DC Machines can be tested by
three different methods namely Direct Method, Indirect Method and Regenerative
Method. Direct Method of testing of DC Machine, also known as Brake Test (if
carried out for a DC Motor) will be discussed in this post.

Direct method is suitable
for small DC machines. In Direct Method, the DC machine is subjected to rated
load and the entire output power is wasted. The ratio of output power to the
input power gives the Efficiency of DC Machine. For a DC Generator the output
power is wasted in resistor.

Direct Method of testing
when conducted on a motor is also known as Brake Test. Brake Test of DC Motor
is carried out as shown in figure below.

A belt around the air cooled
pulley has its end attached to the spring balance S1 and S2. Using belt
tightening hand wheels H1 and H2, the load of motor is adjusted to its rated
value. Assuming the spring balance to be calibrated in kilogram, then rated
load on the DC motor is given as

Motor
Output Power = Torque x Angular Speed

= (Force x Radius) x
Angular Speed

As the torque because of
force F1 and F2 are opposing each other, therefore net torque will be subtraction
of torque because of F1 and F2.

Therefore,

Motor Output = ω (S1 – S2) x
r x9.8 Watt

Now assuming the terminal
voltage of DC Motor to be V

_{t}and I_{L}to be the load current then,
Power input to the DC Motor
= V

_{t}I_{L}_{}

Thus the efficiency of DC
Motor can be calculated as below.

Efficiency = Output / Input

= [ω (S1 – S2) x r x9.8
Watt] / V

_{t}I_{L}_{}

For conducting Brake Test on
DC Series Motor, it must be ensured that belt is sufficiently tight before the
motor is switched on to the sully as DC Series Motor shall not be started at no load.

**Disadvantages of Brake Test:**

1) The
Spring Balance Readings are not stable rather it fluctuates.

2) Output
power is wasted.

3) The
frictional torque at a particular setting of Hand wheel H1 and H2 do not remain
constant.

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