Motor Losses In Terms Of Load

The motor nameplate should give the motor's efficiency. For a 50 HP (37 KW) motor, the efficiency should be something like 90-95%, which means that the losses would be 5-10%, or 1.85-3.70 KW; i.e. 1850-3700 watts. At part loads, these losses may be a little less, but they don't drop off very much. (In other words, at light loads the motor is less efficient. This is typically accompanied by lower power factor also.)

The foregoing assumes that the motor still runs at full speed, with the partial loadings accomplished by solenoid or slide valves or the like. If the lighter loads are achieved by VFD (slowing the motor down), the losses drop off somewhat more, which improves your situation, but I don't know exactly how much. The VFD data may have further information on this.

Hmm. 25 + 5 + 35 + 25 + 10 = 100% losses.

From that I would say you have a 0% efficient motor.

Tornado has the right idea. First you must know the efficiency of the motor because that tells you the TOTAL losses at full load. The percentages you stated are all based on full load and track pretty much the same as load decreases, however the Core Losses, probably better known as magnetic losses, are ralatively constant regardless of load. So as load decreases, the magnetic losses make up a greater PERCENTAGE, but the value (kW) stays the same.

The total losses in a high efficiency motor at full load will be about 5-6%. So in a 50 hp motor, the wattage input is 39kW. The losses are 39kW - 37.3kW, so about 1.7kW. Split this up per your percentages.

At the other loads, you would need to look at the performance curves for a particular motor, but I would not be surprised it the same motor would have fallen to 80% efficiency at 25% load.

The motor manufacturer can provide this information with almost 100% certainty. A telephone call to the technical helpline, perhaps?

As long as it is not an old motor that they don't have data on. Probably anything made in the 1980's or newer will have been required to have efficiency data available, maybe 1970s? But I know that on older motors, nobody kept that information, I've tried to get it to no avail.