Please please help a complete novice trying to rebuild a moped for my son!!!!!!!

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jdnimmo
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Joined: Monday, August 29, 2011 - 03:24
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Please please help a complete novice trying to rebuild a moped for my son!!!!!!!

Cards on the table - Im a novice at this!!!

Here's the problem. My son got a hold of an old electric moped and wanted to do it up. When we charged the batteries and turned it on it ran at 25mph constant and after reading up on it, I disconnected the throttle and it still ran at full speed indicating that the speed controller had gone.

I purchased cheap 800w controller but it came with no wiring diagram! However I found a posting here showing an exact copy 500w controller by the same manufacturer with exactly the same wiring config and colours so connected up the 800w controller in exactly the same way as the details shown for the 500w.

I have no idea what motor is in it - its a hub motor with only two wires so Im assuming its a DC brushed motor. I got an electrician freind of a freind to give me instructions how to calculate the approximate wattage of the motor by measuring resistance and calculating based on voltage/current and came up with 270W.

This sound about right as there is a legislative limit of 250W in the UK for a 14 year old to ride a moped with no MOT/liscence etc and given that it ran at 25mph which is also the limit for that class of moped, I thought this sounds about right to cope with hills.

It has three 12V 10AH batteries connected in series making it 36V 10amp ( correct me if at any point I'm wrong!)

I connected it up as advised but turning the throttle has no effect on the motor - I have noticed that if I connect with power on there is a slight buzz from the motor and if you try to turn the wheel there is more resistance to turning it than you get with no power conected. It is therefore getting some kind of power through the wiring.

I used a multi meter and connected this to the motor leads and turned the throttle and measured both voltage and current. The voltage I expected was 36V but I get 38 - 39V ( this could be because the batteries were fully charged) but Im not worried about that ( should I be?)

When I measure current ( now this is where I struggle) with the digital meter set at 10A I see 0.13 - 0.14
which ( depending upon I have all the wires connected properly) rises to 0.20 - 0.22 when I turn the throttle.

I then disconnected everything and measured the current generated by the motor when I simply turn the wheel by hand using the multi meter on the same settings and turning it slowly shows the current rises dramatically to 4.0 - 6.0 etc.

From this I thought that the problem may be that there is insufficient current getting to the motor to drive it - I measured the current across the batteries and it comes in at between 0.20 and 0.22

I shorted the thin and thick red wires together and grounded the brown wire. My throttle grip has black red and another wire. The controller has black red and another wire ( Im typing this away from the thing) so I bypassed the wiring hardness which has completely different colours and connected them up the only way the two plugs go together and red=red black=black and the other two connect so Im happy that the throttle is connected correctly.

I did try disconnecting the throttle and shorting the connection at the controller to see if the throttle has a problem but no joy there. I read that if the wheel moves when you do this the problem may be with the throttle.

I thought the batteries are not producing enough current but then they were able to drive the wheel appropriately 25mph using the old burnt out controller so the batteries should be moving it. The only thing I can think of is that somehow the brake circuit isn't right and is acting as if the brake has been pressed?

So I suspect that I have made a mistake with the choice of controller....it was onlt 20 quid as opposed to twice that from other sources....perhaps I should have gone for something different?

Thanks for you help and I hope Ive given enough info for you to think about what the issue could be....

LeftieBiker
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Last seen: 11 months 3 days ago
Joined: Saturday, July 9, 2011 - 04:36
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Re: Please please help a complete novice trying to rebuild a ...

I'm not an expert, but check to see if there are brake light microswitches activated by the brake levers. These are usually also used to kill power to the motor while braking, and can cause a no-go condition if one or both are always engaged.

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