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-   -   hub motor watt capacity (http://www.bikeforums.net/electric-bikes/936056-hub-motor-watt-capacity.html)

lesleeharris 02-28-14 04:00 AM

hub motor watt capacity
 
I have a 36v 460w hub motor. I'm wondering if I sent about 1000w @ 36v to the motor if I would damage it. I know the answer if this was an a/c motor, but don't have as much experience with d/c motors.

Thank you my brothers and sisters.

no1mad 02-28-14 04:09 AM

Welcome to the Forums :)

I moved this thread from Introductions in the hopes that you will get a quicker, more informative response.

DrkAngel 02-28-14 05:36 AM

Quote:

Originally Posted by lesleeharris (Post 16534452)
I have a 36v 460w hub motor. I'm wondering if I sent about 1000w @ 36v to the motor if I would damage it. I know the answer if this was an a/c motor, but don't have as much experience with d/c motors.

Thank you my brothers and sisters.

460w motor is 460w peak output.
460w peak output requires 920w input, so, ... 920w/36V = 25.5A ... 25A minimum controller required to meet oem rated, 30 amp probably acceptable.
1000w sound about right.
More than 1000w electrical input would only help acceleration at low speed ... but also increase damaging heat.

lesleeharris 02-28-14 10:19 AM

I'm thinking about the resistance from the motor. I can hook a 7.5amp a/c motor up to a 200amp sub-panel main and the motor will only allow 7.5amps through. So would a brushless hub do the same? I'm really green about this stuff with bike motors. Sometimes I look at motor specs and Amp doesn't =w/v. I think I have a lot more homework to do.

Thanks for the reply, it gave me purpose and direction.
Because I'm soooooooooo lost ............

turbo1889 02-28-14 06:34 PM

Most e-bike motors are not self limiting by their own resistance to keep them from burning themselves up. That design methodology works great for mounted electric motors and is quite common for regular old household use electric motors like in a fan or a heater or a blender, etc. . .

But with an e-bike its all about efficiency and reduced weight so most e-bike motors if you put a good load on them and give them an open circuit where they can pull as many amps as they want they are fully capable of burning themselves up because they are not a self limiting design (which is less efficient and weighs a lot more for the same amount of output power).

Wishes 03-01-14 01:14 AM

As member turbo1889 mentioned, ebike DC motors do not have any limiting capacity. If you feed them enough current, they are capable of melting themselves. The limiting is done by the controller.

Every DC motor has an efficiency point, and beyond that much of the energy is wasted into heat. Hence why they over heat. As a general rule of thumb, most -+ 500 watt motors can handle 1000w.

AMPs will cause more heat than increasing voltages. The weakest point is usually the wires running from the controller to the motor. And those are usually the first things that melt, short and even sometimes fuse together in the motor, when you over amp them.

The stock controller is probably a 15amp, so they are probably using 14 gauge wire at best. Pumping 30 amps through those will heat them up good. However increase your voltage to 48 but keep the amps at 15, or get a 20 amp controller which will give you close to your 1000 watt and the wires will survive.

DIY ebike modders will open hub motors and replace the 3 phase wires (brushless) or the 2 wires in a brushed motor for higher gauge, just to enable them to increase the AMPs.

I hope that helps.

Wishes

DrkAngel 03-01-14 08:43 AM

Quote:

Originally Posted by lesleeharris (Post 16535274)
I'm thinking about the resistance from the motor. I can hook a 7.5amp a/c motor up to a 200amp sub-panel main and the motor will only allow 7.5amps through. So would a brushless hub do the same? I'm really green about this stuff with bike motors. Sometimes I look at motor specs and Amp doesn't =w/v. I think I have a lot more homework to do.

Thanks for the reply, it gave me purpose and direction.
Because I'm soooooooooo lost ............

460w rated output motor.
Motor attains peak watt output at ~40% of no load speed.
This "peak" motor output point coincides with a 50% efficiency.
At peak output point, 36V input, motor will accept only 25.5A.
Below this speed, motor will accept much more amperage!
... Most of it wasted as damaging heat!

http://endless-sphere.com/w/images/e..._Watts_out.jpg

Limiting controller amperage to 25A would be optimal ... for you. IMO

See - ES Wiki - Efficiency?

Ypedal 03-05-14 10:37 AM

Without knowing EXACTLY what motor you are talking about.. all the discussion above is pointless.

Is it a geared motor ? . Direct Drive ?.. brand, model, rim size, rider weight... fill in those blanks and get proper advice.

turbo1889 03-06-14 12:08 AM

@ Ypedal

That is why I said "Most e-bike motors . . ." in my reply. Specifics would help of course but even if we knew the exact motor unless one of us has it and knows the exact internal specs and have tested it on open circuit ultra high amperage controller even then we wouldn't be able to give an exact response. For my part I know that most e-bike motors I have encountered have very low internal resistance that combined with their low thermal mass and limited cooling they are not self limiting and if given an infinite power source (or high enough to be several orders of magnitude over the rating such as a 200-Amp controller on a motor only rated for 15-Amps) and a heavy load they will not self limit and will burn themselves out.

I have some experience with both AC and DC motors that were of a self limiting type wound with many many turns of fine wire such that their internal resistance was high enough that they were self limiting and would not burn themselves out even when connected to a high power source and given a heavy load. Such motors tend to be inefficient and quite heavy for their power output as a result of that type of design. For e-bike use you don't want a heavy low power motor that has a low efficiency. This is why I believe such self limiting designs are not used in e-bikes, at least not normally.

Ypedal 03-06-14 07:05 AM

1000w at 36v is aprox 30 amps or less... most Direct Drive hub motors will handle this without any problems.... Geared motors vary.. the larger BMC/MAC/eZee sized geared units will hold up as long as the rider is not too heavy and terrain not too rough, the smaller geared motors ie : Cute, small Bafangs, 8Fun, mxus will melt at 1000w very easily, 1000w peak for a few seconds here and there should work ok.. but not sustained.... while something like a 9C or Clyte DD motor can tollerage 1000w + until your battery runs out without any problem at all..

With brushless hub motors, all controllers on the market have current limiting that i know of, and 1000w is far from extreme power, you can easily strap an 18fet controller on a fair sized DD motor and give it 3000~5000w if it's in a 20" rim with sub-200 lbs rider on fairly flat terrain.

it all comes down to the motor of choice.. without knowing that specific item. advice is nothing but a meaningless opinion.

turbo1889 03-06-14 07:42 AM

Your overview of approximately what the various types of hubs can handle I can't see any flaws in from my point of view. And you are correct that to answer the specific question of the OP in his first post starting this thread some idea of the motor and conditions are necessary. I was trying to address the more general question the OP was asking in his second post on this thread (post #4 ) as to whether e-bike motors will self limit themselves or not by the motors own resistance sufficiently to prevent melt-down. In my experience the answer to that is almost always a NO and the lighter weight, higher power, higher efficiency motors used for e-bikes are fully capable of melting themselves down and are not self limiting by their basic design as many (but not all) AC household use motors and some other DC motors are.


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