Field Test

Field Test

This is one of the methods of testing the d.c. series motors. Unlike shunt motors, the series motor cannot be tested by the methods which are available for shunt motors as it is impossible to run the motor no load. It may run at dangerously high speed on no load. In case of small series motors brake test may be employed.

The series motors are usually tested in pairs. The field test is applied to two similar series motors which are coupled mechanically. The connection diagram for the test is shown in the Fig. 5.9.1.

As shown in the Fig. 5.9.1 one machine is made to run as a motor while the other as a generator which is separately excited. The fields of the two machines are connected in series so that both the machines are equally excited. This will make iron losses same for the two machines. The two machines are running at the same speed. The generator output is given to the variable resistance R.

The resistance R is changed until the current taken by motor reaches full load value. This will be indicated by ammeter A1. The other readings of different meters are then recorded.

Let V = Supply voltage,

I₁ = Current taken by motor

I₂ = Load current,

V₂ = Terminal p.d. of generator

R_a, R_se = Armature and series field resistance of each machine.

Power taken from supply = V I₁

Output obtained from generator = V₂ I₂

Total losses in both the machines, W_T = V I₁ V₂ I₂

Armature copper and field losses,

Since the two machines are equally excited and are running at same speed the stray losses are equally divided.

For motor :

 For generator :

Efficiency of generator is of little importance because it is running under conditions of separate 16 excitation. Still it can be found as follows.

The important point to be noted is that this is not regenerative method though the two machines are mechanically coupled because the generator output is not fed back to the motor as in case of Hopkinson's test but it is wasted in load resistance.

Ex. 5.9.1 A Field's test on two mechanically coupled similar motors with their fields connected in series and with one machine running as motor and the other as a generator gave the following data:

Motor: Armature current 40 A, armature voltage 200 V, the drop across its field winding 15 V.

Generator: Armature current 32 A, armature voltage 160 V, the drop across its field winding 15 V.

 The resistance of each armature is 0.4 2. Calculate the efficiency of each machine at this load.

Sol. : The connections and various voltages and currents are as shown in the Fig. 5.9.2.

Ex. 5.9.2 A test on two coupled similar tram way motors, with their fields connected in series, gave the following results when one machine acted as a motor and the other as a generator.

Motor:

Armature current = 56 A, Armature voltage = 590 V. Voltage drop across field winding = 40 V.

Generator:

Armature current = 44 A, Armature voltage = 400 V. Field voltage drop = 40 V. Resistance of each armature = 0.3 Ω.

Calculate the efficiency of the motor and generator at this load.

Sol. : The connections and various voltages with currents are as shown in the Fig. 5.9.3.

Review Questions

1. Explain how will you obtain the efficiency of a D.C. series machine, by conducting the field test.

2. A test on two coupled similar tramway motors, with their fields connected in series, gave the following results when one machine acted as a motor and the other as a generator. Calculate the efficiency of motor and generator. Motor: Armature Current: 56 A, Armature Voltage: 590 V,

Voltage drop across field winding: 40 V

Generator: Armature current: 44 A

Armature voltage: 400 V

Field voltage drop: 40 V

Resistance of each armature: 0.3 Ω

(Ans.: 72.7%, 67.02%)