Motor Choice

There are lots of electric motors out there and there’s a lot to consider when picking one out.  Most people, I think, have a misunderstanding of what will be best for them.  Here’s a compilation of what I’ve learned and how I chose the WarP 9” motor.

Brief Overview of Motor Types:

Series Wound DC Motors – These motors are by far the most popular among EV conversions.  The reason for this is that they are relatively cheap and produce a large amount of torque for a given amount of current.  This means you need less current to get your car moving.  Since current is somewhat of a commodity in the EV world, you get more bang for the buck with this motor.  In addition, a relatively inexpensive and simple PWM controller is used to drive this motor.  These motors are basically the same as the motors used in forklifts except that they have been modified for higher voltage and higher RPMs.  Some people even get used forklift motors for their car since they can be quite cheap.

DC Permanent Magnet Motors – These motors operate similarly to the series wound motors except that they have permanent magnets for the stator instead of a coil.  They do not produce as much torque for a given current when compared to series would motors.  They also are probably more susceptible to high temperatures than series would motors, though I don’t know that for sure. 

AC Motors – These motors are driven off an alternating current.  You achieve greater efficiency with these motors and can easily incorporate regenerative braking, which is why I think they are used on most hybrid cars.  The downside is the controller for this motor is complex and expensive.

Comparison of Series Wound Motors:

I picked the series would motor type due to the cost involved and the high torque output vs current as mentioned above.  The next question is what brand and size.  I’ll assume that all manufacturer’s motors are equivalent in quality, reliability, etc, so this question is more about what size and winding parameters do I want.  I’m going to compare 4 motors that are popular among EV conversions: Impulse 9, Warp 9, Warp 11, and ADC 9.  The three 9 inch motors are approximately the same diameter but are wound differently.  The Warp 9 and 11 are supposedly wound similarly but are different diameters.  One thing to remember is that the power you get in is the power you will get out (neglecting variations in efficiency, which are similar for all motors). 

9 Inch Motors - Changing the windings affects the speed (RPMs) of the motor for a given voltage and current.  This means that one motor will spin fast with low torque and another will spin slow with high torque since the power in has to equal the power out.  FYI, power in is the voltage multiplied by the current and power out is equal to the torque multiplied by the rotational speed.  Depending on how much voltage and/or current you can supply to the motor in addition to how quick you want to accelerate and the desired top speed will affect which of these is best for you.  The Impulse 9 is wound for higher speed and lower torque, the ADC 9 is wound for higher torque and lower speed, and the Warp 9 is somewhere in the middle.

Warp 9 vs Warp 11 – Supposedly these two motors are identically wound.  If that is so, then for a given force between the windings (the field and armature) will produce a different torque because of the different moment arm (diameter of the motor).  This means for a given internal force, the Warp 11 will produce around 20% more torque than the Warp 9.  Consequently due to the conservation of power rule, the speed of this motor will be slower in the 11” motor than that of the 9”.

The chart below shows a comparison of motor RPM vs Current at a constant voltage.  Up to about 500 amps this is based on manufacturer’s published data.  More info on extrapolation later.

The next chart shows a comparison of torque vs RPM at a constant voltage.  Again, up to about 500 amps this is based on manufacturer’s published data.

 And one more chart that shows a comparison of torque vs current for each of the motors.

 Most people look at this data and say I have a 500 amp controller and I can get 154 ft-lbf of torque from a Warp 11.  I’ll buy that one because it’ll give me the best acceleration.  Well, not exactly.  This is a correct statement technically speaking but it may not really be what you want.  For many people, I think this is common mistake made when designing your car and here’s why. 

I’ll start with a typical conversion setup.  144 volts is a common battery pack size because it allows you to use most of the base model controllers and is easily achieved with 6 or 12 volt lead acid batteries.  We’ll also assume you have a 500 amp controller and the batteries are sized to allow you to drive 500 amps to the motor.  Many people would assume you can get 154 ft-lbf for as long as you keep your foot on the pedal.  If this were correct, you could make it from 0-60 mph in a blazing 4.75 seconds.  I see a lot of people on evalbums.com who are building their car with very similar specs and expecting 0-60 in under 6 seconds.  I’m sorry to tell you that this is not going to happen.

Here's what is more likely to happen.  With 154 ft-lbf of torque coming from the motor, you can easily peel out in 1^st gear, but it won't last for long.  I’ll assume your pack voltage sags to 120 volts at 100% duty cycle from the controller.  At this point you have 60 kW of power in and we’ll assume you get the same power out.  At 154 ft-lbf, the maximum motor speed is 2300 RPM before you reach your maximum power output.  If your overall gear ratio is 14:1, you are only traveling 12 mph when the torque will begin to taper off.  So you change into second and get a new burst of speed, but that gear starts to runs out at 21 mph.  There are a lot more factors than I previously mentioned, but you’ll find that you can easily spin the wheels in 1^st gear, but it takes you about 9 seconds to get to 45 mph (if you start in 2^nd gear) and an excruciatingly long 20+ seconds to get to 60 mph.  Here’s a plot of a simulated run of the first 10 seconds if you shift at 3000 rpm.  I’ll describe more of the parameters I used in a future post on performance predictions.

So now what do you do?  Well, what this means is that you need to determine what’s most important to you.  The only way to get those super fast speeds like you see with the White Zombie drag racer is to have lots of current, and more importantly, very high voltage (higher voltage means more peak horsepower, which in turn means you get max torque up to a higher motor speed).  I’ll talk more about that in the future post on performance.  For now, we’ll assume you’re on a budget and want to stick to a low voltage (160v nominal or less) controller and battery pack.  The problem we ran into in the example above is that the battery pack reached peak power output at a low motor speed.  This causes you to shift early and reduce your gear ratio (i.e. less torque at the wheels and less acceleration).  If you trade some low end torque for higher motor speed, you will get that max torque output for a greater RPM range.  The extreme case for this (in our set of 4 motors) is the Impulse 9.  This motor is a good choice for people with lower battery pack voltage.  You can see from one of the charts above that it only gets 70 ft-lbf of torque at 450 amps, but at 120 volts you will get that same torque all the way up to 4650 RPM, or around 24 mph.  With this motor and the same example as above, you would be able to accelerate to 45 mph in 8 seconds and to 60 in about 14 seconds.  This performance is comparable to that of a normal car.

The Warp 9 was redesigned in 2010 and I’ve seen groaning on some forums from people who didn't like that Netgain lowered the output torque for a given current.  Well, they did it for a reason, and I presume it’s exactly the sort of thing I described above.  Targeting the largest audience who use a low voltage controller, the newer designed Warp 9 will give you a good balance of peak and high speed torque. 

So what can we conclude from all this data?  If you are not running a high voltage system, you will probably be disappointed with the performance of a large motor.  It also tells you that you need to do some math if you really want a high performing vehicle.  Just putting a big motor (or lots of batteries for that matter) in your car will not make it fast if you don't apply it properly.

For me, I want more of a hot rod, meaning more off the line torque.  That’s why I picked the Warp 9 over the Impulse 9.  But in order to meet my 0-60 goals, I was forced to consider a higher than typical battery pack voltage and controller current.  After figuring out what was economically feasible for me, this motor gave me the fastest 0-60 time when compared against the ADC 9 and Impulse 9.  If I were using a different battery pack or controller, that may be different.  Like I’ve said before, more info to come in a future post on performance predictions.

New Seats

There comes a point in every car's life when the seats get a little worn down.  That point had well been reached long ago for my porsche...

I drilled the first holes in the Porsche over the weekend and started adding new seats.  I picked up a pair of racing seats from summit racing and they fit really well and look great!

 
I also started working on the back seats.  They're essentially a vinyl sheet that's glued to a piece of molded foam that is glued into the back.  It was essentially peeling up all around the sides and I didn't think it would glue back down so I made some sheet metal brackets to cinch it down.  Here's what the seat looked like when I got it.

 Here I've peeled it out and cleaned the sheet metal up a bit.

And here are the brackets I made.

 Back in place and now you can see the brackets clipped to the underside of the seat.  I screwed those into the sheet metal to tighten it down, and you can see the final result below.  I still have some work to do on that center section, but it's looking a lot better. 

 Next step for the back seats are some seat belts.  The problem I'm having there is finding belts with very short ends.  Normally they attach somewhere near the floor and extend up around the seat.  Here, they mount slightly higher than the top of the cushion (you can see a little hole in the carpet where they bolt on), so I need to find some that are only about 6 inches long.

Motor Coupler Part 2 - Fabrication

The last post I told you about the coupler I designed.  This next section is how it was made.  As an engineer, I have a lot of experience putting lines on a paper.  Turning that into a real object is an entirely different thing.

The only way to make this kind of part is on a lathe.  My friend, Larry, graciously let me use his lathe and provided all the advice and help I needed to get it done.  Here's a picture of the setup he has, plus a reminder of what the part looks like that we're making.

The first tidbit of info is that stainless steel is easier to machine than low carbon steel and easier to get a good, smooth, surface finish.  The same goes for aluminum.  I didn't want to make my part from aluminum for a couple reasons, and had already bought a big block of cold rolled low carbon steel.  Oh well.

We started by making a dummy shaft that's the same size as the motor shaft that the coupler fits on.  Since the motor weighs 150 lb and is pretty big, it's a lot easier to test fit this piece instead of the motor.  The final pass put the diameter within 0.0002" of the motor diameter.  If I accidentally oversize the hole that the motor shaft goes in by just a little bit, it'll be game over and try again, which will be tough to swallow since it's the last dimension we'll cut.

The remainder of the 3 hour first day was spent cutting my 4 inch diameter by 9 inch long bar of steel in half, aligning it to minimize runout, and machining a center on one end.  A lot of that time was spent just thinking of how small we could cut it and still be able to machine all the surfaces in the right order.  Here's a picture of what the part started out as after being cut in half, though this is actually the half we didn't use.

Ok so the plan of attack is to rough cut all the dimensions, but leave a little extra material for later.  The reason for this is after extended periods of heavy cutting, it's possible that the part will move in the chuck.  If you do a precision cut of one surface and the part shifts, you will no longer have the concentricity or perpendicularity that you need.  If you do all the finishing cuts, which are lighter loading on the part, you will have a much better result.

The next day we spent another 3 hours machining just one feature.  You can see the 1.675 diameter on the right side of dimensioned picture (above), I spent the whole evening knocking down that diameter.  Instead of being 0.600 inches long it’s more like 2.5 inches long.  We had to make it that long so the chuck could hold it well and still fit a tool between the chuck and the .00000 surface in the next setup. 

The next day we met, we spent another 4 hours and made a lot more progress.  The part was flipped around so the chuck is holding onto the 1.625 diameter.  First I faced off the end (left side in the picture), then hogged out the 1.875 diameter on the left side.  Third we bored out the 0.875 diameter.  It was surprisingly quick to make a ¾ inch hole 2.5 inches deep in a piece of steel.  Now that 80% of the material was gone it was time to retighten the chuck and make some final cuts.  The 0.875 diameter was opened up.  Then the .00000 face was skimmed off.  Then the 3.740 diameter was cut.  I had the flywheel there, so we were able to check that it was a tight fit.  And that’s where we stopped for the night.

Finally we're to the last critical dimension, which is the hole that the motor shaft fits into.  You can see me cutting that dimension in this picture using a boring tool.  Being in inner dimension, it is tough to get a good surface finish and tough to check the diameter accurately.  You can use a micrometer, and unless you have some really specialized tools you can only use calipers to measure down to 0.001" accuracy.  That's where the gauge pin I made earlier helps out.  Once the hole is nearly large enough, you run the tool in repeatedly without changing the radial position.  This skims off more and more of the high spots on each pass and the hole will slowly get bigger.  This happens because the cutter is somewhat V shaped, so it leave small grooves.  On the outside dimensions I used a flat tool for the final cuts, so you don't see much of this behavior.  After taking a long time to close in on the final dimension, the gauge pin is very tight, but fits.  Here you can see the final part and a few pictures of it installed on the motor and the flywheel.  The next step will be to have the keyway broached at a machine shop.

Motor Coupler Part 1 - Design

This post really shows off my engineering side.  All day long I help design parts and create drawings for them.  This project is an opportunity for me to do all that plus figure out how to actually make the part.  This sort of project always gives me a deeper appreciation for the people who bring form to things I look at all day on a computer screen.  As a warning to all of you, this could get boring.  Then again, if you're like me this could be pretty interesting...or maybe still boring.  Enough said.

The electric motor has to somehow transfer power to the transmission.  Some people choose to set up this connection clutchless.  In this case, the flywheel, pressure plate, and clutch disk are removed and a single coupler attaches to the motor shaft and the transmission shaft.  Benefits to this system include simplicity and low rotating mass (i.e. more apparent torque for acceleration).  The downfall here is that changing between gears could be slow.  To other option, of course, is to retain functionality of the clutch, in which case you need a coupler that attaches the motor shaft to the flywheel.  The benefit here are that you shift as quickly as you would have been able to with the gas engine.  The downside is that the coupler design and fabrication is much more challenging and you have higher rotating mass (i.e. some of the torque that the motor produces is used accelerating the mass of the rotating components and not as much is available to accelerate the car).  Based on my friend's experience, I decided to incorporate the clutch.  And here it is.

In this next picture you can see how it fits in with the motor and transmission.  The motor is on the left.  You can see the coupler attached to the shaft and the flywheel attached to the coupler.  You can also see the transmission (or in my car's case the midshaft) that is piloted to the coupler.  This shaft must be supported with a pilot bearing so that it does not flex when you disengage the clutch, which could cause all kinds of problems.

So we have a couple important features here.  First, the ID of the coupler must be a very close fit with the motor shaft or it will cause vibration.  To improve the fit, I've added a slot in the coupler and a bolt that will clamp it to the motor shaft for a slop free fit.  Second, the face that the flywheel mounts to must be perpendicular to the motor shaft.  If it isn't, the flywheel will wobble causing bad wear on the clutch parts.  Third, the OD of the coupler pilots to the flywheel.  This must be a very tight fit or the flywheel will be unbalanced.  And fourth, the pilot bushing must be concentric with the motor shaft so that the transmission shaft is concentric with motor shaft.  Still with me?  Here are all the dimensions I used.

You can pay someone to build this for you for $500-600, but part of the fun for me is to figure out how to do this yourself.  Luckily I have some very generous friends who have the equipment needed to make it and were willing to help me out.  I have to give a big thanks to Larry for supplying me with around 12 hours of time on his lathe and just as much time in advice.

So now that we've got a design, it's time to start cutting chips.  I went to the local metal store and picked up a 4 inch diameter bar of cold rolled (CR) steel.  I got 9 inches figuring that would be enough to have two tries at getting it right.  I probably spent at least and hour discussing this machining process with my friend to make sure everything turns out the way I want it.  Considering how long this is going to take, this long conversation was well worth it.  After all this discussion we cut it down to about 4.25 inches long.  The shorter the piece, the less problem you'll have with wobbling, vibrating, etc.  Plus, even at that length, the starting piece weighs over 15 lb.

Next up, how to make it...

Everything is out

Everything I need/want to remove from the car is finally out.  The last parts to go were the fuel lines and some pieces that I later determined were fuel pressure accumulators.  I can now say with complete confidence that I'm 98% sure I cannot spill any more gasoline on the floor of the garage. 

It's time to pick the next part of the project to work on.  Since most things are best done after the motor adapters to be made and I'm at the mercy and schedule of my gracious friends who are helping to make those, I will probably start doing some interior work at this point.  I have my eye on a few seats and some carpeting, and I'm going to try and reglue the rear seats since they're peeling up.  Once this is done, the wife will be a lot happier!

On a side note, this fuel system in this car is an engineering marvel.  Modern cars have a bunch of sensors to monitor data all over the car and a computer calculates the perfect amount of fuel to inject into each cylinder.  Back in the mid-seventies, they didn't have computers but they managed to figure out how to do this mechanically.  A look at the Haynes manual showed me the schematic and description of how they used a mass airflow sensor, warm up valve, idle bypass, and "wine glass" shaped throats to create a non-linear fuel/airflow ratio that compensates for when you accelerate, engine temperature, and any number of other factors.  It's amazing they figured out how to do this.  It also makes me appreciate the fact that I won't ever have to figure out how to get this car through emissions.

Removing the fuel tank and transaxle

The last major piece of the car that is taking up space and weight is the fuel tank.  Some of the batteries will eventually reside here so it's gotta go.  I pull out my Haynes manual for help on how to take it out.  Step 1: remove the transaxle...crap!  Unfortunately, the fuel tank is situated right above the transaxle and just below the floor of the trunk.  I considered cutting it out, but it's full of fuel vapors so that's not going to work either.  After contemplating this for 3-4 weeks I bite the bullet and go to work.

Being a rear mounted manual transmission, there is some linkage going from the shifter to the back of the car.  This all has to be disconnected and removed.  I was mostly worried about that, but it turned out to be the simplest part of the project.

The shaft of the transmission is connected to a midshaft that links the transmission and engine with a slip fit spline and a clamp.  The instructions tell you to loosen the clamp and it will slide off.  Unfortunately, the bolt on the clamp is keyed to the shaft so you actually have to completely remove the bolt.  Not realizing this, I parted the transmission from the rear bellhousing and the midshaft slipped through its bearings and moved with the transmission.  There is only about 1 inch of space to back up the transmission, and with this slippage there is no longer enough clearance to get the shafts disconnected from each other.  Long story short, after about 3 hours of just messing with the shaft connection, the transmission is out.

The fuel tank was a bit stubborn as well, but no where near as bad as the transmission was.  There are two straps that hold it in and are easy to take off.  You also have to disconnect the fuel sending unit wires that are accessible through a panel in the trunk floor.  I got all that disconnected, and the next step in the instructions is to just remove it, but it's a little stubborn.  It's kinds of wedged between this crossbar that the transaxle hangs off of and the trunk floor.  There's foam that kind of locks it into place and it wasn't clear how to slip it out.  Eventually I just pulled really hard on the back side and it just popped out.  Not that it matters to me, but it doesn't seem like anything was damaged in the process, so I assume that's how it's supposed to come out.  Now I can see how oddly shaped it really is.

Reconnecting the transmission was no simple task either.  I thought that I now had the mechanics all figured out it would go relatively smoothly, but it took just as long to hook up as it did to take off.  12 hours in 5 days of working on it plus a bonus bandaged hand and the fuel tank is out and transmission back and hooked up.  Hopefully it works correctly when the car is moving again.  An additional bonus to this job is the CV joins were likely contaminated with grime in the process, so I plan to disassemble the axle assemblies to clean and regrease them some other day.

Work Completed to Date

Since I didn't start this blog until a month after I started working on it, here's a little catch-up on what I've been doing since I picked up the car.

Nov 29th: Bought the car and brought it home.  She's in pretty good shape for a 35 year old vehicle.  The exterior is in good condition with only a few dings.  The paint is decent except for a few chips.  The interior is where most of the problems are.  A set of new seats, new tires, and a little TLC and this will be a nice car.

 As you can see, it needs new seats.

Dec 10th: Find out a brake bleeder screw is broken.  I'll have to take the wheel cylinder out and disassemble it so I can do some careful cutting and heating to get it out.  More on that later.

Dec 17th: The car has moved under gas power for the last time. Started taking the engine out today.My 7 year old  helped me bleed the brakes and check the compression!

 
Dec 19th: Fixing Brakes Step 1 part 1: To remove axle nut, use special tool A-147. If you don't have that, make your own tool to keep the brake drum from spinning. If your homemade tool is not stiff enough, clamp anti-buclking board to it. Attach 4 foot breaker bar to the 36mm socket and remove. It is possible that the anti-buckling board may snap in two. If so, go to part 2...

Fixing Brakes Step 1 part 2: If your custom tool broke, weld more metal to it and try again. Note, the breaker bar may still not have enough leverage to take the nut off. Also note, your 1/2 inch drive ratchet may now be broken. Stay tuned for the completion of step 1.

Dec 23rd: A new WarP 9 motor arrived and is now waiting for a vacancy under the hood.
 
Dec 29th: Today is the day to remove the engine. We were surprised to find that 2 or 3 of the bellhousing bolts were loose! I guess that's why there are 6? Here we are pondering how to get the last bolt out that's invariably not easy to reach and quite tight. The best part about all this is I had a guy lined up to pick up the engine one day later so it didn't have to take up half my garage for a month!

Here's my nephew helping out with the engine removal.

320 lb of motor.  That's about the same weight as my future battery pack.  Amazingly, I had a guy lined up to pick it up the next day!  He'd paid me a deposit a week earlier.  Not having to store this thing in my garage for a month put a big smile on my face.  Thanks to my brother, Cameron, my friend, Nick, and my wife for helping me out here!

Dec 31st: Realized the front wheel bearings are loose so I'll add that to the "to do" list.
 
Jan 1st: Brake repair Step 1 part 3: Need more leverage? Buy a 25 inch breaker bar! Ok, got that and it was too flexible. Felt like it was going to snap and damage me and the car in the process. A week of penetrating oil, heat, banging, and bouncing on it any the nut is still stuck.
Step 1 part 4: My dad found a 24 inch pipe wrench hiding in the barn. After 17 days of messing with this it finally came off with the help of an 18 inch pipe extension! Now what was I doing that needed that nut off?
Oh yeah, I finally got the rear brakes apart so I can hopefully extract that bleeder screw.
 
Jan 2nd: Bleeder screw is out and a new one is in there. Hopefully the brakes will be in good order after bleeding them one last time.

Jan 7th: Readjusted and repacked the wheel bearings. Also, now BOTH of my ratchets are broken!

Jan 6th: I think I have the motor coupler all figured out. Now I just have to buy a block of steel and relearn how to use a lathe. The first of many trips to the metal store is on the horizon!




Jan 12th: You would never have imagined this possible, but I found a guy on craigslist who wanted to trade the exhaust for 35 feet of 2/0 power cable and a bag of terminals that he had lying around!  I've sold everything but the radiator and the fuel tank at this point.


Jan 14th: I've started working on the motor coupler.  This piece is interesting enough (to me) that I think I'll make an entire post to talk about it.  It's amazing the amount of thought that has to go into making this piece.  Stay tuned for more on this one.


Jan 15th: I finally started working on removing the fuel tank.  I avoided it until now because it requires removing the transaxle.  For those of you who are like me and didn't know what that is, if the transmission and differential are combined into one housing it's called a transaxle.  I had to stop working on removing the axles because they require a special 12 point torx bit.  Luckily my neighbor is going to bring me one from work tomorrow.