Occasionally we are asked the question about the ability to over-speed motors of water pumps using variable frequency drives (VFD) / variable speed drives (VSD) Constant Pressure Booster Pump Inverter. This question usually comes up when there are capacity issues with a mechanical system and the water pump motor is being relied upon to compensate.
In general, the answer to the question is yes – a motor can be run at higher than its nameplate speed. However, there is a trade-off between operating at higher speeds and the resulting torque (rotational force) capability of the motor. Typically, in most common commercial applications with a constant supply voltage such as 220/240vac the relationship between torque and speed is such that the torque is reduced as the motor speed is increased. There is a direct relationship between speed and electrical AC operating frequency. For example, 0 Hz is equal to 0 rpm and 60 Hz is equal to full speed. Hence, 120 Hz would be the equivalent of double speed.
Rules of thumb- a 3600 rpm motor can be run safely up to about 75 Hz (or 25% above its rated speed).
The real limiting factors for over-speeding a motor are not so much electrical as they are mechanical. Bearing wear and rotational instability resulting from changes in mechanical balancing requirements at speeds above the nominal can lead to motor failure and/or unacceptable vibrations when operating a motor continuously at higher than its rated top speed.
The golden rule to remember is that it is critical to confer with the motor manufacturer before attempting to over-speed any motor.