# Questions raised by Bob's VFD experiment

• posted

After reading Bob Engalhardt's post about running his motor at about 6 times the design speed by increasing the frequency to 360 Hz.

In school they talk about an induction motor causing the voltage to lead the current, but in the situation where the frequency is increased, does the phenomenon ever cause the voltage to come back into phase with the current albeit one cycle out?

If so does the motor then run more efficiently at that frequency?

• posted

Any phase angle difference is always at a maximum of 90 degrees out.

• posted

Any phase angle difference is always at a maximum of 90 degrees out.

What stops it at 90 degrees?

• posted

I wonder if the motor's HP output starts to drop significantly at somewhere around or over 4x the rated RPM. I suppose that the HP ouput is much lower at higher speeds.

Although the motor continues to run at many times the rated speed, I suspect that it would fail (smoke, not explode) at or before 4x maybe if a reasonable load (of 1/2 to 1 HP) was being driven.

I'm not suggesting that Bob connect the motor to a load since the original experiment is dangerous enough.

Being able to see the results of such experiments performed in appropriate protective enclosures would be ineresting (or at an isolated location like where Mythbusters did the water heater experiments). Very high speed video cameras would most likely make some enjoyable videos.

There must be some occupations where rotating mechanisms to the point of destruction was a fairly common event, but I suspect that much of that type of analysis is done with computers now, not actually scattering parts.

• posted

Very inspiring and commendable.

I will set out to acquire a free or almost free 3600 RPM motor for a similar experiment.

I actually have such a motor (3 HP), but it is too nice to ruin.

i
• posted

Something interesting happened last night, that may or may not be relevant to your question.

DoN wondered whether it would even start with an initial freq of 360, so I reduced to "accel" time to 1/10 sec (with output freq of 360 Hz). When I pushed "start" the speed snapped up, but not to 10700 (2400 IIRC) & there was a different sound from the motor windings.

As I turned down the speed pot at some point the motor started speeding up & then down as I continued. I may not have the behavior exactly right, but it was quite peculiar. This strangeness was present as I increased the accel time until I got to 6 sec. Then it was fine.

So I expect that I'm going to be learning something new about motors, Bob

• posted

Almost free "inverter capable" motor I can't help with, but if you want to work with a brand new one and make a nice video:

Automationdirect.com MTR-P33-3BD36 - \$70 + shipping

Free shipping if you order more than \$300 worth of stuff from them.

• posted

This would be mean spirited, destroying a brand new motor.

I will look for a free used one in bad shape.

i

• posted

Not if it were posted on a business web site or YouTubbe to generate business leads to your company... Or educate young minds (regardless of the age of the body...)

I'll still look for the results and video. :)

• posted

Oh, yeah! A 3600 rpm motor on 360 Hz -> 21,000 rpm. That would be too cool.

And then there's that much more mass to wreak havoc if things go bad .

Bob

• posted

In general, an inductor will never become a capacitor, ie. develop a phase shift where current leads voltage. it IS possible, in an inductor with a large number of turns of very fine wire where the interwinding capacitance will overtake inductance, but you are usually talking about RF not 400 Hz where that happens. With an ideal inductor, a full 360 degree phase lag is impossible. With a real inductor with winding capacitance, too, then the phase lag will go to 90 degrees, stay there until this crossover frequency (at which point the capacitive reactance approaches the inductive reactance) is reached, and then will head back to zero degrees. When the cap. and inductive reactances are equal, the phase angle will be zero - this is parallel resonance, and so the winding current would be maximum, but the impedance of the winding would be ZERO, ie. short circuit! This would be hard on your power supply. it would also be at low RF frequencies, I would expect. WAY faster than you could spin the motor.

No, the motor efficiency would drop to zero, as all the current would be feeding the winding capacitance, and never be available to do work.

Jon

• posted

Not necessarily. I heard about a wood shop that ran all their motors on

800 Hz power. They were using conventional 50 Hz iron but rewound for higher poles, I believe. So, they were not running plain 2-pole and 4-pole motors at 800 Hz, but they were still running them QUITE fast, perhaps 12,000 RPM for an 8-pole motor. You definitely can get more HP out of them by running the RPM up, pretty much on a linear scale, except for windage losses and hysteresis losses. You can compensate for the hysteresis loss by reducing stator excitation just a little bit, sacrificing just a little bit of torque. You have to be careful balancing eddy losses against frequency, though. For a proper ground-up design of a high-frequency motor, the stator laminations should be made thinner. At 400 Hz, you'd want to go to .001" laminations or so, and then the varnish would be thicker than the iron, and really sabotage the whole effort. If you can't change the laminations, then the stator iron is going to get hot for sure, and I don't know how you combat that. but, whoever rewound the motors for that wood shop figured all this out and apparently made it work.

The guys who build the smaller turbine engines have to do this routinely. They generally use spin test pits, rotating the device on a vertical shaft down in the pit. One popular scheme for making Titanium turbine rotors is to machine them undersize, then run them up to 2X normal operating RPM which stretches the metal in the direction of stress. Somehow they figure out when the stretching has completed and slow it down for balancing.

Anyway, testing to failure on new designs is certainly used less than before, but testing high-speed rotating machinery for material defects is a practice that still has to be done. You sure don't want to put untested rotors in a \$1 million++ engine and have it fly apart.

Jon

• posted

With a pure resistor, the current is in phase with the voltage.

With a pure inductor, the current is 90 degrees behind the voltage.

With a pure capacitance, the current is 90 degrees ahead of the voltage.

With a mixture, you get somewhere in between.

If it helps visualize it ,mechanically, think of:-

Voltage=force (Still alternating :) Current=speed resistance=friction inductance=inertia capacitance=elasticity

it gives a feel for the effects

regards Mark Rand RTFM

• posted

We had several spin pits at NASA's Glenn Research Center for advanced flywheel testing. The highest rpm flywheels were made with titanium rotors, then wound with carbon fiber. The fiber was wound so tight it would compresss the hub to within a high percentage of its failure at the highest stress points. Then we spun until the forces cancelled out the compression, and the hub was averaging neutral stress. Then went higher until the tension forces were too high.

I helped design a containment system using 12x121x12 cubetainers filled with water to aborb an explosion so we could test above ground. Here is a pdf describing the system. On page 7 is a photo of my test setup when I used my Garand to fire blunted 30-06 bullets into the water boxes, great fun!

• posted

I read your report. You clearly had a lot of fun. Nowadays they could even fire you for "bringing a weapon to work".

i
• posted

At normal supply frequencies an induction motor looks like a lossy inductance to the supply so the supply current phase angle lags the supply voltage. The loss component is the combined effect of mechanical and electrical losses plus the power delivered to the load.

With small, not very efficient, motors the phase lag can be large but can never reach 90 deg because this would correspond to a pure inductance. A pure inductance has zero loss component so it cannot draw power from the supply!

The inductive component of a motor load is the combined effect of the shunt magnetising inductance and the series leakage inductance . In conjunction with the internal winding stray capacitance this results in parallel resonance (very high impedance) at a fairly high frequency and series resonance (very low impedance) at a much higher frequency. With typical motors both these frequencies are vastly higher than any likely supply frequency and can be ignored.

Small 3 phase motors will cheerfully run at many times their design speed. Apart from obvious mechanical limitations the main limiting factors are leakage inductance and overheating.

If the frequency is raised, but at the original nominal voltage, a perfect motor would deliver the same HP independent of speed. With a real motor the leakage inductance at the increased frequency reduces the effective supply voltage so the deliverable HP will decrease. The reduction is very dependent on detail motor design.

If the flux density is maintained constant by increasing the supply voltage at the same rate as the supply frequency the iron losses increase rapidly. In addition, if advantage is taken of the increased available HP, copper losses will increase. Overheating then limits the available HP.

Jim

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They could have even then, it wasn't that long ago. But that testing was done at my local range.

We used to have a gun club at NASA, in fact the range is still there. We used to bring in weapons all the time, you just needed to have your name on a list. Management succeeded in shutting down the range by saying to was a hazard to the day care center. At some point they removed all the shooting related trophies from the glass case in the employee center.

• posted

Back before the days of electronic speed controls ( a very long time ago ) I worked at a furniture factory where they used a motor/ generator set to produce 400 hz 3 phase power to operate the motors on the wood shapers. These motors operated at 10,800 rpm.

• posted

ELI the ICE Man do you mean ? :-)

Voltage leads Current in Inductive circuit.

Current leads voltage in capacitive circuit.

Mart> After reading Bob Engalhardt's post about running his motor at about 6 times

• posted

Well said Mark.

Beyond 90 degrees - consider 4 quadrants - resistance is to the right. Inductance goes up and capacitance goes down. Rational values of the combination or Z the AC impedance - is between -90 zero and +90 degrees. beyond +90 and before 270 (or -90) is imaginary values. Remember the root of -1 ? or "i"

Mart> >

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