edit: Please see the comments at the bottom of the page. Apparently, nothing is simple when everybody wants to be an expert – even people with no actual experience with a product. Suffice to say, I’ll stick with talking to the people I know are experts.
If you’re using a wiring diagram you’ve downloaded from Alltrax recently, you may notice a little text box that wasn’t on the older wiring diagrams. Looks like this:
How cool is that, that they’re acknowledging the existence of us motorcycle builder guys, and not just figuring the only people using their controllers are running golf carts? But wuuuut? I’m pretty sure my motor wires are shorter than 3′, in fact, I tried to make them as short as possible to keep the current flowing without heat from resistance. As if I’m even going to get resistance from AWG 2.0 cable, but still, be neat, right?
Well, I have a friend who is at Alltrax* and I asked him what the deal was with this. Was it correct? …and why? Here’s the response I got:
Ok Let’s see if I can put this in plain English.
This is one of those complex topics that has 3 or 4 different things all contributing to it. The Controller is a PWM (pulse-width-modulation) switcher so the transistors turn on and off at some frequency, in our case 18kHz or 5 microseconds, as a way to vary the voltage. The length of time the transistors are on or off determines the average voltage across the motor. PWM is measured in the amount of time the transistors are on, or “duty cycle”. So at 50% duty cycle, the transistors are on half the time, 80% is on 80% of the time and so on.
Part two – motors are big electro-magnets. When you apply voltage and current to a motor field you get a magnetic field (based on the wire turns, current and a bunch of other factors). The inductance of the motor depends on a lot of different things but, bottom line, each motor acts like a small inductor. I explain inductors to people as capacitors for magnetism. It is not a perfect analogy but its close enough to explain what is going on here.
So when the PWM is ON, power flows to the motor and creates a magnetic field in the motor. The interactions of this field with the field or stator creates motion.
When the PWM turns off, that’s when the fun starts. You now have a lot of energy stored up in the coils of wire in the motor (inductance). With no more power being supplied to keep the magnetic field going, it under goes a field collapse, commonly called Back EMF. This pulse of voltage and current can be a large spike that the controller sees. A motor with low inductance will want to discharge all of its energy at once, while one with a higher inductance will bleed off a little more slowly, and the faster it discharges, the higher the change in current over time (di/dt).
When the AXE controller was designed, PM motors had inductances of about 5 uH (micro Henries) at their lowest, so the current limiting was designed to handle that. As time went on materials and design got better and some motors got down to about 1uH. This causes the di/dt to be 4x times higher than other motors and it causes the controller to current limit on each PWM cycle, but it’s not real current going to the motor. It’s basically noise coming from the motor. The real current going to the motor might be 100A, but the inductive spikes might be upwards of 300-500A for less than 0.5 microseconds.
The coils of motor wire in the loop act like a small inductor that slows down the field collapse rate and snubs the inductive spikes to a level that won’t trip the current limiting on the controller.
The newer controllers the SPM and SR Families of Series/PM controllers won’t have this issue as the current limit was designed to handle these inductive spikes and ignore them.
Hell. I even understood that.
By the way, if you’re looking for that wiring diagram, the link is here: Alltrax AXE to PM Motor
* It’s come to my attention that some online pundits don’t seem to consider my “friend from Alltrax” to be a particularly knowledgeable or reliable source. For the sake of his inbox I have not released his name, but suffice to say he knows what he’s talking about. He is upper-level management, helped design the AXE controllers, and isn’t some help-desk support guy answering emails from customers – or in this case, not even a customer. mmmmK? Sorry if this seems snarky, but the whole point of this post was to separate the speculation and bullshit from the facts.
In any case, this doesn’t seem to be a huge deal – and a comment on the diagram that is attempting to provide a solution for a wide array of applications with a single, simple suggestion.
Also, somehow some people have questions on what will happen if you don’t wire this the way Alltrax suggested, in spite of the fact it was pretty clear in the above explanation. Here’s what was said regarding that: “…it causes the controller to current limit on each PWM cycle…”. So. You get the controller thinking there’s more current than there really is, and it will “trip the current limiting on the controller”. So, do it right, use moar wires (in a coil), get moar current. Right?