1.5kW Induction motor overclock

[Stiive]

Hi,
I finally decided to overclock my small 1.5kW 4-pole IM before I move onto something larger.

The current stator windings are as follows


Now you may notice that there are 4 pole windings (no consequential poles), however 2 of the poles have a double winding... (as seen in the diagram above by parallel lines at the bottom).

Here is a pic of the double pole (left) and then single pole (right) for phase U. The double pole has the same amount of turns in each winding.


Now I'm a bit at odds whether to put the double windings in parallel, or leave them in series. I'm doing parallel atm. I've already snipped all the phase W connections. I'm running all wires back to the terminal box and its already crowded just after phase W (12 wires per phase!).

Seems a bit weird one pole pair in each phase is half the strength of the other? Guess it evens out as each double pole is overlapping with another phase's.

Pretty excited to get this running, was getting about 2kW out of it @ 100V, but the stator resistance was too high to get any more with my voltage restriction.

Question is, now that I'm paralleling all 6 windings per phase, is this a 1/6th voltage clock?

[weber]

Now I'm a bit at odds whether to put the double windings in parallel, or leave them in series. I'm doing parallel atm.

Any paths that you parallel should have the same number of turns to avoid circulating currents. As it is, there will still be circulating harmonic currents because of the different spans of the coils. The only configuration that would avoid those, would be one that is only 1/2 voltage.

Seems a bit weird one pole pair in each phase is half the strength of the other? Guess it evens out as each double pole is overlapping with another phase's.

It's kind of "semi-consequent" poles.

Question is, now that I'm paralleling all 6 windings per phase, is this a 1/6th voltage clock?

Yes.

[Stiive]

Any paths that you parallel should have the same number of turns to avoid circulating currents. As it is, there will still be circulating harmonic currents because of the different spans of the coils. The only configuration that would avoid those, would be one that is only 1/2 voltage.

Well that pole already had double the amount of turns since the windings are all in series, not paralleling it would be maintaining the status quo.... My original thought was if I left it in series, the pole would have double the amount of turns, and double the amount of resistance. This would lead to half the amount of current and double the turns, yielding the same ampere*turns as the single poles.
But I guess the magnetic field is also dependent on the length of wire, which would be double for the double pole in series, giving a weaker field.

In the case of the double pole when the whole motor phase is left in series (original winding), the double turns would cancel out the double length, giving that pole a similar (identical apart from coil span) magnetic field to the single winding poles.

So in order to maintain status quo, i need to keep the magnetic field strength the same as the single poles. However if i parallel the double poles, i get double the magnetic field of the single poles... If i leave them in series, i get half... hmm

It's kind of "semi-consequent" poles.

Possibly... I guess the consequent part could be a pole superimposed over the single pole creating 2 double strength pole pairs? If so, we're really getting into the magic here.
But then again, from above it doesn't look like the double pole was infact double strength. May soon be though

Question is, now that I'm paralleling all 6 windings per phase, is this a 1/6th voltage clock?

Yes.

Thought so... Woo! Lets see if i can get the windings to glow.

I'm bringing all the wires out to the terminal box like acmotor did so I can experiment anyway

[Stiive]

*DELETED REPEAT POST*

[Johny]

Referring to the opening diagram in this thread. Did that neutral connection come out to the terminals or is it an internal only connection?

[Stiive]

Referring to the opening diagram in this thread. Did that neutral connection come out to the terminals or is it an internal only connection?

Internal only. Sorry for the ambiguity

[Stiive]

Okay i'm bored... garage time

[weber]

Well that pole already had double the amount of turns since the windings are all in series, not paralleling it would be maintaining the status quo.... My original thought was if I left it in series, the pole would have double the amount of turns, and double the amount of resistance. This would lead to half the amount of current and double the turns, yielding the same ampere*turns as the single poles.
But I guess the magnetic field is also dependent on the length of wire, which would be double for the double pole in series, giving a weaker field.

No the magnetic field is not dependent on the length of wire.

First lets use standard terminology. Each phase of your motor has a winding consisting of 6 coils arranged in four pole-phase-groups (PPGs). Two of those PPGs have 2 concentric coils and one PPG has 1 coil.

Imagine the phase being energised with DC. It's irrelevant to the total flux whether a coil is around a north pole or a south pole. Every pole gets the benefit of 3 coils. The only difference it makes is in the fine details of how that flux is distributed across the pole face. We'd like it to be distributed sinusoidally, more dense near the middle. The pole face with the two concentric coils will have a better approximation to that.

In the case of the double pole when the whole motor phase is left in series (original winding), the double turns would cancel out the double length, giving that pole a similar (identical apart from coil span) magnetic field to the single winding poles.

Right result. Wrong reason. Everything adds here, there's no cancelling. Each single-coil pole gets a 50% contribution from each of the double-coil PPGs as well as its own single coil (total of 3 coils-worth). Each double-coil pole gets a 50% contribution from each of the single-coil PPGs as well as its own two coils (total of 3 coils-worth).

So in order to maintain status quo, i need to keep the magnetic field strength the same as the single poles.

No. You need to keep the volts-per-turn the same for all coils.

Possibly... I guess the consequent part could be a pole superimposed over the single pole creating 2 double strength pole pairs? If so, we're really getting into the magic here.

No. It just means that each single-coil pole is partly a consequence of its own coil and partly a consequence of the coils of the other poles of opposite polarity.

Thought so... Woo! Lets see if i can get the windings to glow.

I'm bringing all the wires out to the terminal box like acmotor did so I can experiment anyway

And if you change from star to delta you'll have 1/10 volts per hertz.

[Stiive]

No the magnetic field is not dependent on the length of wire.

So whats the equation then?

Here are some extracts from wiki:

This equation is for a solenoid with no core. The inclusion of a ferromagnetic core, such as iron, increases the magnitude of the magnetic flux density in the solenoid. This is expressed by the formula

where μ0 is the magnetic constant, N the number of turns, i the current and l the length of the coil. Ignoring end effects, the total magnetic flux through the coil is obtained by multiplying the flux density B by the cross-section area A:

it follows that the inductance of a solenoid is given by:



.....



Closed magnetic circuit
For a closed magnetic circuit (no air gap), such as would be found in an electromagnet lifting a piece of iron bridged across its poles, equation (1) becomes:



and....


Force between electromagnets

The above methods are inapplicable when most of the magnetic field path is outside the core. For electromagnets (or permanent magnets) with well defined 'poles' where the field lines emerge from the core, the force between two electromagnets can be found using the 'Gilbert model' which assumes the magnetic field is produced by fictitious 'magnetic charges' on the surface of the poles, with pole strength m and units of Ampere-turn meter. Magnetic pole strength of electromagnets can be found from:



and of course the original F=iBl.



I'm not saying your wrong, you may well be right, but i'd like to know what the correct formula is.

[weber]

No the magnetic field is not dependent on the length of wire.

So whats the equation then?

Those formulas are all good when applied to the cases they say they apply to. I note that the lengths they mention are not the length of the wire but lengths in the "magnetic circuit", i.e. the closed paths that the flux goes around.

There is a straightforward analogy with Ohms law for electrical circuits. Instead of EMF in volts we have MMF in amp-turns. Instead of current in amps we have flux in webers. Instead of resistance in ohms we have reluctance in amp-turns per weber.

Flux = MMF / Reluctance
phi = script-F / script-R

MMF = number_of_turns x current
script-F = NI

Reluctance can only be calculated by a simple formula in cases with very simple geometry and a uniform material, where it is

Reluctance = Length_of_flux_path /
          (permeability_of_material x cross_sectional_Area_of flux_path)
script-R = L / (mu x A)

All those formulae you gave can be seen to have this general form.

In a motor, a first approximation is to ignore the iron and consider only the length and area of the air-gaps between rotor and stator. But to do it properly you need finite element analysis (FEA) software.

[Edit: Deleted unfinished sentence. Corrected "omega" to "phi".]

[Stiive]

In a motor, a first approximation is to ignore the iron and consider only the length and area of the air-gaps between rotor and stator. But to do it properly you need finite element analysis (FEA) software.

So what you're saying is I should model it on RMXpert or Maxwell....
pfft, lets just see what happens.


Started phase V and realised bringing all the connections out to the terminal box isn't going to work at all, theres just soo many!! gonna now undo all my hard work and make the connections inside the motor.

You'd better be right weber!!!!

[Stiive]

Sucks to be me....

Spent ~$30 on wire, heatshrink, ring terminals etc (3x the cost of the motor!), plus 2 half days of soldering and snipping - finally complete it.

Try to test with before the sun goes down, drop the rotor in with the stupid big exhaust fan attached - and its too heavy to let it down slowly to check clearances and get wires out of the way etc... Give her a gentle spin, and one of the rotor fins catches a winding and it gets stuck.... argh, bout 1/2 hour trying to remove the rotor again from the snagged wires, only to find one wire has snapped so low its gonna be almost impossible to solder it back together - let alone the damage to wire insulation i caused from the screwdriver trying to unsnag the damn thing...

moral of the story - never get exited and rush at the end. Haha i always suffer from this. I'll try salvage it tomorrow, secure the windings properly this time AND grind down the fins - but think it might be for the bin It was all looking so promising too!! Phase resistance were all balanced nicely.. But very very crowded.

[Stiive]

Well, fixed it up, and its all working fine. Phases are all nicely balanced and each read 0.4ohm.

Problem is my power source is limited to 25A so its not much more impressive than before. I'm only running ~140V too so that's only 3.5kW. gah

Need moar batteries

[Canberra32]

.......... Soooooooo what your telling me is it is possible to overclock the bejebus out of a motor.... So overclocking a 12kw brushless could possibly net me ?kw
Because if this is the case then I may have some work for people

[Tritium_James]

Probably not. Induction motors are rated on continuous power, and very conservatively too. They'll do that power 24/7 for 15-20 years, in 50°C ambient temps. They are absolutely guaranteed to do what it says on the nameplate. So there's lots of possibility to thrash them much harder (10x) for short periods of time.

Brushless motors are almost always rated on peak power, and they'll do that for a couple of minutes. There's a lot of inflated numbers out there for these, especially in the hobby RC stuff and e-bike areas. Kind of like power numbers for stereo systems...

[Canberra32]

I'm talking the 12kw cont 30kw peak mars brushless bebeh the ME0913.
I mean I know bearing upgrades are a good start on any cheap motor but trying to get at least double out of one would be great

[Stiive]

James is right, your brushless motor should already be rated peak power. You cant push them beyond this extent for risk of demagnetising the magenets.

You may be able to put the coils in parallel to reduce the voltage rating, but you should stick to the maximum power input

[Canberra32]

Damn.... That's fine I will just go back to running two then.
If the light can't burn twice as bright for half as long then ill just add more lights.
Try as I may my search for a 300-350mm pancake brushless has not gone well other than an interesting frameless torque motor at 500mm