Another very noob question

I'm searching online for a motor to use on a mountain bike. I have found several that might do the job but I am clueless when it comes to watts and amps.

I find a nice 600 watt motor at 36 volts 36 amps, then I find another 600 watt motor 36 volts but at 21.3 amps, then I see a 1000 watt motor 60 volts at 19.4 amps with the same rpm as the others.

I'm really confused. I have been assuming that if the wattage goes up then so should the amps??

Maybe I'm wrong.

What i'm mostly interested in is high torque low speed.

Thanks,

Welcome to the forum. The way it works is volts times amps gives you watts. I wanted the same thing and got an Ebikekit 6x10 It is low speed. I run it at 48 v and 22 amps = around 950 watts. Most motors are more like 9x7 = higher speed. These numbers refer to the windings in the motor .. Problem is there doesn't seem to be a unified system so you can tell how fast a motor will run with xx voltage. Also amps alone will not affect speed as much as voltage will. But more amps will help low speed thrust .. Kinda confusing I know, maybe someone could explain this better. Btw larger diameter motors are better at low speed too, because they have more leverage. Eg. The magnets are farther from the axle. If you want the best low end get a Crystalite 53xx series (large diameter and wide magnet) And go for a low speed winding..
I know lots to figure out , Hope I didn't confuse you too much

Thank you that has helped me a lot.

It would be nice still if they would always throw in HP with the other specs too. It would help me understand the power they have.

Yeah .. One horsepower is 746 watts I think. Some manuf. list rpm per volt so you can figure speed at 36 or 48 volts etc. but then different rim sizes make that confusing too. Basically a smaller rim will always get you more thrust at the expense of speed. And a larger diameter motor like 9 continent which is what Ebikekit sells or a Crystalyte 53xx series give good thrust for a given Voltage/amperage. Do some more reading and maybe check out Ebike.Ca simulator. You can punch in different motors,voltage,amperage rim size etc. Keep in mind the simulator shows no load speed, not on road speed under a load. No load speed is approx. 5 mph faster or more.http://www.ebikes.ca/

The relationship between Amps (I) and Watts (W) is one of Current and Power. Each manufacturer will vary to some degree based upon the physical design of their products, and it is not always apples = apples.

From the OP, what you may be reading is a motor matched with a controller. The motors can take a lot of current. Horsepower or wattage-rating depends on use, and is by no means fixed.

Torker is correct:
Motor Width and Radius are important factors in choosing the proper hub as much as the number of Turns/Windings. In that – both the Crystalyte 5X- series and the Nine Continent (9C) hubs are a good choice for MtBs. There are motors out there with gears and brushes too that are perfectly suited to particular applications.

Power-hungry:
However to be clear, Watts consumed is a function of Load or Work on or by the system. In other words, if I am traveling on a flat surface in good weather with no wind on a light bike, the motor will take a lot less current and power to push the bike, especially if I pedal aggressively. Conversely climbing a steep hill will pull a lot of current: Speed, wind, nasty road conditions, and inclines all suck power.

What I’m trying to say is don’t get stuck on the Wattage rating of a motor because they can do wondrous things with voltage, battery capacity, and the right controller. For instance the 9C motor could be rated at 500W using 36V if I clamped the max current to < 14A which is pretty pathetic for hill climbing, but great for racing down level roads.

Thus, choosing the correct motor then becomes less about wattage and current because both the Crystalyte and 9C are similar fruit: Wattage is gated and constrained by your usage and associated equipment.

Environmental Conditions:
If you live in a hilly area then you want more power early for better climbing. I live near Seattle and the hills here are steep and long ~ therefore my current is easily going to be > 20A at 36V if I wish to sustain 20 mph.

If you live in a flat area then you can get by with less power and your battery reserves will last longer.

Decide:

• What is the top speed desired on a flat. Lower turns/windings = faster
• What is the amount of torque/power desired for starting or climbing. Higher turns/windings = stronger
• Voltage = Potential. Higher voltage ultimately translates into more power and faster speed, though also results in slightly less efficiency. Most folks run between 36 to 48V which is fine for typical applications; it’s a great place to start.
• Current demand is proportional to the load. A good set of batteries can make all the difference in the world. Buy cheap and that’s what you’ll have, and the choice of motor will factor less. A strong battery pack with good capacity is best for hill climbing and/or duration.
• Match the controller with the intended use: Flatlander can get by on 20A, but hill climbers will drink up to 40A depending on how aggressive they are. For the most part those motors can take it …and at that level of consumption, I can guarantee it will be > 1000W! :wink:

The eBikes simulator is great for comparing apples to oranges to cherries; it is a good tool if you’re just starting out and making plans, though I wish it were a bit more customizable

Good Hunting! KF