rpm of free wheeling 3 phase AC motor? 1

[kens]

We have 400Hz 115VAC 3 phase AC 16hp motors (12 pole 3800rpm) that drive 2ft diameter fans in ducting behind our air-conditioning condensers to suck air through them while our plane is on the ground.

The AC power is switched off to these fans when we are flying (350mph max) since there is plenty of ram airflow through the outside ducts to provide cooling. Hence these fans/motors "free-wheel" while we are flying.

We've experienced fan blades breaking apart while flying which damage the plane, hence are beginning to think that the free-wheeling fans/motors may actually be spinning much faster than their 3800rpm design?

When the AC power is switched off to these motors during flight, will the motor act like a generator such that I could monitor across one of the three inputs (to the neutral contact) and see a sine wave voltage whose frequency would show me the rpm (i.e. 400Hz at 3800rpm-----etc.)?

 

[buzzp]

I dont see why this wouldn't work, given the equation for the rpm of a induction motor. The amplitude of signal is what I am not sure of but I believe it would not be greater than the nameplate voltage. This would likely have to be determined experimentally unless someone else knows of a way to calculate the amplitude. Your idea will work fine except be aware of the unknown voltage amplitude. I started a design for my previous employer that wanted to do the same thing for submersible pumps/motors when check valves failed or were not in use. They didn't want to start the motor if it was running backwards at a certain RPM (twisted shaft). The competitor units would monitor down to millivolts up to the nameplate voltage of the motor. This is about as far as I got before I left the company. I never found any real data but was planning on conducting some experiments. If this is what you have to do, could you please let me know the results? Thanks and good luck, Buzzp buzzybooper@excite.com

 

[SteveKW]

If there is no residual magnetism in the rotor I do not see how you can get anything from the motor unless you put a small DC voltage on two of the lines.

 

[electricpete]

Steve is right. It will rely on the presence of residual magnetism.

 

[cbarn24050]

Hi, you might try leaving the supply to th emotors on all the time. They would have there speed controlled and would generate power which the plane could use.

 

[buzzp]

I know from experience the motor will produce a small voltage while it is off line if a fan, for example, is turning the motor. This is a given. The motor is acting like an generator. I think what your thinking of doing will work fine, the only question is the amplitude of the signal. I believe you will be looking at small signals (mV). Please post your results. Thanks

 

[GordS]

I think your idea will work. There should be a voltage generated just due to the residual magnetism of the motor. I must admit I used to think many years ago that the generated voltage would be zero. However, about a year ago, one of the electrians I work with was disconnecting a 4 kv, 450 hp motor which was coupled to a pump. Fluid was sill moving in the pipe so the motor was slowly turning. He noticed about 9V on the 4 kv circuit.

The voltage should be propotional to the rate of flux cutting. If you can do a test where you drive your motor at a known speed, then you should be able to infer speed at any measured voltage by using a ratio.

 

[electricpete]

Based on the presentation of PDMA in using their test set for Rotor Influence Check, residual magnetism at a measurable level is present in MOST BUT NOT ALL induction motor rotors. It depends on the rotor material and possibly on the conditions under which the motor was deenergized.

Good suggestion by Gord to use the voltage rather than frequency, I hadn't thought of that but it should be easier.

** Also you might get tripped up in your assumption of number of poles if you use frequency. Rotor during operation will have the same number of poles as the stator. That may not be true of the residual poles left after deenergization.

 

[electricpete]

If there is line of sight visibility to any rotating part, you can check speed very easily with strobotach. You've probably already ruled that out.... but I just wanted to mention it.

 

[Marke]

You will only get an output voltage if there is a residual magnetic field in the rotor. This is probably going to be the case, albeit very small unless the grain orientation of the steel in the rotor is completely symmetrical.
I would suggest that you apply a constant current DC to the stator and monitor the voltage across the stator. This will provide the driving magnetic field to generate the voltage and will also provide a level of drag. If you set the current high enough, you could generate enough breaking torque to limit the speed or even stop the motor. For monitoring purposes only, I would expect that the current level could be quite low. For breaking purposes, the current would need to be in the order of the rated full load current of the motor or possibly higher. Continuous braking current will cause additional heating in the motor and that would need to be considered.

Best regards,
Mark Empson. Mark Empson

http://www.lmphotonics.com

 

[busbar]

I don’t think it takes very much residual magnetism, based on using a small phase sequence indicator (not currently manufactured) that had an input rating of 25mV to 1200V. Without any range changing or other switching, the device could be connected to a live system or deenergized motor to compare phase sequence and predict correct motor rotation before line voltage was applied to the motor. It worked without fail on 1/3-100hp motors. If needed though, would it be that difficult to add a few milliamperes of DC bias to the motor winding?

It would probably be a regulatory nightmare, but using DC to slow/stop the motor may be feasible.

 

[UKpete]

Personally I would connect a scope between any two lines immediately after isolating the motor and look at the frequency. I think there will certainly be a useable voltage until the motor has stopped (I can only envisage it not being there in a motor that is brought slowly to standstill on a VFD).

I used to do a lot of type-tests on locomotive fans (3-phase IM, typ. 10-100kW) where we had to find the final winding temperature using the resistance/cooling curve method, but the problem was that I couldn't get a reliable resistance until the motor had completely stopped, because of the back-emf. I would monitor the back-emf until it had fallen to zero to confirm that the motor had stopped, and it never got down into the mV range until right at the end.

 

[buzzp]

Busbar brings up an important note. Phase sequence meters rely on hand turning a motor to determine the phasing of the motor leads. Do these apply a DC voltage? If not, then it would seem that this would always work on induction motors. Does anyone know if these meters are applying a DC voltage?

 

[xixixi]

If you leave the motor connected and it is oversped you will have what is called a synchronous generator. The power produced from this can be used to supplement other electrical loads on the plane.

 

[busbar]

buzzp, I measured voltage of the leads on the (not currently manufactured) phase-sequence indicator with the input rating of >25mV and it was below 3mV, so I don't think it applies any intentional bias to excite the motor windings. However, the Biddle Motor Rotation Tester--having separate motor and live-line testers in one case--has a small battery set as part of a bridge that is comprised of the motor windings and a nulling potentiometer with a zero-center milliammeter. The motor under test has a few volts DC on its stator during the rotation test.

 

[buzzp]

Thanks for that info busbar. So maybe the rule is if there are seperate leads for off-line motor rotation verification, then it is likely applying a DC voltage for excitation and if the same leads are used for line voltage and motor rotation then it is likely not applying a DC voltage. Agree?

 

[busbar]

10-4. There are a number of "2-part" {6-lead} testers on the market, but the one "3-lead" [25mV-1200V] device seems to be unique.