3 PHASE INDUCTION MOTOR DESIGNING FOR LOW FREQUENCY OPERATION

Why did the current *rise* over the original value? Normally the current will be LESS because the winding resistance. In fact, AC motor speed control designers put in a boost factor that increases the voltage at low RPM to compensate for that (at least that's what I have done).

Best regards,

Best regards, Spehro Pefhany

There are DC motors that operate at those speeds -- I work with a system that has one. In general, though, with either a DC motor or an induction motor with a reasonable number of poles you must get very good shaft feedback and control the motor torque, and you need to view the motor as a torque source and not a power source. You'll end up dissipating all your "torque generation energy" as the same amount of power as if the motor were operating full speed (which is why other responders have mentioned cooling).

Was your V/F drive perhaps changing the frequency but not the drive voltage, or did it have a minimum drive voltage below which it could not go? Certainly trying to control your motor torque by giving it a too-large voltage and controlling torque by lowering the slip would heat things up in a hurry.

Nasir -

A motor wants a _constant_ Volts-per-Hertz. If your four-pole motor operated properly at 50 Hertz, it was rotating at an unloaded (approximately synchronous) speed of 120*50/4 or 1500 RPM. To get down to 20 RPM, you must reduce the frequency to 50*20/1500 or 0.66667 Hertz. The voltage must also be reduced by the same ratio, approximately). The reason I say approximately is that a voltage boost could be applied to overcome Voltage drop in the windings at low speed and high load.

If the voltage is not reduced along with the frequency, you will saturate the iron. You know what happens when you saturate a transformer. A motor is really quite similar to a transformer except you are extracting mechanical energy rather than electrical energy. If a transformer is designed for 100 V at 50 Hz, you can apply 200 V at 100 Hz with no problem assuming there is sufficient insulation to support the higher voltage. This is true for motors as well.

I have operated a 4-pole, 40 hp, 230 V motor as an 80 hp, 460 V motor. Of course the 80 hp is available only at about 3600 RPM rather than the name-plated 1800 RPM.

You can go the other way, too, and that is what VFDs do for you. They reduce the frequency and voltage while maintaining a constant Volts/Hertz ratio.

If you were controlling your motor with a VFD, try turning off the voltage boost. Be sure to set the VFD for your design V/F. Pay very close attention to the VFD setup instructions such as base speed and number of poles. There are a number of setup parameters which will affect your V/F.

Good luck.

John

----------- At 220V, 50Hz, 4 poles you have a motor which has a synchronous speed of

1500rpm. You are trying to run this at 20 to 50 rpm with V/f control. The impedance of the motor at the lower frequency will be low.You have saturation -probably because you can't reduce the voltage in line with the frequency and there is a resultant flux increase- both this and the high current seem to imply a higher voltage than you should have at low frequency and is likely a result of saturation. You have indicated nothing about the basic R/X ratio of the rotor and that will have an effect. The idea of designing a machine is to design one for the speed range for which you most commonly use it. That is -design on the basis of a synchronous speed of the order of 50-75rpm- with what ever frequency you have (50Hz is a bit extreme for this-can you get down a bit more?). The resultant machine will be big and bulky because of poles/iron requirements. It can be done but there are better options available at the torque and speed range that you want- seriously look at a brushless DC type of machine-as used in some turntables for records.

For the same torque output, the current should be the same at both frequencies if the volt/hz ratio is kept constant.

You mention saturation at the lower frequency. Can you tell us what the RMS voltage was at the motor for the 50 Hz test comparted to the lower frequency? Can you vary the frequency between these two points?

It seems that your ratio between the voltage and frequency is changing as the frequency is decreased. Is the frequency generated by a design of yours or is it a commercial product that you have purchased? The odd part of this would be the fact that you do not have the torque requirement at the lower frequency even with the saturation.

I am more familiar with the motor side of this and look at the drive as a black box that provides the motor with what is required. If the motor will perform at 50 Hz, it should be capable of what you are wanting at the lower freqency providing it is provided with the proper inputs.

Good Luck,

Ed

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If you want to apply a low frequency sine wave to a typical induction motor, first try applying it to an iron core transformer and observe the output versus input waveforms on an oscilloscope.

The results may affect your design.

Rob.

At one place I worked, they had a 5 HP 3-phase motor and a VFD. When you cranked up the VFD, you could hear its oscillators whining. It was very weird, because the pitch would increase, and from the timbre it sounded like a variable-width pulse train. And as you continued to increase the setting, the whine would reset in freq. to the low end, and increase again, but with a little different timbre, like the pulses were wider or something.

Kinda like a dragster going through the gears - same freq. band, different timbre.

So I don't think you want to drive your motor with anything like a 2Hz sine wave!

Presumably, they have a 3-phase oscillator that's locked to some harmonic of the natural rotor frequency, and presumably pulsed or PWM, but the fundamental freq. sounded like it was in the 400Hz ~ 1KHz range.

Hope This Helps!

Considering the number of contributions posted to this thread, it is surprising that nobody had explicitly made the point that motor torque depends on the current through the coils, and the voltage applied across the coils determines the torque only as far as it determines the current.

IIRR, induction motors are not suited for very low speed operation, because they rely on the current induced in the rotor by the driving currents in the static coils, and with very low drive frequencies, you can't put enough current through the static coils to generate enough dI/dt at the rotor to generate enough current in the rotor.

Synchronous motors don't have this problem. which is why stepper motors - which are the devices used when you want really low rotation rates - are actually synchronous motors.

Most stepper motors are two-phase synchronous motors, but there are some three-phase parts out there - usually described as 5-step steppers - which could offer smoother rotation, though a micro-stepping stepper drive should be adequate for most applications.

------- Bill Sloman, Nijmegen

Sincere thanks to all of you for your kind support and help.

I have already mentioned my application but here is some additonal information.

--Application is such that use of brushes (hence dc motor) is not possible.

--If i keep the V/f ratio constant then torque achieved is very very low, so that's why i increased the voltage and this four times increase current is at maximum voltage that is 220 V. Even at this voltage i was unable to achieve the required torque.

--50 Hz motor is actually built and i simulating it on software for low speed operation so i can implement any suggestion.

--Alse please keep in mind the large airgap effect and dimension problem while suggesting.

regaeds, Nasir.

The combination of large air gap and low speed both imply a large diameter rotor will be needed. And, of course, the more poles the better. Have you considered a permanent magnet, synchronous motor? I would suggest a large size stepper motor, but their air gaps are typically less than a millimeter.

Yes, surprising indeed. You really are going to have to get the currents under control If you insist on a 3 phase induction motor I will tell you that it can be done. You will run into some difficult, but not insurmountable problems. A different motor would probably be my solution as well. Not sure of the point Bill was trying to make about low frequency currents, but the frequency of interest may be the slip frequency. The difference between We and Wm -electrical freq and mechanical frequency. That is what the rotor bars will see. If you got 220v to play with then you should be able to design in plenty of control headroom - Big current slewing no problem, but doesn't sound like you will need much dynamic control.

Dimension constraints sound like they are pushing you into custom motor rather than off shelf. (Why the airgap specification? Your professor throw that in for good measure?)

Have you defined the speed regulation? VFD is basically useless where you intend to operate anyway. Simple flux control is an option where magnetizing flux vector is controlled to some degree. Abbondanti wrote paper many years ago on an analog implementaton he had done. Worked well. If speed control isn't a major issue then this may work well for you. I think it maybe simple field orientation control. Open loop vector (sensorless vector) control is a method where speed is estimated and appropriate manipulation of pwm is used in order to control direct and quadrature axis. DQ is just stator current broken down into the discreet components with a transformation from 3 phase coordinate system into a rectangular system. Components are the flux producing current and the torque producing current. You try to control them independantly and maintain an orthogonal system to produce maximum torque per amp. Brush DC motors do this by way of commutator. Closed loop implementations are a step ahead where flux, speed, position information is feedback to controller in order to precisley control the process. You can design some smokin control loops and make the motor dance if that gets you off.

So there ya go!

Good luck with project.

regards, Bob

PS. The fella who mentioned the weird sounding drive - Sounds like synchronous carrier. The carrier is kept in synchronism with the fundamental output frequency. You then have a discreet number of modulation pulses per output cycle. As the motor accelerates the carrier slides up as well. Shift points are used to keep the carrier within a reasonable range. Yes sounds cool.

True.

------ The limitation on current at low frequencies is due to the dominance of resistance if one tries to keep the V/f ratio constant. If you don't then saturation becomes a problem. I have to disagree with the dI/dt statement above. Currents don't induce currents. The rotor current is due to the rotor induced voltage and this in turn depends on d(flux)/dt not di/dt. For AC the peak flux density is determined by the applied voltage, turns and frequency-not current. The exciting current is then determined by the flux and the magnetic path and material so its di/dt will depend on the frequency and the peak flux . The rotor doesn't know this- it only sees the flux produced. The induction motor is simply a transformer with a rotating secondary and the saturation and exciting conditions are the same for both. It is certainly possible to build an induction motor for low speed, low frequency operation. It will be huge and multi-poled and difficult to cool. Gears become a viable option and steppers, if they have enough muscle are another alternative.

----------- > Synchronous motors don't have this problem. which is why stepper

------------ True

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-------------- Do they have enough muscle?

Too true. There has been some experimentation with low speed AC motors for submarine propulsion. The units were very large and peaked at around 180 RPM. IIRC, they had quite a large number of poles and separately driven 'blowers' to circulate cooling air through the units. I don't recall what frequency they used at top speed, but I'm sure it was 20 hz or more.

The idea was to eliminate reduction gears. DC drives had been used, but high maintenance was involved with that. Also brush noise was an issue for submarine use.

daestrom

Once again sincere thanks to all of you.

--Some of you have suggested use of synchronous motors. Could you eleaborate that whether i should use switched reluctance motor or synchronous reluctance motor or some other type?

--Regarding three phase induction motor I will try to implement a) multi pole design b) vector control(Have any idea on some web resources dealing with this) Would post the results a and b in few days.

regards, Nasir.

Look into permanent magnet synchronous motors. I think that these are more efficient (more torque per pound of motor) than the two you list, above.

----- The Swiss use(d?) a 16-2/3 Hz system - and wound rotor induction motors with resistance control would work nicely at low speed -if they had enough poles. The problem is that at any frequency there is a flux density constraint-so pole span must be sufficient and as well the speed requirement means many poles (20 for about 100 rpm synchronous speed) Room for poles means a large diameter. Reducing voltage would help but then there is an additional need for space for larger conductors. Where the saw-off point is is a matter for more detailed analysis but a meter diameter 1 HP machine does present some physical problems (based on a breakfast table estimate before second cup of coffee).