Motor overload

Tell us about the project you do with the open inverter
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pxxmo
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Motor overload

Post by pxxmo »

How much do you think you can squeeze out of the Kia Niro ev 150kW engine? How much can you overload it? I assume that its nominal power will be about 70kW.
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mjc506
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Re: Motor overload

Post by mjc506 »

Motor ratings will probably be thermal.

You will have a max shaft torque, based on the mechanical strength of the rotor/windings/any couplings/gearboxes/etc. Unlikely to ever reach this...
You will have a max shaft speed, based on bearing ratings, and hoop stress on the rotor (spin it fast enough, centripetal force will overcome the strength or the rotor and it'll either expand enough to rub on the stator (bad) or it'll just explode (more bad) :-) This is something to watch out for, but mostly for top speed limits.
Then you have electrical ratings.
Max voltage will be down to the insulation on the windings. Probably don't have to worry about this (inverter/IGBT max voltage will almost certainly be lower)
Max current is essentially a thermal limit. Get it hot enough, things (winding insulation) will start to melt and burn.
The most current you'll be able to push through the motor is theoretically your pack voltage (DC) divided by the winding resistance (usually something like 0.2ohms) 360V/0.2oohms = 1800A!!! (Reality will be less than this. You have IGBT voltage drops, resistive voltage drops through cabling, battery voltage will sag under heavy current draw, plus the inverter will be PWMing most of the time, so you'll get some back voltage from the inductance of the motor. Plus as soon as the motor starts moving it creates back emf)
But you also get magnetic saturation - Field strength (and hence torque) is proportional to current, until the motor saturates. Above this point, more current will most produce more magnetic field strength, but additional heat... Max field strength is more difficult to calculate.

However...
You have a peak rating for your motor (150kW). This will be what it can (conservatively) survive for some short period of time. Maybe a couple of seconds, maybe 10 seconds, maybe a minute, maybe 2ms... But given this is an automotive product with a warranty, it's likely to be quite conservative. Either way, if you can get enough 'cold' through the motor to keep it cool, there's no reason why you can't sustain 150kW continuously. (I'm not saying that getting that much coolant through a motor is trivial - you have to cool the rotor, who's only contact with the casing is through the bearings, and you have to cool the windings, which don't have a fantastic thermal path either - long thin wires, coated in epoxy, all bundled up in a dense lump.) If you've got temp sensors embedded in the windings, and keep an eye on coolant levels/temperatures, you can make sure to derate before things get melty.

That said, continuous rating probably isn't an issue. You'll have your inverter to take care of, and also your battery, both of which will have some peak/continuous limits. If you're driving on public roads, I can't think of many places you'll be able to dump that much power for very long at all...
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pxxmo
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Re: Motor overload

Post by pxxmo »

thanks for the extensive explanation. This theory is familiar to me, but I mean whether I can overload such a motor for a few seconds, say up to 250 kW. What are your experiences?
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Re: Motor overload

Post by SuperV8 »

pxxmo wrote: Thu Oct 27, 2022 5:44 am thanks for the extensive explanation. This theory is familiar to me, but I mean whether I can overload such a motor for a few seconds, say up to 250 kW. What are your experiences?
I think i'd be checking on the differences between the 100kw & 150kw versions. If they are the same motors - then Kia may already be 'eating' into the headroom?
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pxxmo
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Re: Motor overload

Post by pxxmo »

You're right, it's the same motor powered by a smaller battery
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