How much juice can a 300w motor take?

[wheelbender6]

There are a lot of inexpensive ebikes and ebike kits that are 24 volt and 250 to 350 watts. You can’t increase the wattage of the motor, but you can upgrade the controller and batteries.

Would I be able to use a 36v 20AH controller with 20 AH batteries on a motor rated at 300 watts or so (brushless controller & motor)?

How could I calculate the max volts and AH I could use on an e-motor if I know the wattage? Is there a reference table where I could look it up?

 

[Mark_A_W]

It's about heat.

My "250 watt" motor took 1500w peaks and 1000w continuous no problemo - barely got warm.

 

[dogman dan]

Not that I know of. But most of the 250 watt motors can take 500 watts ok. The problem is duration. so you could very likely run 36v 20 amps through em for rides of 10 miles or so, full throttle. Putting a really big hill on the route, or going 25 miles though, then you might need cool weather. The one modifcation the motor might need is thicker phase wires, if they are really thin on the 250 watt motor. When experimenting a way to tell the motor heat, like a remote thermometer or infared thermometer is a good idea.

 

[Demosthenes]

My "500 watt" motor can handle 2000 peak and ~1400 continuous without warming up at all. Sometimes I worry about the phase wires, but I've been running it at those watts for 5 months now. Not a problem.

There really is no way to calculate the max volts or amps you can use with a particular motor. Those ratings they give are really not much more than suggestions. If you were to assume that 300 watts is the max, at 24 volts the most you could run it with is a 12.5amp controller. At 36V, 8.3amps. But again, that's really not true. 36V and 20A is not going to damage a 300 watt motor unless it's absolute rubbish, and not even those cheap ebay kits should be that bad.

 

[Russell]

There are a lot of inexpensive ebikes and ebike kits that are 24 volt and 250 to 350 watts. You can’t increase the wattage of the motor, but you can upgrade the controller and batteries.

Would I be able to use a 36v 20AH controller with 20 AH batteries on a motor rated at 300 watts or so (brushless controller & motor)?

How could I calculate the max volts and AH I could use on an e-motor if I know the wattage? Is there a reference table where I could look it up?

I've used a 36V 250V Bafang geared hub motor for just under 4,000 miles and most of the time that has been at 48V with a 15A controller. At 3,000 miles a hall sensor and wiring was damaged so I yanked it all out and have been running sensorless with a 21A controller for the past 900+ miles. I had the motor apart today and the nylon gears still look great. I do run this motor conservatively however and even with a 48V LiFepo4 pack it topped out at 23 mph and does 21.5 mph unassisted now that I'm down to 15 cells. At those speeds on the flats it's not drawing all that much power so the motor stays healthy. Think twice about over-volting a 24V motor to 48V however since the resulting speeds require more power than the motors can handle for too long...it is a blast though.

While we're talking power; the maximum power a motor will draw from the battery has little to do with the motor rating but rather is simply the voltage of the battery (under load) x the maximum current allowed by the controller (normally on a sticker on the controller). So with the 15A controller my power would peak around 750W and with the second controller it's about 1,000W. You'll pretty much hit the peak power levels at least momentarily when accelerating from a stop or for much of the time you're climbing a steep hill.

How well a little 250W motor stands up to running at the higher power depends on numerous factors. Anyone that tells you they're running high levels of power through a small 250W motor I guarantee you is not doing it for long. The little motors cannot shed heat nearly as fast as you can put it in so they can get hotter and hotter and hotter as time goes on. There have been some folks that early on really pushed Bafangs to crazy power levels (1.5KW+) and quickly either turned the gears to mush or burnt the windings. I had a 24V 250W geared motor get so hot the first time out running on 48V/22A that the insulation on a pair of hall wires melted and fused together. I wasn't riding at anywhere near full throttle for most of the trip but I'm sure it was heating up then at about 24 miles into the ride I get to a series of steep hills where the power peaked at over 1,000W. The motor failed shortly thereafter leaving me to pedal the last 4 miles. Hills really are motor killers.

My recommendation then is if you want to overvolt a 250W geared motor you take it easy; 48V with a 15A controller is a good place to start with a 36V motor or 36V/20A for a 24V motor. Also even over-volted a 250W motor is still an assist motor so you should pedal along as even a little effort helps a great deal on hills. If you want to ride your electric bike like a moped get a big hefty direct drive motor.

-R

 

[amberwolf]

I used to run my little DGA's friction drive off two paralleled 12V motors designed to run radiator fans, at what I'm sure was to be pretty low current (1 or 2 amps?). Yet I was able to continuously run 36V at (guessing, didn't measure much at the time) 10 or 20A, maybe more, thru the pair of them. That'd be something like 360W each, which was probably 15-30 times the original power rating. They did get awfully hot (instantly boil icewater poured on them into steam) but no apparent damage to brushes, comms, magnets, or windings, even after a summer of this in Phoenix.

But a little 250W Unite motor off a scooter didn't even take a sustained 300W before actually vaporizing windings after maybe a few minutes.

It's all in the design, and the build quality. And in how much influence marketing department had on the ratings label.

 

[Lock]

...And in how much influence marketing department had on the ratings label.

And Finance Departments... more cost effective just to produce one design for all markets, able to handle all legal ratings but "de-rated" as desired with labels and controllers... I doubt there are any true 200W motors out there.
About heat, yes, but especially about how efficient the design is (how much input energy is wasted as heat) and how well the design dissipates the waste heat over time.
loCK

 

[dogman dan]

I think Russel just hit it on the head, Go as high as 48v for the top speed you'll get, but keep amps low at 15-20. How fast the motor eventually melts will be a factor of weight carried, how steep the hills are and how long, and weather. You can get away with murder at 80F and below. Above 90F, carefull, Above 100 anything can happen in less than 10 miles. This applies just as much to the 1000 watt motors when used with 72v 50 amps.

The generic infineon 36v 20 amp controller should be safe enough if you live in flatter land.

I think though, that hub motors seem to come in three classes in general.

Class one 200-500 watts. Often gearmotors, these are common on the cheapest prebuilt ebikes. Bafangs are a good example.

Class two 300-1000 watts. Often direct drive hubmotors, often sold in kits to put on bikes. Clyte 407, WE BD 36, golden motor etc. Often run at 1500 -2000 watts with no problems other than heating faster.

Class three. Larger hubmotors with huge wattage capacities. Clyte 5304, larger scooter and motorcycle hubmotors.

 

[wheelbender6]

A member on another forum told me that Currie motors cant tolerate being overvolted pretty well. Perhaps it's because of the way they are mounted. The dropout mount puts them more in the slipstream than a hub motor.