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Lunchbag
20 November 2014, 1721
On the Alltrax AXE to PM motor wiring diagram there is a note that reads:

"Note for motorcycle use: * Motor wires need to be 3-5' long, coil extra wire into 4" loops and zip tie together."

I asked a tech rep for the reason and the answer is, "The reason for needing the extra length is due to inductance. In the simplest terms, the controller needs time to handle the energy from the motor and this extra length of wire gives it that necessary time."

Any other opinions on this?

podolefsky
23 November 2014, 0945
I would assume they know what they're talking about and follow the instructions. Do they mean 3-5' total, including both + and - cables? In that case, it shouldn't be hard to come up with 3' total.

As far as why, here's my guess:

When you get off the throttle, the motor is still spinning and producing BEMF. That BEMF will result in a voltage at the motor terminals. The controller needs to change states to be ready for the voltage to appear - basically to switch the transistors to an off state - and that takes a bit of time. Since the motor is an inductor, that voltage appears at the motor terminals IMMEDIATELY, but the current lags. If there is a resistance between the motor terminals and controller, the voltage will show up at the controller slightly delayed.

A few feet of heavy gauge cable is probably only going to add a few microseconds (maybe less), but that might be just enough time for the transistors to switch off.

Stevo
24 November 2014, 1137
I did not wire mine with extra cable. Seems to work ok without it. Anybody here try it both ways???

Lunchbag
26 November 2014, 1932
Thanks for the great info. I assumed it meant 3-5' each cable, but I'm not certain. The note is only on the diagram for PM, not for series motors. Does that square with the theory?

podolefsky
26 November 2014, 2104
It makes sense it would be on PM and not series. A series motor will not generate BEMF when there is no current from the controller, since the field is created by that current. A PM motor will generate BEMF even without power, since the field is from permanent magnets.

lugnut
28 November 2014, 0713
Thanks for the great info. I assumed it meant 3-5' each cable, but I'm not certain. The note is only on the diagram for PM, not for series motors. Does that square with the theory?

Yes, because PM motors will have significantly lower inductance than series wound motors. Such a low inductance load will allow current to rise faster than the controller can tolerate so extra load inductance is introduced via the cables between the controller and PM motor. The cables don't need to be equal length and don't loop the pair in a coil as the fields tend to cancel. Best to fashion an air core coil in just one of the two motor leads, or use 3 foot leads with some distance between the two. Might be a couple of extra pounds of cable but your mosfets will be happier;-)

podolefsky
28 November 2014, 0859
If it's to add inductance, then why would you only need to do this for a motorcycle? If the cables are run straight, like in a golf cart, they won't add any more inductance.

Frank
28 November 2014, 1125
Does this help?

https://evmc2.wordpress.com/2014/11/24/wiring-up-the-alltrax-axe-moar-wire/

Stevo
28 November 2014, 1151
Bottom line... in plain english, what is the noticed effect of this amperage "spike"? Maybe the cheapo ME0909 motor I used has a high enough inductance that having a short lead is not noticeable
on my build. But when I finally upgrade my battery pack and motor, I will be building a 5' lead on one side and try a test with both short and long leads. Also, it's unclear on whether or not both + & - leads need to be long coiled leads, or just one side needs it.

lugnut
28 November 2014, 1152
If it's to add inductance, then why would you only need to do this for a motorcycle? If the cables are run straight, like in a golf cart, they won't add any more inductance.

Few, if any, golf carts use PM motors. The note doesn't say for motorcycles _only_. I think the motorcycle was seen as the most likely place a low inductance PM motor would be used. I think you'd benefit from the suggestion in whatever application the PM motor was used with that particular controller.

lugnut
28 November 2014, 1155
Bottom line... in plain english, what is the noticed effect of this amperage "spike"?

Maybe nothing. Maybe shorter life of the mosfets. Maybe instant mosfet failure. Could be they added that note for a reason.

frodus
28 November 2014, 1211
In talking to John at motenergy a while back, the reason is to add impedance. These motors done have enough impedance for the controllers to function correctly. It adds some inductance and some resistance, aka impedance.

podolefsky
28 November 2014, 1241
Bottom line... in plain english, what is the noticed effect of this amperage "spike"? Maybe the cheapo ME0909 motor I used has a high enough inductance that having a short lead is not noticeable
on my build. But when I finally upgrade my battery pack and motor, I will be building a 5' lead on one side and try a test with both short and long leads. Also, it's unclear on whether or not both + & - leads need to be long coiled leads, or just one side needs it.

If it is to add inductance (or impedance), you only need it on one side.

A Motenergy ME0708 has an inductance of 60 uH (http://www.motenergy.com/me0708.html). The description Ted's friend provided said 5 uH was fine, and you start having trouble at 1 uH.



Few, if any, golf carts use PM motors. The note doesn't say for motorcycles _only_. I think the motorcycle was seen as the most likely place a low inductance PM motor would be used. I think you'd benefit from the suggestion in whatever application the PM motor was used with that particular controller.

I agree, like I said in my first post if they say to do it, they're probably right.

A single loop of 2AWG cable, 4" diameter is about 4 uH. That should do it according to the guy from Alltrax. But then they should say you need 1-2 loops, not 3-5' of wire. You could have 5' of straight wire and it would make no difference in the inductance.

podolefsky
01 December 2014, 0929
This is the response I just got from Allrax:

"Adding a total of just 3-5’ of wire (the amount of coils are not important) allows the controller to have time to react to the back EMF being generated by the motor."


I asked if it would be OK to have no loops, response was:

"The coiling of the wire does help but isn’t absolutely necessary."


In other words, you don't need to add any inductance. It can help, but what is really needed is to is add resistance.

This is different from what Ted's friend was saying, so I'm not sure what to think. It does fit more with the explanation I gave in post #2 (http://www.elmoto.net/showthread.php?3815-AXE-Wiring-Note&p=45797&viewfull=1#post45797).

lugnut
02 December 2014, 0702
The 3 to 5 feet of cable will have inductance. While it can be argued that a straight length of wire has zero inductance, that is not the case here. There is a circuit made with the cable and that circuit will have inductance and generally speaking, longer cable will increase that inductance.

This is true on the source side of the controller as well; between the controller and battery. The inductance of the supply cables can cause voltage spikes on the controller bus. For this reason, controller manuals often recommend keeping the cables from the battery short, routed near each other and even twisted together in some cases.

So on the motor side of the controller where you want inductance, long cables help and should not be routed together.

podolefsky
02 December 2014, 0900
Interesting, I never thought about that. Turns out 3' of typical motor cable will have an inductance of about 1 uH. I figured that would be negligible compared to a motor, but the guy in Ted's post was saying some motors are as low as 1 uH.

Which PMDC motors out there are that low? Motenergy 0708 specs says 60 uH, and 1003 specs say 93 uH.

lugnut
02 December 2014, 1020
Which PMDC motors out there are that low?

No personal experience but I've heard bitching about low inductance on large Agni, Lemco, Lynch, etc. And if any were to be found, ironless would likely be low on the list.

TonyHelms
02 December 2014, 1054
Which PMDC motors out there are that low? Motenergy 0708 specs says 60 uH, and 1003 specs say 93 uH.

So with the Montenergy motors being this high already is there any reason to add 3~5uH by adding wire and/or coils?

podolefsky
02 December 2014, 1127
No personal experience but I've heard bitching about low inductance on large Agni, Lemco, Lynch, etc. And if any were to be found, ironless would likely be low on the list.

Makes sense. Lynch D126 (http://lynchmotors.co.uk/pdfs/lmc-lem-200.pdf) is 5 uH. The rest of their motors are higher. I couldn't find Agni or Lemco, I guess it's possible one of those is around 1 uH.

It would be nice to know, since adding several feet of cable is going to be a bit of a pain. Since there are tons of people running Alltrax / Motenergy and everyone says the combo is rock solid, I would guess it's fine.