MP5 motor gets pretty hot

[Wklatt]

I have a rear MP5 conversion on a 26" full suspension mtn bike. It worked well on the street hills back home, and I am now using it at our mtn cabin on rough steep trails. It is fine if I can keep the speed up (> 10 kph) on a hill. But that isn't always possible, and I find that if I am on a steep, slow uphill too long the motor really heats up (even with hard pedaling).

So, how hot is too hot? I don't want to burn out my expensive new motor. Will it shut down automatically if it gets too hot? Will repeated hot running reduce its reliability or life span? It has reached the point where the hub gets almost too hot to touch, but has never shut down. My battery (52V) gets a little warm, too, but nothing like the hub.

[Bikemad]

Hi Walter,

I believe the controller has a built in temperature sensor, but this would only monitor the temperature of the controller itself and not the motor windings.

If it hasn't cut out, I would say that the controller is not getting hot enough to activate the safety cut-out function. When it does become hot enough to cut the power, it apparently takes several minutes before it cools down enough to allow the controller to start working again.

If you are concerned about the amount of heat build up in the motor, you could try reducing the maximum current setting to see if it helps, but the maximum power (and climbing speed on steep hills) will also be greatly reduced at the same time.

Unfortunately, the motor's efficiency will drop quite dramatically when the rpm is dragged down under heavy load, and this is when the motor will start to generate heat relatively quickly. I actually run my standard Pie at the maximum 30 Amps setting, as I find it is often better to have more power to enable a higher average speed on steep hills, and therefore climb up them much quicker.
My modified Pie would draw over 50 Amps at maximum load but would easily maintain 20 mph (~32 km/h) on steep hills without having to pedal.

Either your hills are very steep (greater than 25%) and very long, or your Pie is pulling a lot more weight than mine does, as my standard Magic Pie never slows down below 10km/h on what I consider to be reasonably steep hills (even without pedalling).

Dual Pies (or even triple) would be a better solution for climbing really steep hills.



Alan
 

[Wklatt]

Thanks for the reply, Alan. Hmmm, yeah, maybe I need a double pie. As it happens, I have a spare SP5 (not MP5) 26" rear hub. Could it be adapted to use as a front wheel? The extra weight there would also help with keeping the front wheel on the ground. I really have to lean forward (high seat) on the steep climbs. And how would the throttle be wired? Do you use 2 then?

Yes, some of these mtn trails are very steep (not sure what the grade % is), and because they are also very rough (washouts, etc), can't go fast. They are not easy going down either, having to hang your butt way back over the rear wheel. This is where the regen really helps, though, as it also saves my brake pads.

But I do worry about the motor overheating. But from what you are saying, though, it is protected by a heat sensor, so I should be OK. Like I said, I don't know how hot these things can get. I can still touch it, but barely.

I have my amps set to 25. 30 might be too hard on my battery (52V ncrb 13.5 Ah).

[Bikemad]

Unfortunately, the width of the rear axle will be much too wide to fit within the front fork dropouts. 

With a dual Pie setup, the two control harnesses are simply wired in parallel so that one set of controls (cruise, brakes and throttle) will simultaneously control both motors.
I would also stagger the joins in the wires (to reduce the possibility of the soldered joints wearing through the insulation and touching against each other) similar to the extended cable joint shown here:



With dual motors and a single battery, your maximum battery current (30A) would have to be shared between the two motors (each motor would need to be set to 15A maximum).
If you fitted dual motors and dual batteries, each motor could then be run at up to 30A.

I've just had a quick look at the specifications of your battery pack and it states 30A output, so it should be able to handle the controller's battery current if set to the maximum 30A.

Alan
 

[Wklatt]

Thanks, Alan, that's very interesting about how to hook up multiple pies. I assume you could mix SPs and MPs? Maybe then set the SP to 10 and the MP to 20?

With the battery, they offer 2 types in that size. The PF is for high power and can sustain 30 amps continuous. But the NCRB is for range and is only supposed to handle 30 amps for a short period (small hill). That's why I left my MP5 controller setting at 25.

It's not just that the MP5 power is too low (except when the battery is down), but it heats up with the slow speed. With practice (takes a bit of getting used to maneuvering a heavy MTB) I might be able to increase my speed a bit on the uphills. Keeping that front wheel down and not losing balance (thereby going off track) is the biggest challenge.

I just don't want to damage my motor. Have you been able to confirm that it will indeed cut out?

[Bikemad]

I assume you could mix SPs and MPs? Maybe then set the SP to 10 and the MP to 20?

Yes, as the Smart Pies and Magic Pie Edges uses the same harness and controls as the Magic Pies, they can also be used in a dual motor setup.
Earlier MPIII harnesses are slightly different to the later Magic Pies, so it may not be quite as straightforward as joining together the same coloured wires in both harnesses as the wire colours or their associated functions may differ. 

You might need to reduce the maximum rpm setting of the faster wheel to ensure that both wheels share the workload and run at the same speed.

I just don't want to damage my motor. Have you been able to confirm that it will indeed cut out?

I have just received a response to an email I sent to GM China and here is the reply:

For MP5, there isn't any temperature sensor for the motor, because mostly the motor can't reach such high temperature. But there is temperature sensor in the controller to cut off the power when the controller temperature is too high to protect the controller.

Although there isn't a temperature sensor on the motor's stator assembly, I would expect the windings and the Hall sensors to be able to withstand temperatures of up to 150 °C before any damage would occur. Motor temperatures in excess of 200°C could also permanently damage the magnetic strength of the neodymium magnets.

I have not yet seen (or heard of) a Magic Pie whose windings have been damaged by severe overheating, and I suspect that the Hall sensors would fail (causing the motor to stop) before this could occur.

Alan
 

[Wklatt]

Thanks, Alan. Your answers and support are outstanding. I feel better now about my MP5, knowing that it will cut out before damage occurs.