Welder plug installed, brakes make more sense now

 Welder:
$110 later, I have a 40A circuit hooked up for my welder. Plugged her in, and she welded first try.
Of course I checked voltages with a meter first :) I'm just glad the 240 to 208 conversion I did last week worked properly.

Brakes:
In other news, I've been bugging people on the forum diygokarts.com and I've gotten some great help in the hydraulic brakes area. More specifically, how to take hand brakes from a motorcycle, removing the hand lever and replacing it with a different lever, cable-driven by the brake pedal. Perfect!
Now I definitely know what I want to do, I just have to buy some brakes. The person who helped me out might also sell me some, or eBay has a decent selection.

Tires:
Still fuzzy in the tire selection arena. I only have a week left on that Princess Auto sale. Someone on the forum is using that particular kind of tire, but they installed tubes in the tubeless trailer tire, so that he could run a low pressure setup, compensating for having no suspension. According to him, these tires are capable of bending the go kart frame when off-roading at the recommended inflation pressure. Crazy! I have asked for clarification, we'll see what he says.

Work proceeding slowly

So I was home on the weekend for Fathers day, it was a great rib fest - all ten of us!
While I was there I borrowed some of my Dad's tools and picked up some of my own.

Now I know what you're thinking, stealing from my Dad on Fathers day, right? The thing is, he has all the tools he needs up at the cottage right now, the rest were in 'storage' not being used. Except the chop saw my sister and her husband were using.. sorry guys!

Anyways, I have picked up:
Lincoln Electric 'Mig Pak 15' welder
Bosch chop saw
Mikita angle grinder
Welding mask, tool boxes, misc.
Even more electronics junk

Welding to do:
Now I have to get my new 'shop' (read: workplace - thanks for letting me build this at work guys) set up to weld.
Of course the welder is set up for single phase 220, and work has three phase 208, so today I opened the guts of the welder, and swapped over the terminals.

While I was in there, I disconnected the gas solenoid - I use innershield flux-cored welding wire rather than solid wire with CO2 gas. No point in running the solenoid all the time if I never use it!

Now I have to go buy 18 feet of 8AWG four-conductor wire - preferably armoured but I don't think I'm going to spend that much money for something temporary (and in a zero-traffic environment). Also need a two-pole 30A breaker and a box for the receptacle (that I already have)
I hope finding the breaker won't be difficult - it's not a standard home-type breaker with the little metal peg at the bottom, it has a slot that the breaker box pushes inside (so female instead of male). A Cutler-Hammer unit.

Princess auto trip
I live in Toronto, Ontario right now. There are three Princess Auto stores relatively nearby to me. Wouldn't you know that all three stores are equally distant, 36, 36, and 37 minutes travel time - in different directions. Could be worse.

They have some trailer-tire assemblies on sale right now for $50 for tire+rim.
The tire is 16.5 x 6.50-8
PA: 8227175
Of course you should *never* use a trailer tire for a drive or steering wheel on a car - but the go-kart will weight much less than a car, so (at least I'm hoping) the load on the tire isn't too high, and the tire gets a sufficiently good grip on the pavement. They should also ride a little softer than car tires - most trailers have minimal suspension like my kart will, so they have softer sidewalls to compensate.

Other tire alternative:
I'm looking at a wrecking yard close to home in Scarborough, and another much further away. With any luck I'll be able to get a set of used tires from a small crappy car, with rims also already attached. If I'm desperate/lucky I can ever steal the steering knuckle and repurpose it.

Axle attachment:
For the front wheels I'm thinking of getting a trailer stub axle from Princess Auto or the wreckers.
PA: 8217242 1,000lb Posi-Lube Stub-Axle. Has a 4x4" bolt pattern to match the PA tires I was looking at.
For the rear axle, I'm thinking of using the same type of weld-on hub that I'm using for the sprockets
PA: 3845351
then welding a disc onto the hub, turning it down with the lathe, etc. I'm also debating saving time (read: lazy) by using an actual sprocket designed for these hubs, and just drilling holes in it to bolt to the hub, just ignoring the sprocket teeth. That eliminates a lot of machining time -  the hub and spocket have pretty small tolerances where they mate.

So that's four weld-on hubs.
Motor sprocket (18 teeth)
Axle sprocket (60 teeth)
2x tire rims (however many teeth I need to get a size large enough to drill holes through the sprocket and small enough the teeth don't hit the curved part of the rim. Going to decide in-store)

Still need a 1" axle and pillow blocks to go with that.
And a brake!

Brake hunt:
Princess Auto carries cable-powered disc brakes, but they are very expensive for the whole kit.
eBay sellers carry the same thing for half the price or less.

The current alternative I'm pursuing is a hydraulic brake assembly from a motorcycle. I'm hoping to snag the whole thing from a motorcycle wrecker in one piece, so I don't even have to bleed the system. Just have to figure out how to attach the master cylinder to the brake pedal instead of of the brake handle.

Electronics junk:
I brought up my 4x20 character LCD screen so I can have a display for the go kart. RPM, heatsink temperature, current, that kind of thing. The controller board that I'm using (which I designed for work) has a lot of flexibility, including a ribbon cable connector designed for a - you guessed it - LCD screen and some buttons. Note: This was a requirement of the design spec for the board, not something I added on for my own purpose to a work project!

So I'm going to get a ribbon and make up a little board with the LCD, and tack it into the software.
The great part is this ribbon cable has everything, power, I/O and even a PWM output so I can dim the backlight of the LCD screen effortlessly. Gosh I am happy about that.

I need to pick up some metal enclosure for the electronics - the logic board, main contactor, aux relay, precharge resistor, and 18V linear supply. It'd be nice to have this protected from the elements, at least somewhat. I would seal it in a weatherproof plastic box but the linear supply has to dissapate (probably worst case) 10V*0.1A = 1W. Alternatively, just stick 14g of aluminum on the linear supply and it will heat up by 100°C after 20 minutes of runtime too. I kind of hate having to do that twice, the kettle is bad enough. Metal enclosure means I have no time limit here :)
 The IGBTs on the aluminum block will have to remain exposed, maybe a small rain/mud shield.

Kettle design:
As I said in the video, the IGBTs will need water cooling. This project does not call for the complexity of a pump, radiator, etc. so I have decided to use boiling-water cooling. For a current of 200A, 1.5V, 15 minutes that is a grand total of 270kJ.

From the thermal enthalpy of vapourization of water, that equates to 0.12kg of water boiled, or 120mL. 30cm of 1" diameter copper pipe is long enough to contain enough liquid; I will solder the copper pipe to a copper sheet, which is in turn bolted to the back of the aluminum block.
You have to add more pipe than optimally required, since the soldering will involve flattening most of the copper pipe.

I will add a shutoff valves at the bottom of the kettle. This will allow me to drain excess water when I am done driving. To keep bugs out of the kettle during storage, I will either use metal screening to cover the hole, or insert a stopper of some kind.

Summary:
I've been doing a lot of thinking an research for the project, but haven't been posting regularly enough. I'll try to post more often so I don't generate these unreadable walls of text.
For this I am truly sorry.

William

Photo/Video Update

Hi all;

Here are some more photos, but most importantly the video I've promised.

IGBT Motor Controller - Youtube

IGBT Controller with Capacitor bank and wiring

IGBT Controller gate drive circuitry.

Motor upgrade ($$$ ouch!)

A picture is worth a thousand words.. so I'll show you the upgrade first

As a reminder, the old motor on the right was the car starter from the 1999 Sunfire.

The previously-loved motor on the left is from a place in Brampton Ontario.

Anyways, she's a 10" long 7" diameter 24V, 1725RPM motor.

Not quite the beast I was hoping for, but I'm planning on overvolting to 36 to get a higher locked-rotor torque. I think I'll have to be careful to limit the RPM somehow, I don't know if she will survive wide-open-throttle on flat ground (The armature expands and slams into the stator. Poof goes your money.)

Apparently though, everyone in the go-kart hobby arena overvolts their motors - it's just a question of how long you want the motor to last...

I've tested her on 12V using the IGBT controller I haven't even posted on my blog yet. I really need to put batteries in my video camera and actually do some filming.

On 12V the motor doesn't do anything all that interesting.. It's "slow" even at full speed, and doesn't have enough starting torque to even jump a little when I go from 0% to 100% throttle instantly.

Since it is a series-wound motor I'm sure she'll take off with a higher voltage, but I still was hoping for more... These things are supposed to wind up like crazy in a no-load situation.

Onwards!

IGBT Upgrade

It's been a very long time since my last post - I'm no longer broke and was able to purchase some new toys.
I've decided MOSFETs aren't the way to go, because they are too easy to break! I know it is possible to use them, it's been done a million times before, but soldering the SMT mosfets to a copper buss bar with a torch takes a long time, and I've melted the plastic that the buss bars were bolted to.

IGBTs are much more rugged - basically IGBTs are the ruggedness of a BJT glued to the easy gate-drive of a MOSFET.

So I bought a pair of single IGBTs off of ebay
Mistsubishi MG400Q1US41 N-channel IGBT - 1200V 400A

That's right, 400A of switching goodness. Now I need a place to mount them!

So I went to McMaster-Carr's website  to buy some metal. 
IGBT bricks need a fairly flat surface to mount them on, for heat transfer purposes. So I bought a quarter-inch thick, six inch square of aluminum, precision ground to a high degree of flatness
(6.35x152x152mm for the imperially-challenged)
Any warps in the metal means there will be an air gap between the IGBT and the 'heat sink', so the IGBT will be unable to dissipate heat properly.
It is very flat, has very sharp edges, and is one stiff piece of metal - perfect, since it is the structural support for the whole controller.

Then I need some more copper, to connect the IGBTs to each-other, and to the wires. (my mosfet controller used all the copper buss bar I scrounged from home)
The main concern was being able to fit the bar inside the LEM current sensor, its opening is only 10.4mmx20.4mm, so 3/8"x3/4". Of course no-one sells this size, but 1/4x3/4 is easy to find. I got a three foot length.

My apologies for not having pictures of all these components separately, but I hope these make up for it:

The blue brick is an IGBT snubber, a 0.68uF cap, 1250V designed for this application. You mount it directly on the IGBT, across the switch. It dampens the energy spikes, preventing the voltages from getting too high (well not preventing, just ... damping) although it wastes energy every single cycle (0.5C*V^2 joules - 8.5W at 50V and 10kHz). That's a lot for a cap, but nowhere near the power getting to the motor (fingers crossed).


The silver strapping on each IGBT is shorting the Base (Gate) to the Emitter lead - thus turning off the transistor, as well as protecting the gate from ESD - I would cry if I broke one. The gate on modules as rugged as this is still *very* fragile. 20V and *poof* when it can withstand 1200 on the other two, pretty crazy.

Circuit:
Exactly the same concept as before. One transistor that pulls the M- bar to B- and one diode that allows freewheel current from M- to B+ when the transistor is off.
IGBTs have a reverse diode like a power mosfet does, but it is much a better diode, rated at the full 400A, with a pretty low voltage drop (0.3 on a meter test, but that means squat in the hundreds of amps)

Gotta run! Expect more frequent posts in the near future.

Motor Controller Youtube Video

Great success!

I have determined why my mosfets were failing and found a solution.

Thanks to the friendly people over at ecomodder.com.
Permalink to my forum thread: http://ecomodder.com/forum/showthread.php/why-my-homebrew-controller-blowing-mosfets-solved-13805-2.html#post183057

There's a fair bit of detail covering a few weeks of work on the project there. Pretty pictures of my oscilloscope, too.

The reason my mosfets were failing was a large voltage spike on the drain (M- buss bar) causing a high voltage V_DS across the fet. This particular fet (IRLS4030-7PPBF) has a breakdown V_DS (aka V_(BR)DSS) of 100V. Now some sources I have read say this may not kill the fet. Since it seems to be the direct cause, I tend to disagree.

To reiterate, the failure mode is a direct short across drain and source. Once, the gate also shorted to the other two blowing the mosfet driver in turn.

The voltage spike is simply caused by V = L * di/dt. The inductance should be fairly low; the circuit is built with heavy wires and huge buss bars. As it turns out though, you need only a small inductance to generate huge voltages when large currents are involved.

Historically I have been switching 380 amps in 30ns to 50ns, or 12.6A/ns to 7.6A/ns.
I have read that a good buss bar system will have 10nH per inch of bar. Even with just one inch of bar I would get a spike of (12.6A/ns*10nH) = 126V.

To reduce the voltage spike, the answer is simple - reduce dt - by slowing down the mosfet switching.
I put a 100ohm pot between the mosfet driver and the mosfet's gate. Using an oscilloscope to watch the fall time of the drain terminal I could see exactly how fast the mosfet was turning on.
(Turning on the mosfet shorts M- to B- thus powering the motor. This is why the drain terminal falls when the mosfet turns on)
I tuned the pot to get a falltime of 200ns. This gives a di/dt of 1.9A/ns.

It is important to note that I needed to perform this tuning with the starter motor connected! Remember before, turning the controller on with the starter connected would fry the mosfet. I needed to be able to make tiny tiny pulses of power to the motor, so I replaced the microcontroller software with a test program! Instead of outputting PWM to the mosfet, I drove the line high, performed 'x' "nop" opcodes to make a short delay, and drove the line low again, then going to an infinite while loop with the line held low.
This allowed me to pulse the motor, and ensure I was not going to hit a spike high enough to fry the fet.
The initial experiments were only microseconds long, not even enough time for the motor current to be established - preventing me from getting the full brunt of di/dt since the current was too small :)

Eventually I worked my way slowly increasing 'x' for longer pulses of power, all the way to 6 milliseconds. The motor would rotate slightly with this long of a pulse, so I then tried 6ms on, 6ms off, 6ms on again. Long story short, I verified that no pulse would (probably) fry the mosfet, so I reprogrammed my motor controller application to the uC.

Lo and behold, the motor worked first try! At 20% duty cycle the motor begins to turn, and it runs merrily all the way up to full speed at 100%, screaming like a fury :)



At this point I still have only one mosfet installed, which will provide 120A continuous. However the fet is capable of handling much higher currents as long as the duration is short. That's why I'm able to start a motor that draws 380A under locked-rotor conditions (i.e. starting from stationary) When it is turning full speed only draws 70A. The fet would fry and probably explode if I actually locked the rotor before turning the power on. Now I need to add the remaining parts - two mosfets and one diode.

The B+ and M- rails were noticably warmer after running the starter for 30 seconds or so. I'll have to watch for overheating and possibly add forced air :(