Why a 1 HP Motor Draws More Than 746 Watts (Efficiency Math)
Calculate input electrical watts required to output mechanical torque horsepower.
Understanding Electric Motor Efficiency
When you see a motor rated at 1 Horsepower (HP), it means the motor produces 746 Watts of mechanical output work. However, to produce those 746 watts of work, the motor must pull more than 746 watts of electrical power from the wall. This difference between input power and output power is defined by the motor's efficiency.
Where Does the Lost Energy Go?
No machine is perfectly efficient. In electric motors, power is lost primarily through:
- I²R Losses: Electrical resistance in the copper windings generates heat.
- Core Losses: Magnetic hysteresis and eddy currents within the steel core generate heat.
- Friction & Windage: The bearings and the cooling fan mounted on the shaft create mechanical drag.
The Motor Efficiency Formula
To calculate the true input power (in Watts) required to drive a motor, use the following formula:
Where Efficiency is expressed as a decimal (e.g., 85% = 0.85).
Worked Example: Calculating Input Watts
Imagine you have a 5 HP industrial fan motor with a rated efficiency of 88%.
- Calculate theoretical output watts: 5 HP × 746 = 3,730 Watts.
- Divide by efficiency: 3,730 / 0.88 = 4,238 Watts.
The motor requires 4,238 Watts of electrical input to deliver 5 HP of mechanical power.
Apparent Power (VA) Calculation
If you need to size a generator or a circuit breaker, you need to calculate the Apparent Power (Volt-Amps). The formula expands to:
Let's use our 5 HP motor again, assuming an 88% efficiency and an 80% (0.80) power factor.
VA = 3730 / (0.88 × 0.80) = 3730 / 0.704 = 5,298 VA.
This means you need a power source capable of at least 5,300 Volt-Amps (roughly equivalent to sizing for 5,300 Watts on a generator) to run this 5 HP motor smoothly.