How low can you go.

Suzuki4evr

Suzuki4evr

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Hallo guys.

I know that a motor needs speed for the motor fan to keep it cool,but as you know your motor speed falls as you go down with the frequency to controle spindle on your mill using the vfd. Is there a dangerously low frequency you can run on a vfd. My motor runs at 50.00Hz at full tilt. I gather that at a short period of time it wil be ok if you run it at say 20.00Hz. So basically what is the lowest frequency and for how long can you run it at that frequency without risking motor damage?

-Michael-
 
Most of my VFD equiped machines do not go slow long enough to have a great concern, so far just feeling the motor every so often has been ok. On my South Bend, when I get it on line, I plan to add a big muffin fan to blow into the motor. It will be setup to run anytime the motor is running. I just figure it will not hurt to run with the motor all the time, and is simple to wire a relay off the VFD to turn it on. Since the motor is not easy to get at, I have also been considering a temp gauge so I can monitor the temps.
 
Depends on whether or not the motor is rated for invertor use. Can always add a separate fan to help cool the motor as stated above.
Pierre
 
Monitor the motor temperature. As long as you can keep your hand on the motor continuously you have not pushed it too hard. Just be aware that it takes some time for heat to soak out to the case.
 
In school, they said don't run a vfd below half rated frequency. But I took it to mean we were protecting the VFD. Perhaps I misremember.
 
jwmay said:
In school, they said don't run a vfd below half rated frequency. But I took it to mean we were protecting the VFD. Perhaps I misremember.
Click to expand...
That is what I was told,not to go below 50%,and the motor temp was refured to. But if this is the case,then why does the vfd manual not mention this.
 
The VFD doesn't care until you start getting down to 1 or 2 Hertz. If your motor is cooled by a shaft-mounted fan its cooling will suffer as you slow down. Whether that matters depends on how heavily the motor is loaded.
 
Suzuki4evr said:
Hallo guys.

I know that a motor needs speed for the motor fan to keep it cool,but as you know your motor speed falls as you go down with the frequency to controle spindle on your mill using the vfd. Is there a dangerously low frequency you can run on a vfd. My motor runs at 50.00Hz at full tilt. I gather that at a short period of time it wil be ok if you run it at say 20.00Hz. So basically what is the lowest frequency and for how long can you run it at that frequency without risking motor damage?

-Michael-
Click to expand...
With an AC servomotor, I have taken it all the way to zero, acting as a big brake, holding up half a ton of satellite tracking dish in positional control. The AC motor had a smaller little motor mounted on it piggyback, and the task of the little motor was just to supply the cooling for the bigger one.

More general speed control "VFD" drives aimed at AC induction motors conversion might not be able to go all the way to stall, nor creep around with little movements at full torque. If you install a VFD, and the machine is an older one with gears or reduction pulley drives, it helps to use all the drive train in as low gear as possible, to let the motor run as fast as possible for a given spindle speed. Even with a 50Hz motor, you should be able to make the spindle roll around real slow. Cool the motor, even if you have to re-purpose a few computer fans to do it!
 
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