hubmotor pros/cons

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Vanarian   10 kW

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Re: hubmotor pros/cons

Post by Vanarian » Nov 11 2015 7:40am

LEVer wrote:My SkatEPods have freewheel bearings and mechanical brakes, but no one would buy it.
Hey I need to know how you succeeded in adding brakes :P ain't it too wide?
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vedder   100 W

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Re: hubmotor pros/cons

Post by vedder » Nov 11 2015 8:58am

Vanarian wrote:Now talking again only about pure hubs without gearing systems. Hills will of course generate the same load on both high Kv and low Kv motors with same amount of copper. Then if we say that both motors have systems designed to run identical power at identical RPM, one being 50Kv and the other 100kv, will both motors produce the same amount of heat? I think not. Main reason is due to eddy currents, which should already be numerous times higher inside the high Kv motor. Torque should be almost the same (lessened by losses) but heat should prove to be worst on the 100Kv version.
This is where you are wrong. Eddy currents and hysteresis losses occur in the stator with the skin depth at the frequency the motor spins at, not in the windings. If they were in the windings themselves, windings can be made with several smaller parallel strands (as is already done with hobby motors if you take them apart and have a look at the windings). One area where lower kv will give a slight improvement is where the windings are connected between the coils in the stator, but that difference will be minor.

You have to understand this properly. Ultra low kv will not do any magic. For hub motors, you need larger motors in order to climb hills. The main difference I see with LEVers hub motors is that they are larger than other peoples hub motors, which is why they perform well. Then, of course, you should adapt the KV of the motor to the electrical system. The vesc for example will be more efficient if you run hub motors with e.g. 100kv and 10s than if you run 200kv motors with 5s, but the motors themselves will generate the exact same amount of heat under both configurations.

There shouldn't be any doubt that for a given size motor class and voltage a low Kv wind can climb steeper hills than a high Kv wind, this is because a lower Kv wind produces more torque at lower rpm and can crawl up the hill at a "lower speed". I think using 'given speed' in a hill climb discussion can be misleading because there will be a point where the practical ability of getting up the hill is forgotten. A 200Kv motor may be able to climb a 1 in 10 rise at a 20km/h whereas a 100Kv motor could climb a 1 in 5 rise at 10km/h. The given speed is different but the work done is similar and the hill climbed by the lower Kv motor is much steeper.
In this case the 100kv motor will get much warmer because it has to endure the high torque that generates heat for a longer time will less airflow :-) A quite bad example...
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Hummina Shadeeba   1 MW

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Re: hubmotor pros/cons

Post by Hummina Shadeeba » Nov 11 2015 11:09am

eddy currents flow through any conductor near a magnetic field but they produce more loses in the stator

The biggest obstacle or source of inefficiency in a hub motor, I think, comes from the stator being saturated with flux due to its size not being up to the challenge of the load and it's low kv..there's more magnetism being asked of the ferromagnet than it can hold..and then not only will it start to draw more amps as it tries to overcome a load that is requiring more torque and magnetism than the small stator can hold,it will also suffer a much worse hysterisis loop due to having been saturated with flux.

Pedrodemio   10 W

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Re: hubmotor pros/cons

Post by Pedrodemio » Nov 11 2015 12:05pm

So, if i change a hubmotor winding configuration from delta to wye, lowering kv by 1.73 factor maintaining the same electrical system, i gain nothing? or because the lower kv i have a little bigger torque to climb a hill at a lower speed?

Hummina Shadeeba   1 MW

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Re: hubmotor pros/cons

Post by Hummina Shadeeba » Nov 11 2015 12:20pm

With lower kv u get more torque per amp within the limit of your stator size. if u change from delta to wye it might be better or worse depending on the load and your battery voltage. But..it is true that people
Are less likely to melt down/burn up higher kv motors with their thicker windings.

As an example(don't know if it's what ur looking for though: if u took a 1000kv motor and tried to run it on a board that was typically geared and with a typical 22 volts.. U would suck a lot of amps to produce ur torque and ultimately suck a lot
Of battery. You could get maybe a 2 volt battery with the same watt hours as the 22, so same size, more amps less voltage and it would be more appropriate.
I still don't understand what happens to the volts in the situation where you've got maybe a 1000kv motor running on 22 volts, or even worse say at 50 volts: if volts x amps equals watts yet they will produce the same heat to produce the same amount of torque regardless of kv. Where the seemingly wasted voltage going? Or is the voltage x amps =watts equation only valid when time is included for the motor to spin up those volts and then it's torque x speed = power. Is time necessary to make volts x amps = watts valid?

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LEVer   10 kW

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Re: hubmotor pros/cons

Post by LEVer » Nov 11 2015 2:48pm

Vanarian wrote:...

Hey I need to know how you succeeded in adding brakes :P ain't it too wide?
I use mountainboard trucks and wheels on SkatEPods and use the MBS braking system.

vedder   100 W

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Re: hubmotor pros/cons

Post by vedder » Nov 13 2015 6:05am

Hummina Shadeeba wrote:eddy currents flow through any conductor near a magnetic field but they produce more loses in the stator

The biggest obstacle or source of inefficiency in a hub motor, I think, comes from the stator being saturated with flux due to its size not being up to the challenge of the load and it's low kv..there's more magnetism being asked of the ferromagnet than it can hold..and then not only will it start to draw more amps as it tries to overcome a load that is requiring more torque and magnetism than the small stator can hold,it will also suffer a much worse hysterisis loop due to having been saturated with flux.
You are right about that. And if you run on high current vs motor resistance, the resistive losses also become large since they grow with the square of the current. Since the motor resistance for a given amount of copper decreases with the square of KV, and the resistive losses increase with the square of current, the resistive losses for a given torque with a given amount of copper don't change with the motor kv.
Where the seemingly wasted voltage going? Or is the voltage x amps =watts equation only valid when time is included for the motor to spin up those volts and then it's torque x speed = power. Is time necessary to make volts x amps = watts valid?
When you run a high kv motor on high voltage at low speed the duty cycle will be low, so you are actually drawing much lower current from the battery. If you have a 50V battery and run at 10% duty cycle (=5V output voltage) and the motor draws 100A, you will only draw 10A from the battery since the battery is connected to the motor only 10% of the time. The current ripple on the motor side is smoothed by the motor inductance and on the input side it is smoothed by the bus capacitors.
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