17a or 20a controller?

I am rebuilding an old bike with a BFUN 250w rear hub motor to run on a 48v battery.

I was told to get a 17a (350w) KT controller from PSW Power. But I see the size of the next one up 20a (500w) is the same and will fit on the bike.

Would that also work? Pros/cons?

Thanks

Chris



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A little more power thru a motor it can usually handle, evne a little geared one.

But twice the power in that situation, especially if it's not just occasional, can overheat and damage it.


But....even the 17A controller, at 48v, is over 800w of power, not 350, so it's possible (likely in many situations) that you'll smoke that motor soon.

If you monitor and limit your usage based on the core motor temperature (you'd probably have to install a sensor), it could survive...but it would be better to get a controller that limits to something close to the power levels that motor was "rated" for.

At 48v, to get say, 350w, you'd need a controller limited to about 7A.


Also note that if the system was originally 24v, the motor is going to spin twice as fast for the same throttle setting.

If it was 36v, it'll be about one and a half times as fast.

Ok so 17a would be safer then?


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Safer is relative....17A, about 850w at 48v, is still more than twice the amount of power I'd want to try running thru that little motor.

But isn’t that the max output? So that would be occasionally. I was told by someone much more savvy than me this would be ok. I think those motors marked 250 often actually are more in reality?


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Not that much more, they're small geared hubmotors that can't shed heat very well.

They also have plastic gears that can only take so much torque. Too much current means too much torque, and they can strip or break.

Same with their built in clutch that lets the wheel freewheel around the motor when you're not powering it, so it doesn't drag and slow you down. That can only handle so much torque.


larger DD hubmotors can cool themselves better, so you could put twice the power than its' rated for, in relatively short bursts, and it'd do just fine. No gears, no clutch, so they won't break those from torque.

But the small geared hubs can't handle that much more than their ratings.


To avoid overheating the hub, you may have to stay at slower (normal bicycle pedalling) speeds, so air resistance wouldn't force you to use more power.

Going up a hill, or riding into a strong headwind, could take enough power to overheat the hub.

Similarly, lots of stop and start traffic could do it too, especially if the bike, including rider and anything you're carrying, is heavy.

Or it might work fine. Depends on conditions and how you use it.


I'd still recommend some sort of temperature sensor, even if it's just a BBQ thermometer with the sensor stuffed into the axle between the metal and the wires. If you don't have a way to monitor things, then usually the first time you know it's too much for it is when it stops working.


I was going to suggest checking out the http://ebikes.ca/simulator to see what happens in various situations. but I didn't see a motor listed that might be like yours, and I couldn't setup an "accurate" basic simulation with other motors since we have no info in this thread on your bike/etc., or terrain, wind, weight, traffic, or other conditions.

Something to consider is the fact the KT controller, if it's equipped with a decent LCD display, can be configured to limit available power to the motor pretty easily. It should have no trouble keeping that motor happy, from the controller's perspective anyway. You still have the same ability to burn the motor down by spending too much time climbing with it for instance.

Not pretending to be a electronics guru, but in my experience a little controller capacity "overhead" is not a bad thing, if you keep in mind it doesn't make the motor any more heat resistant. My opinion, FWIW.

Power delivered to motor has little to do with controller's max amp rating......it is a function of motor's KV, battery's impedance, motor&controller's impedance, battery voltage, and finally waveform delivered to motor by controller.
If you take a controller rated at 5000amps with 36V input, power output to identical motor system will only be slightly higher (due to decreased conduction resistance of additional MOSFETs) than that of a a controller rated at 15amps with same 36V input.

That said, go with the higher rated controller for reasons already pointed out by AHicks.
PS: I'm running a controller rated at 40amps for a 500w motor. My motor only draws 15amps under full load at 36v.

Is it Safe? No.
Will it work? Yes.
Will it be more fun? Yes.
Will it melt my motor faster? Yes.
Is this OK because I only have a 250w motor and really want something bigger and more fun? Yes.


17A is already going to be too much power. 20A is 18% more power. it will give you a 18% bigger grin with every throttle twist. it will also melt the gears that much faster. However, your motor is doomed anyway because you found it boring enough to want to upgrade controllers. Might as well use it up before you replace it. Who knows, it might last years before it finally gives up. Or might give up on the first ride.

Awesome. 18% grin is def worth 18% more wear on the gears. And Maybe the LCD3 actually does have the ability to turn the power down in case I’m grinning too much. 20A it is then and maybe that bbq thermometer? Anyone installed that? Which kind?


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A 17A controller probably delivers 8A continuous. A 20A controller probably 10A.
Upping to 48V... That's 380W and 480W of power continous... But 800W to 960W burst...

I would do it... Your original conttoller on the 250W probably already had burst power of 540W...

Should survive no prob. Just dont go 48V at 30A... You will cook it for sure, especially if your a heavy guy

I've been using both the 17A and the 20A controllers from PSW Power on Q100's (Cutes) for years and my experience with them are;
Note; They are the 9-FET Non-sine wave versions with a simple LED display (that I prefer).
The Q100's are about the same size as that Bafang mini, if anything, slightly smaller.
First off, there is little difference in the way they feel or perform.
For 90% of mini user's and their conditions, the risk from too many Amps is at the take-off. Given enough Amps, they tend to "hammer" (A series of shocks felt in the motor) , which if is continued for long, may damage the nylon gears. This, I suspect is an artifact of the Square-wave function, as I don't recall experiencing it with the KT controller.
The fact is, these PSW controllers are "soft start" in nature, even the Square-waves.
I "shunt modified" one of the PSW's to about 22 Amps and started to detect a slight amount of hammering. By way of comparison, a 20 Amp Grineon (An Infineon from Grin) would hammer pretty good and I had to be careful. For a while, I used a 6-FET Lynn Mini Monster and it really was a monster. I could almost feel the gears flexing, but this was at a time when I was seeing how much power I could run thru a Q100H and was also upping the Voltage to as high as 56 Volts. It was a front mounted motor and it was all too easy for the tire to spin, but the reason I backed it down was I knew I would melt something if I continued.
The other area where Amps become a factor is climbing hills, always a challenge for mini's. Even a steep hill, if it's not too long, a mini can handle. But if it's more than several blocks and steep enough it would be tiring to walk up, a mini will likely run out of steam. As counter-intuitive as it sounds, the more Amps the better (up to a point) to maintain momentum. But one has to know when it's time to jump off and push, To "lug down" a mini is an invitation to start melting stuff, in this order;
1) the phase wire connections
2) the phase wires
3)Something in the controller
4)The windings in the motor

Matador is probably right. PSWpower is selling the max current rating. Here's the inside of my 20A sinewave model. and there's only 6 MOSFET's. I've got the S12S, rated at continuous 25A, and I believe there are 12 MOSFET's in it.



In any case. I've also been running the very mild mannered 100H with the 20A sinewave model, and it rarely shows over 400W on the LCD3 on 36V. I would suggest the 20A version.

Well, there is always the possibility to "upgrade" a smaller controller: better FETs, beefed up power traces and larger shunt can help a lot, especially when space is scarce, and you need to stick with a small controller case (like integrated controller in hailong cases).