Testing of DC machines: retardation vs Swinburne's test

Necessity of testing DC machines

Testing is done to find the performance of a DC machine without (or before) putting it in service:

• To find losses (constant and variable) and hence efficiency at any load.
• To check temperature rise and heating under load, which decides the rating.
• To check commutation and sparking at the brushes.
• To find characteristics (OCC, load and speed-torque curves) and confirm the design and name-plate values.
• For acceptance and quality control by manufacturer and buyer.

Direct loading of large machines wastes much energy and needs a large load, so indirect tests such as Swinburne's (no-load) test and the retardation test are used.

Swinburne's test

The machine runs as a shunt motor on no load at rated voltage and speed. The input $V I_0$ minus the small no-load armature copper loss gives the constant losses (iron + friction and windage + shunt field copper loss) directly. The efficiency at any load is then calculated.

Retardation (running-down) test

The machine is run above rated speed and the supply to the armature is cut off (field kept excited for iron loss). The armature slows down as its stored kinetic energy supplies the rotational losses. The time taken to fall through a known speed range (say 1.1 $N$ to 0.9 $N$) is measured:

$$P_{rot} = \omega J \frac{d\omega}{dt}$$

$J$ is found by repeating the test with a known extra load (or an added flywheel). Running down with the field off gives the mechanical loss alone, so the iron loss can be separated.

Difference