# how do motors scale up?



## aeroscott (Jan 5, 2008)

If speed is the same, 2X torque is 2X hp. or torque X rpm= hp



no question to gain understanding is dumb.


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## lithiumlogic (Aug 24, 2011)

Well, if you connect two batteries in parallel you get double the AH, but voltage stays the same. Since energy = voltage x current, you get double the available energy too, thanks to the doubling of current, even though voltage hasn't changed.

With two motors in parallel, you get double the torque and since horsepower = rpm x torque, you get double the power too even though rpm hasn't changed.

This analogy falls down when you talk about series. You CAN connect batteries in series to double your voltage. But you can't connect motors in series to double your rpm.


This is all highly theoretical. There's so many real world considerations. To start

1) A motor will take as many volts and as many amps as the controller pushes into it, until it blows up. In most EVs the voltage and current limits of your controller set the performance envelope. Running two motors off the same controller won't double the performance, though the motors will run cooler since each is handling half as much power as before. Efficiency suffers at very high currents so you MIGHT gain another horsepower or two even though watts coming from the battery is the same. But you'll also heavier so acceleration could in fact be worse. I guess you're talking about doubling the number of motors and controllers, like the HPEVS AC35 x 2 kit.


2) I'm not going any further without caffeine, and i can't have caffeine because i have an early start tomorrow. I've probably started talking garbage already so i'll stop here.


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## CKidder (Dec 12, 2009)

lithiumlogic said:


> Well, if you connect two batteries in parallel you get double the AH, but voltage stays the same. Since energy = voltage x current, you get double the available energy too, thanks to the doubling of current, even though voltage hasn't changed.
> 
> With two motors in parallel, you get double the torque and since horsepower = rpm x torque, you get double the power too even though rpm hasn't changed.
> 
> This analogy falls down when you talk about series. You CAN connect batteries in series to double your voltage. But you can't connect motors in series to double your rpm.


It's even worse than that. Keep in mind that two items in series share the voltage across the loop (Kirchhoff's voltage law) If they are identical loads then they'll each take half of the voltage. Thus, both motors will want to run half as fast as if you hadn't wired them in series.



> In most EVs the voltage and current limits of your controller set the performance envelope. Running two motors off the same controller won't double the performance, though the motors will run cooler since each is handling half as much power as before. Efficiency suffers at very high currents so you MIGHT gain another horsepower or two even though watts coming from the battery is the same. But you'll also heavier so acceleration could in fact be worse. I guess you're talking about doubling the number of motors and controllers, like the HPEVS AC35 x 2 kit.


If you think about it you should get half the power by using two motors on one controller. As I said above, each should get about half the voltage. The controller is also limiting current so it'll chop you off at the total current so all things being the same each motor will get about half the current too. Half the voltage * half the current is a quarter the power. You'd have to up both the voltage and current a lot to compensate and then you would need a really powerful controller.


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## lithiumlogic (Aug 24, 2011)

CKidder said:


> It's even worse than that. Keep in mind that two items in series share the voltage across the loop (Kirchhoff's voltage law) If they are identical loads then they'll each take half of the voltage. Thus, both motors will want to run half as fast as if you hadn't wired them in series.
> 
> 
> 
> If you think about it you should get half the power by using two motors on one controller. As I said above, each should get about half the voltage. The controller is also limiting current so it'll chop you off at the total current so all things being the same each motor will get about half the current too. Half the voltage * half the current is a quarter the power. You'd have to up both the voltage and current a lot to compensate and then you would need a really powerful controller.


Aha, i was wrong then, you CAN connect motors in series (at least, DC motors). But the effect is kind of the opposite you get with putting batteries in series.

Eg. you have a DC motor controller capable of handling 200V and 500Amps.

One motor sees the full 200V and 500Amps.

If you connected two identical DC motors across this controller in series.

Both motors would see the 500Amps still. 
But each would only get half the voltage.

Since Current = Torque
and 
Voltage = RPM

torque output of each motor at low RPM would be unchanged, and since the whole system now has twice as many motors, total system torque output is doubled.

BUT

Wheras the motor might be able to hold full torque to 4000 RPM with 200V, on 100V , the torque will start to fall off at half that RPM.

So power hasn't really increased at all, you've just shifted it to lower in the RPM range. 


I have seen at least one EV drag racer that used a similar setup, so as to get by on a single speed transmission.

Recipie 

1 - One GODLIKE controller
2 - two motors on a common shaft

At low speeds, the two motors are connected in series, to double the total torque of the system, at the expense of RPM.

Once the dragster reaches a certain speed, the driver momentarily lifts off the accelerator , and operates a series of relays to put the two motors in parallel.

to use the numbers from my above example (200v, 500A controller), with the motors in parallel, both would see the full 200V so would peak at the full 4000rpm. Unfortunately, the 500A current is now shared between them, so each one only gets 250A (torque is halved, compared to the series arrangement)


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