Wednesday, January 30, 2013

13,000 EV Miles


No doubt the 2 or 3 people that read this blog have long lost interest since I haven't posted anything for 3 1/2 months.  But as they say, no news is good news, and I would say that by and large, that applies here.  I pulled into the garage yesterday after running a quick errand (a Frosty for a sick child), and I took note of the odometer reading.  I delivered the Frosty and then entered the data in my spreadsheet and was surprised to see that the running total of EV miles I've driven added up to exactly 13,000.  Now the 3 year anniversary of the car hitting the road is in a month and 1/2 or so, but this seemed noteworthy enough that I thought I'd post an update of how things are going and include some numbers for those who enjoy statistics.

Keen readers will remember the episode I went through, just about a year ago, with a high voltage leak to the chassis, which I traced back to a frame leak through the motor and ultimately resulted in getting a new motor from Netgain.  George Hamstra was simply sick of repairing it and sending it back to me.  On a side note, George told me he has that motor in his garage and intends to put it in his Bricklin conversion.  I hope it does well for him.  At any rate, in addition to putting the new motor in, George also sent me some new brushes to try out, ones he felt would produce less carbon dust and so were better suited for my application.  He sent me (at his expense mind you) some Helwig Red Top brushes that are made of a harder compound than the standard brushes the Netgain motors come with.

You have to understand, Netgain Motors was born out of the EV drag racing community.  The earlier drag racers kept burning up their GE motors.  George and others saw different opportunities to improve the motors here and there.  They were working with Warfield Electric on building motors that incorporated their changes.  Before long they had a completely new beast on their hands and decided to start selling them.  The brushes they chose were of a softer carbon compound, ideal for pushing high current to the armature   Under drag racing conditions, the motor would generally fail for some other reason long before the brushes did.  But under low current applications, like driving to the store for milk and such, the brushes tended to wear out much faster and give off a lot of carbon dust.  The Helwig brushes have worked out so well for them that they've made them available on new motors.

Understanding that, we get back to my car and the unexpected change I noticed after I installed the new motor with the new Helwig brushes.  Suddenly, the car was more efficient.  About 14% more efficient to be precise.  Jack Rickard of EVTV ran experiments with an old WarP 9 motor he had at his shop, and found the same improvement.  Well, a great deal of time has passed, and I've put many more miles on the car, 4797 to be precise.  I thought I'd take a look at the numbers to see if the increase in efficiency has held true.  The thing is, I realized that I'd been doing the math wrong.  Well, the truth is, I knew I was doing it wrong, but I was too lazy to fix it, thinking the difference would be minimal.  To sum up, I was taking all the trips I made, and averaging the Watt hours per mile used.  The trouble is, the 275 Watt hours per mile used on a 35 mile trip was being averaged equally with the 378 Watt hours per mile on a 4 mile trip.  Anyone can see, those numbers needed to be weighted differently since the distance was different.  The solution is easy: add up all the miles driven and divide it by the sum of kilowatt/hours used.

What did we get?  As I mentioned, I've driven 4797 miles since the brush replacement, and I've used a total of 1526.46 Kilowatt/hours.  That works out to 318 Watt hours per mile.  In comparison, I grabbed a 4804 mile sample of data from before the brush swap and noted that I'd used 1779.72 Kilowatt/hours, which works out to 370 Watt hours per mile.  So does the 14% efficiency gain still hold water?  370 - 14%(51.8) = 318.2.  Apparently so.  I knew the way I was calculating the data before wasn't going to be wildly inaccurate, or even mildly for that matter.  But it's encouraging to see that numbers still hold to be true.

Moving on.  A couple weeks ago I plugged the car in, turned on the charger and heard a familiar "pop" come from the front of the car.  I recognized it immediately as the same "pop" I'd heard before when the motor had become caked with carbon dust and there was a mini flash over in the motor.  What the...?!!!  The only way this would happen was if the carbon dust given off by the brushes wasn't being blown out.  Since the Manzanita charger is grounded to the chassis, when you turn on the charger, if it senses a path from the chassis to the battery pack, it will either throw the circuit breaker in the charger itself, or blow up.  I've had it do both.  For the gory details of each, feel free to look further back in the blog.  Suffice it to say, you don't want to accidentally touch a battery terminal and the chassis with a multi-meter probe at the same time when you're charging.  Anyway, clearly I had a build up of carbon again!  How could this happen?  I've got a fan forcing air into the motor for chrissake! It should be blowing all that dust out.

I checked the fan, and it was running, but the output was very low.  I looked at the filter and found that it was caked with dirt.  Ahhhh.  I'd cleaned it about 6 months earlier, but clearly there's a problem.  Well the problem is really quite simple.  Having no way to locate the fan and the filter higher up in the motor compartment, I mounted it on a tray right next to the motor.  Meaning that the fan and the filter are about 9" off the ground, right were the dirtiest air is.  You may have noticed that every manufacturer locates the air intake for their cars as high up in the engine compartment as possible.  I'm thinking that may be intentional.  I took the filter off and the flow of air was much better, like I expected it to be.

So now I'm left with two problems.  First, the motor is full of dust and I need to blow it out. Second, I need to figure out what to do about this filter location; clearly where it's at is not the best place for it.  Last time I had a dust problem with the motor, I used compressed air to blow it out.  That worked pretty well as a huge plume of dust came out when I aimed the air gun into it.  The problem is, I have to take the motor's shroud off, and to do that I have to take out the batteries over the motor and to do that I have to ...  you get the picture.  Curse the idiot that designed this!!!  But then I had a flash of inspiration.  What If I simply hook up my leaf blower to the inlet hose, the one that runs from the fan to the motor.  Leaf blowers put out a crazy amount of air.  Oh, now this will be good.  Or at least fun.

It wasn't easy, but I was able to get to the inlet hose and basically taped the end of my leaf blower to it using my favorite tape in the world, gaffer's tape.  It's basically black duct tape, but easier to tear.  I had my daughter sit in the car and rev the motor up to about 3500 RPM, and I turned the blower on.  I was very pleased to see a plume of black dust come out the back of the motor (and also a bit disturbed).  I let it run this way for three or four minutes until I was confident it had expelled all the free carbon it could.  The great thing was, I could put my hand at the back of the motor and feel a tremendous volume of air coming out.  I started to wonder, could I some how use this leaf blower as my motor fan?  Apart from it being impractical, it would make the Z3 the noisiest EV on the road, ever.  No, what I really need to do is sort out the filter location problem.

With the fan located low like it was, it's collecting dust at a much faster rate, and ultimately choking the motor.  I looked, thought, measured, looked, thought some more and came up with nothing.  There is simply nowhere else to locate that filter.  Then I had a thought.  what if I eliminated the filter completely?  Stick with me for a moment.  The Netgain motors come with a screen covering the front bell housing, and the brushes.  People run them like this all the time.  The problem is some dirt and sand will inevitably get in and get on the armature and brushes cause them to wear prematurely.  OK, so that's the trade off.  But now, what if the inlet for that air came from a place that was less prone to see dirt and sand in the first place?  After all, the front of the motor sits right where you would expect most of the dust and grime to be, so if I could find a place for the inlet that would provide cleaner air than that, I should be good.  What I found was that there was a way to run a 3" hose from the fan inlet up in front of the firewall and just behind the large battery pack that's over the motor.  It's shielded from the main airflow when the car is moving down the road, yet it's got an open area large enough that it can easily get all the air it needs.  I checked and the output of the fan was terrific.  As far as I'm concerned, problem solved.  It's true that a bit of dirt may be blown into the motor, but I think this is a far better solution than cleaning the filter weekly to ensure good airflow through the motor.

The truth is, I'd love to get a fan that would push more air into the motor.  The one I have pushes 120 CFM.  George even said that's likely too low.  The problem again is one of space.  The fans I've found that push more air won't fit in the space I have.  I keep an open eye out for one, from time to time I comb through the interwebs looking for a suitable candidate.  So far, no luck.

That's all for now, but check back in early March on the car's 3rd anniversary as I'll post some more stats.


3 comments:

Brad Woodcock said...

Clearly you need to build a big dyson fan around the thing and turn the whole motor into a giant fan. You'll get a bit of power boost too!

Joey said...

Tim, I haven't lost interest. The contrary is true. Most blogs drop off after the car is complete, and I'm pleased to hear about how it is going once the machine is on the road. Your reports have lead me to seek an isolated battey charger. Until your new inlet location proves out, maybe you should use the leaf blower every 3000 miles, and extend the interval if no dust is expelled.

Tim Catellier said...

That's an excellent point Joey. When I was at EVCCON, I got a chance to talk to several others about their DC motors and frame leaks. As it turns out, nearly everyone has a small frame leak, and in every case, it's through the motor. During George's presentation, he gave us all a pretty good idea of what's going on and some insight into motor design.

The fact is that if you have a brushed DC motor, you will over time find there is a "leak" of current from the field windings to the case. It's inevitable. It all has to do with how efficient (or inefficient) the fan is at blowing the dust out of the motor.

All motors have an internal fan. He told us that European motor manufacturers tend to put the fan up near the commutator. It pulls air in from the back of the motor and vents it out the front. Consequently, the carbon dust doesn't travel through the motor and since the air vents out of the front it carries the dust with it.

I heard this and thought "Why doesn't Netgain do that?" As it turns out, there's a trade off with that design. Because the fresh cool air comes in the back, the system is very inefficient at cooling the commutator, which is where almost all of the heat is generated in a DC motor. Instead, Netgain, and most North American manufacturers pull air in from the front of the motor so that when it's coolest, it's flowing past the commutator and cooling it more effectively. The down side is that all the carbon dust given off by the brushes is forced to move through the motor, hopefully venting out the back. But as we've seen, that doesn't work with 100% efficiency, so we end up with a buildup of carbon in the motor.

As it turns out, the only way to tell if the build up of carbon has become a problem is if the resistance from the field windings to the case drops. George said that there will always be a small amount of conductivity. Warfield Electric doesn't consider it a problem until you can pass enough current through it to make a light bulb glow.

What that means to all of us with DC systems is that there will always be a shock hazard if you're touching the chassis and the battery pack at the same time. Worse, if you have a charger that isn't isolated, you may experience what I have.

But like I said, you bring up an excellent point. It really is a good idea to blow the motor out on a regular basis. Plus it would be a good idea to track on the resistance between the battery and the chassis to get an idea of how much carbon build up is occurring. Now that I have that inlet hose in an area where I can reach it, I should be able to hook the leaf blower up to it without too much difficulty. Of course I'll be blowing it through the external fan which will over-rev it like crazy. That's not all that bad for that little motor, right? :)