Need to limit 1000w system to 750w easily.

[drutledge]

I currently have a brushless motor rated for 1000w on a homebuilt tricycle. It has a controller rated for 28 amps and 3-22ah 12V batteries wired in series for 36V.

Is my thinking correct that the easiest way to limit the output of this setup to 750 watts is to swap out the controller to something rated at say 20 amps? If I took a battery off to cut the voltage to 24v that would trigger a low-voltage cut-off in the current controller I'm sure.

This is to get the bike legal with the local police. I don't really care about the lower output since it's really more than enough right now at 1000 watts. I don't want to switch the motor which, as I understand it, wouldn't make a difference anyway. (Correct?) Right now I have a potentiometer that limits the throttle signal when kids (or adults who look too confident) ride the bike; I'd like to be fully 'legal' though since the trike looks like a cupcake and has attracted the local police's attention.

 

[vanilla ice]

Is that a standard or reverse trike?

Did you put a meter on that cupcake? You might already be putting down less than 750..

 

[dogman dan]

20 amp controller should get you into the ballpark. Often, they are actually 22 amp. Another option could be to snip a shunt wire, if it has two or more.

Lastly, installing the throttle in such a way as to prevent use of 100% throttle can be possible, with thumb throttles.


But really, you can't just tell em it's a 750 watt? Will they have a wattmeter?

 

[drutledge]

The trike is in a standard configuration, one front - two rear.

It has a "Watts-Up" meter that peaks at about 1500 watts under full acceleration, fully laden with the 28 amp controller - which begs the question, how do they rate these motors/controllers? Not peak watts or amps obviously. Is it under no load, full throttle?

The bike may have to go to the Secretary of State's testing facility which will have the means to actually test the wattage evidently.
Unlike most eBikes, this one does not fly under the radar and now must be deemed legal by the powers-that-be if I want to ride it to and from my friends'.

Just to clarify: motor ratings are fairly meaningless right? You can over volt/amp a motor within reason and it won't blow up anytime soon. The motor on the bike is sold as a 1000W motor. I wanted it more for the nice mounting bracket it had than the wattage. I'm guessing that if it has 36V and a sub 20amp controller and thus shows <750 watts of 'nominal' output that I'll pass at least that test.

 

[John in CR]

To start with, just put a 750W sticker on it. When you go to have it tested, just bring it in slightly above the LVC of your controller.

Screw those silly rules anyway. Cars aren't limited by hp, just the speed limit. Don't accept being singled out differently without at least some fight.

 

[dogman dan]

Ok, gonna be tested. Glad I don't live there. So get a weak controller and put it on the bike for the test. Something like a 36v 15 amp should do the trick for sure and pass the test. After the test of course, a 5 min job to return it to 28 amp controller.

I don't know if the testers would accept a CA limiting the watts, or a euro wattage jumper used that you simply unplug later. A weak controller is likely to be cheaper than a CA or any other method of limiting, except for snipping a shunt. That is still an option. If it has two shunts, snip one near the end. Then solder it back later.

But a weak controller may be easiest, and quickest to go back and forth to as you require it for the dumb vehicle dept.

 

[tuxman]

Love your cup cake. Do you have a matching helmet?

I see two options.

1) swap out the controller. Your motor is rated up to 1,000w nominal and will work fine at 750w.
2) add tail lights, head lights, and turn signals and call it an electric moped. Depending on where you live, you may need a tag, drivers license, and/or insurance.

 

[vanilla ice]

Where do they measure the power? At the rear wheel(s)?

What state are you in? Have you checked the laws? There may be other limits like mph also.

 

[tuxman]

Where do they measure the power? At the rear wheel(s)?

What state are you in? Have you checked the laws? There may be other limits like mph also.

and brake inhibits too.

 

[drutledge]

Thanks guys. Just ordered a smaller controller to swap out for the 28 amp one.

In answer to the other questions:
-The cupcake only goes 16.5mph now, well under the 20mph limit on ebikes here. Has brake inhibit.
-Don't have a matching helmet.
-No idea how they measure the wattage, probably just with a simple meter off the motor wire is my guess.
-I'm trying to keep it as 'cupcake-y' as possible so lights and such are out. Unless I could make them very temporary.

Thanks.

P.S. Can anyone point me to how motors are actually rated? Is there some sort of standard such as 'at XXX watts a motor should run continuously for at least 10,000 hours' etc. What determines a motor's rating. Manufacturer's best guess/marketing?

 

[jonathanm]

The bike may have to go to the Secretary of State's testing facility which will have the means to actually test the wattage evidently.
Unlike most eBikes, this one does not fly under the radar and now must be deemed legal by the powers-that-be if I want to ride it to and from my friends'.

wow, bad luck....how did this situation come about?

EDIT: oh, I thought you were joking with the cupcake until I reread the post, lol

 

[grindz145]

A CUPCAKE! Don't bother with the 750W thing, find a sticker that says 750w and stick it on the motor. It's close enough. It's not like you're hot rodding it like crazy anyway. The police really don't know what the power is, and if you're reasonable about the way you ride (or whatever) that cupcake, you have nothing to worry about.

 

[ColinB]

Motors are typically rated by how much power they can dissipate. It is my understanding that you can keep feeding more and more power to a motor until it:
- overheats, cooking the magnets, wires, etc
- spins too fast.

So one manufacturer may be very conservative, and others may be less so. Which makes it tough to compare. The best thing is when a manufacturer says motor x makes X Watts when run at x current.

I think for your testing, you will have to:
- switch to a lower rated controller
- switch to some tired batteries (maybe old lead acid?) that can't deliver much current
- apply some other type of current limiting
- switch to a different motor

 

[Miles]

P.S. Can anyone point me to how motors are actually rated? Is there some sort of standard such as 'at XXX watts a motor should run continuously for at least 10,000 hours' etc. What determines a motor's rating. Manufacturer's best guess/marketing?

 

[drutledge]

Ok so I got my cheap 700 watt controller from monsterscooterparts.com (rated 14 amps/36volts which seems more like 500watts but whatever) and attached it to my cupcake and took it for a test drive and...

No difference. Speed, as measured by a bicycle computer, is exactly the same. At peak it pulls 1270 watts at 36 amps which rings a bell as being the same as before; unfortunately I didn't write down the old max amps/watts, but either way it's still way too much.

These controllers are meant for things like Razor scooters. They might not have a LVC. Perhaps if I just went to 24V that would do the trick; or maybe it would just increase the current the motor pulls.

So what's up? Is it not the controller that limits the current going to the motor? Will a '1000 watt' motor always pull about 1000 watts and using a smaller rated controller only risks frying it? If so, the only easy solution might be to 'cheat' and sneak a resistor into the throttle voltage wire so it doesn't go to max signal voltage. I have no desire to actually switch motors.

Any ideas? I'm very confused here. If you can add more voltage/current/wattage to a motor than its rated for (within reason), can you not limit a motor's output to less than rated output?

 

[liveforphysics]

Your state dept and/or police can't test your bike for anything but top speed. They do not have the equipment.


If you're in the USA, it's done by the continuous output rating of the motor. This means at low RPMs, you can dump 10's of KW's into the hub and be perfectly legal, as the output won't be close to 750w. Secondly, it just says the rating of the motor, and mentions nothing of what you send into it for get out of it for your application, just what it's rated for. So, in the USA, if you put a 200w controller on a bike or a 5,000w controller on a bike, it has no effect on the motors output rating.

 

[TylerDurden]

If you can add more voltage/current/wattage to a motor than its rated for (within reason), can you not limit a motor's output to less than rated output?

You can llimit the power into the motor. Your smaller controller should have done that. :?

So, throttle hack will work, or take a segment out of your pack.

 

[Tommy]

Maybe the cupcake should be a birthday cake, complete with banner that reads something like
"Don't Support Terrorism Go Electric"
or
"Do Your Part! Go Electric"

no red necked cop would mess with you.

I could be wrong.
Tommy

 

[Pure]

I'm dying to know why you chose to ride a cupcake?

 

[TylerDurden]

I'm dying to know why you chose to ride a cupcake?

[youtube]LR2LtV6RRTc[/youtube]

 

[DrkAngel]

The "watt" rating, on an electric motor, is a measure of a motors power output!
Not the input watts!!!

A "750w motor" is equal to 1 horse power. (750w = 1hp)
The motor might require 1000w input to produce 750w output, this would mean that the motor is 75% efficient!
Various motors attain between 75-90% "peak" efficiency!

Controllers vary widely in efficiency, also!
"Peak" efficiency is between 80-95%.

Batteries vary widely in efficiency, also!
eg. SLA batteries expended at 1C are 60% of rated, Li-ion at .35C are about 100%+.

750w output might require

750w/75% motor efficiency = 1000w/80% controller efficiency = 1250/60% SLA efficiency = 2083 watts used!

 

[TylerDurden]

750w/75% motor efficiency = 1000w/80% controller efficiency = 1250/60% SLA efficiency = 2083 watts used!

I rekon battery "efficiency" only matters when accounting for the capacity... Not really applicable to a "legality" discussion.

 

[DrkAngel]

750w/75% motor efficiency = 1000w/80% controller efficiency = 1250/60% SLA efficiency = 2083 watts used!

I rekon battery "efficiency" only matters when accounting for the capacity... Not really applicable to a "legality" discussion.

"Legality" is only a function of the motor output.
If the manufactures rating, for the motor, is 750w, the controller and the battery voltage is oem, then the input watts, is irrelevant!

I can't believe any police dept putting an ebike on a dynamometer.
Explain to them that 1000w input, into a 75% efficient motor, produces the legal 750w motor output.

Yeah I just got carried away with demonstrating the power losses due to the inefficiencies of ebike components.

 

[drutledge]

For what it's worth, the Illinois law (where I am) is amended to match US Federal definitions of a 'low-speed electric bike":
Defines a low-speed electric bicycle as a 2 or 3-wheeled device with fully operable pedals and an electric motor of less than 750 watts (one horsepower), whose maximum speed on a paved level surface, when powered solely by such a motor while ridden by an operator who weighs 170 pounds, is less than 20 miles per hour. Excludes low-speed electric bicycles from the definition of "motor vehicle".

And just to verify, my 36V / 14Amp controller SHOULD have throttled back the readings I was getting, right?
I could use an FAQ for noobs on controllers. Can someone point me to one here or on another site? I keeps seeing people refer to Mosfets (?) and other things.

 

[John in CR]

That's a brushed motor, right. That 1200+W draw that you're measuring from the batteries is nowhere near 750W output at the wheels, especially at start-up where efficiency is well below 50%. That's not including losses in the drive line after the motor and a few % in the controller, so you're actually already legal. Add a 750W sticker like I said before.

If you're really that worried about it, bring it in for testing with a low battery, which will deliver the current at a lower voltage.

Another idea is to open up the controller. If one of them has multiple legs on the shunt, which is likely at that current level, just clip off one of the shunt legs and that will drop the current by that percentage. ie going from two down to one shunt wire will cut the power draw allowed by the controller in half.

16mph seems a bit fast for a cupcake, so maybe gear the motor down a bit for lower speed and less attention.