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Motor And Adapter Plate

This page covers the process of determining which electric motor I wanted to use, and how I am installing it into the Toyota MR2 EV.

motor and transmission assembly, ready to be installed
motor and transmission assembly, ready to be installed
motor and transmission assembly installed into car.
motor and transmission assembly installed into car.
Motor in the car for the first time for test fitting.  (bottom view)
Motor in the car for the first time for test fitting. (bottom view)

The above pictures are of the motor and transmission after final assembly, and after final installation. The assembly weighs about 250 pounds, so I used the shop crane to move it into position to install it. The mounts I made include lifting eyelets on both ends, to allow easy lifting with the shop crane. The engine compartment of the car was modified as well (see the Chassis Modifications page) as were the motor mounts (See the Custom Motor Mounts page) as part of the work for preparing to install the motor and transmission.


Advanced DC 9 inch motor as purchased for Toyota MR2 EV
Advanced DC 9 inch motor as purchased for Toyota MR2 EV

Advanced DC 9" Motor

I am using an Advanced DC 9" Motor in my Toyota MR2 EV conversion. This motor is a series-wound, direct-current motor. This is the most common type used in EV conversions. Its maximum input voltage is rated at 120V, though everybody in the EV community says 144v is fine. I'll be using 136 volts. The maximum safe RPM is probably around 5000 RPM. This is a bit lower than the redline on the original engine, but still plenty good enough.

The Advanced DC 9" is a rather large motor for the size of the car (This motor can be used in full sized cars and pickup trucks). I have my reasons for doing this, as detailed below. Advanced DC makes several other smaller models of motors as well. Another common maker of motors for EV conversions is Netgain/Warp (See the EV Reference Material page for links). Those are basically slightly souped up versions of the Advanced DC models however. Motors made by GE, Kostov, and a bunch of other makers have also been used in EV's, but these days there are enough purpose-built models that I think one should really stick to them.

Depending on what you are converting, you may have space limitations or other requirements that limit your choices of motor. In my case, there is plenty of room in the MR2 to install basically whatever motor I wanted.

My Motor has a tailshaft, which may be useful in the future for driving an accessory or installing a kicker motor that is more efficient for low power operation, or a regen system, sensors, or whatever.

Why This Motor?

If you've been reading anything technical about EV's (including my own rules of thumb listed on the EV Performance Analysis page) you might have concluded that I should be using an Advanced DC 8" or some other similar smaller motor. Indeed, that motor would perform quite well in this application, and I would have saved about 20 pounds of weight by using it. Why then am I going for the larger, heavier Advanced DC 9" motor? There are several reasons I decided to go this route:

  • The Toyota MR2 is a sports car. With the ADC 9", the car should perform quite well. (Assuming I get the remainder of the electrics and the suspension right) In fact, if I put too big of a controller into the car, I could probably over-torque the transmission and break something with this motor.
  • The ADC 9" has a higher torque constant than the smaller 8" motors. This means that for a given amount of torque output, I am pulling fewer amps from the batteries. This makes sense if you think about it because with a larger diameter armature, (The "spinny" part of the motor) the magnetic fields created have a longer lever arm with which to exert force on the drive shaft.
  • The ADC 9" motor has a better efficiency curve than the 8" motors. It is better in two ways: First, the peak efficiency is a bit higher, but maybe only by a percent or two over the ADC 8" motor. But also, the curve is flatter, meaning the motor retains higher operating efficiency at both higher RPMs, and at greater torque outputs than the smaller motor. Granted, the differences are fairly small over the entire curve between these motors, but my calculations suggested that even with the additional weight of the motor, the overall range of the car would be slightly better.
  • The price of the larger motor is only a few hundred dollars more than the smaller motors go for. So the incremental cost in going with the bigger motor when you figure in the total cost of the project is minimal.

Finding One

EV parts suppliers all sell a variety of motors. These include DC motors like mine, AC motors, Permanent magnet motors, and specialty motors. Keep an eye out on ebay and other internet venues for a motor. Any industrial parts supplier might have a motor kicking around, but make sure it will work for you if you buy one from one of these guys. And of course, don't forget to poll the local EV community and find out who has a garage full of parts to sell. The last method is how I got mine. The last method has a major advantage in that you get to lay your hands on a motor before buying it (especially important for used parts), and there are no shipping fees if you can pick it up yourself. These motors are heavy so shipping one is going to be expensive.

Preparing It For Use

My motor is used, but it does not appear to have very much mileage. The seller insisted it had only ever been bench run. While I agree that the usage is minimal, I suspect it has a bit more usage than that. Nonetheless, there is nothing wrong with it, so I took it. I will clean it up a bit and repaint the housing as it is a bit weathered. I will take out all the brushes, clean the commutator, and put everything back together using conductive grease where appropriate. Other than that, there is not much to do with the motor. Especially compared to the sort of maintenance you would expect to do to a gas engine.

Other Motor Types

You can also get AC (Alternating current) motors and permanent magnet motors that may be suitable for EV usage. I'll touch on them briefly here, but I'm not using these so there won't be much detail other than the reasons why I did not use these.

AC motors work great in EV's but are much more expensive than their DC counterparts, and they require complicated, specifically-designed controllers, so they are usually sold together with those. AC motors are what is typically used in factory-built EV's. If you want to build a direct drive car (Eliminating the original transmission) then you will want to go with an AC drive system. AC motors function well at the high RPMs (up to 10000 RPM) necessary to allow for direct drive to be practical.

Most permanent magnet motors one can find aren't big enough to propel a full sized car at freeway speed. There may be other drawbacks. For example, the permanent magnets will continuously collect ferrous junk that blows into the motor. With an series wound motor, as soon as you start coasting all magnetism goes away and any ferrous dust will get blown out. Some permanent magnet motor designs such as "pancake" motors (used on electric motocross bikes) are very powerful for their size, and extremely efficient, but they have a low RPM limit of about 3000 RPM due to their armature design which uses very little or no iron. I initially considered a direct drive design using two Perm 132 pancake motors, but I abandoned plan this due to the RPM limitations of the motors which would have forced me to accept either very poor acceleration and no hill climbing ability, or adequate acceleration, but a top speed of 35mph. Also, most permanent magnet motors aren't rated at as high of an input voltage compared to series DC motors for whatever reason.

Adapter Assembly

This is a custom part (actually two parts) that must be created to attach the chosen motor to the transmission of the car. The EV adapter plate is usually the only high-precision custom part that must be made when performing a typical EV conversion. Therefore, it is critical that this part be made correctly. This part will have to be fabricated specifically for your combination of motor and transmission, unless you are converting a commonly done car, in which case it will be possible to order off for a plate. Expect to spend a fair amount effort here to get it right, and unless you are a machinist, plan on having somebody else do the work.

The Plate

The plate is a sturdy, usually flat metal plate that mates the electric motor of the car to the original transmission by matching the bolt pattern of the transmission on one side, and that of the motor on the other side. It must hold the motor shaft in exact alignment with the transmission shaft.

adapter plate hub parts
adapter plate hub parts

The Hub

A typical motor used in EV conversions (such my ADC 9") will have a 1.125" straight shaft with a keyway. The adapter hub is necessary to mate the straight shaft of the motor to the flywheel, which was originally intended to bolt to a flange on the end of the crankshaft of the engine. The hub must mimic the back of the crankshaft, and sit at the same position relative to the transmission as the original crankshaft flange did. The picture to the right is of the adapter hub that I have. Its parts include the main flange which the flywheel will bolt to, the lock ring, retainer screws, and shaft key. The short shaft at the top is not part of the hub, but was used to mount the hub in a lathe for making the needed repairs. Not shown is the shim that will align the flywheel on the motor shaft, which will stick through the end of the hub when installed.

Design considerations

There are a bunch of design considerations that go into creating an adapter plate for an EV conversion:

  • The adapter plate must be strong enough to handle all the torque generated by the motor, and it must be strong enough to support the very heavy motor and transmission connection. Mine was made from 5/8" aluminum plate, which should be solid enough.
  • The plate must provide sufficient depth to allow the end shaft of the motor to clear the input shaft of the transmission, and to allow the adapter hub to be positioned in the correct spot. This may mean a thicker plate, or (as with mine) spacers between the plate and the motor to move it back from the transmission.
  • The plate must align the motor shaft with the input shaft from the transmission. This is a critical alignment since if it is off, it will force the clutch to slip continuously as the assembly turns. This will waste energy and cause excessive wear. For this reason, a good plate will use the same alignment mechanism that the engine and transmission originally used to stay in position relative to each other. Usually this is a pair of pins pressed into the engine block that tightly mate to matching holes in the transmission. The plate must also ensure that the motor is held firmly in place. My plate does this using tapered head screws which, once tightened, cannot slide around inside their holes. This is an acceptable method of alignment for keeping the motor in position relative to the plate. However, My plate does not use any alignment method other than the (loose-fitting) mounting bolts for aligning the plate to the tranmsission, which disappointed me. More on this later.
  • The adapter plate must hold the motor in a proper orientation to allow electrical hookups and other mounting points to be in the best positions possible. My motor has two mounting holes at the back of the motor which I wanted at the top of the assembly. This puts two of the four electrical hookups to the motor down close to one of the driveshafts (I forgot to consider this) but not close enough to be an issue, luckily.
  • The adapter hub must be designed to securely attach the flywheel to the motor shaft. Typically this means a taper-lock mechanism that attaches the hub to the motor shaft, and a flange that is machined and drilled to allow the flywheel to be bolted to it. The flywheel will align not on the bolts, but on a central flange that is identical to what the crankshaft provided. This hub must be made to very close tolerances, as if the flywheel mounting surface is uneven the flywheel may wobble and this would be very bad. The adapter hub that I was provided has this problem and needs to be reworked. More on this later.
  • The adapter hub must allow a pilot bearing to be inserted in the same manner that it would have been with the crankshaft, if the transmission originally required one. Most do, but some transaxles (such as mine) do not, which made things a bit simpler.

Finding One

There are only a couple options here. Make one, or have one made for you. Buying one off the shelf will only be possible if you are converting a very commonly done car such as a Geo Metro or Volkswagen Rabbit. The first option here is really only practical if you are a machinist. There are a few operations around the continent that make adapter plates. Electro Automotive Inc is one. There is a company in canada that apparently does this as well. Any decent machine shop could do it, but if it isn't something they are used to doing it would probably take longer and cost more, but if you were clever enough to manufacture everything you could yourself, and leave the critical alignment steps to the machinist, you might be able to work something out.

In my case, the same person whom I bought my motor from offered to make an adapter plate for me. His price was the best I had seen, I knew he had done many of these before, and he came well recommended, so I agreed.

adapter hub before being repaired
adapter hub before being repaired
adapter hub being machined by my uncle
adapter hub being machined by my uncle
adapter hub after being machined true
adapter hub after being machined true
alignment dowels added to keep adapter plate in place
alignment dowels added to keep adapter plate in place

Problems!

Unfortunately there were some issues with the plate that I commissioned to be made. It appears that it was a bit of a rush job as there were other higher profit things to get back to. When I pointed out some issues with the plate and hub upon delivery, it basically came down to the fact that this person felt they had not charged me enough and the job was more complex than they had expected, and thus did not want to work on it any more. I was rather annoyed but took it anyway at a reduced price. The plate and hub will be fine with some rework, but this sort of thing was what I was hoping to avoid by paying somebody to do this who had done it before. Oh well.

There were three main issues with my plate:

  • The first was that the plate itself did not align against the original alignment holes in the transmission. There should have been some pins fitted to the plate that mimic those in the back of the engine block to hold the plate in proper position by engaging those holes in the transmission. The mounting bolts fit too loosely to do this properly. I fixed this by bench-running the whole assembly while standing on end with loose bolts so that it would self-center, then tightening down the bolts to hold it, and drilling a couple more holes in the adapter plate through from existing holes in the transmission. I went slightly oversize (drilling out the holes in the transmission a little too) to ensure perfect alignment. I then pressed through two hollow roll pins that engages both the tranmsission and plate. This mades for a zero-slop installation, and it also makes disassembly and reassembly easier.
  • I also had to trim the adapter plate's edge a bit to allow the clutch slave cylinder to install properly. I can't blame the fellow who made it for this one, as I had not installed the cylinder when I gave the transmisison to him to make the adapter, so he could not have forseen the clearance issues. This work was easy enough, I just traced the outline of the edge of the transmission onto the plate, and then sawed and filed down the plate until it matched the profile of the transmission in the area where clearance was an issue.
  • The last, and most severe problem was that the adapter hub did not hold the flywheel in proper alignment. While the flywheel was properly balanced radially, the "runout" (vertical wobble of the friction surface) was about 0.030" (nearly a millimeter) at the outer edge of the friction surface. The flywheel visibly wobbled as the motor turned. This became very obvious when we spun up the motor with flywheel attached on a 12v battery. For comparison, the MR2 service manual I have states that the maximum allowable runout is 0.004". Initally the flywheel was suspect but it became obvious that the hub flange itself was, for whatever reason, not perpendicular to the rotation axis of the motor shaft.

    To fix this, I enlisted the aid of my Uncle Frank, who lives close enough by that I was able to take an afternoon and go down and visit and get some free machine shop time. It was simply a matter of mounting the hub in the lathe (using a stub shaft borrowed from the fellow who made the hub in the first place for precisely this purpose) and turning down the flange surface until it was true. The only issue that came up is some welded spots on the hub were much harder metal than the rest, and were still high spots after the machine work. These cleaned up with some very gentle grinding with a dremel tool after the fact. After that I sanded the surface by sandwiching some sandpaper between the hub face and the table on my drill press (a flat, machined surface) and sanding it, rotating it frequently. The result is still not perfect, but it is now within .001 of being true, whereas before the hub was at about .010. This is acceptable.

Given that the adapter plate for an EV is nearly always going to be a custom job, some issues are always going to pop up. I haven't been at this for very long and I've already heard lots of stories about adapter plate problems. So, it is very wise to always inspect what you are getting carefully, ideally before rendering final payment. This may not be possible if you are dealing with someone remotely; in this case make darn sure they have a return policy. If you are dealing with someone in person, don't accept an assembled unit, insist on taking everything apart and looking at it. Without doing this, I would never have found the flywheel problem until I had the unit at home and would have been unable to prove that there were pre-existing issues with it. If you can find somebody who has made this part before for your type of car, they are the best option to use. If somebody else has a conversion of the same car that you are doing, talk to them about who they used and what problems they encountered.

Final Assembly

After making the necessary fixes to the adapter plate, I was able to actually put the whole assembly together for good. This involved:

  1. Installing the taperlock hub onto the motor shaft. This involved getting the height right (I messed this up and had to tear everything apart and reposition it 1/8" lower) and then using a gear puller to get the taperlock ring as tight as possible. Just using the tiny screws to pull it up tight would not have been sufficient in my opinion. Once in place however, the screws will keep it from loosening. I also used some medium-strength loctite on the threads here to keep stuff from coming loose.
  2. Putting the flywheel shims onto the motor shaft. Since the motor shaft is different diameter from the flywheel, this shim keeps the flywheel centered on the motor shaft, just as the flange on the original crankshaft had done.
  3. Installing the adapter plate. Again, I used loctite to keep stuff from coming loose.
  4. Installing the flywheel. More loctite, and modified bolts (I bought longer bolts that would normally fit, so that they would have an unthreaded shoulder, and then trimmed the threaded section down to the right length. This way the torque from the motor will be transmitted to the flywheel through unthreaded sections of the bolts, instead of through threads. Specially shaped washers keep the shims and woodruff key in place.
  5. Just like with a regular car, the pressure plate and clutch disk get installed onto the flywheel
  6. Mating the motor to the transmission. This was clumsy as both pieces are very heavy, but it went fairly smoothly (in other words, I didn't drop anything on my foot).
  7. Installing the new slave cylinder (the old one was junk) was straightforward except that I rerouted the hard lines, requiring making new ones. Doing this makes more room for electronics or whatever next to the transmission in the engine bay.
  8. Final paint work and a cover for the motor tailshaft. I made a cover for the rear tailshaft of the motor out of a tin can. It will later house a sender for a tachometer, but for now it is just to keep it from rusting and getting dirty. I also painted the motor itself as it was a bit dingy and had some minor rust spots where the paint had peeled off.

These steps are summarized in the following sequence of pictures.

step 1: bare motor
step 1: bare motor
step 2: install taperlock hub onto shaft
step 2: install taperlock hub onto shaft
step 3: hub installed, flywheel shims in place
step 3: hub installed, flywheel shims in place
step 4: install adapter plate onto motor
step 4: install adapter plate onto motor
step 5: mount flywheel to hub
step 5: mount flywheel to hub
step 6: install pressure plate onto flywheel
step 6: install pressure plate onto flywheel
step 7: install clutch slave cylinder and lines
step 7: install clutch slave cylinder and lines
motor and transmission assembly, ready to be installed
motor and transmission assembly, ready to be installed