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How fast can a motor be driven

dazz

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Aug 20, 2006
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New Zealand
Hello

I have a Brook Crompton 3ph/1.26kw/2 pole motor that would normally do 3450rpm at 60Hz. The VSD I have can drive the speed up to about 80Hz.

How fast can a motor be driven above 60Hz, or is that the safe limit?
 
I'd probably limit the time @ 80hz.

The PROBLEM with overspeeding motors is you won't know if it's bad for it until it's too late. I know the motor on my milling machine is probably OK up to 1800 rpm insted of the rated 1200 rpm. The motor on my buddy's lathe, that looks identical, probably has the same bearings, and the same gearbox on the front of it. The difference is his is a 3hp 1800rpm motor. I don't need to overspeed it, but I could if I wanted to. 3450 rpm motors are allready about as fast as that style of motor is typically arranged. If it were an 1800 rpm motor, and a company made a 3600 rpm motor in the same basic shell, I'd say push it to 120 hz! You'd be pushing 4800 RPM @ 80 hz, so check to see if it's geting hod, and listen for funny noises. Don't overspeed it unless it's actually needed. :cheers:
 
I am about to do a VSD conversion on my lathe. At present, it has four speed selection by shifting the belt on the pulleys.

As part of the conversion, I am going to fit a poly Vee belt. I am aiming for a two speed selection based on pulleys. I am trying to optimise pulley selection to maintain much the same torque that I currently have. I don't want to over or under power the lathe.

So if I allowed 10% overspeed (66Hz), this would decrease the number of belt shifts I would have to do.
 
10% is no big deal.

You're looking at +345 rpm, so I doubt that would hurt the motor @3795 rpm. If you started pushing it over 25% faster, you might have problems. Even 70hz shouldn't hurt it, allthough you never know. Just make sure the motor doesn't get hot. Another thing you have to worry about with motors on VFDs is running them at low speeds. The internal fan can't blow enough air to cool them. If you plan on running the motor real slow for long periods, you MAY want to consider adding an external fan to blow air into the motor. :cheers:
 
Hi

I was going to add a fan anyway to cool the VSD. The aim is to mount it within the lathe stand to keep the whole installation tidy.
 
Hello

I have a Brook Crompton 3ph/1.26kw/2 pole motor that would normally do 3450rpm at 60Hz. The VSD I have can drive the speed up to about 80Hz.

How fast can a motor be driven above 60Hz, or is that the safe limit?

There was a thread a few months ago regarding this very same question.

http://www.practicalmachinist.com/vb/showthread.php?t=101940


In it Forrest Addy said....

"In years past I looked into this. I experimentally ran an unloaded 5 HP 3450 RPM motor to 400 Hz for a few minutes until something inductive happened and the motor fell out of sync. That's - um - 23,000 RPM. I recall when I dismantled the motor that there were no indications of failure, there were no cracks or yielding of the cast-in armature fan, radial distortion of the assembly, the sqirrel cage passed the growler, etc.

Remember that induction motor armatures are of nearly solid steel construction, its squirrel cage is high pressure cast aluminum, and the laminations are in the plane of rotaton. Thus the typical squirrel cage armature structure is quite robust especially when compared to a wound DC armature or an internal combustion engine crankshaft and flywheel with all the associated reciprocating mechanism.

The steel itself isn't high strength by any means but I think it's 35,000 PSI yield or there abouts. I failed to note the armature diameter when I ran the test and I since reassembled it and put it back on the shelf. Without taking the motor apart and by peeking in the ODP vents I would guess the armature is about 4 1/2" is dia.

Maybe someone with an engineering bent would shed a little analysis on the topic of the centrifugal strength of induction motor armatures.

If we over-speed the motor we have to consider what happens if something fails at high RPM. Given the fully housed construction of a typical motor and the strangth of the stator I would think that even an utter and explosive failure of the armature would be fully contained. The motor itself might jump if not restrained but there would be practically zero chance of flying debris. Consequent electrical casualty in the motor would be mitigated by the VFD's fault protection. There's an excellent chance that a motor subject to such a failure would be a complete write-off.

All is subject to experimental verification, of course. On the basis of my crude experiment I'd suggest that induction motor armatures may be presumed safe to 10,000 peripheral feet per minute which is about 1/2 the speed I took the 5 HP armature to.

Since radial stress goes up as the square of the RPM increase the 10,000 ft per minute figure would suggest an interim safety factor of roughly 4 to 1 better than the crude experiment I ran."


Motion Guru also said "I have operated a 50HP Reliance motor on the 230 volt windings on a 200HP Vector drive that operated at 480V and run the motor at 120Hz at 100HP continuously and 240Hz (7200 rpm) at up to 150HP for minutes at a time and 10,000 rpm for extended periods at 50HP - 75HP. This was a dynomometer application that I did for Borg Warner about 15 years ago - to my knowledge the dyne is still operating.

The only modification to the motor was the removal of the fan and the installation of an incremental encoder. Bearing grease was flushed from the bearings and a lighter grease was used."
 
That is suprising.

Overspeeding a motor to 10k rpm is suprising. Other motors may not be so robust though. My concerns would be with a motor having a bearing failure, especially with it allready being a 2 pole motor. If you put a large load on it, AND overspeed it, you might get some problems. Now I seriously doubt he would have a catastrophic failure, BUT I could see it affecting lifespan. Being a 2 pole motor, I'd limit the overspeed to 70hz or 75hz depending on the load. That's just me, I don't see much point on beating on things. :cheers:
 
Many times the same bearings used in motors are used in applications like wood shapers which has speeds reaching 10,000 to 12,000 RPM. Grizzly is one that uses a common 6204 bearing in their shaper spindles.

So that same 6204 bearing in a motor could run at 175 hz (10,000 rpm) with no issues.
 
My lathe is only used for home workshop stuff. It spends very little time at any speed, yet alone full speed. Even if the service life of the motor was reduced by 90%, it would probably last 100 years. I think I will just wind up the speed on the VSD to the max. It might mean I will only need one pulley pair.
 
I know a few of the higher-end, specific inverter-duty motors can take a lot more than a lot of your average motors: I've got some 1800 rpm marathons that have a max rpm stamped on the nameplate of 5400 rpm. I think a lot of it is probably simply an issue of whether it's got high speed bearings more than anything else....
 
I have VFD's on 2 mills, a lathe, a drill press, and 2 band saws. I set the max freq to 90 hz (for a 50% increase in speed) and the min freq to 20 Hz (for 1/3 speed). This gives me about a 5:1 speed range with the VFD's. Still have the step pulleys in case I need a speed above or below what the VFD can provide.

I wouldn't have any reservations about taking an 1800 rpm motor to 3600 rpm, but would be hesitant about taking a 3600 rpm motor to 7200 rpm.
 
Many times the same bearings used in motors are used in applications like wood shapers which has speeds reaching 10,000 to 12,000 RPM. Grizzly is one that uses a common 6204 bearing in their shaper spindles.

So that same 6204 bearing in a motor could run at 175 hz (10,000 rpm) with no issues.

The bearings are speed/load /life limited. Each loading has a number of rotations before fatigue failure.

Faster just gets there faster, at reasonable speeds. assuming you don't heat it by packing it with heavy grease. Below 10,000 for smaller motor bearings is almost certainly fine, especially if not heavily loaded.

If you want to get to 20,000 or 60,000 you would certainly need some "engineering" to be done.
 
The bearings are speed/load /life limited. Each loading has a number of rotations before fatigue failure.

Faster just gets there faster, at reasonable speeds. assuming you don't heat it by packing it with heavy grease. Below 10,000 for smaller motor bearings is almost certainly fine, especially if not heavily loaded.

If you want to get to 20,000 or 60,000 you would certainly need some "engineering" to be done.

Another thing is rotor stress. The centrifigal force acting on the rotor is MV^2/R. If you increase the speed 25%, the stress on the rotor is increased by 60% as well as vibration.

The power consumption of propeller is cube root, so with 25% over speed, the cooling fan drag on the motor will double. On a 1800RPM motor is designed with an integral fan to cool under full load at 1800 is used at 5400RPM(as someone else mentioned) the fan is going to cost quite a bit of motor power since it's going to use 27x the power (3^3).
 
That I'd have to see to beleive!

The fan thing makes sense, it gets to a point where you get diminishing returns from a motor because of the fan. A quick question on that though, isn't there a point where fans spin so fast they stop producing more airflow?? Or at least they don't push air arround as efficiently, and don't continue to load the motor as much?? This is a rather stupid thing to do, but I put a set of fan blades from a 12v fan like the school busses had on a sewing machine motor when I was younger (same shaft size)! It made HELLA airflow, but if it did load the motor according to that fromula, it would have NEVER turned as fast as it did. It spun near 10k rpm (yes I put a tach on it motor spun 15k no load) , and it was a .1hp motor. It made more noise than anything else, seems like when I spun it between 4 and 5k (variac) it didn't make as much noise, and didn't seem to make much more airflow when I spun it faster. This is all from memory when I was younger screwing around with that DEATH MACHINE! I'm LUCKY the metal blades that were RIVITTED to the hub stayed put!!! I've seen what a flex fan on a V8 does when it flies apart around 7 grand, blades go THROUGH THE HOOD!

Bearing life of 3,000 years is kinda unbeleivable, maybe THEORETICAL, but not in real life. I could see bearings long outlasting a motor, and even tolerating speeds of 3x the rated. A 3450 rpm motor is allready near the top of it's class though. I doubt it would have problems even at 80hz, but better safe than sorry. :cheers:
 
The fan thing makes sense, it gets to a point where you get diminishing returns from a motor because of the fan. A quick question on that though, isn't there a point where fans spin so fast they stop producing more airflow?? Or at least they don't push air arround as efficiently, and don't continue to load the motor as much??

Yes it's called cavitation, but I don't know when it would occur. I don't know how much useful power overspeeding fan can waste, but a Cummins study shows a stuck fan clutch for engine-driven fan on a semi-truck saps fuel big time.
http://www.everytime.cummins.com/every/pdf/MPG_Secrets_Whitepaper.pdf

This is a rather stupid thing to do, but I put a set of fan blades from a 12v fan like the school busses had on a sewing machine motor when I was younger (same shaft size)! It made HELLA airflow, but if it did load the motor according to that fromula, it would have NEVER turned as fast as it did. It spun near 10k rpm (yes I put a tach on it motor spun 15k no load) , and it was a .1hp motor.
Well, the diameter is a big factor. I'm not sure how big the fan was, but a motor driven motor vehicle radiator fan spins quite fast. 0.1hp means that it's design power is 0.1hp but that doesn't mean it can't produce more. You'd simply end up with a burned up motor.

Bearing life of 3,000 years is kinda unbeleivable, maybe THEORETICAL, but not in real life. I could see bearings long outlasting a motor, and even tolerating speeds of 3x the rated. A 3450 rpm motor is allready near the top of it's class though. I doubt it would have problems even at 80hz, but better safe than sorry. :cheers:

VFD is hard on bearings for a different reason as well.
You can extend the bearing life by putting on a pair of slip rings on the shaft and grounding it to the frame. VFD induces voltage between the shaft and the frame, which shorts through the bearing creating microscopic sparks between the balls and the race.
http://www.greenheck.com/technical/tech_detail.php?display=files/Product_guide/fa117_03
 
With VSD, at low speeds, the fan will run too slow to supply adequate cooling air. At high speeds, it is inefficient and energy sapping. It seems to me that the way to solve the problem with the fan is to remove it.


What about fitting a separate, constant speed electric fan in a simple shroud. A standard axial flow fan would provide sufficient air flow regardless of motor speed.
 
I think it's a good idea.

Removing the fan, and putting an external fan on it is a good idea. Just make sure the fan is big enough to cool it by itself. Make sure the air can actually flow through the motor. You'll probably want at least 100 cfm. Surplus center has some really cheap cooling fans that should work well for that. :cheers:
 
Probably a lot of the criteria is rotor balance, a strange anomaly is that relatively cheap wound rotors as used in Universal motors, especially used in such as router applications etc reach 10k-15k rpm, I would assume they are not using super costly bearings in these motors.
I realize they do not often have the mass as many 2/4 pole induction motors, but 15krpm is high for a wound rotor.
M.
 
Many times the same bearings used in motors are used in applications like wood shapers which has speeds reaching 10,000 to 12,000 RPM. Grizzly is one that uses a common 6204 bearing in their shaper spindles.
So that same 6204 bearing in a motor could run at 175 hz (10,000 rpm) with no issues.
There's certainly no basis for that assumption.

It's far more likely that they use cheap bearings at higher speeds because they don't care if they fail.

- Leigh
 








 
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