When is a 100 hp motor not really a 100 hp motor?

Variable frequency drives (VFDs) or variable speed drives (VSDs) are excellent electrical devices that offer great process control, as well as save energy compared to non-variable drives. For example, the advantage of speed control on a fan is that it will deliver the required flow necessary for the process without wasteful over-delivering, provided that the fan and VFD are sized correctly. VFDs work by changing the frequency of the alternating current that the motor sees. In Canada and the US, the alternating current operates at 60 Hz or cycles per second (in many other countries such as South America and Europe, the electrical grid operates at 50 Hz). If the motor shows 1800 rpm on the nameplate, that indicates that the motor is a four-pole motor and will operate at 1800 rpm — when connected directly to an electrical motor control panel. (Actually, it will run at a slightly lower speed — somewhere between 1750 and 1795 rpm — due to slippage, but for our purposes we will ignore this).

If you put a VFD in the line, you can adjust the frequency to anything between 10 Hz and 90 Hz and the drive will operate in a linear fashion between the two ranges. Note that lower frequency limits exist, as heat generation at the motor can only be dissipated by operating at a certain minimum speed.

As an example, if the VFD is set to deliver a frequency of 30 Hz instead of 60 Hz, the 1800-rpm motor will run at half its nameplate speed or 900 rpm. Similarly, if 45 Hz is set, then 3/4 the operating speed or 1350 rpm is produced. This is pretty straightforward and most people know this.

But here’s the thing.

Although a motor can run at different speeds, the horsepower the motor generates is also proportional to the speed between the lower limit and its and nameplate speed (also called its synchronous speed).

This is where you can run into problems. A standard industrial induction motor generates a constant torque within its operating range (somewhere around 20% to 100% of synchronous speed). As motor horsepower is the result of torque and speed, and while torque may be constant, because the speed varies so does the ability to deliver horsepower.

Only when you hit 100% of the grid frequency (e. g. 60 Hz) can the motor develop the full 100 hp stamped on it.

So in all practicality, if you have a 100-hp motor as per the nameplate and the VFD is supplying power to the motor with a frequency less than 60 Hz, you don’t have a 100-hp motor any more, regardless of what it says on the nameplate. You have reduced its ability to deliver that horsepower by slowing it down through frequency control.

This may be tricky to grasp because the nameplate clearly says 100 hp. If you think of every frequency pulse as an energy shot similar to the combustion stroke in a car engine, and if you have half the pulses, you have only half the energy shots to do useful work.

When the required horsepower is higher than the available hp, it usually shows up by tripping the starter fuses, or as a current reading showing the overloading of your motor or motor control centre (MCC).

Your first comment might be, “The fan is drawing too much power,” when it would be much more accurate to say, “The motor isn’t delivering enough horsepower.”

Here is an example. If you require 80 hp at 1300 rpm for a process, and if a VFD is speed-controlling a 100 hp/1800 rpm motor, then at 1300 rpm the available power will only be 72 hp (1300/1800 x 100 = 72).

The motor may in fact carry the load despite this because many motors have a 15% service factor, but the amperage will be higher than the amperage on the nameplate because the motor needs to take in more current to meet the horsepower requirement.

If you have this situation, then what? Here are some remedies. One is to increase the size of the 1800 rpm motor to a larger size. In this example, a 125-hp motor will do, because 1300/1800 x 125 = 90 hp and this is more than enough to carry the load. But you will also need to upsize the fuses and possibly the VFD too.

Another way to have enough available hp is to keep the same hp but change the motor to a lower speed. A 1200-rpm motor will generate its nameplate hp at its nameplate speed and not lose any power as it goes above this point. So at 1300 rpm, the motor will still generate 100 hp, which will be more than enough to carry the 80 hp requirement. In this way, installing larger fuses isn’t required.

There are also limits to over-speeding, including mechanical — such as the motor bearing life — and there is some loss of hp drop-off with higher speeds. So it is best to confer with the motor and VFD vendor to clarify what hp and speeds are available. This being said, over-speeding is becoming more commonplace in industry as people become comfortable about using the full potential of VFDs.

Another way to deliver the required hp at reduced fan speeds is to make proper use of mechanical advantage. If the fan is driven through a belt drive, then simply ensure that the motor-fan drive ratio is set up to permit the motor to run fast enough in order to develop the necessary hp at the required fan speed. In this example, to deliver 80 hp at 1300 rpm for the fan using a 100-hp, 1800-rpm motor, the motor must be turning at least 80% of 1800 rpm or 1440 rpm.

Jim Wywrot, P. Eng., is a partner at CB Power and Industrial Equipment, Cambridge, ON. For more information, visit www.cbpowerandindustrial.com.