If I run a motor from a VFD at a higher than 60 Hz frequency, it would decrease its output power. This is all fine, but I would like to see some graphs as to how much it loses its power if I overspeed it by 2 times (such as running a 1800 RPM motor at 3600 RPM). Does anyone have a such a graph?
There's no loss in power above 60Hz with a volts/Hz VFD set up normally. Below 60Hz power is proportional to RPM, above 60Hz power is constant. In other words, torque is constant below 60Hz, HP is constant above.
It might be wise to contact the motor manufacture and verify that the motor design is capable of operating above the rpm name plate rating. You dont want a catastrophic failure of the rotor.
On Wed, 06 Jan 2010 14:15:20 -0600, the infamous Ignoramus5111 scrawled the following:
Holy Shit, Ig. Got flak jackets and armored viewing ports?
P.S: You're trolling, aren't you? Shameful.
-- We rightly care about the environment. But our neurotic obsession with carbon betrays an inability to distinguish between pollution and the stuff of life itself. --Bret Stephens, WSJ 1/5/10
Probably not, but I'd hate to find out the hard way. Most inverter duty motors have a max rpm listed on the nameplate, which I like to see to know the safe range. I've used plenty with max rpm about 4300. No way I'd run 120 Hz on a 3600 rpm motor, though. You might look on line at the motor manufacturer's web site for max speed data.
Also, consider that since the horsepower stays constant above 60 Hz, the torque is dropping proportionally to the speed increase.
I was told of the aftermath of a sudden stoppage on a slow speed 2300 V. 100 hp. pump motor in a plant I worked in. Part of the plant lore. Apparently something really big dropped into the impeller. This was on a production scale caustic evaporator circulation pump. The motor (3' in diameter IIRC) tore off its foundation and wrapped the pump shaft into a corkscrew as it rolled across the ground floor of the plant, coming to rest outside in a ditch. Since it would have dumped the whole evaporator of hot caustic, glad I wasn't around to see it. Throwing the armature windings would cause a similar sudden stoppage.
A failure is possible both mechanically and electrically - Inverter Duty motors have to handle much higher voltage spikes coming from the inverter than they would ever see from mains power, and you can easily flashover and short or ground the windings on an old motor. Or to put it in simpler terms, the Magic Smoke escapes.
This is why you do not run a very expensive or irreplaceable old motor on a VFD. Stuff like the purpose built motors that are a part of the machine casting (old Bridgeport vertical mills, some Shapers, etc.) that will cost you a bloody fortune to tear the motor out and get it rebuilt, and FWIH many of the old ones are not inverter rated.
You use the VFD on inverter duty rated motors, or on something that uses a cheap and easily replaced motor like 1 HP 56-C mount. You can get a new motor for pennies that are inverter rated if that old one goes kablooey.
I know it goes against the r.c.m credo, but Real Men do in fact read the (friendly) manual first. At least a quick once-over.
And last I checked, 3600 is more than 2750. You might be able to get a few percent over, but anything past 3000 is a crap-shoot.
They usually set those limits because the test motor started to come apart at 3600 but held long-term at 2750, and they left a cushion in the max speed ratings.
I doubt it. 3600 rpm motors are usually max'ed at 5400 & it's unlikely that 1750 motors are much differently constructed. I don't think that I'd run a 1750 at 5400, but I would & do run 1750 at 3600 (on DP with VFD).
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.