over clocking 3 phase motors

I have the VFD set for a max frequency of 120 Hz (so 2.4 times rated speed) on my ML-7; seems to work a treat. The motor hasn't died yet...

Regards, Tony

sounds good to me

Thanks Andrew

Frequency wise I'd agree - 120hz seems fine. However If you're doing that to a 2 pole (3000rpm) it'll be whirring along at 6000rpm. Personally I'd be frightened at that. A 4 pole (1500rpm) motor running at 3000rpm is probably OK since it's the same mechanical starting point as the 2 pole one.

Charles

Depends on the motor, age and construction. I once tool a 1HP brooke Gryphon, the type that was very popular a few years ago, and operspeeded it to see how fast it would go. Standard 1425 rev 4 poler, up to 90 Hz it was Ok but after that it got lumpy, and started to make a noise like a phase down, at 120 hz it was actually slowing down and getting hot.

It's all to do with the % of iron to copper and saturation, ear of bat, tongue of newt etc.

Modern metric frame motors don't have the problem.

As a rider to overspeeding I have converted six or seven 3/4Hp 2 polers to run at 12,000 with no problems, it took a new end shield with double bearings and a rewind but they are now running as spindle motors on CNC routers.

John S.

Mine's a 4-pole.

Regards, Tony

ok its a four pole and about 3 years old so I should be able to run at

120 hz whats the groups opinion on the low end 25hz

thanks for all the help andrew bishop

My setup is a Newton-Tesla conversion (but with the top frequency set to 120Hz as a special). From the chart on their website, here:

it looks like the bottom frequency they are using is aboout 12.5Hz.

Regards, Tony

Hi, Sorry to top post, but frankly those if you who are overspeeding a motor on a VSD are insane. Motors are designed to operate at the frequency on their rating plate,no higher. You are risking a motor explosion. I have seen the after effects of a motor letting go and I promise you it is like a bomb going off. You don't have to go a lot higher to be in the dangerous region.

Just so you know my background is motor speed controllers and control systems and have seen the results of endeavours like this.

Most sincerely,

Robert.

Hi Robert -

As it happens, the installation I have was supplied pre-wired and pre-configured to those settings by Newton-Tesla, so I presume they checked that the motor theuy supplied was good for 2.4 times over-speed. However, I take your point that in general, you can't rely on a motor being safe at more than its name-plate RPM unless you have other information from the manufacturer that tells you different.

Regards, Tony

Well Robert I do a lot of work on electric motors, in fact three of my biggest 4 customers are rewind and motor people. Presumably you have never heard of VDF rated motors ? Why do VDF manufacturers make them able to go to 120 Hz on the normal range because although I have seen special motors that run on 200 Hz and 400 Hz I have never seen a 120 Hz motor. [ Quick phone call to one of the motor people and he's not got a clue what I'm talking about ]

The rotor of a 2 pole motor is identical to a 4 pole rotor, it's the stator that's different so if a 4 pole runs at 1425 and 2 pole runs at

2800 how do you differentiate what rotor of the two identical ones to fit.

Last item have you seen a rotor off a modern metric framed motor lately ? Gone are the helical grooves filled with cast alloy. They are still there but it's all internal, the outside is one complete smooth surface with nothing to snag or cause centrifugal force to act on it.

Want proof ?

I have converted 6 or 7 3/4Hp 2 pole motors to run at 12,000 with the expert help of the motor people. These run 8 hours a day 4 -1/2 days a week continuous and have done for the last 4 or 5 years. Only one has given problems and that was caused by crashing the head into the frame of the machine which bent the spindle.

I have seen motors that have exploded but not for many years and it's usually been another fault that caused the problem. Like someone dropping a length of chain into a fork truck motor so they can get off work early [ true ]

Most applications that will come into use here is on machine tools which also need a low speed so the norm is to use 4 pole motors to get the low speed [ many shaper etc even use 6 pole motors for the low speed application ] and over speed to get higher which still doesn't exceed fitting a 2 pole motor anyway.

Personally and from many years hands on experience I think you are over reacting.

John S.

Hi John, I agree with you that i'm probably over reacting but I thought it important to make the point because no-one else had. If you have the blessing from the manufacturer then it's a different game as you say, but I doubt many of the other contributirs have gone to that kind of lengths to be sure about what they're doing.

Am I being naive here?

Sincerely,

Rob.

snipped-for-privacy@stevens> >> Andrew Bish>>> what sort of maximum frequency can I run a 3 phase 3 hp motor at

Robert

I don't think that you're over-reacting. Just expressing the caution that an expert professional would be expected to deliver. Thankfully I'm not an expert professional but you've said more forcefully what I suggested earlier - Beware the basic meachanics of the arrangement.

By the way about a year ago I took your advice on running a 415volt three phase inverter on single phase 415volt. Your rule of thumb of 3 times the rating was spot on. It took a 10HP inverter to run a 3HP motor in this fashion.

Regards

Charles

Hi Charles, Thanks for the kind words. Having said that a 3 times rule of thumb is true for dumb V to F drives, the same is not true of open loop vector drives which actually look at the power factor of the motor in real time, which changes with load. If you overly lightly load a drive like this then it will tend to cause lots of instability. You will find that stability improves as the torque load gets heavier.

I lurk around these parts (being an ex sheffield SMEE chap) and always look for inverter qiestions that I could help with.

Cheers,

Rob.

Charles P>

Low end speed is frequently limited by motor cooling. Most standard motors have integral fans wich are designed to cool at their rated speed. If you drop the speed cooling drops proportionally more. I can't remember the relationship but think it's the cube of the speed. Fit a seperate fan and you remove this limitation. Of course if you overspeed, the fan absorbs more power so you're getting diminishing returns. Eventually all the power gets taken by the fan if it's still in one piece.

John

Hi, Yes that's true because the current will be same to deliver the same torque as at rated speed, although the freq and volts will be lower to deliver the correct speed withouy saturating to rotor. So the I^2R losses will be the same in the windings. The motor will also be magetised to a similar level but I do seem to remember that motors tend to be less efficient when run at low speed with a drive. Can't remember exactly why though right now.

Thinking about it the core losses will be lower because of the reduced fundamental frequency. This does not consider PWM frequency though which can add extra heating to the rotor, hence why non-inverter rated motors tend to require filtering with L and C components before delivery to the motor.

I still think you would require extra cooling as John says.

Rob.

John wrote:

On or around Tue, 17 Jun 2008 13:29:37 +0100, Robert Wilson enlightened us thusly:

no, I don't think so. It is critical to know WHAT motor you're dealing with.

many years ago, while I was still at school, the tech people acquired from somewhere a small Aveling Barford roller, with a knackered engine. They fitted a different engine, and the teacher in charge got on to Aveling about how fast the engine should run and was told that if you go over 1800 engine rpm there's a good chance of the transmission exploding. Father tells of a tank transporter many years ago which he heard about, had a total brake failure going down a steep hill, and eventually the engine revs got so high the flywheel exploded.

No doubt some motors, especially modern ones designed to run with a VFD, are made to cope with the revs that result. However it would be folly indeed to assume that ANY motor can safely be run over rated speed.

Another thing to consider is the machine to which it's attached. I could, for example, put a 2800 rpm motor on my Student, which would allow it to run up to 2400 spindle RPM. The machine was designed for 1200 RPM and as such all the bearings, gears and so on are rated at that speed. I've an idea some were made with 2-speed motors and ran up to 1500, but that's a bit different to 2800. I doubt it'd be safe at 2800.