Re: VFD help

Jim B. <btdtrf@...>

With an electric motor running off a VFD, the Torque is constant at all practical speeds. The Power varies linearly with RPM. 
Thus at 30 Hz or 1/2 speed you will have only 1/4 HP. 
So how little HP can you live with?
Further the VFD lowers voltage as the frequency (RPM) is decreased. 
It also does not increase Voltage over 60HZ. So there is no increase in input power and hence the motor power will be constant over 60 HZ driving frequency. 

There seems to be no problem driving a 60 Hz motor at 90 Hz. 
Thus your 1800 RPM motor could run with acceptable power and constant torque between 900 and 2700 RPM. 
To maximize this range you would like the spindle to turn at about 1000 RPM when the motor is at 90 Hz. 
I would guess this might be with the belt on the middle step. 

I run a 3/4 HP motor on a band saw. 
The actual HP needed to cut metal is low. 
Even thick metal. 
If I don't push it I can reduce the frequency to 7 Hz. I replaced the (nominal) 1800 RPM motor with a 1150 RPM motor. 
At 7 Hz I cut steel. At 90 HZ I cut wood. 

Jim B,

On Jul 15, 2016, at 8:56 PM, 'John Fischer' n2nu@... [SOUTHBENDLATHE] <SOUTHBENDLATHE@...> wrote:




You will get much more torque at low speeds if you use the pulleys and belts.  That said, sometimes it is nice to be able to just turn a control to tweak the speed.


I still remember the machine shop course I took where the instructor put a 2x4 sideways in the 3 jaw so that the 4 inch dimension would bear against the ways.  In back gear, the lathe didn’t even hesitate…





Sent: Friday, July 15, 2016 8:49 PM



I just bought a SB 9A with a 1/2 horsepower 3 phase motor. I ordered VFD to power it off of the 110/220 in my shop. The VFD has a motor speed control.


My question is - is it better to put the belt and pulleys on the lathe to the highest speed position and control the speed of the lathe with the VFD, or place the VFD on 100% and control the lathe speed with the pulleys?

Would they both have the same power at slow,back gear speed?


Thanks, John.


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