Electric Motor Horsepower Rating for Automobile Use

In my first blog post I looked at the horsepower rating for the killer app of cars, the Model T, and found that it took America by storm on 20 HP. That’s a lot of horses of the flesh-and-blood variety, but not so many by automobile standards, where most vehicles produced have at least 100 under the hood. When I started running web searches on motors for use in electric cars, I found that they their power ratings tend to be in KW, with the simple conversion factor that . 1 hp = .746 kW. However, the more I read about motors for electric and hybrid vehicles on the web, the more I see references to the rated horsepower – and ignoring it. The logic runs something like this.

Electric motors are rated for continuous output, connected to a power source, they should be able to deliver their rated horsepower until the cows come home. In order to compare them with internal combustion engines, which are rated for peak power, you have to know what maximum horsepower an electric motor can put forth for a short period of time without melting something or catching fire. I know that sounds like a pretty inexact description, but I haven’t come across any sites giving a rationalized rating system, like (the following is made up)

Rated 10 HP
Delivers 20 HP for two minutes
Delivers 30 HP for one minute
Delivers 40 HP for 20 seconds

Not to mention a nice curve showing horsepower vs time through the longest range you’d want to consider. While two minutes is much longer than you’d hope to need for acceleration when entering a highway, it’s nothing when you consider some of the hills you encounter even just in New England. I’m going to keep searching, but I’m getting tired of seeing rule-of-thumb statements that vary from "Electric motors can deliver over twice their rated horsepower for short periods of time" to "… five time their rated horsepower…" That’s a big spread.

As to how the motors are coaxed into delivering the extra horsepower, that’s entirely in the hands of the control circuitry. I’m as innocent about solid state power controls as I am about electric motors, so I have no idea if some of them do this through current regulation or if it’s a simple question of upping the voltage. Maybe in motors that use electromagnets rather than permanent magnets, they overdrive the electromagnet windings rather than the armature windings. I’m not even sure if the failure mechanism is always heat, it could be that the insulators fail and you get arcing at higher voltages, or maybe the air-gap is idealized for a lower voltage. Much more research to do, but I hope I find some primary electric motor vendors who have better specs.