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brushless dc motor drive from the mains

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3l3ctrici7y

Sun Sept 25 2011, 04:07AM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

Hi,

I am thinking about how I could use three half bridges to run a three phase motor directly off of the mains. The idea is that I would rectify and then filter the mains with a large electrolytic cap and get 170v of mostly DC at around 15 amps. Most regrettably, the motor I intend to run is rated at 37 volts and 60 amps with a "resistance" of 28 mOhms. The 1800 watts of mains power available is not enough to max out the motor, but I am OK with getting only 1800 watts (minus losses) out of the motor. We are also going to have a 120hz torque ripple which is not a concern.

I have two thoughts about how this might work;

1) The switch will apply 170 volts to the motor and it will have to shut off once the measured current reaches 15 amps. This huge current fluctuation will back feed all kinds of noise into the mains without proper filtering, and provide me with only a small portion of the motor's rated power.

2) Rely on the large electrolytic capacitor to filter the noise and provide the 60 amps that the mains cannot. Then I can maintain 170 volts across the phase until the current reaches the specified 60 amps, then shut it off. Proper sizing of the capacitor will be crucial and will likely be undesirably large.

These two thoughts are incompatible and cannot both be correct, but could certainly both be wrong. I fear that #1 is correct. Am I way off in the weeds here, or am I even close to where I need to be?

Thank you.

edit; thread title changed at Bored Chemist's suggestion.

Dr. Slack

Sun Sept 25 2011, 06:03AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The motor may be *rated* at 37v 60A, but it will only draw 60A on a heavy load.

If you are chopping 170v into the motor, then there is no problem synthesising a 37v sine wave into the windings, more or less indepednant of the current that it's drawing. The magic of SMPS means that less than line voltage into the motor wil give you more than line current.

Do beware some 3 phase motors which are designed to be used with high slip frequencies. Paradoxically, these overheat if run at rated input with no load. What happens is the slip frequency is to low, and the armature saturates, drawing excess current. The cure is to drop the applied winding voltage.

Basically ratings are to be regarded as maximums. So with a motor load, you need to undestand the physics of what's happening and tune your drive accordingly, and not exceed the rated maxima.

Mattski

Sun Sept 25 2011, 06:27AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

It's also worth considering the power factor of this circuit. If you are running a bridge rectifier with filter capacitor on a 15 amp mains circuit then the breaker will trip well before you hit 15 amps average current. This is because the current will only be drawn in short pulses at the peaks of the AC waveform, the pulse time gets shorter and height gets higher as you increase filter capacitor size. This in turn increases the RMS current draw, which heats up the mains wire more and causes the breaker to trip sooner.

3l3ctrici7y

Sun Sept 25 2011, 06:35AM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

Dr. Slack;
Thank you so much :)

Mattski;
I think I might understand what you're saying. Does it have to do with the voltage that the input capacitor discharges to under load and that no current is drawn from the mains until the point in the waveform where it exceeds the discharged voltage of the cap at which point, now current is drawn out of the mains into the cap? Make the cap bigger and more current will be drawn to charge it back up? What sort of PFC is indicated in this situation? Maybe a properly sized inductor on the input, matched to the value of the capacitor?

Is the capacitor what damaged the power factor in the first place? I still have a bit of trouble wrapping my head around this, but it seems to me that without a cap, the situation would be worse.

What you say reminds me of something that I noticed about my bench supply. It appears to trip the breaker at a certain output current regardless of output voltage. It is very old and uses an SCR to switch the mains into a huge transformer, then rectifies, and filters the output.

---
Ouch. I did a bit of research about PFC in SMPS devices, and its not good :(
There went the cheap simplicity of that I was planning.

Thanks again :)

Mattski

Sun Sept 25 2011, 05:30PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

wrote ...
I think I might understand what you're saying. Does it have to do with the voltage that the input capacitor discharges to under load and that no current is drawn from the mains until the point in the waveform where it exceeds the discharged voltage of the cap at which point, now current is drawn out of the mains into the cap?

That's precisely what's happening. With a full-wave rectifier the period is 8.3ms. If you are drawing 15 amps average from the filter capacitor, but the capacitor is charging from mains only for 1ms, then the peak current must but on the order of (8.3-1)*15=110A. Since power dissipation through a resistor (which is effectively what your mains wire and mains breaker are) is equal to I^2*R at every instant of time, the fact that you have such a large current peak leads to a much higher total power dissipation, even when you average out the 1ms of power dissipation over the 8.3ms period.

The problem is, as the filter capacitor size increases, your voltage ripple decreases, so there is a smaller and smaller amount of time when the mains voltage is higher than capacitor voltage, so power factor gets worse.

You can to a certain extent smooth out these current pulses with an inductor, and this is done in some switching power supplies as a simple way to raise power factor, though I'm not sure offhand how to choose the value of the inductor.

The best (and most difficult) method is active power factor correction which is frequently done by using a boost converter which is designed to draw current in a sine wave (or various approximations thereof) to get very close to unity power factor.

3l3ctrici7y

Sun Sept 25 2011, 11:50PM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

What I got from reading wiki was that it is not possible to effectively correct SMPS power factor problems with passive components, and that an active circuit is required. It kind of makes sense from what i read (to me anyway, and maybe I'm wrong). A motor/transformer with bad power factor is an L, so correct it with a C. An SMPS is neither and would need like-kind correction; another SMPS intended for PFC. I have studied active PFC circuits, and they're not trivial.

Now, what I read on wiki not withstanding, It seems to me like a properly sized inductor would be reasonably effective. With a 1ms period and 0-110-0 amps, the frequency (dI/dt) of that pulse would be huge; perhaps on the order of something a reasonable inductor could help with. I was strongly trying to avoid needing a transformer because those are so hard to acquire with generic parameters. Whereas a plain single circuit inductor can easily be acquired in a wide range pf parameters.

Thank you.

Sulaiman

Mon Sept 26 2011, 04:24AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I repair motor inverters (all brands) almost every week so I strongly recommend that you don't build your own,
I'd get a used one from eBay.

Almost all inverters can drive a much larger power motor than they are rated for, up to their output current limit.

If you find a few that interest you I can advise on their suitability.

Mattski

Mon Sept 26 2011, 04:59AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

wrote ...
What I got from reading wiki was that it is not possible to effectively correct SMPS power factor problems with passive components, and that an active circuit is required. It kind of makes sense from what i read (to me anyway, and maybe I'm wrong). A motor/transformer with bad power factor is an L, so correct it with a C. An SMPS is neither and would need like-kind correction; another SMPS intended for PFC. I have studied active PFC circuits, and they're not trivial.

True, a single inductor will not be able to completely improve the power factor, but it can improve it somewhat. I think a 60Hz bandpass filter (or maybe low-pass) could in theory bring your power factor quite close to 1, but creating a filter with that low of a frequency will be quite large, expensive, and probably inefficient to boot.

Dr. Slack

Mon Sept 26 2011, 06:57AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Don't let the idea of perfection stop you from doing something simple that's good enough.

While you can't *correct* the bad power factor of a recitifer/capacitor input with a single fixed inductor, you can improve it a great deal, basically taking the sting out of the short current pulses, forcing them to be be smaller and flow for longer. Pick your inductor value for maximum load (not forgetting to keep it out of saturation) and it will still help at lower loads, where the problem is not as bad.

3l3ctrici7y

Mon Sept 26 2011, 07:04AM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

Sulaiman;
Thank you for your input and your offer. Like those building induction heaters, part of the goal was to do this myself rather than buying an existing device.. If I cannot do it myself, then I have much less interest in doing it at all.

Mattski;
Thank you.

Dr. Slack;
If we assume that the RMS current from the mains is <= 15 amps, can we use an inductor rated for 15 amps and expect it to stay out of saturation? That is; we have a very tall peak, but the total integrated area under the curve is equivalent to a 15 amp resistive load? (not to suggest that I expect to look resistive, but rather conceptually for purposes of sizing the inductor).

I am thinking bad power factor is more of a thing of bad engineering form and perhaps stepping on electrical codes, rather than a complete show stopper. A test circuit could be built to evaluate the performance of different circuit designs and component parameters to achieve a sufficiently high power factor (.8 perhaps?)

One could probably minimize the problem with a few worse case assumptions, and compute the power factor, or at least run a SPICE simulation.

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