Electric Car Motors Shari Prange c. 1993 Shari Prange "Pssst! Hey, buddy, over here! Wanna buy an aircraft generator?" If you meet this fellow in an alley, don't walk, run in the other direction. He will bring your electric car nothing but grief. The motor is the center of the electric car's universe. It is the first component you need to install. It has a pre-determined position in the center of the engine compartment, and all other components must fit in around it. The type of motor you use will determine what other components you use, and what performance you will get. It is not the place to skimp. The Early Days In the rugged early days of the modern electric car (meaning the late '70s), the aircraft generator was the most common motor used simply because there were few options available. It made a great aircraft generator, but a marginal electric car motor. It was rated at 24 Volts, but for automotive use it was operated at 48 to 72 Volts. Not surprisingly, the generator often refused to take such abuse and died. Also, the rpm band of the generator was not suitable for a car, causing it to suck up great rivers of current, which translated to a short driving range. Today, these generators are still emerging from basements with "For Sale" signs, but they are less appropriate than ever. For one thing, the shaft has a 16-tooth spline that is very expensive to mate to an adaptor. For another thing, they are incompatible with modern motor controllers. Another motor of yesteryear was the Baldor. These had a disturbing habit of spontaneously disassembling themselves. An interim motor of the mid-'80s was the China Motor. Designed as an improved version of a generator, it operated well at 72 Volts, but not higher, and it had the same spline problem. Today's Motor The motor of choice today is the series DC motor. The most popular is manufactured by Advanced DC Motors. It is efficient, reliable, readily available from various distributors, affordable, and most important, it is designed to be an electric car motor. This motor dominated the 1992 Solar & Electric 500 in Phoenix. Other usable motors are the Prestolite and G.E. The Prestolite is the forerunner of the Advanced DC motor. It is a little larger, heavier, less efficient, and less well ventilated. If cost is a factor and a used Prestolite is available, it will serve you well. This motor is no longer in production. The G.E. motor (not the same as a G.E. aircraft generator!) is also adequate, and has approximately the same slight drawbacks as the Prestolite. If you are buying a used G.E. motor, be sure to check the rpm rating. It should be 5100. There were some in circulation that were only rated at 3900 rpm. They had lots of torque, but no top end speed. Modern Motor Types What about other types of electric motors: shunt, compound, permanent magnet, brushless DC, or ac? All of these suffer from limited or non-existent availability in sizes suitable for cars. Shunt and compound motors have less acceleration than series motors. Permanent magnet motors have a narrower efficiency vs. rpm band than series motors. This means that permanent magnet motors are most efficient at a constant speed. In normal driving, however, speed varies greatly. Permanent magnet motors are least efficient at the low rpm used most in stop-and-go driving. Brushless DC and ac motors are available primarily from a company called Solectria in Massachusetts. While brushless motors are slightly more efficient than series motors, their control systems are much more expensive. Then there are the rare earth motors, and similar exotics. These are at the opposite end of the spectrum from generators. These are the unproven dreams of tomorrow. They are laboratory animals, available only as hand-built test prototypes. They are still being debugged, and are not commercially available in regular production. Don't design your electric car around something that is still a half-formed pipe dream. Honda Alert If you are converting a Honda, be sure to inform your supplier when you buy your motor. Hondas rotate in the opposite direction from everything else. Some motors can run in either direction. On the Advanced DC motor, there is an adjustment the supplier makes to make the motor most efficient in the right direction. Some motors will not work in this reverse application. Motor Installation Tips Once you have your motor, you need to install it in the car. When you disassembled the car, you should have taken some measurements to lock in the original position of the transmission. You want to duplicate the transmission's position when you install the electric motor. In order to do this, you will need to design your motor mount precisely. This is essential for proper shift linkage and driveshaft angularity, among other things. Any change in the position of the transmission will impair smooth efficient performance. In air-cooled Volkswagens, no motor mount is necessary. The electric motor hangs off the transmission, just like the original gas engine did. For all other cars, read on. Motor Mounts Install the motor on the transmission with an adaptor plate. Use a floor jack to raise the motor until the transmission is back in its original position. Now design your motor mount to fill that space between the motor and the chassis. There are two types of common motor mounts. One is a cradle that supports the motor around its middle, with a strap over the top. This is appropriate for a car with an in-line engine. This type of mount arrangement requires a torque rod to brace the motor against spinning in its cradle. For a transverse engine car, use a plate which mounts to the anti-drive end of the motor and extends to a mounting bracket. There are some bolt heads exposed on the case of the motor. Do not loosen these or in any way try to use them for your mount. They hold the field coils in place inside the motor, and they are installed at a specified torque. Loosening them will cause damage to the motor. The original motor mounts are "metalastic" pieces. That is, they have a rubbery part enclosed in a metal frame. This is great for reducing vibration and noise, so you should use the same technology. Design your mount to attach to these mounts. If the original mounts aren't too misshapen, use them for your design phase, then buy new ones for the actual installation. If the original style of mount won't work, try to find another style from a different model of car at your auto parts store. Keep it Clean and Cool Keep the motor's commutator covered with a cloth or masking tape while you are working on the car, to prevent any tiny chips or debris from falling into it and damaging it. Normal road splash will not be a problem, but don't try fording puddles that might actually immerse the motor. A small amount of belly shielding is recommended, especially in areas where the roads are salted. Be sure the shielding does not obstruct airflow for cooling. If it does, duct air from another area to the motor. Prestolite and G.E. motors should have an external cooling fan. This is not necessary on the Advanced DC motor. The electric motor beautifully simplifies a car. If it is chosen, installed, and driven properly it will give many years of reliable, smooth, quiet performance. Access Shari Prange, POB 1113, Felton, CA 95018 ù (408) 429-1989