Speed Control of the DC Shunt Motor

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

• Supply voltage
• Flux per pole ( Flux control )
• Armature resistance control ( OR Rheostatic control )

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

As

( N₂ / N₁ ) = ( E_b2 / E_b1 )

( N₂ / N₁ ) = [ V – I_a2 ( R_a + R ) ] / [ V – I_a1 R_a ]

If we consider no load speed N₀

( N / N₀ ) = [ V – I_a ( R_a + R ) ] / [ V – I_a0 R_a ]

Neglecting I_a0 R_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

• This method is wasteful because large power loss in the armature circuit.
• Poor voltage regulation is obtained particularly at lower speed

( 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.

Advantages

• Smooth speed control
• Wide range of speed control from maximum to minimum
• Good speed regulation is achieved

Disadvantages

• Two extra machines require therefore the initial cost is high
• Low efficiency of the system particularly at light load
• More space requires as there are two extra machines require

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