Paul and Sabrina's Cheap 3 Phase Inverter (AC Controller) with Field Oriented Control

[thingstodo]

Originally Posted by e*clipse

What's the inductance on the big ( 75hp ) motors, in general? Isn't motor inductance lower at higher power levels ( in general ) It seems as the inductance decreases, the smoothing would work better at higher frequencies. True??

Not sure - I've never had the need to measure it.

We regularly check phase resistance to determine if there has been damage to the motor windings (too much heat, coil insulation breaks down, resistance is no longer the same between phases)

Most of them are Y connected, 3 lug motors. If there is an easy way to measure (I don't know of one) - I have access to a scopemeter, multimeter ... variac ... not much else at work. At home I have more equipment but the motors are not at home

I would expect that the motor is a better filter as the motor HP goes up and the inductance goes down. The inductance needs to go down, or the motor current would not go higher .. and you would not get more HP.

[thingstodo]

Originally Posted by e*clipse

The best info I can find is the inductance for the motors I'm using is very low - 1.4mH to about 3mH ( It varies due to phase angle, I think )

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?

[thingstodo]

Update for Sep 16

Got the encoder mount modified, screwed down, and connected to the motor.

The motor output shaft is a spline, but DUCT TAPE to the rescue! With enough wraps of duct tape, the encoder adapter seems to work fine. And I lucked out with the alignment - no wobble that I can feel!

Got the firmware installed, powered it up and did a run-rotor-test.

Results in foc-test-10 . I recognized a test without encoder feedback. It took a while to run but came back .. not with 0's but a few 4's and 8's ?

Checked out the encoder and one wire fell off the encoder connector. So I stripped back the wire and put it temporarily on.

foc-test-11 is run-rotor-test with the temporary encoder channel.

The test seemed successful until it hit max speed and shut down.

foc-test-12 is run-rotor-test with the max-speed set to 9700 rpm (the motor lists 10,000 rpm as max)

The test went well until neat the end when it began to surge and hunt. There was regen involved and I had to keep a hand on the motor to make sure it stayed where it was (turns out I had the DC motor bolted down, and the AC motor bolted to the DC motor ... so other than the fact it weighs 150 lbs and is kinda square, there was nothing holding down the AC motor)

After the test concluded I had good numbers. But the motor was rotating (??) Index 13 was the best so I chose that one rotor-constant-index 13.

The rotor sped up at that point (?)

The raw throttle is 0. I tried 1 a few times and got it to minimum speed .. or a low speed, anyway. 2 a few times started a run-away acceleration so I put in a few 1's again.

Then the controller rebooted and I got the AC controller welcome message.

I shut it down and started to document the tests. I have video but it won't likely be posted until the weekend when I have some time.

We may want to discuss what I can and cannot monitor on video, with a good meter, a decent meter and a crappy meter. I can use a gopro to document some other stuff if we can come up with what to monitor and how to measure it.

EDIT - video links, if anyone is interested. As usual, mostly raw footage so there is long breaks between interesting stuff

foc 10 video https://youtu.be/6HhOfa1qjxU
encoder adjustment https://youtu.be/Rz8oAMRFOOg
foc 11 video https://youtu.be/Fm_ZrTHpcO0
foc 12 video https://youtu.be/P_gxP6swOsY

[MPaulHolmes]

Interesting. When you were doing 1 1 1 and it seemed to be running away, what was really happening was it was trying to command a torque and a flux as follows:

id & iq = 10
id & iq = 20
...30
40

So, the controller is controlling torque, and not RPM. So even though it seemed like something was going wrong, it was actually doing what it was supposed to do. A DC motor behaves exactly this way when you control torque rather than rpm. IF you held the motor post so it couldn't spin, Each time you would hit 1, it would put just a tiny bit more torque (and flux) through the motor.

It's interesting that the hardware overcurrent is tripping. My guess is that at high speed, when the rotor flux angle gets out of sync, it viciously commands current in the wrong orientation and there's a current spike that trips the hardware. I think we need to get some data of Id and Iq and Vd and Vq and a bunch of other stuff at high rpm. The rotor flux angle may get out of sync because the rotor time constant changes as a function of RPM. I just learned today that as frequency goes up, the rotor inductance and stator inductance go down. I think we should get a set of data points for the phase current and phase voltage, and then plug the stuff into a formula and get the stator inductance at several rpms. The rotor inductance has the same percent of variation as the stator inductance. Then we can update the inductance for the rotor (and the rotor time constant) in real time, based on RPM. I think we also might need to one of those run pi tests for Id. I just assumed that the response for iq was the same as for Id, just because it was for my motor.

[thingstodo]

Originally Posted by MPaulHolmes

So, the controller is controlling torque, and not RPM. So even though it seemed like something was going wrong, it was actually doing what it was supposed to do. A DC motor behaves exactly this way when you control torque rather than rpm. IF you held the motor post so it couldn't spin, Each time you would hit 1, it would put just a tiny bit more torque (and flux) through the motor.

OK - so I should hit 2, then pause to see the reaction, then 2 again ... so that it does not speed up and sound like a run-away.

Same deal with the series of 1s.

Decrementing the Id and Iq should have put it into regen and stopped it .. right? That's not what I saw.

It's interesting that the hardware overcurrent is tripping. My guess is that at high speed, when the rotor flux angle gets out of sync, it viciously commands current in the wrong orientation and there's a current spike that trips the hardware.

Hmm. foc test 10 stopped normally - no encoder input
foc test 11 the run-rotor-test shut down on overspeed.
foc test 12 ran to completion and I used 1 and 2 (apparently too fast) to control speed. Then the controller wake-up message came on - no fault there but maybe there should have been?

Hardware overcurrent?

Do you mean when the controller is turned off? I thought that was a signal that needed to be ignored when you saw the 24V input voltage drop below minimum, or maybe the 5V regulator voltage. Not sure if I'm off base here?

Every time I turn off the 12V battery, I get part of the fault as the last thing that happens before the controller shuts down. You don't even get ALL of the message - the controller doesn't have enough power left to finish logging the message ...

I think we need to get some data of Id and Iq and Vd and Vq and a bunch of other stuff at high rpm. The rotor flux angle may get out of sync because the rotor time constant changes as a function of RPM. I just learned today that as frequency goes up, the rotor inductance and stator inductance go down. I think we should get a set of data points for the phase current and phase voltage, and then plug the stuff into a formula and get the stator inductance at several rpms. The rotor inductance has the same percent of variation as the stator inductance. Then we can update the inductance for the rotor (and the rotor time constant) in real time, based on RPM. I think we also might need to one of those run pi tests for Id. I just assumed that the response for iq was the same as for Id, just because it was for my motor.

I think that the motor is more controllable when it is cool. All of the test sessions I have seen where the motor hunts are at the end of maybe 30 minutes or an hour of on-and-off testing. I can't locate my infra-red gun to see the motor temperature. Perhaps I should hook the motor up to a garden hose and just run water through it to keep the temperature relatively consistent?

It would also be great to locate my hand-held tachometer. A confirmation of the motor rpm would let me know if the encoder is operating as it should.

When you talk about capturing phase current and phase voltage - is that a stream of data from the controller?

I have no way of taking dynamic readings of 3 phase voltage and 3 phase current - but I could command a torque, measure one phase voltage and current as the motor accelerates, then check voltage and current on each phase in a semi-static condition. The torque command could then be changed and the readings taken again ... slow and painful but it should work.

[MPaulHolmes]

I have a command on there (well, I'll send another hex file, because I don't remember if it's on there or not) that captures Va and Ia (phase A current and voltage) at 10KHz. IT only captures about 250 data points for each of them, but it's enough for quite a few cycles. Then from that you just look at the peaks of the voltage and current, and the RPM, which is also outputted, and then plug them into some random formula that tells you the stator inductance. You can run that test several times as the motor speed gradually increases, and it will give the stator inductance at several RPMs. From that we can see how the curve behaves as rpm changes.

If you hit 1 a couple times, it will spin the motor one way. If you hit 2, it will slow the motor down, and then eventually spin the motor the other way. At least in theory. haha. that's what my motor does.

There are several interesting things going on that we need to get to the bottom of. The hardware overcurrent protection is basically a comparator and latching circuit that turns the IGBTs off if the current in any of the 3 phases goes outside of -600amp to 600amp (even for a couple microseconds). That's separate from the undervoltage protection circuit. The undervoltage protection disables the IGBTs if the 5v supply goes below like 4.5v and the 24v supply goes below maybe 22.5v or something like that.

[thingstodo]

Originally Posted by MPaulHolmes

The hardware overcurrent protection is basically a comparator and latching circuit that turns the IGBTs off if the current in any of the 3 phases goes outside of -600amp to 600amp (even for a couple microseconds). That's separate from the undervoltage protection circuit. The undervoltage protection disables the IGBTs if the 5v supply goes below like 4.5v and the 24v supply goes below maybe 22.5v or something like that.

When I get the video posted, you will likely watch it and say 'Oh, you are turning things off in THAT order' or something like that.

I think I have done things in different orders - high voltage off first, then 12V, 12V first while leaving the high voltage on ... I get the hardware overcurrent fault each time I turn off the 12V .. and the fault message never completes before the controller turns off.

If it the overcurrent latches before your undervoltage circuit triggers?

I'm quite certain that it is NOT an overcurrent. I think I've gotten the fault without the high voltage on at all, after doing the firmware upgrade and cycling 12V power to make sure that the output contactors cycle as they should

[MPaulHolmes]

OK then things are much more manageable than I was thinking. It could just be the current sensor output. Its behavior is unknown to me below the minimum required 4.5v supply on the current sensor. The microcontroller can run down to like 1.8v I think, so it's still doing its thing, long after the current sensor went by by. haha.

[e*clipse]

Originally Posted by thingstodo

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.

[MPaulHolmes]

Do you have a link to an equivalent circuit for this motor?