## 27/11/2020

### 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 RL.

• 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 RL 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

Ra = Hot armature resistance

Rse = Hot series field resistance

Now

Input of both machines = VI1

Output of both machines = VI2

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

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

Field losses of both machines = 2I12 × Rse

Stray losses per machine ( Ws )

= Wt – [ (( I1 + I2 )2 × Ra ) + 2I12 × Rse ] / 2

Motor efficiency

Motor input = V1I1

Motor losses = ( I1 + I2 )2 × Ra + Ws

Motor output = Motor input – Motor losses

= V1I1 – { ( I1 + I2 )2 × Ra + Ws }

Motor efficiency

= { Output power / Input power } × 100%

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

Generator efficiency

Generator output = V2I2

Generator losses = I12 Rse + I22 Ra + Ws

Generator efficiency

= { Output power / Input power } × 100%

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

= { V2I2 / ( V2I2 + I12 Rse + I22 Ra + Ws ) } × 100%

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

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