Low power from 5kw motor

[Willyrwalker]

HI all

I have recently converted a kids quad to electric using a 48v 5kw motor and HPC300 controller from golden motor. previously I had the kit on a aprilla rs50 motorcyle.

The quad has low torq and a low top speed( about 25mph) , I am running Lead acid batteries with 75Amp hr, previosly I had 26amp hr lithium.

The controller gets hot after say 2 minutes and then has even less power barely moves so the controlller is overheating( I will fit a large heat sink) .

This 5kw motor and controller seems to have less power than a 2kw conversion I did some years ago and on a small quad should this should have loads of torq as the motor and controller configuration is more than the 110cc engine that was originally fitted to the quad and more than the 50cc engine fitted to the motorcycle.

I have a sprocket on the motor with a chain drive to a much larger sprocket on the quad probably 10 times bigger.

Any help would be much apreciated.

Thanks

William

[Bikemad]

Hi William andto the forum.

Presumably, you had more torque and speed when the motor was installed on the motorbike? If so, it must be something that you have altered during the swap.

Are you sure that the Yellow, Green and Blue phase wires are still in the same positions as they were before, or could they have accidentally been refitted in a different order or position on the motor or controller terminals?

Do you have any means of checking how much current is being drawn by the controller, or how low the voltage is dropping under load?

I have a sprocket on the motor with a chain drive to a much larger sprocket on the quad probably 10 times bigger.

I would be surprised if it is as high as a 10:1 gear ratio. The following diagram shows the size comparison of a 5:1 gear ratio:



If you count the number of teeth (or measure the PCD of the chain) on both of the sprockets, and also measure the outer diameter of the tyre, it should be possible to roughly calculate the expected speed and driving force produced at the wheel using the 5kW motor.

I guess the quad will be quite a fair bit heavier than it was before, as it is now having to carry four 75Ah lead acid batteries.

Alan
 

[Willyrwalker]

HI Alan

The front sprocket is 17 tooth with a diameter of 70mm, the rear is a 78 tooth with a diameter of 330mm. the wheel diameter including tyre is 500mm.

The quad with the lithium batteries barely made a difference to the acceleration and top speed, the same sprocket set up was used on the motorbike, and both have low torq.

The phase wires are the same as on the motorbike, I have tried swapping over the green and yellow wires and this just makes the motor judder. The motor does work and pulls away just seems not what I would expect from a 5kw motor.

Thanks

William

[Bikemad]

According to the datasheet, the 5Kw Motor requires 125A @ 48V to produce an output of just over 5kW @ ~3500rpm with 13.92Nm of torque.

A 17 tooth motor sprocket driving a 78 tooth axle sprocket provides a gear reduction of 4.588:1.

At 3500rpm, the rear wheels should be turning at ~763rpm, which should produce a theoretical speed of 44.7mph with a 500mm tyre.

The torque available at the rear axle should be ~63Nm (13.92x4.588), which should also produce a forward force of 56lbs (25.5kg) with a typical chain drive mechanism (98% efficiency).

If the vehicle and rider combined weighed 500lbs (227kg) it would just about manage to climb up a 5° slope (8.75% gradient) with 63Nm of axle torque.

I suspect your quad is producing a lot more torque than this at lower speeds, which might explain why the controller gets hot so quickly,  but I'm unsure as to why your top speed is quite so slow (about 25mph) with a 4.588:1 gear ratio.
According to my calculations, you should still be able to achieve 25mph with an 8:1 gear reduction, but the torque available at the axle would increase from 63Nm to over 111Nm (76% more!).

This would produce much improved acceleration and the ability to climb steeper hills (10° slope or 17.6% gradient) with the same 13.92Nm of motor torque.

Unfortunately, an 8:1 gear ratio may not be feasible with a single stage reduction, as the rear sprocket would probably be larger than your 500mm tyre diameter. 

Alan
 

[Willyrwalker]

HI Alan

Many thanks for your detailed calculations, I could possibly increase the tire diameter by having larger wheels however from your cals the increased torq would be suttle not dramatic .Do you know of a supplier where a may purchase a simple reduction gearbox to achieve the 8:1 gear reduction with the current set up.

Thanks

William

[Bikemad]

Hi William,

Increasing the tyre diameter by having larger wheels would not increase the torque, it would basically reduce the available forward force and put more load on the motor. 

If you have room to install a separate jackshaft arrangement, you could use another 17T sprocket and a 30T sprocket  mounted on the jackshaft to provide a two stage gear reduction with an overall gear ratio of ~8.1:1.



The existing 17T motor sprocket would drive the 30T sprocket on the jackshaft, and then the 17T sprocket on the jackshaft would drive the 78T sprocket on the axle.

By increasing your overall gear ratio from 4.588:1 to 8.1:1, the torque on the rear axle would jump from 63Nm to 112.7Nm which should produce nearly 79% more driving force.

I would describe an increase in the available axle torque of 79% as substantial rather than subtle.
Applying a forward force of 99lbs (440N) should be noticeably different to 56lbs (250N).

Time for some Physics:

Force (N) = Mass (kg) x Acceleration (m/s), therefore:  Acceleration = Force divided by Mass (a=F/m)

Acceleration with existing 4.588:1 gear reduction= 250N/227kg = 1.1m/s² 

Acceleration with proposed 8.1:1 gear reduction = 440N/227kg = 1.94m/s²

As Time = (Final Velocity - Initial Velocity) / Acceleration, where Initial Velocity = 0m/s (0mph) and Final Velocity = 11.176m/s (25mph) then (11.176-0)/1.1 = 10.16 and (11.176-0)/1.94 = 5.76. 

A constant motor torque of 13.92N driving through your existing gearing of 4.588:1 would take 10.16 seconds to accelerate a 227kg mass from zero to 25mph (over a distance of 56.77m).

The same constant motor torque of 13.92N driving through the proposed 8.1:1 gearing would take 5.76 seconds to accelerate the same 227kg mass from zero to 25mph (over a distance of 32.18m).

In my opinion, an 8:1 ratio would make a significant difference to the acceleration. 

Alan
 

[Willyrwalker]

HI Alan

Many thanks, the subtle reference was refering to the change in wheel size  not the change in sprocket size as I wronglly presumed that a larger wheel diameter would slighlty increase the torq

Kind regards

William