I found this today, when trying to understand electric motor horsepower ratings compared to gas engines. I kind of thought it was a dumb question. Apparently not.
Motor horsepower is the most misunderstood (and misused) electric motor rating. Neither motor, universal or induction, produces usable horsepower unless it is slowed down (by applied mechanical load) from no-load speed. For induction motors, this slowdown is called “slip”, and the horsepower “developed” by a motor increases with slip (to a simple approximation). This is why induction motors are typically rated at 3450 rpm (two pole motor) or 1750 rpm (four pole motor). The rating speed allows for slip from the “synchronous” speeds of 3600 and 1800 rpm, respectively. Universal motors do not have a synchronous speed, but have a maximum no-load speed that depends upon the voltage applied to the motor.
Most motors can put out a lot more maximum horsepower than they can sustain continuously. By forcing more mechanical load on the motor, slowdown is increased and so therefore is the output horsepower. Mechanically, horsepower is torque times rpm, and increasing the mechanical load means that the rpm is slowed slightly and the drag torque is increased to obtain more torque times rpm. Electrically, horsepower is volts times amps, and by conservation of energy, the mechanical output horsepower must be balanced by electrical input horsepower. Since the voltage is relatively constant, this means that as a motor is loaded, the input current increases. But the electrical winding impedance has a resistive component, so that higher current means more power dissipated in the windings. In fact, the motor windings heat up proportional to the square of the motor current. Except for specially designed motors, the current that a motor can sustain continuously without burning out its windings is a fraction of the current at maximum load.”
…wish I could quote more, but take a look at the article here.
Now. About gasoline engines…. Here’s a nice, simple explain on the Briggs & Stratten site on HP ratings: Engine Power Rating Information
“The gross power rating for individual gas engine models is labeled in accordance with SAE (Society of Automotive Engineers) code J1940 (Small Engine Power & Torque Rating Procedure), and rating performance has been obtained and corrected in accordance with SAE J1995 (Revision 2002-05). Torque values are derived at 3060 RPM; horsepower values are derived at 3600 RPM.”
So. Now. Why the Big Difference in horsepower ratings between electric motors and gasoline engines?
Let’s go back to Mr. Roche’s article:
“The Rated HP is typically the torque level at which the motor can be run continuously without exceeding the temperature at which the winding insulation beaks down.”
Compared to gasoline:
“Torque values are derived at 3060 RPM; horsepower values are derived at 3600 RPM.”
Here’s another breadcrumb along the path, a great, short explanation from FastElectrics.com (with pictures). Here, in one sentence, is the crux of the matter: “When the load on a petrol motor increases and rpms drop, torque also falls. For an electric motor torque increases.”
A gasoline motor has a peak torque value for a given RPM (3600). It’s easy to peg that as a HP rating – measured torque at a fixed RPM. An electric motor produces torque in response to load. You load it, the RPM drops, and it draws more current and produces more torque. It’s more of a moving target, so you have to rate it, rather than the actual horsepower it produces, the horsepower it can produce at a given load before it melts.
I think it goes back to the whole idea of a gas motor “producing” power and an electric motor “handling” power. Since an electric motor responds to load by drawing more current, it really is a completely different animal, ratings-wise, than internal combustion.