Testing of Dc machines - 2

Retardation or Running Down Test

This method is applicable to shunt motors and generators and is used for finding stray losses. Then,
knowing the armature and shunt Cu losses at a given load current, efficiency can be calculated.
The machine under test is speeded up slightly beyond its normal speed and then supply is cut off from
the armature while keeping the field excited. Consequently, the armature slows down and its kinetic energy
is used to meet the rotational losses i.e. friction, windage and iron losses.*
Kinetic energy of the armature is K.E

where I = moment of inertia of the armature and w= angular velocity
 Rotational losses, W = Rate of loss of K.E.

Finding Moment of Inertia (I)

(a) First Method–where I is calculated.
First, slowing down curve is drawn with armature alone. Next, a fly-wheel of known moment of inertia
I1 is keyed onto the shaft and slowing down curve is drawn again. Obviously, slowing down time will be
longer due to combined increased moment of inertia of the two. For any given speed,
(dN/dt1) and (dN/dt2) are determined as before. It should be noted that the losses in both cases would be
almost the same, because addition of a fly-wheel will not make much difference to the losses.
Hence, from equation (ii) above


(b) Second Method–where I is eliminated.

In this method, first, time taken to slow down, say by 5%, is noted with armature alone. Next, a
retarding torque–mechanical or preferably electrical, is applied to the armature and again time is noted.
The method using electrical torque is shown in Fig. 31.12. The double-throw switch S while cutting off
the armature from supply, automatically joins it to a non-inductive resistance R as shown. The power
drawn by this resistance acts as a retarding torque on the armature, thereby making it slow down
comparatively quickly. The additional loss is Ia
2 (Ra + R) or VIa ,
where Ia = average current through R ; V = average voltage across R.


Field’s Test for Series Motor
This test is applicable to two similar
series motors. Series motors which are
mainly used for traction work are easily
available in pairs. The two machines are
coupled mechanically.
One machine runs normally as a
motor and drives generator whose output
is wasted in a variable load R (Fig.
31.14). Iron and friction losses of two
machines are made equal (i) by joining
the series field winding of the generator
in the motor armature circuit so that both
machines are equally excited and (ii) by running them at
equal speed. Load resistance R is varied till the motor
current reaches its full-load value indicated by ammeter
A1. After this adjustment for full-load current, different
ammeter and voltmeter readings are noted.
Let V = supply voltage ; I1 = motor current ; V2 = terminal
p.d. of generator ; I2 = load current.
 Intake of the whole set = VI1 ; output = V2I2.
Total losses in the set, Wt= VI1 ? V2I2
Armature and field Cu losses Wcu = (Ra + 2Rse)I1
2 + I2
2Ra
where Ra = hot armature resistance of each machine
Rse = hot series field resistance of each machine