# How to calculate large DC motor efficiency?

## Main Question or Discussion Point

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

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billy_joule
Experimentally and analytically.

Experimentally and analytically.

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.

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

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