three phase motor current

[Paul BarterParticipant @paulbarter66156]

Hello all

This might be a simple misconception on my part, but whilst messing about with two three phase motors both being run from an IMO 750 Watt inverter(not at the same time).On measuring the phase amps I discovered that raising the freaquency and thus rpm led to a decrease in phase amps. Surely more rpm even on a motor running on no load means more power and as volts remain the same then amps should go up?I am using a rather crude clamp on ammeter but this effect is shown on both the quarter horse power motor and the one horse power motor.

Thanks in advance to anyone who can put me straight

Paul

[Paul BarterParticipant @paulbarter66156]

current from inverter decreases as frequency increases

[Stuart BridgerParticipant @stuartbridger82290]

A couple of points come to mind

1) Are you sure that the voltage is constant? Most inverters vary both voltage and frequency.

2) what is the frequency response of your clamp on meter, it may only be accurate at 50-60Hz

[Paul BarterParticipant @paulbarter66156]

Hi Stuart

Thanks for your reply. I must admit I assumed that voltage would remain at 240 as both motors are delta connected and the meter is a cheap one but the frequency range I was experimenting with was 50 to 65 Hz so hopefully within its range.

Paul

 

Edited By Paul Barter on 13/07/2016 22:18:53

[oldveloParticipant @oldvelo]

Hi

Sorry to answer with more question so what is the reading on the mains to inverter leads.

Would this not be more accurate.

Eric

[Stuart BridgerParticipant @stuartbridger82290]

Hi Paul,

Check out https://inverterdrive.com/HowTo/inv/

and https://inverterdrive.com/HowTo/240V-Supply-to-a-400V-AC-Motor/

For some very good explanations as to how inverters work

As an aside, I run the original 400V motor on my Chipmaster using a 240V inverter based on the technique describe in the second article.

Stuart

 

Edited By John Stevenson on 13/07/2016 23:06:00

[SkarvenParticipant @skarven]

Hi

It is the inductance of the motor that limits the current. A 60Hz motor will at 50Hz draw as much current as it can handle even with very little load. My US 60Hz bandsaw is run with a transformer at 190V 50Hz and gives about 80% of its rated power. At 240V 50Hz this motor run very hot even with no load.

As the frequency increases, you vould have to increase the Voltage to keep the current constant, but this is not possible with an inverter giving only 240V. You will have to live with the fact that increasing the frequency will limit the current, and with it, the torque. The motors construction will be the limiting factor for the current and of course the torque. Of course, as the RPM increases, this will give an increase in power.

That said, if you are running a 50Hz motor at 60Hz, the current is still limited by the inductance, but this will decrease with load, so you might still be able to run the motor at full torque, and 20% higher RPM, ie. 20% more power.

Kai

[Paul BarterParticipant @paulbarter66156]

Hi Stuart

Thanks, As you pointed out on measuring the voltage with a decent Fluke meter, surprise surprise the voltage goes up with the frequency, so no Nobel prize for free energy discovery, more a case of never assume! Thank you also for the two links, they were most informative.

Paul

[Paul BarterParticipant @paulbarter66156]

Hello Eik and Skarven

Thank you both for your replies,My tiny store of knowledge has been increased and my curiosity satisfied, I am most grateful to the menmbers of this forum for the knowledge and experience they are so willing to to take time and trouble to share with complete strangers.

Paul

[Anonymous]

I'm afraid it's little to do with the motor inductance. Instead it is to do with the back emf generated in the windings by the magnetic field generated by the current in the rotor, which in turn is induced by the rotating magnetic field in the windings.

If you keep the voltage constant and increase the frequency then the speed of the motor will increase. That in turn increases the back emf voltage, which opposes the applied voltage, so there is less voltage driving the current, which then decreases.

As a thought experiment what would happen if one applied voltages to the windings but locked the rotor so it didn't turn and hence didn't generate any back emf?

Andrew

[SkarvenParticipant @skarven]

Andrew

I completely agree, but the back emf is generated by the magnetic field from the rotor rotating in the stator winding.

The back emf will depend on the number of turns in the winding, and the inductance. A motor made for 50Hz will have more turns in the windings than a 60Hz one.

Kai

[not done it yetParticipant @notdoneityet]

Resistance, inductance, reactance, back emf, Flemming's left and right hand rules, magnetic field strength and saturation – and possibly a few others – all inter-react dependent on speed.

Andrew's thought experiment emulates starting current required for these motors. Until it gets going, Ohms law applies initially (apart from not being at constant temperature) along with building and collapsing induced magnetic field (due to A/C frequency), dependent on the structure. Hopefuly zero rotation does not persist for long or the dreaded smoke will escape!

Motors are designed with all things in balance at their operating conditions. One certainly cannot continue to increase the frequency ad infinitum and expect to extract useful power…. One might even think of an extreme, like induction furnaces, where the electrical energy is coupled to a small lump of metal which simply heats up, when the induced fields are concentrated on the target.

[SillyOldDufferModerator @sillyoldduffer]

Just a thought, but most AC meters only work properly with sinusoidal currents. As the waveform coming out of an inverter is unlikely to be sinusoidal, the meter might be thoroughly confused! That's in addition to the possibilities mentioned by others.

[John OlsenParticipant @johnolsen79199]

At any particular speed, the current the motor draws will depend on the load on the motor. When unloaded the current will be low, and it will increase as the load is increased.

The inverter reduces the voltage applied at low frequencies, since the reactance of the windings is less. To a first approximation this is a linear function, eg at half the frequency the voltage would be halved, but many inverters are a bit more sophisticated and apply a little more voltage at low speeds to compensate for the fact that although the reactance of the windings is less, the resistance is the same. So if they did not compensate, the torque available would be less at lower speeds. This compensation should ideally be set up to suit the individual motor.

When you run at speeds over the normal maximum, eg 50 Hz, then the usual inverter cannot increase the voltage any more. So the available torque will fall off. This is not usually a problem, since the sort of things we do at maximum speed don't usually require high torque. So for instance I can run my little Unimat 3 at up to 8000 rpm, which is good for things like the little circular saw attachment. That means setting the frequency to 100Hz, and the motor runs at twice nominal speed.

The University of Canterbury tested ordinary production motors back in the 80's and found that they were OK for balance at up to 10,000 rpm, which is also about the speed that the bearings were rated for. So you could have safely gone to 150Hz with those motors, if you needed to go really fast. If your inverter could supply the right voltage, that would also give you three times the normal rated power. (Provided the insulation was also up to the increased voltage!) Apart from insulation limitations, there will also be an increase in eddy current losses as the frequency goes up, so you can't increase the frequency indefinitely. However, three times normal speed is certainly possible.

It is not just Ohms law when starting. The motor can be modeled as a transformer with a shorted secondary, but since the coupling between primary and secondary will not be perfect there will also be some leakage reactance. That also helps to limit the starting surge.

John

[AjohnwParticipant @ajohnw51620]

Clamp on amp meters don't usually have much of a frequency range. They are generally aimed at mains frequency and I assume near sinusoidal wave forms. There will be some scope on wave forms as power factors can vary.

What is actually happening when the frequency exceeds the motors normal frequency range is that it's effectively moving into a constant HP region. The speed is increasing but the torque is dropping off. This tend to balance so that HP remains fairly constant.

John

–

[Anonymous]

Posted by John Olsen on 14/07/2016 22:25:57:

It is not just Ohms law when starting. The motor can be modeled as a transformer with a shorted secondary, but since the coupling between primary and secondary will not be perfect there will also be some leakage reactance. That also helps to limit the starting surge.

So, to test my understanding, if the motor can be modelled as a transformer with a shorted secondary presumably the start up current is determined by the winding resistance and the reactance of the leakage inductance rather than the inductance of the winding? It's been a good few years since I designed 'transformers' for flyback switchers, but I recall that leakage inductance was measured across the primary winding terminals with the secondary shorted.

Much has been bandied about concerning the role of the winding inductance, but what rough value is the inductance? I have a vague recollection of measuring winding inductance values of low hundreds of microhenries on a three phase coolant pump motor.

Andrew

[MuzzerParticipant @muzzer]

An induction motor is rather like a transformer with a variable frequency secondary. The secondary ("rotor" ) has a fixed resistance across it (a shorted turn with finite resistance – "squirrel cage" etc). So it's not very straightforward to intuit the workings. However, in simple terms, the torque generated (due to the induced rotor current) is proportional to the slip frequency ie the difference between the rotational speed of the field (eg 1500rpm for a 50Hz 4 pole machine) and the rotational speed of the rotor. So you can pretty much generate the same torque at stall as you can at any other speed. A VFD can generate the required slip frequency at stall in a way that a direct mains connection can't, by applying a low frequency voltage – it knows the required slip frequency from the nameplate data you enter during setup.

The magnetic flux generated in the stator is proportional to the volt-second product applied to the motor terminals (as the impedence is dominated by the winding inductance). So simple (old fashioned) inverters tended to use a "constant V/F" to control this crudely. This flux isn't directly generating any torque but you need to avoid saturating the stator by applying too much voltage for too long.

You wouldn't measure the primary inductance by sorting out the secondary. As you say, this would give an indication of the leakage inductance. For an induction motor, the rotor resistance isn't a short but its presence makes measurement of the inductance more complicated. If you know the no-load running current at a given frequency, you could probably make a reasonable estimate of the motor inductance.

Edited By Muzzer on 15/07/2016 22:01:55

[John OlsenParticipant @johnolsen79199]

Andrew…yes, the simple model (at stall) is an inductance in series with a resistance. The inductance is mostly the leakage inductance, and the resistance is a composite of the main winding resistance, the squirrel cage resistance reflected back into the primary, and any other losses. As Muzzer points out, the rotor is not strictly a short, it does have resistance and this is actually critical to the design, since if it is too low the starting current will be too high. Larger motors have wound rotors with slip rings and switch in resistance for starting to control this.

My father came across a case where a motor had a squirrel cage that extended out each end of the iron a couple of inches, being shorted at the ends. This partly served as a useful fan for cooling. However, one of the motors got these broken to some degree, and the electricians decided to fix it by cutting them back closer to the iron and brazing on a shorting ring closer to the iron. The result was that the starting current was greatly increased and the motor was unusable because it threw out the breaker every time a start was attempted. So although those bars might look like a short, they actually do have some resistance, enough to be significant at the low voltages and high currents in a single turn secondary.

I don't think it is usually necessary to know the winding inductance, but I think the best way to measure it would be to drive the motor at synchronous speed and measure the current with the mains voltage applied. This would remove the effect of the secondary, since the slip frequency would go to zero.

John

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