Balancing speed and efficiency in motor selection
Motor speed can be just as important as efficiency in selecting the most economical induction motor to drive so-called ``cube-law`` loads such as HVAC fans or centrifugal pumps. The amount of energy used in these loads is very sensitive to rotational speed. An increase in motor efficiency can be quickly negated by the extra energy consumed by a faster turning cube-law load. For induction motors, operating speed is always less than the synchronous speed, the familiar round number usually associated with motors (1,200 rpm, 1,800 rpm, 3,600 rpm and so on). The difference between the synchronous speed of an induction motor -- how fast it would spin if the rotor kept perfect pace with the rotating magnetic field that it is chasing -- and the rotor`s actual speed of rotation is called ``slip.`` Slip ranges from about half a percent of the synchronous speed in large motors to over 5 percent in small units; the higher the slip, the slower the motor. Induction motors of identical synchronous speed can differ significantly in operating speed. Most utility motor programs provide incentives simply on the basis of motor efficiency, ignoring the potential impact of a motor`s speed on the system`s energy use. In many cases, then, utilities are overestimating the energy savings in motor installations on cube-law loads.
- OSTI ID:
- 55537
- Country of Publication:
- United States
- Language:
- English
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