Wednesday, October 13, 2010

The Difference in Motor Power – 250w 500w 750w

The Difference in Motor Power – 250w 500w 750w
Light Electric Vehicle Association (LEVA) Founder, Ed Benjamin, explains what it means in terms of electric bike performance for motors having varying degrees of output, from 250w-500w-750w:
A “bicycle” is a very privileged vehicle. It can use the roadway, or the bike path. There is no need for a driver’s license, license plate, or insurance. Taxes are limited to sales tax.
One of the key issues in defining an electric assisted bicycle as a “bicycle” is the power of the motor. The general idea is that if the bike has a motor that is “too powerful” then it is really a moped or motorcycle. So most laws that create and define the category of electric bicycle worldwide have a limitation on the power of the motor – with the idea that the ebike should have similar speed and performance to a normal bike.
That, by the way, is a pretty broad range of speed. Normal bikes can travel as fast as 30 MPH with a strong rider, and they can climb nearly any grade.
But in general, many nations have adopted laws that define a bike that uses a relatively low-powered motor, with a limited speed, as an electric bike – with the same privileges as a normal bicycle.
So what about motor output?
There are a lot of factors to consider in motor output choices for an electric bike. Here are some of them:
1. Legality. Different jurisdictions have different laws about motor output for a vehicle that can still be considered an electric bike. In the EU, Japan, China, and other places, the power limit is 250 watts. In the USA it is 750 watts.
2. How that power is measured. An argument can be made to measure power in these ways:
A. Electric current into the motor.
B. Mechanical power output at the “shaft”. (but if it is a hub motor…do we measure at the hub flange or the rim / tire?)
C. Power in, less the efficiency losses of that motor. (Complicated.) And more, whose machine and which method do we use to measure that power? In the EU, there are detailed regulations about how to measure power. In the USA, it is pretty much what the maker says it is, with no testing method described or required.
3. Do we use peak power (the amount of power that the motor is capable of producing under maximum effort for a short period before overheating) or do we use continuous power?
4. How much power can the battery support? There is a balance of cost, weight, and energy storage in the decisions about the motor power, battery size, etc.
This is not a simple subject. But I will offer my advice:
Most 250 watt systems are satisfactory for pedelecs (where the rider is pedaling and thus adding in his energy / effort).
For throttle-controlled, or power-on-demand systems where the rider is not pedaling, 350 to 500 watt systems are a better choice.
750 watts seems attractive, but this requires a big battery – and the combination of cost and weight is not that attractive. This combination will get better as technology improves, but at this time, 500 watts may be a better choice in many cases.
Climbing hills on any of these will require the rider to add in some muscle power – but not a lot.
In all cases, the rider will enjoy the ride, sweat a lot less, and have less fatigue and go farther, faster.