How to Calculate Motor Sizing
What is Motor Sizing?
Motor sizing matches horsepower to load requirements. Over-sizing wastes energy; under-sizing risks overheating and failure.
Formula
Calculate: P = (T × RPM) / 5252 for mechanical power
- P
- (T × RPM) / 5252 for mechanical power — (T × RPM) / 5252 for mechanical power
- T
- T value — Variable used in the calculation
- RPM
- RPM value — Variable used in the calculation
Step-by-Step Guide
- 1Calculate: P = (T × RPM) / 5252 for mechanical power
- 2Apply service factor 1.15-1.25 for continuous duty
- 3Select standard motor size meeting service factor requirement
Worked Examples
Input
Pump load 7.5 kW continuous, service factor 1.25
Result
Motor = 7.5 × 1.25 = 9.4 kW, select 10 kW standard motor
Next size up
Common Mistakes to Avoid
- ✕Using continuous rating for intermittent duty
- ✕Neglecting starting current requirements
Frequently Asked Questions
What is service factor?
Allowable overload margin (typically 1.15-1.25); accounts for load variations and starting surges.
Why not over-size motors?
Larger motors are less efficient at partial load; match size to duty point.
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