by Joe Evans, Ph.D. PumpTech Inc., P&S Editorial Advisory Board

Higher Efficiency Versus Motor Speed

The synchronous speed of a four-pole motor is 1,800 rpm. This is the speed at which the magnetic field rotates about the stator. The actual rotational speed of the rotor (slip speed) will always be less. The actual slip speed depends on the design. Because of the design characteristics of premium-efficiency motors, their slip speed will be higher than most motors produced prior to EISA. Usually, this is not a problem, but in some applications, it should be considered. For example, if a pump is tested in the factory at 1,725 rpm and it operates at 1,760 rpm, the speed increase is only 2 percent. However, because horsepower varies as the cube of the change in speed, it could increase by 6 percent. Slip speed is also sensitive to the terminal voltage. It increases with an increase in voltage and decreases with a drop in voltage.

As a final note, three-phase voltage unbalance reduces both insulation life and motor efficiency. An unbalance of only 2 percent can reduce the useful life by 50 percent and efficiency by 2 percent. Testing for voltage unbalance, finding the cause and fixing it are important. If not, you probably will not be able to benefit from improved nameplate efficiency.