Occasionally we are asked the question about the ability to over-speed a motor using a variable frequency drive (VFD). This question usually comes up when there are capacity issues with a mechanical system and the fan or 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 460/480vac 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). An 1800 rpm motor can be run safely up to 120 Hz or 200% of its rated speed but…as the accompanying graph indicates, at double speed, the motor can only operate with 25% of its nominal full speed torque.
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.
The following information discusses in some detail what happens to the Volts/Hz ratio and also what the motor torque implications are when operating a motor at higher speeds than its nameplate speed.
AC MOTOR OPERATION ABOVE BASE SPEED
Maximum Motor Speed
Standard AC motors rated for 60Hz operation may be run at higher frequencies when powered by an AC drive. The top speed depends upon the voltage limits of the motor and its mechanical balancing. 230V and 460V motors normally employ insulation rated for as much as 1600V, so the voltage limit is not usually a problem. Average 2 pole industrial motors can safely exceed its base speed by 25%. Many manufacturers balance their 2 pole and 4 pole rotors to the same speed – 25% over the 2 pole base speed. A 2 pole motor may therefore operate up to 125% over base speed before reaching its balance limit. A 60hz 4-pole motor may have the ability to run up to 135Hz, whereas a 60 Hz 2-pole motor would reach its balance limit at 75Hz. Both motors would run at the same rpm. Always contact your motor’s manufacturer if you plan to operate at these speeds.
Constant Voltage Operation
What happens to the volts per hertz ratio above a rated frequency? If output frequency is increased to 120hz with 100% voltage applied to the motor; the Volts per Hertz ratio of the drive is no longer 7.6 but rather 3.83. The same Volts per Hertz ratio results when a line started motor is operated at 60Hz with only 50% voltage applied (for reduced voltage starting). As might be expected the effect on torque is the same. Recall that torque varies as the square of the applied voltage when using a reduced voltage starter:
Torque = (Start Voltage E2 / Base Voltage E1) quantity squared.
As such, the maximum torque at 120Hz is only 25% of the maximum torque at 60Hz. If the VFD output frequency is reduced from 120hz to 90hz at a constant voltage, the Volts per Hertz ratio improves from 3.83 to 5.1V/Hz. This is the same as providing 66% voltage at 60Hz to a line-started motor. Torque will be 0.662 or 44% of the full voltage torque at 60Hz. The graph illustrates the peak torque curve for constant voltage operation from base speed to 4 times base speed.
Motor Torque Above Base Frequency
Since the voltage, in reality, is not changing above the base speed, it is more appropriate to define torque in terms of frequency or RPM change instead of voltage change. It can be stated then that torque above base speed drops as the square of the frequency – doubling the frequency, quarters the available torque. Applied frequency and synchronous speed are equivalent, so going a step further; torque may be defined in terms of speed. In the constant voltage range then, motor torque drops off as the inverse of synchronous speed squared, or 1/(N squared). This is shown in the accompanying graph.
Torque = (line frequency/extended frequency)squared
Torque = (base speed/extended speed)squared
Many machine applications are constant horsepower in their load characteristics. As speed increases, the torque drops off as the inverse of the speed, or 1/N. The torque drop-off is not as severe as the motor’s peak torque, 1/(Nsquared). The accompanying graph compares peak torque to rated torque.