3/4 HP enough for RF30 on VFD?

martik777

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I have an old gryphon 3/4 3 phase motor that I'd like to use on my Taiwanese RF31 with a VFD. Will have to mod the frame to bracket plate and make a 5/8" to 7/8" shim for the shaft but I'm wondering if 3/4HP will be adequate. According to ME consultant the max RPM I need for the work I do is 1/4HP. The original motor is 1.5HP but I doubt it puts out more than 1
 
That's exactly what I am thinking about doing. I have two 3/4 hp 3 phase motors but one (baldor) is tiny compared to the ge industrial 3/4 I have.
I think the sheer mass of the industrial will keep it moving though most anything I would do. The smaller one will go back on the heavy 10 it came off of. Curious what the gryphon looks like? I just posted pics of my ge in my thread.
 
Here it is. likely from the 1950's: I used it for my RPC before I got a VFD

I plan to use this VFD which I already have on my lathe:

gryphon.jpggryphon2.jpggryphon2.jpg
 
That's a cool motor, I have not seen one of them before.
Just going by what 3/4 does on the lathe I would think it should be fine as long as your vfd handles the load well.
Hopefully some people who have used one on a mill will chime in. I'll be testing mine out next week at the earliest.
 
Remember the hp goes down as speed goes down - constant torque. So at 1/2 speed you have 3/8 hp. etc.

Bigger motors are installed to get enough power to run the machine at slower and slower speeds.

So your question should be, how low an RPM can I run this motor on this machine?
 
I agree with Karl, the usual recommendation when upgrading to a VFD is to use a motor around 1.5-2X the stock motor Hp if you plan to use the VFD variable speed. The other concerns with the motor you have are the age of the motor/insulation rating and also cooling. The stock RF31 motor is rated as 1.5/2.0Hp per the manual, so going with a 3/4Hp with a VFD is going to be pretty wimpy. You would be better off with something like a 2Hp TEFC/TENV type of motor that is inverter rated. The newer motors will also can be used with higher carrier frequency so less motor whine and you can usually use them out to 2X their base speed. This would give you a much wider speed range, similar to the 2 speed motors offered with current RF31's. You can often pick up motors inexpensively via auctions.

Although you often are using a fraction of the rated Hp, if you are drilling with a large bit or something like a hole saw then you would want a bigger motor.
 
Good points in support of a larger motor, I could always go back to the belt changes for those very rare slow speed jobs but I'll keep a lookout for a larger motor.
 
I thought most current VFDs had solved the torque problem but maybe not?
I have not noticed much loss of power even when crawling at 10 hz although I have not done extensive testing.
This is with a lenze vfd.
 
I get usable torque down to 15hz which is 100rpm on the lowest belt setting, gives me a range of 100-600rpm max 80hz
 
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