When is a 100 horsepower motor not a 100 horsepower motor?

Photo by Gunnar Þór Gunnarsson

Variable Frequency drives (VFDs), or Variable Speed Drives (VSDs) are excellent electrical devices that offer greater process control, as well as save energy.

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 (note 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 “4-pole motor” and will operate at 1800 rpm1. – when connected directly to the electrical motor control panel.

If however you put a VFD in the line you can adjust the frequency to anything between 10 hz2. and 90 Hz and the VFD will operate in a linear fashion between the two ranges.

As an example if the VFD is set to deliver a frequency of 30 Hz the motor will run at half its nameplate speed or 900 rpm. Similarly if 45 Hz then 3/4 the operating speed or 1350 rpm. 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 synchronous speed).

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

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

So in all practicality if you have a 100 hp motor as per the motor 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 anymore, regardless of what it says on the nameplate. You have reduced its ability to deliver that hp by slowing it down through frequency control.

This may be tricky to grasp because the nameplate clearly says 100 hp.

But if you think of every frequency pulse as an energy shot similar to 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 hp is higher than the available hp it usually shows up as tripping of the starter fuses, or an amp reading showing overloading of your MCC/motor.

The 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”.

Let’s give 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. Because1300/1800 x 100 = 72 hp.

The motor may in fact carry the load 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 amperage to meet the hp requirement.

Say you are in this situation, then what?

Okay here are some remedies to correct this. 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 90 hp 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 horsepower 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 and 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 speed. 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 ratios 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.

And always if any questions feel free to put a comment in and we will reply.

1.Actually it will run slightly lower – somewhere between 1750 and 1795 rpm due to “slippage” but for the purposes of this entry we will ignore that.

2.Lower limits exist as heat generation at the motor can only be dissipated by operating at a minimum speed.

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2 responses to “When is a 100 horsepower motor not a 100 horsepower motor?

  1. What will be the results in terms of over amps if running a VFD at 1970 rpm and you need 102 hp out of a 100 hp motor. You are well within the service factor but will the motor have enough torque?

  2. First we will assume a 1750 RPM motor and not a 3550 RPM motor.
    Horsepower remains constant above overspeed, therefore you will be able to get the 102 hp from the motor easily enough.
    Speed increases but the torque drops which creates more or less a constant output.
    If you were working with a fan or pump (variable torque) the amps would be 2% higher than name plate at 100 hp.

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