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.

field-test-on-dc-series-motor


  • 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.

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 


No comments:

Post a comment