Limiting max speed by voltage or current limit???

I'm looking to maximize efficiency of a hub motor that goes too fast at 72V, but it's easier for me if I maintain that voltage. The factory rep of the motor factory suggested limiting the current to limit the speed. The controllers have two bars, so halving the current is easy, and after thinking about it I wonder if there is really any difference between limiting the current in that manner vs changing my packs and changing the controller's voltage regulator section to run on 36V.

Assuming I get the speed I want running WOT using the same controller at 36V or limiting the current to half and running at 72V, how will the motor react in terms of efficiency? At the same speed the motor's BEMF will be identical, so I think an identical amount of power will be drawn by the controller. The brushless commutation should be identical, and at WOT I think the controller in either setup will run at full duty cycle, so identical efficiency, but I'm unsure of that aspect. I'm also unsure if the motor will really see anything different, and if so, run at a different efficiency.

What do you guys think? Am I likely to get identical power in and identical power out?

Happy holidays!

John

Controlling speed with current won't work.

I'm Oversimplifying a bit, but a motor's rpm is determined by voltage, regardless of current. Current gives the motor it's thrust, for acceleration and hill climbing. Unless your controller is grossly under rated, you won't be running at max current at max speed. halving the current will likely only result in poor acceleration and hill climbing.

But halving the voltage would work fine. The controller runs more efficently at max throttle. and while motors run more efficent at higher voltage, That realy only applies at max speed as well.

John,

At a given high speed what you need is POWER.. not only high voltage or high current.

Let say you have a 5305 on a 26" wheel at 100V and 100A and reach 75kmh

In this case if you want to go faster you will need higher voltage. That 5305 is so powerfull then even though you are at 75kmh, th BEMF is still close to the battery voltage!

But if you use a 5302 in the same conditions, you will reach the 100A Before reaching 75kmh so you will need more current

It depend on the motor winding! or... KV

with minimal load it's the voltage that determine speed.

When I say you need POWER instead of just voltage or current, I mean that what make the current or voltage perfect match for your desired speed is the motor winding (KV)

(High KV motor + 100A and 24V) = ( Low KV motor + 24A and 100V)
Doc

Limiting current can be used to reduce top speed; for example when I ran my 24V GM mini at 48V with a 22A controller I could hit 29 mph but with a 15A controller the bike topped out at 24 mph. I ran some simulations at ebikes.ca using an eZee 20" motor, the closest thing to my 24V Mini, and it doesn't look like efficiency suffers, the curves all just get really flat. That said the reduced current will of course decrease your low-end torque therefore you might consider a throttle limiter, which could be electrical or mechanical, and you'd still have full power from a stop but top speed would be limited.

-R

There will be some loss of efficiency running the motor at reduced speed, but it's not too bad. Going downhill, you'll be able to reach a much higher speed.

fechter said:

There will be some loss of efficiency running the motor at reduced speed, but it's not too bad. Going downhill, you'll be able to reach a much higher speed.

Click to expand...

Thanks Fechter,

I'd like to understand the why. I understand that the efficiency drops for speeds below the maximum efficiency point for a given voltage, but that assumes you allow the current the motor asks for to flow (reduced bemf, so more current). This is quite different. Assuming I hit the target speed on flat road with either cutting the voltage in half or limiting the current by half (maybe a false assumption), is the controller going to do something different that the motor "sees" as a real difference? The brushless commutation will be the same, the bemf will be the same, so will the pulses of electricity coming from the controller be different? If I'm able to get to identical power from the battery for equal speed (identical work), where are the extra losses using current limiting as the speed limiter? Does the controller lose more (a % guesstimate???), is the motor less efficient, both, or maybe neither?

My motor gets me past 50mph with my current packs, but I want to reduce that to about 25mph for a long-range attempt. A few percent difference won't matter, but a 10% or more difference would, so I want to understand the potential effects of my choices. The higher voltage would be nice, because then toward the later stages I could increase speed (using a secondary controller) if I've overestimated wind and mountain climbing energy requirements, or underestimated regen on the downhills (I'm budgeting 0 for regen).

John

Do you have an infenion? If so program it to where it limits the WOT to a certain percentage. At 88v on my GM 60% max throttle programmed value gives me 25-30mph depending on SOC of my battery. It limits current when going full speed in that programmed mode and you have full torque on start/accceleration and on hills also.

If you dont haven infenion it maybe worth the time to set up some resistor switch box that pulls down the throttle voltage to a certain level which will do the same a programmed controller does. I think this should/could be a marketable mod that steveo or methods could make quite easily. Maybe pot this in some epoxy. And put some connectors on it. Have 33/66/100 throttle resistor combinations and have it on a single pole triple throw switch to select you mode. This can help control high power setup.

Then again this could introduce slew of problems for other people. Good alternative to the cycle analyst tho for controlling speed and amps for literally less than 5 bucks in parts. Its not automatic but its easier than trying to regulate a throttle vs just going WOT and have the resistor mod limit your WOT speed.