How to limit speed or wattage?

[rg12]

Hey Dudes,

I want to sell a few custom bikes (high wattage) and I want to do what stealth bikes does with the bomber where they sell the bikes with a jumper wire to remove the 250w limit.

Is there a way to do this without a cycle analyst? either to limit to 25kmh or to limit the wattage to 250w.
Maybe some king of a resistor in the controller?

 

[flangefrog]

Yeah, just add a resistor that changes the voltage divider from the shunt to the microcontroller. You could even just plug in a wire that shorts one of two parallel resistors on half of the voltage divider.

There's a voltage divider circuit on this schematic (above "fast current limit" and "average current limit") http://www.avdweb.nl/Article_files/Solarbike/Motor-controller/China-BLDC-motor-controller-36V-250W.pdf

 

[rg12]

Damn, that is Chinese for me...well I know what a resistor and a few other components look like on a schematic but this is a bit complex especially when this is not a schematic of my own (Chinese) controller.
Is there a way to simplify/explain this to a dummy like me? I really need this one...

 

[flangefrog]

The schematic is of a generic chinese controller, I doubt yours is much different. Read up on shunts and voltage dividers:

http://www.rc-electronics-usa.com/current-shunt.html
https://learn.sparkfun.com/tutorials/voltage-dividers

The shunt voltage is scaled by a voltage divider before going into the microcontroller input (CPU-31 and CPU-41). Change the division factor and the input voltage into the microcontroller will be changed by the same amount. This will lower the power (unless the shunt is on a motor phase in which case the phase current would be lowered which is not what you want).

 

[rg12]

Thanks alot, will read this later on tonight.

Why not lower current? it's always better to have a higher voltage and lower current than the opposite right? (heat wise etc...)

 

[flangefrog]

Thanks alot, will read this later on tonight.

Why not lower current? it's always better to have a higher voltage and lower current than the opposite right? (heat wise etc...)

Microcontrollers can only read voltage. This is why we use a shunt to convert the current to a voltage. The voltage is then divided by a certain factor before being fed into the microcontroller. So we're only (directly) changing the voltage of the microcontroller, not the output voltage of the controller. The microcontroller will read the voltage on the input pin and multiply it to get the actual current. Now it knows the current flowing into the controller it can limit it to a certain value.

Higher voltage and lower current is not always best. The mosfets used in controllers are most efficient within a certain voltage range (about 50V to 100V). Use half the voltage and you just need twice as big wires/pcb traces and mosfets with half the resistance (lower voltage mosfets do have less resistance) for the same efficiency. And when talking about the motor, you just need one with half the length of twice as big copper (same copper fill/weight).

The statement is only true when you're talking about a particular object. e.g. for 100V rated controller, it will always be more efficient at 100V than 50V. But a 50V rated controller (with half resistance fets and wires/traces twice the size) will be just as efficient at 50V as the 100V one at 100V.

Even if we were limiting the output voltage of the controller, well, it doesn't really work that way. The controller is sort of like a switch-mode buck converter. It takes a certain voltage and current, and makes a lower voltage and higher current in the motor. The power (A * V) going into or out of the controller is always the same. You can drive a certain motor with 100V at 10A (1000W) or 50V at 20A (1000W) and you'll get exactly the same performance (although half the top speed with 50V, as most controllers only make voltages lower not higher). Note the currents in this paragraph are battery currents. Battery current is the current going into the controller, phase current is the current coming out of the controller. The example in this paragraph could have 20V 50A (1000W) phase voltage and current for both the scenarios. The ratio of phase current to voltage will change based on the motor's RPM.

 

[rg12]

So if the fets work at a certain minimal voltage wouldn't it be a problem lowering a lets say 16s controller to a 3rd of the voltage (that's about what I need)?
Wouldn't it be a problem having about 30A and 22v? heating and stuff...

I looked up the page about the voltage dividers posted flagefrog and it's very helpful (thanks man), and I started messing with calculator and in order to get a 3rd of the voltage I need to have two resistors: one that is a certain resistance and the second one half the resistance of the first.
So should I just choose the ohm rating randomly and get another resistor with half the resistance? like 3300ohm and 1700ohm? which are common ohm numbers so I can get exactly half for the second resistor? (looking up eBay)

How important is the resistor's wattage? I see some 0.25w and some 1w (which seem big)...what exactly do I need?
Are there other things I need to consider while buying resistors? what is all that 1%, 5%, carbon etc?

Now the biggy for me...how do I find the place to stick my "super special voltage divider circuit"? I need to find that IN and OUT area to connect into my controller...
Is there a common physical place that it should be at when looking at the PCB? like a spot near a certain big cap or something?
Let's say this is my controller:
https://i.ytimg.com/vi/1PotpG-cVoU/maxresdefault.jpg
which it isn't but I will go through the hassle of uploading a pic of my controller's PCB if it's important to be very specific about the type of controller etc...

Thanks alot so far,
Roy

 

[dogman dan]

Controller with two speed, or three speed switch.

Back before displays that have 5 speeds,, lots of controllers had a jumper wire to enable European mode, 250w operation. So secret switch on that wire, if you can still find a controller that old school.

No doubt the geniuses here can also put that feature on many stock controllers. But that's beyond me. I'd just look on ebay for a controller with two wire speed limiter plug on it.

Also,, you can just put a switch on the throttle wiring, which puts the throttle signal through a resistor that lowers the voltage. Methods used to sell a three speed throttle tamer that worked that way. simpler that messing with the controller board imo, and completely external so it works with any controller that runs a 5v hall sensor throttle. Methods used adjustable pots for resistors, so it was completely tunable. That was very cool I thought.

 

[rg12]

Most high wattage controllers don't have the speed limit wires.

I do have the 3 speed wires (3 wires), so you say connecting the resistors somehow through those wires?

 

[Voltron]

Do you mean that your controller already has the wires that would hook up to a three speed switch?

 

[rg12]

Yes but it's usually something like that:
1 - 90% top speed with throttle that is easy to control from 0 speed
2 - 100% top speed with a very "jumpy" start from 0 speed (delivers alot of power with the slightest turn of the throttle)
3 - same as 2 but spins at 120% only when the wheel is in the air and not exactly the same as 2 when I ride the bike

 

[dogman dan]

Go back and look up methods throttle limiter setup. Just regulates the throttle signal. Easy to copy. It will be pretty far back, like 2009 or so. I'd hunt in the for sale section.

It's just a switch that selects which resistor the throttle current passes through on the way back to the controller. Simple.

 

[rg12]

So I can just take the let's say "50%" wire out of the 3 wires and add a potentiometer through it and fine tune it to see what is the best resistance I need and then buy a resistor that matches that resistance right?
* I'm not using the 3 speed switch

How important is the resistor's wattage? I see some 0.25w and some 1w (which seem big)...what exactly do I need?
Are there other things I need to consider while buying resistors? what is all that 1%, 5%, carbon etc?

 

[dogman dan]

You aint getting what I'm saying, if that reply was for me.

methods does not use the 3 speed switch wires.

It's just resistors on the wire to the throttle. the throttle signal back to the controller. The throttle still regulates voltage, from 0-100% of 5v. After the throttle, pot resistors cut that signal in half, or whatever you want.

Not my product, I have no idea what he used. Look it up in the archives.

 

[rg12]

I get what you are saying but if the 3 speed wires also cut voltage, why not use them instead in order to have the PAS work with the limit also?

 

[dogman dan]

Yes, if you have controllers with three speed, use that. If you don't I was suggesting another way without a CA.

 

[d8veh]

Just a reminder in case you're in Europe or Auistralia. The limit is not 250w. Instead, you can only use a motor rated at 250w continuous output power, but you can push as much power through it as you want. You cannot use a motor rated at 1000w and then only push 250w through it. That's illegal, so no point in restricting the speed.