Belt grinder question

[Johnny chaos]

I'm planning a 2"x72" belt grinder build using a new 3ph motor that I got for a really good price. I want use a variable frequency drive to get speed adjustability and need some advice. I am hoping to use an existing 220v 50a plug I have in my shop for my vfd input. I will upload a pic of the motor plate. I need some clarification on what amp vfd I should get as I am getting some varied answers on it from the people I have spoken to?

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[bullnerd]

http://dealerselectric.com/

Call this guy

 

[matt_i]

VFDs sized by HP or probably kW which is the same measurement of electromechanical power in different unit systems...correlating to the motor you'd want to drive.

745 Watts per HP, so if you can get a 1kW drive it will easily handle a 1hp motor, probably draw less than 4 amps on 240vac at full load...

Another thing to keep in mind is that you do lose HP as the frequency is turned down, so while speed is variable there's not a vast well of low-end torque to be tapped into. Below 20Hz one can get into motor cooling problems.

 

[catalytic]

Most new VFD's are nameplate rated. So you want:

220v 1phase in
220v 3phase out

rated to whatever horsepower your motor is (so if you have a 2hp motor, get a VFD rated to 2hp). A VFD rated for a larger hp is fine. Smaller VFD is sortof OK -- it will just supply enough amps to 'underpower' your motor. So a 2hp vfd on a 3hp motor will still only give you 2hp.

it USED to be that VFD's were rated for 3phase out only, and you had to de-rate (i.e. size up the VFD) to use on single phase. This is NO LONGER true for most new vfd's.

PS, Go with a NEMA 4 or NEMA 4X vfd. it costs more, but it will keep the metal grinding dust out. Dealerselectric is fine. I have one from them. Nowadays I prefer the KB electronics KBAC and KBDC units -- they are all aluminum NEMA 4X (washdown ok/dustproof), compact, and reliable. Burr King and several other mfg's now use them on the factory variable speed models.

Last note: DONT use an on/off switch between the VFD and the motor. VFD should wire directly into the motor, and use the buttons on the VFD or controls wired into the VFD to control on/off. Switching between the VFD and the motor will fry the VFD.*

* (true of almost all small VFD's... there are a few exceptions if you want to nitpick about fancier industrial drives).

 

[Johnny chaos]

Thanks for all the responses so far. My concern going into this is, lots of people have recommended I size the vfd by amps or kw and not by hp? With that being said this motor shows 7.4 amps in three phase so I assume running it single phase I should look for double the rated amps, so 15 amps? Does that sound correct? Amps go up as volts go down?

 

[exmaxima1]

Thanks for all the responses so far. My concern going into this is, lots of people have recommended I size the vfd by amps or kw and not by hp? With that being said this motor shows 7.4 amps in three phase so I assume running it single phase I should look for double the rated amps, so 15 amps? Does that sound correct? Amps go up as volts go down?

Look at the rated output current of the vfd to match your motor (it can be larger, but not significantly lower). Don't worry about the input current.

 

[Johnny chaos]

The original photo I attached of the motor plate shows 7.4 amps at 230v 3ph so I can get a 7.4 or slightly higher and be ok? I thought because I was using single phase my amperage would have to go up? I would love to use 7-10 because the price is substantially less than if i went 15 or more. Thanks so far for the help I'm def a newb when it comes to vfd's.

 

[catalytic]

I think you are making this more complicated than it needs to be.

What HP is your 3phase motor?

I would just get a good new NEMA 4X VFD with the same size.

For example, if you have a 1hp motor, I would buy a KBAC 24d or KBDC 24d. If you have a 1.5 or 2hp motor, I would buy a KBAC 27d or KBDC 27d. If you have a 3HP motor, I would buy a KBAC or KBDC 29D. This is what the vast majority of high end 2x72 grinder manufacturers use. If you are patient, you can find them NOS on ebay for under $160, at least the 24d and the 29d (27d rarely comes up IMHO).

If you want to get technical, give the mfg a call and talk to engineering. I have a couple of times. They will know exactly how much your particular VFD can actually handle, as ones with the same ratings are slightly different in terms of actual output.

 

[Johnny chaos]

Catalytic...It's a 3 hp motor. I usually do tend to overcomplicate things

"paralysis through analysis" I heard that somewhere once and have adopted it as my cripplying mantra that puts me behind on everything!!!!!

Thanks for the input.

 

[bullnerd]

Also, if you watch the video link in the other thread about building a grinder, he mentions a sealed VFD. Good idea IMO to keep all the grinding dust out.

The link I posted earlier, If the same guy still answers the phone, he knows his stuff and they sell them, done!