Determining Motor Efficiency


The NEMA definition of energy efficiency is the ratio of its useful power output to its total power input and is usually expressed in percentage, as shown in Equation 7.


By definition, a motor of a given rated horsepower is expected to deliver that quantity of power in a mechanical form at the motor shaft.

Figure 5 is a graphical depiction of the process of converting electrical energy to mechanical energy. Motor losses are the difference between the input and output power. Once the motor efficiency has been determined and the input power is known, you can calculate output power.


NEMA design A and B motors up to 500 hp in size are required to have a full-load efficiency value (selected from a table of nominal efficiencies) stamped on the nameplate. Most analyses of motor energy conservation savings assume that the existing motor is operating at its nameplate efficiency. This assumption is reasonable above the 50% load point as motor efficiencies generally peak at around 3/4 load with performance at 50% load almost identical to that at full load. Larger horsepower motors exhibit a relatively flat efficiency curve down to 25% of full load.

It is more difficult to determine the efficiency of a motor that has been in service a long time. It is not uncommon for the nameplate on the motor to be lost or painted over. In that case, it is almost impossible to locate efficiency information. Also, if the motor has been rewound, there is a probability that the motor efficiency has been reduced.

When nameplate efficiency is missing or unreadable, you must determine the efficiency value at the operating load point for the motor. If available, record significant nameplate data and contact the motor manufacturer. With the style, type, and serial number, the manufacturer can identify approximately when the motor was manufactured. Often the manufacturer will have historical records and can supply nominal efficiency values as a function of load for a family of motors.

When the manufacturer cannot provide motor efficiency values, you may use estimates from Attachment C. Attachment C contains nominal efficiency values at full, 75%, 50%, and 25% load for typical standard efficiency motors of various sizes and with synchronous speeds of 900, 1200, 1800, and 3600 rpm. Attachment C indicates “industry average” full- and part-load performance for all standard efficiency motors currently on the market.

Three steps are used to estimate efficiency and load. First, use power, amperage, or slip measurements to identify the load imposed on the operating motor. Second, obtain a motor part-load efficiency value consistent with the approximated load either from the manufacturer or by interpolating from the data supplied in Attachment C. Finally, if direct-read power measurements are available, derive a revised load estimate using both the power measurement at the motor terminals and the part-load efficiency value as shown in Equation 8.


For rewound motors, you should make an adjustment to the efficiency values in Attachment C. Tests of rewound motors show that rewound motor efficiency is less than that of the original motor. To reflect typical rewind losses, you should subtract two points from your standard motor efficiency on smaller motors (<40 hp) and subtract one point for larger motors. Shops with the best quality-control practices can often rewind with no significant efficiency degradation.

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