How to calculate large DC motor efficiency?

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Two primary methods for calculating large DC motor efficiency are experimental and analytical approaches. The analytical method involves recording motor nameplate values, calculating the electrical power for the shunt field and armature, and dividing the mechanical output power by the total electrical power. An example calculation shows that a 400 HP motor yields an efficiency of 93.8%. The experimental method requires a dynamometer to measure output shaft power while recording armature and field voltage and current. Additionally, both Swinburne’s Test and Hopkinson’s Test can be utilized, with Hopkinson's Test providing greater accuracy due to its use of two identical machines.
Ana Mido
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What are the two methods for calculating large DC motor efficiency ?
 
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Experimentally and analytically.
 
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billy_joule said:
Experimentally and analytically.
In detail, please.
 
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.
 
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Asymptotic said:
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?
 
Ana Mido said:
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).
 
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