# How to calculate large DC motor efficiency?

1. May 10, 2017

### Ana Mido

What are the two methods for calculating large DC motor efficiency ?

2. May 10, 2017

### billy_joule

Experimentally and analytically.

3. May 11, 2017

### Ana Mido

4. May 12, 2017

### Asymptotic

For a reasonable estimate at full power rating:
• Record motor nameplate values.
• Calculate electrical power required by the shunt field, and armature power. Add them together.
• Divide mechanical output (shaft) power into the electrical power required to generate it.
Example: 400 HP. 1750 RPM. 500V/633A armature. 300V/4.84A shunt field.
316.5 kW + 1452 watts = 317.952 kW
400 HP = 298.28 kW
298.28/317.95 = 93.8%

Experimentally, design and build a suitably sized dynamometer (or haul the motor to a rewind shop large enough to have the requisite equipment), and couple it to the motor. Measure output shaft power while simultaneously measuring and recording armature and field voltage and current. Do the math.

5. May 12, 2017

### Ana Mido

What about Swinburne’s Test and Hopkinson’s Test ?. Can they be used for calculating large Machine efficiency?

6. May 12, 2017

### Asymptotic

Sure.
To do a Hopkinson you need two identical DC machines, but it's more accurate than a Swinburne (which is run no-load, and doesn't account for several significant effects).