Author Topic: Terrible Idea Boost Converter To Be Tested On Gocart  (Read 3613 times)

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Offline i_am_fubarTopic starter

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Terrible Idea Boost Converter To Be Tested On Gocart
« on: December 24, 2019, 07:58:41 am »
I got an 1800W treadmill motor a while ago and decided to make a go cart with it.

It's a 180v motor.

I've run it off 4 SLA's, but at 48v, the speed is less that impressive. Plus, I think the batteries are almost dead, so really not as fun as I hoped.

Sooooooooooo, I've decided to build a boost converter / motor driver to run treadmill motors (upto 200v) from not 4, not 3, not 2 but 1 SLA battery (probably a big ol' leasure battery).... Just dealing with the fact it'll pull 150A. It's freaking me out a bit :)

Have got prototype working on the bench without load, but wondering if there are any other similar designs out there I could get inspired from?  :-+
« Last Edit: December 25, 2019, 12:24:20 pm by i_am_fubar »
 

Offline richard.cs

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Re: Go cart with treadmill motor
« Reply #1 on: December 24, 2019, 10:23:23 am »
Boosting a 12 V battery up to 180 V for a motor can probably be made to work but for that sort of power levels will almost certainly cost more than buying a lower voltage motor or a higher voltage battery pack. A 150 A input boost converter just isn't that sensible but if you really wanted to then you should look at schematics for a 2 kW inverter minus the output chopper (or just buy one and modify it).

I have a kart project at the moment using a car dynamo as the motor (in seperately excited mode) with a 48 or 60 V battery (not settled yet). This is a very cheap way of getting a brushed motor capable of around a kilowatt.
 

Offline T3sl4co1l

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Re: Go cart with treadmill motor
« Reply #2 on: December 25, 2019, 01:26:12 am »
Just one?  It's not going to run long, that's for sure; that's typical current draw for a starter motor!

Consider using several channels in parallel, and using push-pull forward converters.  This reduces the amount of reactive power required, so you can use smaller inductor, capacitors and transistors, and layout is easier and efficiency is better.

Transformers can be scrapped from car audio amps, rewinding the secondary for the desired output voltage.  The power transistors and controller (usually a TL494) can be reused.  You will need to add an output filter choke, current sensor (can be a shunt resistor or Hall effect sensor), gate drivers and rectifiers; snubbers are a likely addition as well.

The channels can be phase-shifted between each other, to help cancel out some ripple, so you don't need as large capacitors at the input and output.  (The caps from the audio amps would be fine for input, and caps from off-line PSUs would be fine for output.)  You then need one TL494 controller per channel, and a phase shift / clock oscillator to run them all (which I haven't figured out a minimal method for TL494s just yet, but there are some straightforward options to start with).

It's no harder than doing just one channel, it's just more bother, repetition.  Indeed you'll have a higher chance of success with the smaller channels, than with one big one.

Tim
« Last Edit: December 25, 2019, 01:31:02 am by T3sl4co1l »
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Offline i_am_fubarTopic starter

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Re: Go cart with treadmill motor
« Reply #3 on: December 25, 2019, 07:24:52 am »
Yea, the prototype doesn't have a choke or any filtering other than the caps (all elec, no ceramics so RF will be a pig). Will need to add some external filter to the actual switcher. Thankfully I'm not going for EMC tests, but how hard could they be....  :-DD

I've got 1 phase made up at the moment. Another I can make up, buy provision for 4. Using a PIC to generate PWM, won't be smooth as far as far as PSUs go, but it's driving a motor, so not too worried. Plus I can adjust motor speed with finer resolution from the drive PWM.

Have got current and voltage monitoring on input and output. At the moment streams results to PUtty, but got a phone app that will give me a proper dash board in time.

Going to try and attach a picture now.... Seems to be working  :)

Resistors give me 200ohm, so it's enough to mad a 10A bench supply out for now. Once I'm happy with control loops and voltage rating, I'll swap over to a (fused) battery.

Have designed to take up to 36v input, so could knock it down to 50A input which is a much happier current.

Also, I know this isn't an ideal way of doing it....bits just fun :p

Also, proof of concept... On verovoard... That blew up...  :-//
https://youtu.be/SDAV5vs-Nlo
 

Online Zero999

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Re: Go cart with treadmill motor
« Reply #4 on: December 25, 2019, 09:00:47 am »
Your best bet is to scrap the boost converter and use more batteries in series to give 180V. Failing that go with a lower battery voltage, but still higher than 12V, such as 48V or  60V, to keep the current low. It will be easier to construct an efficient boost converter to go from 60V to 180V, than 12V to 180V.
 

Offline i_am_fubarTopic starter

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Re: Go cart with treadmill motor
« Reply #5 on: December 25, 2019, 09:09:38 am »
I know it's not optimal and my efficiency will be lacking. But it's a good learning experience. First time dabbling above 1kW. And not the last by a long shot.

Once I can tick this one off the list, I've got my eye on a 10kW BLDC driver I want to make.
 

Offline MagicSmoker

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Re: Go cart with treadmill motor
« Reply #6 on: December 25, 2019, 11:17:30 am »
Terrible idea for a number of reasons, with perhaps the worst being that a standard boost converter can't deliver an output voltage any lower than the battery voltage because there is only a diode in the path between source and load. This also means there isn't a way to limit, much less regulate, load current, and that is highly desirable both for protection and usability in a traction application. (NB - a synchronous boost can turn off the output, but can't regulate it below the source voltage except in the reverse direction, because then it becomes a synchronous buck)

Third is that lead-acid batteries deliver less and less actual capacity as current draw approaches/exceeds the 1C mark; ie - a drawing 100A from a 100Ah battery might reduce usable capacity to 45-55Ah (see the Peukert Effect).

Fourth is that treadmill motors are mostly rated on their peak power and if from a consumer grade device you can bet they won't withstand even running at 50% of their claimed power rating for more than a few minutes. The continuous power rating of a motor - much like with mains transformers - is rather inextricably linked to weight and size, after all.

 
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Online iMo

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Re: Go cart with treadmill motor
« Reply #7 on: December 25, 2019, 11:18:28 am »
..But it's a good learning experience..
As a child I spent some time with cherry pit spitting training. I did perhaps 5 meters. I knew it was not optimal and a waste of time. But it was a good learning experience too  :D
Readers discretion is advised..
 
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Offline i_am_fubarTopic starter

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Re: Terrible Idea Boost Converter To Be Tested On Gocart
« Reply #8 on: December 25, 2019, 12:39:42 pm »
Is this a forum for ants only good ideas? I know this is very much not the way to do it. Ideally I would want a battery with a minimal full load voltage greater than the motor with an off the shelf controller.....   :=\

BUT! It seemed like a project that has lots of oppertunities for experementing with funcky control loops and high power and some cool flat wire inductors I got.

However, I have re-named the thread to something a bit more accurate (thanks for the inspiration Magic :) )

Also, to address the reasons this is a terrible idea, and maybe make it sound like not-such-a-terrible-idea project......

1) Minimum voltage output
Yup. if i were running the motor directly off it... This would be bad. Buuuuuuuut I have an extra low side switch for fine speed control, right down to 0%.

2) load current regulation
I have current sensing on output and input. I can control my load through adjusting both the boost AND the motor drive FET.

2.5) This is not a fixed output converter
I have a PIC that measures the motors actual speed vs demand. If it starts to run on too low of a drive signal, it can reduce the boost output voltage. Negating a lot of the efficiency issues. this also means I can loose the boost in the future and just have a 180v supply.

3) Flogging lead acid
pB is not ideal. Would rather have Li-ion or Li-po, but on top of the cost, I want to minimise risk of bang bang when charging. If i could run this off a bench supply, at 100%, I would.... But I'm not allowed a 150A bench supply :(

4) treadmill motor
A few minutes of full power would make me very happy :D I'm not doing laps on this thing. If it goes ka-bloom then oh well. I get to buy brushless  :-+ that does remind me though that I should put a temperature sensor on the motor.

So, with all said, I'll try and get some results up at some point :) or pictures of a charred PCB stack.
 

Offline T3sl4co1l

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Re: Terrible Idea Boost Converter To Be Tested On Gocart
« Reply #9 on: December 25, 2019, 12:50:49 pm »
Another advantage to the forward converter: it throttles to zero.

You can do the same with a boost by making it SEPIC instead, but that would be terribly impractical at this level (even more so than boost is already).  It would be better to make it flyback, but it's even better to use forward.

Tim
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Online Zero999

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Re: Go cart with treadmill motor
« Reply #10 on: December 25, 2019, 12:51:23 pm »
Terrible idea for a number of reasons, with perhaps the worst being that a standard boost converter can't deliver an output voltage any lower than the battery voltage because there is only a diode in the path between source and load. This also means there isn't a way to limit, much less regulate, load current, and that is highly desirable both for protection and usability in a traction application. (NB - a synchronous boost can turn off the output, but can't regulate it below the source voltage except in the reverse direction, because then it becomes a synchronous buck)
I don't see at as an issue. Use the boost converter to get a regulated 180V and add the standard motor + MOSFET and freewheeling diode to the output. Now the motor's speed can be PWM'd from 0 to full speed.

Quote
Third is that lead-acid batteries deliver less and less actual capacity as current draw approaches/exceeds the 1C mark; ie - a drawing 100A from a 100Ah battery might reduce usable capacity to 45-55Ah (see the Peukert Effect).

Fourth is that treadmill motors are mostly rated on their peak power and if from a consumer grade device you can bet they won't withstand even running at 50% of their claimed power rating for more than a few minutes. The continuous power rating of a motor - much like with mains transformers - is rather inextricably linked to weight and size, after all.
Yes those are far more important reasons why this is a bad idea. and the one about the lead acid reducing its capacity, at higher loads though is true irrespective of whether many small batteries are connected in series or a boost converter and larger battery is used.
« Last Edit: January 03, 2020, 11:44:24 pm by Zero999 »
 

Offline i_am_fubarTopic starter

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Re: Terrible Idea Boost Converter To Be Tested On Gocart
« Reply #11 on: January 02, 2020, 08:39:07 am »
So far, I've hit a few snags, some real fails (like my floating current monitor IC back to front), some micro controller niggles (such as ADC cross coupling between inputs due to bad timings) and some legitimate gotchas (like noise on encoder screwing up speed readings. However, I've now hit....

PROBLEM THAT I NEED HELP WITH #1
My main boost switching element is a 80A, 250V MOSFET IXFH80N25X3. The gate is driven from a FAN3100TSX 3A gate driver running at 12v. Circuit as follows
900822-0

Unfortunately, I am getting some bad ringing as the gate voltage reduces past VGSon. Will get the scope traces when I get home.

While the circuit mostly works, with the initial setup (gate resistor of 11 ohms) the drive chip was dying, ringing started and just didn't stop until failure, presumably through large negative voltage spikes or very high rate of on/off switching.

Up rated the resistor to 100r and no more dying driver, but very slow on-off speed. Not what I want for a SMPS. Dropped down to 20r to hit speed spec, but ringing back.

The layout is reasonably tight and as far as I can tell has limited inductance.
900818-1

Any thoughts or ideas on how to optimise this gate drive?

Cheers All :)
« Last Edit: January 02, 2020, 08:47:13 am by i_am_fubar »
 

Offline mzzj

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Re: Go cart with treadmill motor
« Reply #12 on: January 02, 2020, 10:04:39 am »
Terrible idea for a number of reasons, with perhaps the worst being that a standard boost converter can't deliver an output voltage any lower than the battery voltage because there is only a diode in the path between source and load. This also means there isn't a way to limit, much less regulate, load current, and that is highly desirable both for protection and usability in a traction application. (NB - a synchronous boost can turn off the output, but can't regulate it below the source voltage except in the reverse direction, because then it becomes a synchronous buck)

Not problem really if you use 180v threadmill motor and 12v battery.
Go-kart would probably barely if at all move with 12v.
 

Offline MagicSmoker

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Re: Go cart with treadmill motor
« Reply #13 on: January 02, 2020, 11:14:03 am »
Not problem really if you use 180v threadmill motor and 12v battery.
Go-kart would probably barely if at all move with 12v.

So letting the motor sit there stalled while the 12V battery continues to dump current into it through the boost diode isn't a problem?

 

Offline fcb

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Re: Terrible Idea Boost Converter To Be Tested On Gocart
« Reply #14 on: January 02, 2020, 01:49:50 pm »
In the mid-90's I built small bost converters that went from 12V to 26V at 10A output for the film industry.  We ended up with a custom planar magnetic inductor which was really expensive but dense. Going to 15x with a boost converter is pretty out-there, you'll need to be able to shut that FET off FAST, that's a heck of di/dt on the gate. Just doubling the number of lead-acids is going to halve your head-ache.

And THE biggest problem (by far) was scoping out the gate drive - all the high currents and fast edges meant you chased ghosts all day long.  Have you got access to an IsoVu?

Good luck - you'll learn alot from the project. Look forward to the next installment.
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Offline i_am_fubarTopic starter

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Re: Terrible Idea Boost Converter To Be Tested On Gocart
« Reply #15 on: January 02, 2020, 02:43:23 pm »
Very fair point on the fast turn off. Hadn't really considered that turn on is much less critical.

Inductor wise, I'm hoping these Wurth ones do the job, the spec is very tidy looking, but I'm also expecting more leakage than on the toroidal one I wound.

I wish I had an Iso probe. Just measuring the ground, that was also the ground point I was grounded to, is giving me high levels of pickup :(

Am going to build up another boost phase over the weekend and play around with different snubber arrangements, decoupling options, slew rate control.... etc. Drive it from Sig-gen as opposed to PIC so I can have a more consistent drive signal.

Got my battery isolator, meter & fuse box on the go to make it a bit safer and be able to operate the whole thing floating.
900990-0

Oh, and I'm slowly relenting..... 10Ah 22.2v Lipo may be a better option for me :p


 

Offline T3sl4co1l

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Re: Terrible Idea Boost Converter To Be Tested On Gocart
« Reply #16 on: January 02, 2020, 03:06:26 pm »
So what, ballpark 20nH there?  Mostly from lead length.  And Coss around 1nF?  Resonates at 35MHz and impedance 4.5 ohms.  So, if turnoff is faster than about 30ns, peak voltage will be noticeable.

Note that the part of turn-off below Miller plateau is what's relevant: Vds has stopped moving (the diode has turned on) and current falls (until this moment, the transistor is still carrying all the inductor current).  So this can still turn off fairly quickly even if your gate resistor is relatively large.

The maximum overshoot is delta V = I_pk * Zo.  So it's pretty significant that the impedance is quite high, compared to the switching impedance (which if it's say 100V and 50A peak or about 2 ohms).  Preferably it should be the other way around (Zo < Zsw).

No, there's no good way to bring it down -- you could load the drain with some capacitance, but for one it's not going to be any "closer" to the transistor itself (there's still the 10nH or so of the TO-247 package), and it's only going to increase losses (namely at turn-on).  Increasing L or C only increases the reactive power and thus switching loss.

Note, no source resistor, or current clamp on the inductor, is a disaster waiting to happen.

Tim
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Offline i_am_fubarTopic starter

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Re: Terrible Idea Boost Converter To Be Tested On Gocart
« Reply #17 on: January 02, 2020, 04:04:37 pm »
Need to have a proper read through this reply when I've got the thing in-front of me.

For reference here, what do you mean by source resistor or current clamp? (in this instance).

Cheers.
 

Offline T3sl4co1l

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Re: Terrible Idea Boost Converter To Be Tested On Gocart
« Reply #18 on: January 02, 2020, 04:12:22 pm »
Anything to sense switch or inductor current.  So, a source shunt resistor, or one referenced to the input (positive or ground, as, and if, applicable).  Or a current sensor, like a Hall effect chip or clamp.  Or etc.

Tim
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Offline i_am_fubarTopic starter

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Re: Terrible Idea Boost Converter To Be Tested On Gocart
« Reply #19 on: January 02, 2020, 05:00:18 pm »
Ahhh, I've got a separate 100A hall sensor on another PCB that also generates the PWM. It's shared across phases but should be able to detect OC and failures to force shutdown.

Ditto a 12A floating shunt on the output, shared across.
 

Online Siwastaja

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Re: Go cart with treadmill motor
« Reply #20 on: January 02, 2020, 05:12:39 pm »
Not problem really if you use 180v threadmill motor and 12v battery.
Go-kart would probably barely if at all move with 12v.

So letting the motor sit there stalled while the 12V battery continues to dump current into it through the boost diode isn't a problem?

With a large mechanical switch or contactor, which likely needs to be there for safety shutdown anyway, maybe it's acceptable behavior if the builder is the only user? Off-state current and torque would be low. It sucks, though.
 

Offline i_am_fubarTopic starter

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Re: Terrible Idea Boost Converter To Be Tested On Gocart
« Reply #21 on: January 02, 2020, 05:45:33 pm »
I did try a basic relay switch. 4x (rather cr4p condition) batteries gave an ok speed. 1 off would be a slow, but not insignificant providing the conductor losses aren't too low.
 

Offline amyk

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Re: Terrible Idea Boost Converter To Be Tested On Gocart
« Reply #22 on: January 02, 2020, 09:38:08 pm »
For a relatively cheap and quick solution I would suggest getting an inverter (one with 120VAC output will be ~170VDC) and either putting a rectifier on the output or tapping into its internal HVDC bus.
 

Offline T3sl4co1l

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Re: Terrible Idea Boost Converter To Be Tested On Gocart
« Reply #23 on: January 03, 2020, 12:57:41 am »
Ahhh, I've got a separate 100A hall sensor on another PCB that also generates the PWM. It's shared across phases but should be able to detect OC and failures to force shutdown.

Ditto a 12A floating shunt on the output, shared across.

You need a per-channel sense.  Think about current sharing, let alone compensation.

Tim
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Offline i_am_fubarTopic starter

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Re: Terrible Idea Boost Converter To Be Tested On Gocart
« Reply #24 on: January 03, 2020, 06:12:17 am »
Assuming ultimate accuracy and voltage stabilty isn't my priority. I figured that hitting each channel with the same PWM duty for a few cycles before readjusting based on average current /voltage would be sufficient to keep the system in it's safe operating perimeters.

Or do you think that the channel characteristics will vary that much?

Re. Inverter. Considered it, but the goal is to get some experience designing high power switching systems. So not really applicable :(
 


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