Whats limiting me, motor or controller

[dcastillo]

Whats the easiest way to determine what is limiting my power to the wheels (stated another way, which should I replace for more power)

I have a 200A controller (80 cont) and a 2kW golfcart motor...

As powerful as it is, I want more... even after hard driving, neither seem to get very warm (though my wires do )

Can a standard DC ammeter read the current to the motor properly sense its pulsed? Or would I need a more complex scope to get an accurate reading?
thanx for any help
Danny

 

[dcastillo]

Forgot to add, its a 36V motor that Im running on 48V at the moment....
Darn thing kills my 19AHr batteries in barely 15min!!!!

 

[itsid]

2000Watts @48V that's not even 42Amps

So the motor can drain as much Amperage as he likes...

You can provide four -almost five- times the Amps with your controller;
enough to power a 9.6kW Motor

'sid

PS Or in short: the motor

 

[dcastillo]

I understand the math, but a little confused on the real world application/results...

I know for a fact Im drawing more then 42 amps... heres a few more bits of info

-one my 10AWG wires going from the batteries to the controller get hot (10AWG rated at 55A)
-the motor doesnt get warm at all no matter how hard I push it
- my cheap 100A digital ammeter from China I mounted in the system hit a hundred and went to dashed lines when in series with the motor
- when in series with the batteries it spikes near 70A on hard accelerations (which isnt perfectly accurate since I has a 60A fuse inline that hasnt blown yet.

I guess what Im not clear on is how the power draw on the motor works in the real world. I know its rated at 2000W, but the EE in me knows its basically a short and has the lowest resistance at start, its definitely drawing more then 42A (2000W/48V) when I floor it... Im just not smart enough to fully understand how that all calculates out

From my simple minded view, its a motor designed to run on 36V and push a 4 person golfcart (probably at least 1200lbs)... I shoudl be able to smoke my tires on demand...
Or am I expecting to much?

As usual, thanx for the insight Itsid

(heres a quick vid driving with my son on wet asphalt, could spin the tires some, but was hoping for more... though they were a little flat and all my weight is in the rear of the cart which doesnt help)

http://youtu.be/DCQGmnWVjAI

also the current drive ratio is over 10... so I may not even have a high enough topend to really spin the tires with that ratio...
Danny

 

[itsid]

Yes, I cheated... motors for electric vehicles are rated in mechanical poweroutput.

So you indeed have 2kw of power on the motor shaft.
(whereas a RC motor for example is rated in electrical power INPUT, the motor draws a max of 2kw of energy from the batteries)

So it's true, that your golfcart motor will in fact draw more like 2.5kw peak and a max of maybe 50-55 Amps;
and convert that additional input into heat.

First beef up your wiring to something rated at least 70Amps to keep it cool.
It's safe to assume that your motor will be around 80% efficient at least (in its recommended range) normally that's more like 85% and peaks at 92% or something.
But 80 is nice to calculate
plus it adds a a bit of safety;
when it comes to wirings, take 150% of what's needed, the hotter the wiring gets the more brittle the copper will become. And you want it to stay soft

Anyways, it's still perfectly okay to calculate motors like yours with the numbers you know.
Most of the time a motor will be well below it's max potential thus draining less than its peak Amps
(that's why I felt confident cheating here )

The moment you really hold it at "WOT" for a period exceeding 5mins straight, that's the moment where you'll need to know it's actual powerdraw.
And if you don't, it's safe to think something about 80% is powerout,
you'll cover almost all ev motors that way.

Small RC motors is a different story...
Some motors are nasty, due to the winding schematics and internal resistance and whatnot they can and will be no more than say 45% efficient at their max speed, more than HALF the provided energy is converted to heat not mechanical power.

But luckily that's not really important for you

'sid

 

[dcastillo]

That for the details
so i guess my golfcart motor, which has a date stamped on it of 1982 (almost as old as me) needs to be replaced then...... guess ill have to keep my eye out for a slightly more powerful motor then... 3kW will probably get my the power Im looking for..
Been planning on beefing everything to either 8AWG or double 10AWG (since my quick disconnects only go to 10AWG)... just havent had time yet...


Just to make sure I understand how this works...
If I have the option between two (say) 3kW motors, but ones a 36V and ones a 48V, I should get the 36V since I can then power it with 48V to get even more power out of it right?

 

[itsid]

That for the details
so i guess my golfcart motor, which has a date stamped on it of 1982 (almost as old as me) needs to be replaced then...... guess ill have to keep my eye out for a slightly more powerful motor then... 3kW will probably get my the power Im looking for..
Been planning on beefing everything to either 8AWG or double 10AWG (since my quick disconnects only go to 10AWG)... just havent had time yet...


Just to make sure I understand how this works...
If I have the option between two (say) 3kW motors, but ones a 36V and ones a 48V, I should get the 36V since I can then power it with 48V to get even more power out of it right?

theoretically yes.

practically well.. uhm

The thing is, a motor intended to run on 48V runs much better than a motor running @48V that's intended to run on 36V.

It's again about efficiency here;
different winding, different awg and wirecount per coil.
I cannot explain the details, mainly because I still do not fully understand myself how to calculate motorwindings properly

What you're after is the mechanical power of course,
Overvolting does not only provide more power, it'll also reduce the efficiency.

Say a 3kW motor for 36V will draw ~104 Amps (80% again, drawing 3.75kW)
and it'll draw about the same 104Amps on 48V;
But instead of the expected 4kW it only produces something like 3.4 kW mechanical power whilst drawing almost 5kW,
reducing the efficiency to about 70%.

That's an example only,
it all depends on the specifics of the actual motor of course
(wire gauge, number of windings, magnets, electrical as well as magnetic resistance etc...)

So yes, most (nearly all) motors can be overvolted without any harm, but it's not the best idea you could have
keep in mind, everything that's not power output is converted to heat and/or mechanical friction.

So if you want to run it at 48V find a 48V motor with a little more powerout instead.


'sid

 

[dcastillo]

That actually competely made sense.... thanx