what happens to voltage when frequency is increased to 100hz in induction motor

artis

few questions

ane) is it true that for a l or 60hz induction motor maximum torque is accomplished from 0hz upward to pattern hz and if an VFD is used and frequency is increased in a higher place 50 or 60hz similar 100hz the rpm will increase only the available torque will subtract?
Is the reason backside this the fact that the motor stator coils are wound with the number of turns that are fixed for a fixed frequency so going beyond this frequency decreases the electric current that tin can flow through the coils and and then the B field is weaker and results in less torque?

The same rule must apply to all inductors, then my next question is this

2) the same must apply to transformers as if a transformer has a fixed winding ratio and then increasing the primary frequency above sure point would result in lowered electric current in the primary and lower B flux in the core resulting in lower secondary voltage/electric current?

essenmein

1) It is non quite true, torque is more or less current dependent assuming y'all tin get the rotor excited properly. Then like most electric machines, once you enter field weakening region you lot are essentially in a constant power mode (at which speed this happens is BEMF dependent, ie straight related to available driving voltage and stator turns), so since power remains constant, speed increases, torque must go down.

ii) a transformer at unlike frequencies with the same applied voltage will generate the same output, within the magnetic limits of the core material (more on this in a scrap). What changing the frequency does is increase or reduce the magnetizing current (higher frequency reduces magnetizing current), this in turn affects the cadre hysteresis losses. As frequency gets too high eddy current losses in the cadre will limit the applied voltage or frequency. Note that these magnetizing losses are present regardless of load (bold applied voltage is always the same). Conversely as frequency goes down, y'all volition somewhen saturate the magnetic core, and it will stop being a very good transformer. Then a transformer will operate identically, independent of frequency, assuming you lot are within the operational range of the core material. Skin effect also plays a role if freq is high enough.

artis

one) ok I guess I needed to specify some details more , assume the rotor is either locked or with a very loftier load on it (at the point where induction motor is most efficient) so at this point it should have torque that is loftier and the highest as nosotros go from zero hz upwardly to rated hz , then above rated hz the torque would decrease , at to the lowest degree this is how I come across it.

2) ok I see the point about the transformer, sure core losses will touch electrical functioning merely leaving them out for the moment , bold nosotros are talking most an air core inductor where cadre losses or limitations don't employ, isn't here also the example that increasing frequency lowers the current given we have a constant voltage/current supply ? In theory.
Or have I got this incorrect and in theory frequency increase doesn't subtract current and this only happens due to real physical limitations surrounding the curl, like core losses, skin effect on coil itself , anterior , capacitive etc?

And then I guess I have to enquire whether a ideal air core coil doesn't take this limitation of electric current vs frequency?

essenmein

ane) ok I guess I needed to specify some details more than , assume the rotor is either locked or with a very high load on information technology (at the point where induction motor is most efficient) so at this point it should accept torque that is loftier and the highest as we go from zero hz upward to rated hz , so in a higher place rated hz the torque would decrease , at to the lowest degree this is how I run across it.
2) ok I see the point about the transformer, sure core losses will impact electric functioning but leaving them out for the moment , assuming we are talking about an air core inductor where core losses or limitations don't apply, isn't here too the case that increasing frequency lowers the current given nosotros have a constant voltage/electric current supply ? In theory.
Or accept I got this wrong and in theory frequency increase doesn't decrease current and this only happens due to real concrete limitations surrounding the curl, like core losses, pare issue on coil itself , inductive , capacitive etc?

And so I guess I have to ask whether a ideal air cadre curl doesn't take this limitation of current vs frequency?

ane) Well equally an assumption, at that place is quite a difference between locked rotor and heavily loaded where its most efficient! I call back "rated speed" is only really useful for line operated machines at line frequency, as soon as you lot control it with a VFD you lot would use a speed torque curve to describe the useful operating range.

As far as induction machines go, they are asynchronous, so at locked rotor, they still crave Air-conditioning on the stator otherwise the rotor field is not created, ie not goose egg Hz at 0rpm. All machines follow a speed torque bend not also dissimilar from the above. There are other considerations, but substantially below the knee speed, system phase electric current limit is what limits torque, above the human knee, system voltage limits how much torque producing current you lot can drive into the car. If the available current is the same, just you lot increase the motorbus voltage on the inverter, the articulatio genus only moves up in speed proportionally to the increase in voltage. Field weakening allows effective loftier speed operation, torque tails off equally speed increases but mechanical power more or less remains constant.

2) As far as I know, ignoring material limitations and with limitless electric current, an air core transformer follows the equations with no apparent limit to frequency (loftier or low) until you become into short enough wavelengths where other furnishings come into play.

Where are you going with this?

artis

quite frankly nowhere, I was just reading some related physics and wanted to exist sure I'm thinking correctly.

Yes as for the ac consecration motor , sad should have said at to the lowest degree 1hz instead of 0, surely without induction taking place the rotor doesn't feel whatsoever force at all.
I retrieve I get the flick , from few hz upward to rated hz the torque increases rapidly and then does rpm , so above rated hz rpm continue to increase merely torque falls off due to the fact that increasing frequency but having constant voltage results in less electric current through the stator coils.

I presume you say that mechanical ability expressed in kW or HP stay near abiding because those are calculated from the production of torque times RPM so when one falls the other increases and the result stays virtually the aforementioned ?
Although I would assume that torque falls faster than RPM increases above a certain frequency.?

essenmein · Feb eighteen, 2020

quite frankly nowhere, I was but reading some related physics and wanted to be sure I'm thinking correctly.
Yes equally for the ac induction motor , pitiful should take said at least 1hz instead of 0, surely without induction taking identify the rotor doesn't feel any force at all.
I think I get the picture , from few hz up to rated hz the torque increases rapidly so does rpm , then higher up rated hz rpm go along to increment but torque falls off due to the fact that increasing frequency but having abiding voltage results in less current through the stator coils.

I assume you say that mechanical power expressed in kW or HP stay almost constant considering those are calculated from the product of torque times RPM so when one falls the other increases and the consequence stays well-nigh the same ?
Although I would assume that torque falls faster than RPM increases above a sure frequency.?

From zero rpm, or a few Hz, to the knee speed, the torque is constant and dependent on electric current, mechanical power increases from nil to maximum at the knee speed.

The genu speed to me is when you demand 100% Iq and the machine will not spin faster, yous have "run out of voltage" at this betoken, increase you double-decker voltage, this speed moves up. If you want to run above this genu speed, you accept to apply Id, or field weakening current, now some of your phase electric current is used to reduce the net back EMF (ie abolish some of the rotor flux), since y'all are all the same stage current limited, this means you have less Iq available for making torque. Rated Hz doesn't brand sense in this context, the motorcar will happily run from aught Hz (in case of synchronous machine) to a maximum frequency adamant by the pole count and mechanical rpm limit of the auto (bearings etc).

There are other things that come into play and as speed goes upwardly really high you do finish being in "constant power", and enter a voltage limited surface area. Frequency does affect your voltages, iwL vector (produced by the stator inductance) is taking voltage without producing torque. However fundamentally these speed limits come almost not because of Hz, but because of Volts, the master one is back EMF.

artis

I remember seeing this clearly how the back EMF works to limit current is when I was on an older electric railroad train equally a kid , a DC train runs from 3kV overhead line and it had an ammeter in the electric utility box nigh the waiting expanse , I watched the meter and everytime from a consummate cease as the DC motors were switched on the meter went to something similar 600A and and so as the speed of the train increased equally the motors spun up the amperage decreased to the betoken where at near max speed the amps were some 50A perhaps , surely those were series synchronous motors and the max speed of the train wasn't probably their max speed and if uncoupled with a lighter load they would probably spin even faster and the amps would go fifty-fifty lower probably to the betoken where the commutator would self destruct due to arcing over or else.

essenmein

Ahem, DC motors are not synchronous machines. o0)

artis

yup my bad in DC at that place is nothing to be synchronous almost, I guess the right term for them is series universal motor as they all have commutators and can run on both Air conditioning and DC.

essenmein

yup my bad in DC there is nothing to be synchronous about, I gauge the correct term for them is series universal motor as they all have commutators and can run on both Ac and DC.

Heh, and universal motors are the odd ball wrt to speed torque curve likewise, they are hilariously non linear, and everything about voltage, speed and current you lot thought you knew with "normal" machines goes out the window.

artis

well 1 thing I know in the midst of the night about them is that they accept IIRC the highest starting torque of all electric motors , and they are dirt cheap and simple in design so all power tools use them.
At present that I think nearly it I am non certain how and what changes in this type of motor if used on Air-conditioning and again voltage/electric current is kept constant but frequency is increased, given they are series wound I would say that the rpm can but be influenced by force of the B field which is itself proportional to electric current through the motor which is then determined by voltage applied, as far equally I know these motors are in the simplest example controlled by voltage.
only then over again the commutator at high rpm'due south is like a switch manner supply of sorts right? and so the back emf becomes larger the college the rpm's get so torque should subtract with increased rpm

essenmein

well ane thing I know in the midst of the night about them is that they take IIRC the highest starting torque of all electric motors , and they are dirt cheap and uncomplicated in pattern so all power tools use them.
Now that I retrieve about information technology I am not sure how and what changes in this type of motor if used on Air-conditioning and again voltage/electric current is kept constant simply frequency is increased, given they are serial wound I would say that the rpm can simply be influenced by force of the B field which is itself proportional to electric current through the motor which is then determined by voltage applied, as far as I know these motors are in the simplest case controlled by voltage.
but so again the commutator at loftier rpm'due south is like a switch mode supply of sorts correct? so the back emf becomes larger the higher the rpm'southward go so torque should decrease with increased rpm

Yup, since these motors are series machines they do deport quite differently. Since both the excitation and torque producing currents are the same (beingness connected in series) means that at any speed everything is dependent on everything... lol

If you await at the one extreme, locked rotor (ie starting torque), naught BEMF, current is only limited by DC resistance of field and armature winding => large currents in both the field and armature = many torques.

As speed increases, BEMF rises, electric current for both the field and armature get down, now your field is weaker and armature currents are lower meaning less torque.

And so every bit speed increases, these machines reduce their field naturally, creating less BEMF, therefore allowing more speed.

The theoretical upper speed is really only limited by friction and windage, well, and mechanical integrity, but basically independent of voltage, due to their self field weakening property.

So at a fixed voltage, speed will change dramatically with load...

what happens to voltage when frequency is increased to 100hz in induction motor

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