arrgh!! wrong motor - now what?

[barod]

So, a Q128 arrived with me today, and despite my believing i'd ordered a 36v 201rpm it's very clearly labelled 48v 328rpm, which is the default setting on BMS's webpage when you want to place an order. I didn't spot it on the order confirmation email either. I really need to pay more attention to detail in future!
I have a couple of questions out of this that I'd welcome views on:
First off, what are BMS like when it comes to sorting something like this out?
Second, how different an experience would it be if I just went ahead with this motor instead of the one originally intended?

 

[Nealh]

BMS aren't very good with comms so you are probably stuck with as you ordered wrongly, it is to costly to send back.

It might be a bit inefficient in running, you should be ok at 36v. At 36v it will run at about 250/260 rpm.

 

[awol]

BMS aren't very good with comms so you are probably stuck with as you ordered wrongly, it is to costly to send back.

It might be a bit inefficient in running, you should be ok at 36v. At 36v it will run at about 250/260 rpm.

What are the chances of a 48v motor having good torque at 36v?
A light 36v 250rpm motor sounds ideal for me in the future.

 

[BornAgainCyclist]

Get yourself a cheap BMX and have a laugh off road, then order the proper one.

 

[Deleted member 4366]

I'm pretty sure that the 48v 328rpm and the 36v 201 rpm are the same motor, so it's no problem at all. Both will run at 260 rpm at 36v and both will run at 328 rpm at 48v.

When you install it, please let us no the no-load speed at whatever voltage you apply, so that we can confirm the above.

 

[danielrlee]

I'm pretty sure that the 48v 328rpm and the 36v 201 rpm are the same motor, so it's no problem at all. Both will run at 260 rpm at 36v and both will run at 328 rpm at 48v.

When you install it, please let us no the no-load speed at whatever voltage you apply, so that we can confirm the above.

This is probably covering old ground, but are they EXACTLY the same though?:

201/36=5.58Kv
328/48=6.83Kv

Calculations above show that the 201RPM@36V motor is wound about 20% slower than the 328RPM@48V.

Or us it a case of inaccurate motor specs from BMS and they are in fact the same motor?

 

[Deleted member 4366]

Those rpms aren't exact, they're only nominal. Basically, 201 rpm is 15.5 mph, 260 rpm is 20 mph, both in 26" wheels, and 328 rpm is 15.5 mph in a 16" wheel. The speeds in the listings are only approximate to show what category they fall into.

I have a 36v "201 rpm" Q128c. It spins the wheel to 29 mph at 48v, which is the speed you'd expect from a 48v 328 rpm one. Therefore, they're the same. That's all that matters.

 

[danielrlee]

How can they publish even 'nominal' ratings that on closer inspection, appear to be 20% out?

I hate motor specs published in RPM at an arbitrary voltage. It can be misleading to the uninformed and leads people into thinking there's such thing as a 24V, 36V or 48V motor. It makes it even worse when the published specs aren't even accurate, which in turn makes it almost impossible to compare motors without prior knowledge.

Kv (RPM/V) rating is a much more useful piece of info and would do away with such confusion. If the published specs were accurate, the true Kv could be calculated, but as you've already pointed out, they're not.

 

[barod]

Thanks for all the replies.

I was inclined to offer it up for sale (would take £75 plus £15 postage), but perhaps in the interests of research for the benefit of the forum as a whole i'll go ahead with it, particularly as d8veh reckons it's probably the same (or near as dammit). If it's not, well then I guess I'll get another motor in due course.

I've already got a 14A controller on the way, but not ordered the battery yet. I was planning on the 48V 12Ah one from Eclipse (with the Samsung 30Q cells) - is that still sensible (probably a dumb question, but better safe than sorry, particularly given the amount at stake).

Finally, if it's found that they are the same motor, then should this recommended kit combination of Q128 36v 201rpm with 48V battery be amended for future converters?

 

[Deleted member 4366]

I'm pretty sure that they are the same, so you have nothing to worry about. The battery and controller will match the motor nicely.

 

[barod]

I'm pretty sure that they are the same, so you have nothing to worry about. The battery and controller will match the motor nicely.

Thank you for the reassurance. I have my friendly local bike mechanic lined up for the week after next to lace the motor into a good strong rim, so one that's done and I've got the battery and controller I can check out the no-load speed and report back

 

[Nealh]

My Q128c labelled 201rpm/36v runs at 21mph no load speed hot off the charger in 700c wheel so well above the 15/16 mph usually stated for a 201 motor.

 

[BornAgainCyclist]

If looking for a 700c rim I used a dt swiss 535 with apline 3 spokes. 2 cross. It works well.

 

[jhruk]

Last year I purchased from BMSB a Q128c 48V 328rpm motor intending to run it at 36V, which I was expecting to be 246rpm. It was rather faster than that and although I initially calculated it at over 300rpm I’ve now measured it and found it to be actually 286rpm at 36V with a KU65 controller.

This gives it a Kv of 7.94 instead of 6.83 which the specs suggest. Like d8veh I have learnt to take the specified rpm figures as nominal but agree with daniel that it is very misleading. Particularly as the figures quoted suggest precision, ie. 201 and 328.

 

[danielrlee]

Like d8veh I have learnt to take the specified rpm figures as nominal but agree with daniel that it is very misleading. Particularly as the figures quoted suggest precision, ie. 201 and 328.

It makes the mind boggle, especially when "getting it right" is often so much easier.

If quoted "xyzRPM @ 36V", it should be at 36V, not a fully charged nominal 36V, which is 42V, which is where I suspect where the discrepancy arises from.

 

[jhruk]

If quoted "xyzRPM @ 36V", it should be at 36V, not a fully charged nominal 36V, which is 42V, which is where I suspect where the discrepancy arises from.

That theory looks good for this motor, but not the Q100c which I also measured. The 328rpm 36V Q100c is more or less spot on spec, 327rpm at 36V with a KU65.

I also measured the same Q100c with a S06S controller and it only manages 310rpm at 36V. This suggests that the controller is also a relevant factor in the specs for these motors.

 

[Deleted member 4366]

Last year I purchased from BMSB a Q128c 48V 328rpm motor intending to run it at 36V, which I was expecting to be 246rpm. It was rather faster than that and although I initially calculated it at over 300rpm I’ve now measured it and found it to be actually 286rpm at 36V with a KU65 controller.

This gives it a Kv of 7.94 instead of 6.83 which the specs suggest. Like d8veh I have learnt to take the specified rpm figures as nominal but agree with daniel that it is very misleading. Particularly as the figures quoted suggest precision, ie. 201 and 328.

.....but was that with a fully-charged battery, which is actually 40.5 when running?

286 x 36/40.5 = 254.2 or working back 286/260 x 36 = 39.6v. Could the battery have been at that voltage?

 

[jhruk]

.....but was that with a fully-charged battery, which is actually 40.5 when running?

286 x 36/40.5 = 254.2 or working back 286/260 x 36 = 39.6v. Could the battery have been at that voltage?

Nope, battery voltage was around 36.3V offload, 36V while testing.

 

[barod]

If looking for a 700c rim I used a dt swiss 535 with apline 3 spokes. 2 cross. It works well.

Thanks for the recommendation, my mechanic was asking if I had any ideas, so I'll pass that on and see what he thinks. He suggested plain gauge spokes though, which the alpine 3s aren't?

 

[BornAgainCyclist]

Thanks for the recommendation, my mechanic was asking if I had any ideas, so I'll pass that on and see what he thinks. He suggested plain gauge spokes though, which the alpine 3s aren't?

They are variable guage. I went wider at the top to get a tighter fit with the spoke holes. My personal choice rather than a recommendation. They are described as suitable for ebikes. Got 255 spokes which fitted but could have gone a mil or 2 shorter (wheel needed a lot of dishing) and 12mm pro lock swiss nipples with rim tape all from rose bikes.

The rim is described as the 36 hole 535 trekking rim 28inch