Field Test

Introduction

• The brake test is applicable only for small DC series motor. 
• The field test is carried out on large DC series motor in order to find out various losses and efficiency.
• There are two similar DC series motor requires in this test. 
• The field winding of both series motors are connected in series in order to make iron losses of both machines are equal. 
• One machine acts as a motor which drive a second machine as a separately excited generator. 
• The output of the generator is connected to variable resistance load R_L.

• The motor M is started with help of starter. The voltage V₁ across the motor terminals is kept equal to rated value. 
• Obviously, the supply voltage V is equal to V₁ plus voltage drop across the field winding of the generator. 
• The load resistance R_L is varies until the ammeter I₁ show the full load current of the motor. 
• Now the readings of the all meters are to be noted. 
• The value of armature resistance is measured by V – I method or by using multi meter.

Let

V = Supply voltage

I₁ = Motor input current

V₁ = Voltage across motor

V₂ = Output voltage of generator

I₂ = Load current of generator

R_a = Hot armature resistance

R_se = Hot series field resistance

Now

Input of both machines = VI₁

Output of both machines = VI₂

Total losses of both machines ( W_t ) = VI₁ – VI₂

Armature copper losses of both machines = ( I₁ + I₂ )² × R_a

Field losses of both machines = 2I₁² × R_se

Stray losses per machine ( W_s )

= W_t – [ (( I₁ + I₂ )² × R_a ) + 2I₁² × R_se ] / 2

Motor efficiency

Motor input = V₁I₁

Motor losses = ( I₁ + I₂ )² × R_a + W_s

Motor output = Motor input – Motor losses

= V₁I₁ – { ( I₁ + I₂ )² × R_a + W_s }

Motor efficiency

= { Output power / Input power } × 100%

=  [ V₁I₁ – { ( I₁ + I₂ )² × R_a + W_s } / V₁I₁ ] × 100%

Generator efficiency

Generator output = V₂I₂

Generator losses = I₁² R_se + I₂² R_a + W_s

Generator efficiency

= { Output power / Input power } × 100%

= { Output power / Output power + Losses } × 100%

= { V₂I₂ / ( V₂I₂ + I₁² R_se + I₂² R_a + W_s ) } × 100%

• This is not regenerative test because the generator output is wasted instead of feedback to input of the motor.

You may also like :

Standard Alternating voltage in India

Compare Aluminum and Copper

Right of way ( ROW ) in the transmission line

Minimum Clearance between transmission line and Ground