Electrical Revolution: 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.

Electrical Revolution