Help choosing a controller

[Liam.great98]

I have two questions that are basically the same. I'm 15 making my first electric bike and I'm using a 24v 250 watt brushed motor (it's the MY1018) and I was originally looking at this controller:
http://www.ebay.com/itm/190898620052?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649
It is cheap, but so am I. Would it be wise to get a controller with a higher wattage than the motor? It says "matching motor: 250 watt brushed" so I'm guessing it would work. But if I was climbing a hill or something could the motor draw more current than the controller is designed for? Or do they design controllers with a margin for error?
Another controller I'm looking at is this:
http://www.ebay.com/itm/Motor-Brush...00W-/171115307262?hash=item27d74460fe&vxp=mtr
It is slightly more expensive but it looks higher quality. Partially because it's in an aluminum heat sink rather than just black plastic or metal. I am leaning towards this one. It says that the matching motor is a 500 watt brushed motor though. Would it still work with a motor of only 250 watts, half of what the controller is designed for? The way I think it works is the controller needs to be the same or greater wattage as the motor, but not less. Am I right?
Thanks

 

[mlt34]

Actually you have the rule backwards Liam. You should spec a controller that is the same as or lower wattage than your motor. It is the controller that limits the power (watts) to the motor. So if you have a 250W controller and 500W motor, the motor will only get 250 watts of power. But if you had a 250W motor and 500W controller, the motor would actually be getting 500W and would be overpowered, risking overheating on hills.

Generally, try to match the controller and motor voltage if you can.

You're right about the plastic vs aluminum controller case and heat reduction, though 250W is pretty low power so it won't generate too much extra heat anyways.

I'd go with the first controller though since the power level is more appropriate for your motor. Good luck with your build!

 

[Liam.great98]

Wow, I never imagined it like that. Thanks. I'll go ahead and order the first one!

 

[John in CR]

With a programmable controller, then it's better to get one that exceeds what the motor can handle as long as you can afford it and fit it. Then not only can your controller grow with you, but you can also push the motor to higher limits once you've learned it's stress levels. I run almost all of my motors at several times or more of their rated power. I can do that without issue because I know what operating conditions create more heat, so I know when I can twist the throttle at will and when load conditions mean I should go easy on the throttle. That comes with experience.

If I tried the opposite approach my motors would blow controllers on a weekly if not daily basis, because I'd push the controllers to their limits trying to get performance from them while the motor is saying "give me more".

My first paragraph is primarily applicable to brushless motors, but the same can hold true with brushed motors. Heat is the enemy and the real indicator of stress in the system components. If you try to avoid repetitive full throttle starts and stops, and avoid heavy loads like riding in sand or up long hills, then your little motor can handle more peak power than it's rating.