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dian
02-06-2013, 06:08 PM
i have this little grinder motor (230v, 50hz, 2900rpm, 200w) that i hooked up to a 750w vfd. (yes, it is designed to work with single phase.) i set the frequency to 400hz and adjusted some parameters. the motor is happy between 50hz and 100hz. at 150hz it starts to struggle, above that it will slow down and at 200hz it stops. why is that?

i know the motor will not last at 9000 rpm, just curios why it will not run faster.

loply
02-06-2013, 06:13 PM
You surely realise that there comes a point where friction and momentum compete with the electrical frequency? Otherwise your motor could be span up to an almost infinite speed...

lakeside53
02-06-2013, 06:58 PM
You have constant torque to your base frequency (say 50hz). Above 50hz you need more voltage to maintain the torque but you don't have it, so you get "constant hp" instead. That's another way of saying your torque degreases with speed above 50hz. It's reasonably linear to 2X base frequency (1/2 torque at 100hz) then losses creep in more and more. At 200hz you only have 1/8th base torque (did I calc that correctly?) and with your example you have insuffient torque to overcome the motor losses. At 400hz, heck, you'd have pretty much zero torque anyhow.

Oh.. this is all explained in your vfd manual ;)

tdmidget
02-06-2013, 07:43 PM
Are you trying to blow up the wheel? Tinkering with the speed of a grinding machine is usually a bad idea, especially with a VFD.

darryl
02-06-2013, 07:59 PM
'are you trying to blow up the wheel' My first thought was, hey, that's clever- blow up the wheel instead of trying to re-invent it. Then I realized that the wheel is the grinding wheel. Agreed- you don't want to overspeed those.

But I like the experimentation aspect. I'm guessing that this is an induction motor and not a universal motor- I'm not surprised then that the higher frequency starts to get lost in the rotor instead of creating a reactive magnetic field in it. You've got a laminated rotor, but then a conductor shorting out induced currents. There's going to be a narrow range of frequency where the reactive magnetic field is induced within any reasonable level of efficiency. The higher the frequency, the more of that induced energy is lost to heat. At some point, all you have there is an induction heater, where the rotor is what's getting baked.

dian
02-07-2013, 03:58 AM
interesting, so how do the motors that you can run at 400hz differ from this one?

btw, its a little benchtop grinder with the wheels off. i even put a little hose clamp on one of the shafts, to get rid of most of the vibrations.

also: can a universal motor be run from a vfd? ok, i could just plug in a mixer and find out.

EVguru
02-07-2013, 04:38 AM
Rule of thumb with induction motors is to keep a constant voltage to frequency ratio.

A 230v 50hz motor is going to need 460V @ 100Hz to produce full rated torque.

A 400Hz 230V motor will be fed reduced voltage when run at lower rpm.

ironmonger
02-07-2013, 07:56 AM
Perhaps a slightly different aspect of the 400 cps needs to be mentioned.
At 50 or 60 cycles per second the motors are essentially running at frequencies that they are designed for. Once you begin moving into higher frequencies the coil reactance, a product of both frequency and the inductors core material begin to rise. Reactance is the same property we use in induction heaters, it's kind of like resistance in DC circuits.

As the frequency rises the reactance also rises. At some higher frequency the core begins to be unable to change magnetic polarities as fast as the signal you are driving it with. At this point the energy that you tired to use to spin the motor begins to just create heat in the core of the motor windings and laminations. This is a good thing in induction heaters... I can melt steel in the induction coil... not so much in your grinder.

Motors that run at 400 cps are designed to do so. The don't much like running at 50 or 60 cps, but those frequencies won't cause them to melt.

VFD's are great tools, the greatest use of which is generating 3 phase power form single phase, soft starting motors and current limiting.

Experiment all you want... physics and motor design has pretty much prevented you from destroying the grinding wheel. Its just not what the VFD was designed to do.


paul

J Tiers
02-07-2013, 08:36 AM
The motor you have is almost certainly a "PSC" motor, a motor with a permanently connected (no centrifugal switch) capacitor to provide the "second phase" and produce torque. This "phase shift" is accomplished by the capacitor, which has properties opposite to an inductor (coil of wire).

The capacitor produces a phase shift due to the nature of its "impedance", its opposition to the flow of current. That is variable with frequency. The value of the capacitor is chosen for best performance at your 50Hz mains frequency. While a PSC motor can be varied in frequency, the fixed capacitor causes it to produce less torque at different frequencies, away from 50 Hz. At higher frequencies, the capacitor impedance is less, at lower frequencies it is larger.

When the impedance is low, the phase shift is typically less unless the rest of the circuit is lowered in impedance also. But the impedance of the windings is going UP as frequency increases, causing the capacitor to have less influence. It is closer to a "short circuit" at higher frequencies, and produces only a small phase shift, reducing the torque, and eventually causing the motor to be unable to turn against the "windage" and other friction.

A related effect occurs at lower frequencies, as the capacitor impedance increases, reducing the current passed by the capacitor, which in turn reduces torque. But there the capacitor is INCREASING the phase shift, which partly offsets the reduced current. And the coil impedance is decreasing, also providing an offset to increased impedance of the capacitor, so torque may not be reduced as fast with change of frequency as it is reduced vs increased.

Your motor would probably run better at the higher frequency with a somewhat reduced value of capacitor. But that wouldn't work as well at normal mains frequency, so the PSC is not as easy to run at speeds too far from the normal mains frequency unless the capacitor value is changed as well as frequency.

The normal 3 phase motor has only inductance as its main property, so the changes vs frequency are slower and the torque does not change as fast.

One other problem with the PSC is that when phase shift changes too much, the capacitor winding is "in the wrong place" phase-wise (in a physical sense), and can start to *oppose* the torque instead of add to it. That means the motor will not run well "single phased", unless you open the capacitor circuit, as is done with a normal so-called "split phase" motor.

Jaakko Fagerlund
02-07-2013, 08:47 AM
Are you trying to blow up the wheel? Tinkering with the speed of a grinding machine is usually a bad idea, especially with a VFD.
Wow...reading between non-existant lines there?

rdhem2
02-07-2013, 12:04 PM
Listen to the ironmonger, he sums it all up pretty well.

When as a young man, I used to try silly stuff like this Dad used to say "What? The engineer who designed it did not know anything?"
Think about it.

dian
02-07-2013, 04:41 PM
thanks alot for the info everybody. but i still wonder: how exactly are the 400 hz motors designed differently? also, if i wanted to run this single cap motor fast, do i change the capacitor (smaller?) or do i disconect it after stat up?

the issue with the cap also explains the fact, thet the motor will stall at 30hz or lower.

interestingly the 550w two cap drill press doesnt take frequencies lower than 590 hz kindly, it makes strange noisel (which the grinder motor doesnt).

maybe it would not be too difficult to have three caps and a switch then?

tdmidget
02-07-2013, 06:20 PM
Wow...reading between non-existant lines there?

Very clear lines, Jaakko. Bench grinders are not designed for 9000rpm and there are no wheels that I ever heard of suitable for a bench grinder at 9000rpm.
The VFD makes it very easy to overspeed something like this without thought about the possible disastrous consequences.

robosilo
02-07-2013, 06:32 PM
I would imagine that a motor designed to run on 400Hz will have coils tuned and designed for proper inductance to run at that frequency. It's not like a servo motor where you have a magnetized core and coils that are bullied into running at what ever frequency you want them to. Those motors too have a steep drop off after a certain RPM. With inductance motors, you're dealing with coils, caps(sometimes) and the cage which are usually designed to operate within a somewhat narrow range.

A lot of CNC lathes switch between 2 coil arrangements depending on if they need high RPM or high torque. They switch between Delta and Wye mode. I'm not sure if they make smaller motors that are capable of this or VFDs for that matter, but if you really need those high RPMs it may be worth looking into, otherwise you may be able to get the RPMs from a DC motor. Mars makes 8, 12 and 20 HP DC motors that can run about 3500 RPM. You could pulley it to get more speed and the motors have a fair amount of torque. Only problem is that the motors run 48 or 72 volts, so start saving up some golf-cart batteries.

J Tiers
02-07-2013, 07:42 PM
A motor for 400 hz will have a smaller core, less turns, and so lower inductance.

Since most will run a lot faster at 400 Hz, unless they have a huge number of poles, they are made proportionately stronger for the higher forces, but since they are always smaller than the equal power 50/60 Hz motor of same poles, that isn't much of a problem.

We have a pair of small AC fans from a client at work. These are 400Hz Rotron fans, maybe 80mm diameter or so. That size fan at 60 hz would spin at perhaps 1750 (4 pole) or 3450 (2 pole) rpm, and move a reasonable amount of air, but probably nothing earth-shattering. It would probably draw maybe 25 VA.

These, however, are 4 pole and spin at 11,000 rpm. They are so powerful that they literally have "thrust power"...... each exerts a very noticeable 'push", and the test fixture has to be clamped down. You can easily feel their output 20 feet away, and they blow the paper off any table within that range. Each fan draws nearly 100 VA, and clearly turns a good bit of that into air movement.

They are PSC motors, with a capacitor and aux winding, similar to the grinder. It can be done, but the motor needs to be designed for the frequency.

Mcgyver
02-07-2013, 07:57 PM
there are no wheels that I ever heard of suitable for a bench grinder at 9000rpm.

mostly depends on wheel diameter

Jaakko Fagerlund
02-07-2013, 11:53 PM
Very clear lines, Jaakko. Bench grinders are not designed for 9000rpm and there are no wheels that I ever heard of suitable for a bench grinder at 9000rpm.
The VFD makes it very easy to overspeed something like this without thought about the possible disastrous consequences.
Again reading between the lines. Or could you point out where there are wheels?

lakeside53
02-08-2013, 02:56 AM
I must have read between the line also - thought the motor was three phase! After all, who would try to run a PSC motor on a vfd? But sometimes you get it right if the OP wasn't clear ;)

darryl
02-08-2013, 05:39 AM
I don't think Dian meant to be using the motor as a grinder- I think he was just experimenting with it and the vfd. He did say he didn't expect the motor to last at 9000 rpm. I once tested a small dc motor, one from a cd player- rated at 5 volts- at 80 volts. I determined that it had spun up to at least 70,000 rpm before the windings came off and it died. It was just a test- I had no use for it in mind at those rpm and voltage levels. It was just interesting to do.

The warning about not exceeding a grinding stones rpm is valid of course.