across resistor increases resulting voltage drop across armature decreases. This will result in speed of the motor decreases.
Greater the value of variable resistor, greater fall in speed.

Let

I_a1 = Armature current in first case

I_a2 = Armature current in second case

N₁ = Speed in first case

N₂= Speed in second case

V = Supply voltage

R_a = Armature resistance in first case

R_a + R = Armature + variable resistor in second case

( N₂ / N₁) = ( E_b2 / E_b1 )

( N₂ / N₁) = [ V – I_a2( R_a + R ) ] / [ V – I_a1R_a ]

If we consider no load speed N₀

( N / N₀) = [ V – I_a( R_a + R ) ] / [ V – I_a0R_a ]

Neglecting I_a0R_a as compared to supply voltage V

( N / N₀) = [ V – I_a( R_a + R ) ] / V

N = N₀[ V – I_a( R_a + R ) / V ]

N = N₀[ 1 – I_a( R_a + R ) / V ] …………. ( 1 )

For a given value of ( R_a + R ) in the armature circuit, the speed is linear function of armature current.

By putting N = 0 in the equation ( 1 )

N₀[ 1 – I_a( R_a + R ) / V ] = 0

I_a = V / ( R_a + R )

This is maximum current and it is known as stalling current.
This method is employed when speed below the normal speed is required for short period duty i.e. printing machine, crane, hoist etc.

Disadvantages

( 3 ) Voltage control ( Ward – Leonard method )

The field winding is permanently connected to fixed supply voltage and voltage across armature varies by means of either variable supply voltage system or motor – generator set.
The speed of motor is approximately proportional to voltage across armature.
The M is main DC shunt motor whose speed control is required as shown in the Figure A.
The field of the motor is connected to exciter.
The variable voltage across armature is supplied through induction motor – DC generator set.
The DC generator is driven by induction motor whose shaft is coupled to an exciter.
The voltage of the generator is varied from maximum to minimum value by means of a field regulator.
The generated voltage is given to the DC shunt motor.
The generator voltage and direction of motor M is reversed by reversing switch RS.
The induction motor – generator set always run in the same direction.