No load power consumption of 9C hub motor + Bafang CST

[fairlyfaded]

Greetings all,

I am in the process of putting together a single wheel electric trailer for hauling goods+dog to and from work. I have two hub motors, a 9C laced to a 20" rim and a 500 watt Bafang CST geared hub motor laced to a 26" rim. For the past month I have been using the Bafang to pull around a kids trailer with my dog in it. Unfortunately I recently toasted a hall sensor inside of the Bafang due to running the thing too hot, so now I am going to switch to a single wheel trailer pusher.

Anyways, I took apart the Bafang motor to try and see what had happened to it. While I had the motor apart, I figured I would measure the no load power consumption of the thing.

With a 44 V battery, I measured 2600 rpm at the motor, and a no load current of around 6 amps (or 264 watts!). I was amazed! First, I was amazed by how fast it was spinning (it was never clear to me what rpm/volt my motor was due to how dubious the website I ordered the thing from was). Second, I was amazed by how much power it was chewing through! 264 watts to do... nothing but make a toasty motor and controller.

Next, I tried the 9c motor. To be fair, the 9c had a 20" rim laced to it (no tire) so presumably suffered from higher windage losses.

With a 44 V battery, I measured 800 rpm at the motor, and a no load current of around 12 amps (or 528 watts!!). Good god!

Do these numbers seem correct? Obviously I'm pushing the motors quite fast due to no load. With a load, the speeds would drop by... 30%? Which should drop the core losses considerably.

Regards,

Michael

P.S. I should note that I am using a 6 fet IR4110 Lyen controller. At no load, the motor controller losses will mostly be from switching. I'm not sure how high the switching losses on one of these guys should be... But I would have guessed that these controllers are in the 95% efficiency range even at low power...? Then again... a few hundred watts isn't that low power.

 

[bowlofsalad]

Have you adjusted the Rshunt value of your watt meter? The rshunt value should be written on your controller housing somewhere.

Are you certain you have the halls connections all correct?

Those numbers are substantially higher than they should be, that is for certain. The cause of getting those numbers is what we need to find out.

 

[fairlyfaded]

I took the measurements on the battery side of the motor controller using a multimeter in line. I tried two different volt meters and got the same result. I'm going to take the whole assembly in to work with me tomorrow where we have cc/cv power supplies and the works to redo the experiment.

Regarding whether I have the hall sensor wires connected correctly, admittedly, I did not do a scientific investigation of all possible permutations of hall sensor/phase wires. I fiddled around with them until the motor spun up smoothly. For the 9c motor, I did the experiment twice with the motor spinning in both directions (not that I think it should mater) and got the same no load power draw.

I wasn't able to find any other data on the no load power consumption of either of these motors, so I have no idea what I should be getting.

Bowlofsalad, have you seen any data?

-Michael

 

[fairlyfaded]

I just started making my way through the many combinations of hall sensor wires and phase sensor wires and it looks like I may well have had them connected wrong! I found a combination that results in much lower current draw, smooth sounding operation, and, interestingly, much lower speed.

Do these controllers apply some amount of phase advance? I'm wondering how an incorrect combination of hall sensor wires would result in the motor spinning faster than expected.

-Michael

 

[fairlyfaded]

Amazing. I used the same color coding that I had found worked with the 9C and tried it on the Bafang motor.

Now the motor spins at 1600 rpm and a cool 0.77 amps at full speed. I'm totally blown away! I think I had been riding it for the last month connected completely wrong! Thanks so much for your help, salad!

I had no idea that an incorrect combination of hall sensor/phase wires could result in what sounded like a reasonably "happy" motor, but with a far higher max rpm (and presumably a far lower torque constant). I can only guess that by connecting the phase wires incorrectly, you can achieve some sort of phase advance (or delay) that results in something like field weakening? If anyone knows the mechanism behind this, please do tell!

-Michael