Mike, is there a problem running the motor too slow? Say, 50hz.
Originally Posted by Mike Henderson
[OP]
Member
Mike, is there a problem running the motor too slow? Say, 50hz.
Contributor
No, below 60 Hz (for a 60 Hz motor) the motor will produce constant torque. Since HP is torque times RPM (times a factor), your motor will put out less HP but it will require less HP for the load because it is running slower (lower RPM, less air moved, less power required).
The only reason I can see for running it slower is to reduce the noise.
Mike
[Oh, one problem with running a standard induction motor too slowly is that there may not be enough air movement to cool the motor. Most induction motors have some way to move air over them - for example a TEFC motor will have a fan on the rear (usually) and it will blow air over the body of the motor. Under certain low speed conditions, the air movement may not be sufficient to keep the motor cool. People who adapt a standard 3 phase induction motor to a lathe (for example) may put an external fan blowing on the motor to make sure it is kept cool when they run it very slowly.]
[Okay, one more comment. When you exceed the rated Hz (and RPM) you move from constant torque to constant HP - that is, the torque decreases as the RPM increases. However, on a fan, as the RPM increases, the required HP goes up by a significant factor (cubed? don't remember) so you can easily exceed the capacity of the motor.]
Last edited by Mike Henderson; 03-28-2015 at 3:49 PM.
Go into the world and do well. But more importantly, go into the world and do good.
[OP]
Member
Thanks again Mike. I have noticed that certain machines seem to have adequate dust collection at 50hz, so why not slow the motor down to run quieter and save energy? Oh, and when you say "exceed the capacity of the motor", if it is running at or under FLA (say ~13.5A @ 65hz for example) how is the motor being overloaded?
Contributor
Joe, running an impeller faster than 60 hz has a couple of limitations. The impeller needs to be radial to benefit from the increase in rpm. A curved blade doesn't increase cfm at greater pressure enough to really warrant the rpm increase. The impeller also needs to be rated for faster speed. The Torit is a heavy design. I would hesitate on most hobby impellers. As the hz increases, the motor needs higher voltage to keep the hp the same. Since your motor is rated to run at 208-230, you can run in the 63-65 range without damaging the motor. My utility power is 240 which give me an extra measure so I can program my output to 240 when I run faster. I only run 65hz for short periods anyway when using the edge sander. Your Torit also runs a good quality Baldor motor which helps too. A little extra speed really benefits those situations when the machine port is a little small and you need the extra cfm at higher pressure. I'd never exceed 66 hz and never for very long but it does make a difference in the right application. Running the system at 50 as an air cleaner is pretty effective and much less horrible for my vertigo. Dave
Contributor
I would expect that if the load increased, the current would increase because it would take more power to meet the load. If your motor stays within the rated amperage you should be okay.
Just an added note, your motor may have a service factor, maybe something like 1.15. That means you can operate the motor at 15% over rated HP. When you do, the motor will get hotter - I believe the specifications say that the motor will increase by 10 degrees C if you were to operate it continuously at 15% over. And an increase of 10 degrees C will cut the life approximately in half. I'm going from memory here but that's what I remember from my studies of motors.
Mike
Go into the world and do well. But more importantly, go into the world and do good.
Contributor
David and Mike have it pretty well explained. I dealt in electrical and then HVAC/R work for years, and got a chance to teach it for a few years before retirement.
Simple thing to remember. Air has weight, and the more you move as CFM, cubic feet per Minute the more power is needed.
Retired Guy- Central Iowa.HVAC/R , Cloudray Galvo Fiber , -Windows 10
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