Motor Operating Horsepower Formula and New Required Horsepower due to RPM Change
Motor Operating HP= Name Plate Horsepower x (Actual Amps)/(Full Load Amps)
Existing Motor Name/Data Plate Information Example:
Horsepower 10 HP
Full Load Amps (FLA) 37.7
230 Volts 3 Phase
1750 Rpm
Power Factor (PF) .91, % Efficiency.88
Service Factor 1.15
Amp Meter Reading 16 Amps at 3000 CFM
Step 1: New HP = 10 HP x (16 RLA)/(37.7 FLA)
Step 2: New HP = 10 HP x .424
Step 3: New HP = 4.2 HP is what it is currently operating at.
Motor HP requirement when Airflow Increases= Exist HP x ((New RPM)/(Exist RPM))^3
For Example: We need to increase the Current RPM of 875 to 1167 RPM to go from 3000 CFM to 4000 CFM. How much more HP will we need to achieve this?
Step 1: New HP= 4.2 HP x ((1167 RPM)/(875 RPM))^3
Step 2: New HP = 4.2 HP x 1.33373
Step 3: New HP = 4.2 HP x 2.3723
Step 4: New HP = 9.96 HP Motor needed to increase the RPM from 875 to 1167.
Horsepower increased by the cube of the Change in RPM. It’s not a linear conversion. The example above 1167 RPM / 875 RPM was 1.3337 or a 133% increase in rpm. The new horsepower 9.96 HP / 4.2 HP is 2.35 or a 235% increase in required Horsepower!