motor power explanation (voltage,current, etc)

[dcastillo]

I feel dumb asking this since im an engineer, but I typically work on much smaller hi tech things, never motors...
A coworker has tried explaining it to me but it just hasnt clicked yet. I learned in the past that having someone else explain things a little differently sometimes helps, so heres my confusion...

the power of the motor...
I understand the power/torque comes form the current and the motor requires the most when accelerating, especially from standstill.
but Im having a hard time incorporating how the voltage affects the equation... I saw a youtube of a guy with, I think a 12v setup but was pulling a ton of amps for power, why 12V over 48V...

maybe someone has a link to a short explanation that might help me understand?

thanx for any help
-Danny

 

[itsid]

Well the basics first:

Voltage * Amperage = Power
so
12V * 100A = 1200W (Watt)
or
24V * 50A = 1200W
or
48V * 25A = 1200W

Now you know the power of the motor, (1200W ~= 1.61 bhp)

The torque comes from the current is partially right.
Why partially? because it comes from the magnetic force it can produce,
a copper wire with a larger diameter can not only withstand a higher current, but it also has a lower resistance, thus generating a greater magnetic force, and as a result in an electric motor produces more torque (with very few exceptions).
That's why you can shorten the term for an electric motor to "more current equals more torque"
(Note: not more torque than an equally powered motor but an equally wound motor with a smaller dia wire)

Why choose 12 or 24V instead of 36 or 48V ?
Well mostly because of batteries!
If you want to apply that much current (say 100A) you can either run ONE 100AH batterie for an hour @ 12V (1200W) or run FOUR 100AH batteries at 48V (4800W);
I think it's clear that you don't want to carry the extra three batteries if you don't have to

Anyways...
Any motor with about the same output power in Watts, should produce about the same max torque too.
more current produces only a wider powerband than more voltage, that's about it.

Almost the same as a bigger cylinder has a wider band than a faster reving smaller cylinder with the same power.

'sid

PS If we're talking about the same guy on youtube he has a 24V truck starter motor with 6.7kW using two 100AH batteries.

 

[dcastillo]

thanx sid....

following your example of 12V at 100A or 24V at 50A producing the same power (which i fully understand), is it the motor you use that then determines how much current it will draw (requires)?

meaning if you took the 24V motor drawing 50A, theres no way to put 12V on it and make it draw 100A correct? That characteristic is based on the motor, which means you need to research the specs on the motor to determine how much current it will/should draw at its rated voltage?

one last question, if you take the 24V 50A motor and put 36V on it, will it then draw more more and thats how you get more torque?

(btw, im trying to figure this out cause I just put the 90V 21A treadmill motor on my cart and will only power it with 48V, trying to figure out the power characteristics of the setup)

 

[itsid]

thanx sid....

following your example of 12V at 100A or 24V at 50A producing the same power (which i fully understand), is it the motor you use that then determines how much current it will draw (requires)?

correct, the motor can draw a specific amount of current based on it's windings (or to be precise: inner resistance)

meaning if you took the 24V motor drawing 50A, theres no way to put 12V on it and make it draw 100A correct? That characteristic is based on the motor, which means you need to research the specs on the motor to determine how much current it will/should draw at its rated voltage?

Exactly, you know the current it can draw if you know it's rated power draw* and voltage based on the same equation
Amps = Power/Voltage.

*rated power can be drawn power or output power which is a little less (parasitic loss)

one last question, if you take the 24V 50A motor and put 36V on it, will it then draw more more and thats how you get more torque?

You can increase the power by adding a higher voltage, since the inner resistance of the motor will remain almost the same.
Almost? That will also produce more heat, thus more resistance and you can (will) end up with slightly less power than expected.
Say a motor rated 20A @24V (480W) will draw slightly less current (hotter = higher resistance) @36V so you might end up with 18A @36V (648W instead of 720W)

(btw, im trying to figure this out cause I just put the 90V 21A treadmill motor on my cart and will only power it with 48V, trying to figure out the power characteristics of the setup)

That might be too little voltage (based on the motor itself) since all DC motors have a threshold at which poiont they start turning. that can be as little as 6V and as high as 60V on a 90V motor, based on how it was wound, what permanent magnets are used, the bearings and bushings etc.
So the only way to know for sure is to test it.

btw 90V sounds like AC... you might need a DC/AC converter.
If it's a DC motor you could also use a DC/DC converter to step up the voltage if it's not running @ 48V (by reducing the current of course) but you still can feed 48V @40A into the converter and get 90V @21A out for 'full power'.

I'd suggest a truck starter motor, they're as cheap as a treadmill motor, but they run at 24V and can be even more powerfull
(they require a bit of work though )

'sid

 

[dcastillo]

Thanx...
not sure I follow this one comment
"Say a motor rated 20A @24V (480W) will draw slightly less current (hotter = higher resistance) @36V so you might end up with 18A @36V (648W instead of 720W)"

if the motor has a characteristic impedance, then wouldnt adding voltage increase the current draw? or is there something else im missing?


Ive already connected it with as little as 12V, which spins it, up to 48V which spins it pretty quickly...
I got it mounted on the cart last night around midnight with a 10t on the motor, 70 on the axel, with 14" tires... on jacks it seemed to spin the tires just fine, tonight Ill try it on the road and see if it has enough torque...
Controller and throttle just arrived so Ill try and install that this weekend if I have time...
If the motor isnt strong enough Ill just order one of the 48V 1000W my1020 motors Ive seen recommended many times on here...

and If i am following everything here... would I be better off buying one of the 36V or 24V 1000W motors giving me the ability to overvolt it for more power?

 

[loud1]

sid,
He is correct about the 90v motor, I worked for a treadmill manufacturer when I was 18/19 and they used 90v leeson motors for their machines. It is easier to control the speed on a dc motor.

 

[itsid]

sid,
He is correct about the 90v motor, I worked for a treadmill manufacturer when I was 18/19 and they used 90v leeson motors for their machines. It is easier to control the speed on a dc motor.

I never doubted that he is, I was just asking if it's a AC or DC motor

Oh and no not really,nowadays most of the controllers for AC and DC are pretty much identical.

Thanx...
not sure I follow this one comment
"Say a motor rated 20A @24V (480W) will draw slightly less current (hotter = higher resistance) @36V so you might end up with 18A @36V (648W instead of 720W)"

if the motor has a characteristic impedance, then wouldnt adding voltage increase the current draw? or is there something else im missing?

Impedance is a "virtual resistance" of AC circuits,
your up against static resistance of a DC circuit.
virtual because it's variing based on the wavestate it's in.

Adding voltage does not increase the current.
The current is limited by the resistance, not the voltage
And that increases with the current (producing heat)
So there are two things that can happen increasing the voltage:
a) you heat up the coils to a point where no additional current can be fed
b) you overheat the coils and you get a short

Ive already connected it with as little as 12V, which spins it, up to 48V which spins it pretty quickly...
I got it mounted on the cart last night around midnight with a 10t on the motor, 70 on the axel, with 14" tires... on jacks it seemed to spin the tires just fine, tonight Ill try it on the road and see if it has enough torque...
Controller and throttle just arrived so Ill try and install that this weekend if I have time...

Great, I'm confident that this will work out well then.
So instead of the 1890W (90V @21A) you will most likely have something in the range of 1000W which should be good enough.

If the motor isnt strong enough Ill just order one of the 48V 1000W my1020 motors Ive seen recommended many times on here...

and If i am following everything here... would I be better off buying one of the 36V or 24V 1000W motors giving me the ability to overvolt it for more power?

Depends!
Yes you can overvolt a motor of course, and most of the times it results in having more power.
But winding a motor for a specific voltage and current is serious craftmanship, I wouldn't underestimate the reason for that.
Frankly, I don't know what's the exact differences, but I think there is a reason for having them.
Spending 100 bucks on a motor and busting it for the fun of a few hundred watts for a few hours...
I don't know.

'sid

 

[dcastillo]

you were correct, powered it for the first time last night and it drove with myself and two kids (about 275lbs in riders)..
seemed to work just fine, had to do a rolling start since I dont have a controllers for it yet... didnt want to draw to much current starting from standstill
hopefully ill have that in this weekend..
thanx again

 

[loud1]

itsid
Quote:
Originally Posted by loud1 View Post
sid,
He is correct about the 90v motor, I worked for a treadmill manufacturer when I was 18/19 and they used 90v leeson motors for their machines. It is easier to control the speed on a dc motor.

I never doubted that he is, I was just asking if it's a AC or DC motor

Oh and no not really,nowadays most of the controllers for AC and DC are pretty much identical.


OK, your right, discreet semi-conductors have gotten much cheaper so higher power triacs would be more affordable, as a matter of fact, many light dimmers are triac controlled instead of rheostat now.

After my treadmill days I spent 15 or so years in silicon wafer manufacturing, from processing to engineering, so I have been out of the treadmill biz for a while.
Just recently however, one of my homebrew mates made a grain mill out of an old treadmill, it had a 90v dc motor.