Pack charging with an AC controller

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coulomb
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Pack charging with an AC controller

Post by coulomb »

Here is my idea for using the often underutilised chopper IGBT (assuming that your industrial controller has one; many do) as a boost converter to charge the pack:

Image

Normally, a non-inductive brake resistor is wired between chop+ and chop-.

In a real implementation, D1 will probably have to be inside the controller, connected with thick straps to minimise inductance.

It may look as though you are only re-using one of the big components (being the switch, inductor, and diode; the bridge is low speed and costs under $10). But the IGBT driver circuit is already present and connected to a port of your controller. Also, the inductor may well come from the mains filter that is disconnected from the mains input filter. That leaves a high current, fast recovery diode as the main component to buy. (There will also be various filter components to keep the PWM spikes from the mains, and interlocks for safety, but these will be minor). If you are using the brake chopper for actual resistive braking, a contactor will also be needed (single pole, single throw; just need to remove the brake resistor when charging).

This circuit will not be isolated, but does not affect the motor wiring. Hopefully, your controller will have facilities for varying the PWM of the brake chopper under software control, such that a suitable transfer function can be arranged so that the harmonic distortion of mains current is minimised (usually stated as power factor being close to unity). It may be necessary to sense the zero transitions of the mains, so the PWM function can stay synchronised with the mains.

I believe that brake choppers are capable of considerable current, comparable to the current to the motor. Hence, this circuit should be capable of substantial charge current. You can program whatever high level algorithm you want (CC high/ CC low/CV with timeout, etc.) on top of the basic PWM foudation, using whatever programming facilities your industrial controller has.

Comments?

Edit: added interlocks
Edit2: industrial controller
Last edited by coulomb on Tue, 18 Aug 2009, 18:29, edited 1 time in total.
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Post by Richo »

Maybe a fuse?
But I guess that would be a given.
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Pack charging with an AC controller

Post by Hemonster »

Are you using the software in the inverter to drive the IGBT? or separate boost controller? If separate, will you be adding hardware to sense battery current? battery voltage? How big is the inductor likely to be (what sort of power?) and at what switching freq. are you likely to run this at?
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Post by coulomb »

Hemonster wrote: Are you using the software in the inverter to drive the IGBT?
Yes, that's the plan.
If separate, will you be adding hardware to sense battery current? battery voltage?
The controller has all that already.
How big is the inductor likely to be (what sort of power?)
The inductors that came with the controller weigh about 20 kg for the three, so about 7 kg. (Edit: they're about 200 mm diameter torroids, with very thick "wire" (square cross section from memory).) That should handle full motor current.
and at what switching freq. are you likely to run this at?

I don't have to worry about that, the controller does it all for me. I assume I'll just call a function that is designed to set the chopper current, and that will set the PWM for me. That will all be called in a loop.

We paid a fair bit extra for our controller to have a secondary micro, so the main micro can be busy doing PWM of 7 transistors, and we can do all our high level loops (speed ramps, adjusting regen, and charging) on the other micro. The software can be saved to a supplied plug-in flash cartridge.
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Post by Tritium_James »

Coulomb, if you want to run just a simple PWM signal for that boost converter (ie PWM = 1/(Vin-Vbatt) or something) then you also need a (large) cap between the bridge rectifier and the inductor. It needs to handle whatever current ripple is caused by your charge power for the intervals where the mains AC is below the voltage on the cap. It will be physically large and expensive.

The other option is to PWM the IGBT to produce a rectified sinewave shaped current waveform in sync with the 100Hz shape you have coming from your rectifier bridge. Then you've made yourself a power-factor corrected boost converter. This is a much better arrangement, it means you don't need the big capacitor, and it also keeps your electricity supplier happy. If you have a current sensor in the DC link to the batteries then you should be able to generate the current shape correctly, then you'll need some voltage sensing around the bridge somewhere to keep it in sync with the grid.
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Pack charging with an AC controller

Post by Hemonster »

coulomb wrote:
Hemonster wrote: Are you using the software in the inverter to drive the IGBT?
Yes, that's the plan.
If separate, will you be adding hardware to sense battery current? battery voltage?
The controller has all that already.

...
We paid a fair bit extra for our controller to have a secondary micro, so the main micro can be busy doing PWM of 7 transistors, and we can do all our high level loops (speed ramps, adjusting regen, and charging) on the other micro. The software can be saved to a supplied plug-in flash cartridge.
What controller are you using again, sounds nifty? And it has battery votlage sensing as well as battery current? I thought most of them only had two current sensors in two of the motor drive phases?


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Post by coulomb »

Tritium_James wrote: The other option is to PWM the IGBT to produce a rectified sinewave shaped current waveform in sync with the 100Hz shape you have coming from your rectifier bridge. Then you've made yourself a power-factor corrected boost converter.
Yes, that's the idea. And surely necessary at > 1kW or so power levels.
...then you'll need some voltage sensing around the bridge somewhere to keep it in sync with the grid.

Yes, so that should be pretty easy - just some resistors in series, driving an opto into a controller digital input.

This was only meant to be a skeleton circuit.
Last edited by coulomb on Wed, 19 Aug 2009, 05:16, edited 1 time in total.
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Post by Tritium_James »

You'll be able to make something that's "good enough" - especially for a one off project - by taking a few shortcuts. Things like assuming the mains is at 50Hz, and actually sinusoidal, and about 230V. Then all you need is a zero-crossing detector on the input, and when it goes, you begin a precomputed PWM output shape, scaled by the current you want into your batteries and the difference between Vbatt and (a possibly assumed constant) Vmains.

Of course, it might be just as easy to actually measure the voltage on the grid as it is to do the zero-crossing detector, in which case you can do everything properly. But it's likely to require isolation, and it's certainly easier to isolate a binary signal from a zero-crossing circuit than it is to isolate an analog voltage signal.
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Post by Johny »

Since you are only using the Gate drivers and single IGBT in the controller and you may not have access to DC Bus current...

How about a stand alone unit that does adjustable voltage brake resistor chopping (many VFDs are not settable) AND is a charger that can handle DC buses from 300 to 750 Volts (configurable).

That unit would have a market for folk using commercial AC systems that do not have brake resistor controllers built in, and provide a charger for the 300 VDC-up Evers (and of course the industrial AC weirdos). Image
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Post by woody »

Chuck in a precharge circuit as many VFDs don't have that either :-)
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Post by coulomb »

Hemonster wrote: What controller are you using again, sounds nifty?
Control Techniques Unidrive SP5402, 75/90 kW VF drive, with
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viewtopic.php?t=980&p=13625#p13625
And it has battery votlage sensing as well as battery current? I thought most of them only had two current sensors in two of the motor drive phases?
Oh. I'm assuming it has, since I'm pretty sure you can limit the DC bus current, but maybe I'm dreaming. I'm not near the manual at the moment, sorry.

Edit: fix quoting
Last edited by coulomb on Sat, 07 Nov 2009, 16:33, edited 1 time in total.
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Post by coulomb »

Tritium_James wrote: ... assuming the mains is at 50Hz, and actually sinusoidal, and about 230V. Then all you need is a zero-crossing detector on the input, and when it goes, you begin a precomputed PWM output shape, scaled by the current you want into your batteries and the difference between Vbatt and (a possibly assumed constant) Vmains.

Of course, it might be just as easy to actually measure the voltage on the grid as it is to do the zero-crossing detector, in which case you can do everything properly. But it's likely to require isolation,
It will certainly require isolation, so I'd prefer the binary zero crossing idea. That will give frequency, right? Though at the resolution it could be measured, assuming 50 Hz might be good enough. Though it may be powered from a generator some day, so I think it's worth measuring and averaging the last few cycles to extrapolate the current frequency.

How about this to not assume the mains is at 230 V (a rather poor assumption, I think). First I assume a high mains, say 265 V. Do a cycle (10 mS) and measure the RMS current. If it's less than the set point, bump up the PWM ratio (say for expected mains at 263 V) and try again. If the current is more than what I want, bump down the PWM ratio (say for 5V more mains voltage, so it cuts back faster than it bumps up the current), and let it settle into a rhythm. When close, do some smoothing so it's not constantly hunting.

Hmmm. I wonder if I need to monitor the instantaneous voltage of the pack for each PWM burst, or at least at several points during the mains half cycle, to cater for varying pack internal resistance. That should be doable too, I hope.
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Post by coulomb »

Hmmm. I've just realised a problem with this scheme; it will charge the battery with half-sine wave pulses of current, not DC. To get DC, I believe that most PF chargers use the split-Pi topology:

Image
(From Wikipedia's Split-Pi page.) S2 and S3 would be diodes for a battery charger.

So the first stage boosts the mains to (usually) about 400 VDC, and the second stage bucks this to the pack voltage. For charging a 700 V pack, the intermediate voltage would have to be some 800 V. When the mains is at a low voltage part of the cycle, the buck stage bucks less as the capacitor discharges, so the charger can provide smooth DC to the pack.

But in an EV, especially one with a low internal resistance pack like lithium iron, pulsing the current from zero to say 5 A peak (3.5 ARMS), the ripple caused by this sort of current would be negligible.

So the simplicity and economy of the single stage would surely win out.

However, I rarely seem to see examples of single stage chargers that use a simple boost topology. Is there some factor I'm overlooking here?

Edit: typo
Last edited by coulomb on Sat, 07 Nov 2009, 17:48, edited 1 time in total.
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Post by Tritium_James »

I'm guessing most chargers are 2-stage because the 2nd stage is the logical place to put an isolation transformer. You can drive it with a full bridge (best topology for high power) at 20kHz or so and get good transformer and silicon utilisation. This also means the PFC stage ALWAYS boosts to around 400V, and you can pick your output voltage by varying turns on the transformer. This means for a high voltage pack you don't need to go to less efficient 1200V silicon on the PFC stage and full bridge, you can stay at 600V. The only 1200V components are the diodes on the output side.

But if you don't need isolation (and I don't think you do) then I think the half-sine (or is it sine-squared?) pulsed current output won't be a problem - at least for a charger in the low kW range. If you're in the 10's of kW range it might become more of an issue.

The other reason I'm guessing you don't see commercial chargers with a single stage is it implies you need a battery pack that goes no lower than 240V * sqrt(2) at 0% SOC, ie 340V min = 136 cell LiFEPO4 pack. Most EVs are a LOT less than this.
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Post by coulomb »

Thanks for your thoughts, TJ.
Tritium_James wrote: The other reason I'm guessing you don't see commercial chargers with a single stage is it implies you need a battery pack that goes no lower than 240V * sqrt(2) at 0% SOC, ie 340V min = 136 cell LiFEPO4 pack. Most EVs are a LOT less than this.

Ok, but can't you use the buck/boost topology? That effectively adds the current (as in present value of) mains voltage to the voltage the input has to boost to. I suppose it's "even more unisolated", but to me that's a bit like "even more pregnant".
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Post by Tritium_James »

I think a buck/boost could probably work in this application. Don't forget it has an inverted output voltage, but for an EV with a floating pack I guess you don't care.
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Post by coulomb »

Here is a brave soul using an SCR based motor drive:

http://www.diyelectriccar.com/forums/sh ... post136028
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Post by p8 »

Hi, I have been thinking of similar idea.
In my area we have 3 phase power supply, so it is easy to get 400V 3phase 16A power from the wall.
This got me thinking - wouldn't it be easier (if at all possible) to connect an AC drive's output to the mains (U,V,W of the drive to the L1, L2, L3 in the outlet on the wall) and then set the AC drive in a constant regen mode.
If the AC drive can regenerate at variety of input voltages and frequencies, maybe it could "regenerate" from normal 3 phase outlet.
Does this make any sense?
If this seems feasible, what about safety?
How do one set voltage on the DC line then?
What kind of AC drive would be best suited for such application?


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Post by Richo »

sounds plausible.

However when a motor is spinning and the controller is not running the bridge there is no voltage on U,V,W. (ie no BEMF)
So the controller may spit the dummy with the voltage there.
Also the controller may not like the variation in set-up from the motor.
It would be hard to "auto-tune" the 3-phase mains.

A custom controller purpose built could do it.

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Post by coulomb »

p8 wrote:If this seems feasible, what about safety?
It should be safe enough; the drive is designed to handle this sort of voltage.
How do one set voltage on the DC line then?
Indirectly, by demanding the appropriate amount of regen torque, using a feedback loop with appropriate PID constants. It might be difficult to implement a very precise CV charging stage, though.
What kind of AC drive would be best suited for such application?
One that you know well, or is very well documented. And preferably are very used to. A tall order, most of the time. [/quote]
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Post by coulomb »

coulomb wrote: Hmmm. I've just realised a problem with this scheme; it will charge the battery with half-sine wave pulses of current, not DC. ...
But in an EV, especially one with a low internal resistance pack like lithium iron, pulsing the current from zero to say 5 A peak (3.5 ARMS), the ripple caused by this sort of current would be negligible.

I just happened to re-read this tonight, and after doing some work on the BMS, I realise that pulsing current isn't all that ideal when bypassing.

Suppose your BMS is capable of dissipating 1 A max. Then if you have a half sinusoid of current at 1 A RMS, the peaks will be about 1.41 A, more than the BMS can handle. So you need to turn the current down to 1 A peak, which is only about 0.707 A RMS, so your finishing charge (while some cells are in bypass) is going to take 41% longer. Sometimes you might want your charge to finish quickly.

Ah but!   Image   Our BMS can actually bypass more peak current if we just used lower value bypass resistors. The resistor heat is proportional to RMS current, not peak current, so half-sinusoidal charging current would actually be OK.

Ah but! Image   Then we rely on the charger never supplying DC current more than the continuous ability of the BMS boards. That means the BMS boards aren't foolproof. And when it comes to a potential fire situation, one needs all the fool proofing that one can, in my humble opinion.

So I suppose that we could have low harmonic distortion ("good power factor correction") for most of the charging, and all of the high power bulk charging. When the first cell starts overvoltaging, the charger flips into a different mode, where the charge current is lower, but is also almost DC. So the charger will use more mains current when the mains is at a low voltage part of the cycle, and less current when the mains is at a high voltage part of the cycle. At least, this is the opposite to most high harmonic ("poor power factor") loads, so the electricity authorities shouldn't be too upset.

We could also clip the peak current to a reasonable value so the distortion isn't too disgusting. So then the charge current would be flat most of the cycle, with dips near the zero crossings of the mains. The duty cycle could still be say 80%, so the finishing time would not need to be extended too long. It could even be a switchable setting, so if you are not in a hurry (most nights), you can let the finishing part of the charge take 41% longer and have low harmonics. If you happen to know you need the car in a hurry, you could flip the switch, get a warning light, and accept the poor harmonics for that charge.

Then again, assuming charging is done at night except for opportunity charging, most bypassing will be at night, so you don't care how long finishing takes, as long as it finishes by the time you want to start driving, so why not just use 0.707 A RMS charging and low current harmonics.

Still, it's a least a slight argument against using an AC controller as a charger: you'll get either poor harmonics ("low power factor"), or extended finishing time.

Edit: I'm assuming that at the end of charge, where bypassing is occurring, higher charge current will result in higher cell voltage, even over the time frame of one half mains cycle. Maybe that's not true; maybe a pulse of 1.41 A peak over 10 ms won't significantly change the voltage of the cell. I guess I should do an experiment, or perhaps someone already knows?
Last edited by coulomb on Sat, 07 Nov 2009, 18:34, edited 1 time in total.
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Post by antiscab »

i know from playing around with lithium cells on the bench,

even when a cell is completely full, it takes seconds for 1A to raise the cell voltage (even on tiny 40Ah cells).

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Post by coulomb »

Ah, thanks Matt. I thought it might be like that, but end of charge is a strange state for lithium iron cells.

That means we can use 1 A RMS during the finish stage on our 1.09 A BMS, and the BMS won't try to follow the current waveform, and should instead just respond to the cell voltage, which should depend on the total charge averaged over many cycles. I hope that holds under all conditions.

One way to find out...
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Post by Electrocycle »

yeah as long as the BMS can handle the RMS current of the balancing charge, the battery will smooth it out (or at least the BMS will have more effect in the lower part of the current waveform, and not do much in the higher part)

I've noticed that at full charge with 60Ah TS cells the voltage rises scarily fast even with 500mA charge current - but it's certainly not instant, and as long as it's kept below the BMS capability it seems to be fine.
I did find though that if you're close to the BMS current limit, it takes a very long time to bring that cell down to the bottom of the clamped voltage range (ie turn off main charge when a cell reaches O/V cutoff, then let the BMS bring it down while charging at balance current).
Obviously if you're charging at 500mA and the BMS can do 600, you're only getting 100mA of clamping, so it's probably good to leave a decent margin there.
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Post by evric »

Electrocycle wrote: ... if you're charging at 500mA and the BMS can do 600, you're only getting 100mA of clamping, so it's probably good to leave a decent margin there.

No, you are clamping at the full 500mA.
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