Mechanical Efficiency and Power Loss Calculator

What is mechanical efficiency?

No machine turns all the power it takes into useful work. Part is lost to friction, churning and heat. Efficiency is the ratio of output power to input power: η = P_out / P_in. It is always below 1, that is, below 100 percent.

The difference is the power loss, which becomes heat: P_loss = P_in − P_out = P_in × (1 − η).

Required input power

If the wanted output is known, input power is P_in = P_out / η. A common mistake is to multiply by the efficiency instead. For 7.5 kW at the output with 90 percent efficiency the input must be 7.5 / 0.9 = 8.33 kW, not 7.5 × 1.1 = 8.25 kW.

Stages in series

When power passes through several elements, the overall efficiency is the product of the individual ones: η_total = η₁ × η₂ × η₃. A 95 percent motor, a 90 percent gearbox and a 98 percent belt drive in series give 0.95 × 0.90 × 0.98 = 0.838, or 83.8 percent. The least efficient element in the chain has the largest effect.

Example

A drive that draws 10 kW and delivers 8.5 kW has an efficiency of 85 percent and a loss of 1.5 kW. That 1.5 kW turns into heat continuously, and in a closed housing it has to be removed.

Limits

Efficiency changes with load and speed. Catalogue values are usually for rated load, and efficiency can drop noticeably at low load. For energy cost estimates use the real operating point and running hours.