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Old 09-17-2015, 04:11 PM   #2029 (permalink)
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Originally Posted by thingstodo View Post
Hmm. The motor manufacturer designed the 8 pole motor for 50 Hz and 1500 rpm (as an example). But you want to run it much faster .. 800 Hz and 12,000 rpm was it? The inductance is a part of the impedance of the motor, but it varies by frequency. In fact, it rises with frequency. The resistance does not change. So you have a proportional but slightly non-linear increase in impedance as your frequency increases.

If you are trying to push rated current .. say 100 amps .. through a motor that is rated for 50 Hz and 750 rpm, but you are using 800 Hz and 12,000 rpm ... I would expect that the rise in impedance would come mostly from the inductance and that your power factor would suffer. Not only would you get less amps into the motor, but the work done by the motor would drop because the power factor is worse.

Are you planning/already have rewound the motor for lower voltage so you can get good current at say 5000 rpm? Is the motor rated for this speed and so you already have the correct applied voltage?

... and ... would the carrier frequency have *ANYTHING* to do with how much current you can put through the motor? Does the motor *SEE* carrier frequency or just the averaged frequency?
Ok, now I'm very confused...
Perhaps you're talking about an induction motor? They do all sorts of things that I honestly don't understand.

The motor that I'm using (the MGR motor) has very similar specifications to the newer (2010) Prius motors. The Prius motor is a slightly larger motor (60kW vs 50kW) with the same number of poles and very similar windings (number of wraps, wire guage, etc) The rotor is nearly identical, just scaled to be slightly larger. I'm not planning to rewind the motor, just use it as it was designed. That's why I'm stuck with the high voltage.
Here are some numbers, as tested by ORNL:
The test controller (not a Prius controller) operated with a switching frequency of 5kHz.
BEMF - linear from 0 to a maximum of 525V @ 14,000 rpm (RMS, line to neutral)
Base speed: About 3500 RPM
Maximum speed tested 13,000 RPM using 650V. Output Torque is about 35Nm @13,000 RPM.
Maximum torque is 200Nm up to 2500RPM and 190Nm up to 3500 RPM.

You can see there's huge amount of BEMF @ 13,000 RPM - about 475V. (BTW - how do you relate RMS line-neutral to the Bus voltage?? ) Say you're supplying 650V, the voltage difference would only be 175V. According to ORNL's locked rotor tests, the amount of current required to get 35Nm of torque ( the motor's aximum output @ 13,0000 RPM ) is about 40A. I know it's not this simple, but just using Ohm's law says the maximum impedance for 40A would be 4.375Ohms.

They did a few more tests that were Bus voltage limited:
@ 500V, Base speed is 3750RPM, Max Torque is 150Nm and max speed is 8000RPM with 45Nm of torque.
@ 250V, Base speed is 1750PRM, Max Torque is 120Nm and max speed is 5000RPM with 25Nm of torque

So, at 8000RPM, the BEMF is 300V; difference of 200V, 45Nm requires 40A, so the impedance should be about 4 Ohms.
at 5000RPM, the BEMF is 190V; difference of 60V, 25Nm requires 25A, impedance of 2.4 Ohms.

So, looking at a large range of operating parameters, we see the impedance increasing from 2.4 Ohms @5000 RPM to 4 Ohms @ 8000 RPM - that's pretty significant. However, the impedance increases only to 4.375Ohms @ 13,000 RPM.

The problem is these numbers really can't be directly compared. The locked rotor tests were done with a rotor speed of zero (duh). The electrical frequency would be 333Hz @ 5000 rpm, and 866Hz @ 13,000 rpm.
Xl = 2*Pi*f*L
So, assuming 1.4mH for the motor inductance (sourced Toyota motor design paper)
@ 5000 RPM, Xl = 2.93 Ohms
@ 13,0000RPM, Xl = 7.62 Ohms

Now here is where I'm really confused - is there an increase in impedance -because of the increased frequency - and/or a decrease in effective voltage that limits the maximum speed? (assuming they won't physically explode) Also, to further complicate things, these motors provide a large % of reluctance torque - greater than 50% at high rpm.

Sorry about the long post - hopefully we can learn something about these motors from the good test info provided by ORNL.
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