Hertz and RPM Relationship

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Tom.Margillo

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Here is my situation, I have a AHU motor that is supposed to run at 1823 RPM to give me the correct CFM. The motor is 480v 100amp 60hz controled by a VFD. The motor is drawing about 70amps on all three phases and the voltage is ok at the motor. The problem is that when I run the VFD at 100%(60hz) I only get 1802RPM. Here is the question I have without sounding to stupid, if I increase the hertz lets say to 61 will the RPM increase? Here is the really crazy question, exactly what is hertz? :confused: :confused: :confused:
 
Re: Hertz and RPM Relationship

Yes but you will have to talk to the motor maker to see if it can be run above the design speed.

It sounds like the motor is not fully loaded, a better option would be to replace the sheave on the motor with a larger one to speed up the fan.
 
Re: Hertz and RPM Relationship

"Hertz" is the unit of AC frequency or cycles per second. Induction motors must "slip" in order to operate, therefore they run a bit slower than a synchronous motor, say 1750RPM as opposed to 1800.

[ February 01, 2006, 07:12 AM: Message edited by: rattus ]
 
Re: Hertz and RPM Relationship

Bob,
It sounds like the motor is not fully loaded, ...
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It seems to me that the motor may be overloaded. The slip is greater than the designed slip and that happens at high loads.
If the motor current is below nameplate, I would have no problem with increasing the hertz to get to design speed or even a little above.
Don
 
Re: Hertz and RPM Relationship

Hz = inverse time, cycles per second. For a four pole motor, 1800 Hz = 60 RPM. You have a four pole motor. For 1823, (1823/1800) gives 60.766 or 61 Hz. So yes to q#2, input 61 Hz to get there. Remarkable how close VFDs get things - with a direct connected motor you'd see that 'slip' turn the motor at an actual value closer to 1700 than 1800. Ignore 'slip' for this VFD; it's something you won't see. If you're overruning past capacity, the current will peg and the motor will never reach that value; 60 to 61 won't make any difference.
I'm a little concerned about this. A VFD would have been specified for either energy savings, or to avoid loading the electrical mains. It sounds as if you're taking on the task of commissioning, for which your questions show that you're really not familiar with VFDs. You may do better getting help from local reps, either the fan or VFD maker. This forum cannot supply specific programming information for this item.
 
Re: Hertz and RPM Relationship

Tom,

What is the nameplate horsepower and RPM of the motor? What is the design bhp? I agree with Bob that it sounds like the motor is not fully loaded. If the motor is not running at design bhp it will not provide design CFM. I think peteo hit it on the head, get other people involved, IMO the AHU supplier first.

Tony
 
Re: Hertz and RPM Relationship

Thanks for all the information. I do have all other responsible parties involved, I didn't want to be totaly lost during conversations between suppliers and commisioning personel. So once again thanks for the help, I owe everyone a beer. :) :) :) :)
 
Re: Hertz and RPM Relationship

Remarkable how close VFDs get things - with a direct connected motor you'd see that 'slip' turn the motor at an actual value closer to 1700 than 1800. Ignore 'slip' for this VFD; it's something you won't see.
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I'm not following that. If it is an induction motor, it should be turning slightly slower than the applied voltage. If the VFD cranks out 60HZ, and its a 4 pole motor, that's 1800 RPM. The motor should turn slower if it is loaded.

Tom: Slip is just what Rattus said. The motor turns slightly slower than the rotating magnetic field created by the voltage. At no load, the motor will turn almost as fast as the magnetic field, and we say the slip is almost zero. As you increase the load on the motor, the motor turns slightly slower. Thats a larger slip. If you way overload the motor, it will stop. That would be infinite slip.

Steve
 
Re: Hertz and RPM Relationship

I think you have a mis-application on your hands. At least, something is not making sense to me. Please note that a motor does not know anything about its power source. The only thing different between a motor designed to work with a VFD and a motor designed to work with a constant 60 hertz supply has to do with bearings and/or grounding of the shaft. The motors themselves work in exactly the same way.

{ASIDE: By the way, if your motor is not designed to operate with a VFD, then you better change it out now. Otherwise, you will be changing out the damaged motor in a few weeks.}

So I have to agree with Steve: The motor can't be designed to run above 1800 RPM with a 60 hertz supply, unless it is designed to run far, far above 1800 RPM (the next step up is 3600 RPM). You might have a motor that is designed to run with a VFD, that needs to run at 1823 RPM to give you your desired CFM, that has 2 poles (not four), and that would run at 3600 RPM if you hit it with 60 hertz, and that therefore would normally run nearer to 30.4 hertz.

Finally, keep in mind that the motor does not care what you are hoping to get, in terms of CFM. It takes incoming power, and it pushes its load in circles. If you aren't getting the CFM you want, it's not the motor's fault. It's the fault of the designer(s) who selected that particular motor and fan combination.
 
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