Effects of volts and current on motor performance

[TRS300]

Hi all. This might sound like a strange question but here it goes... First off, I understand power (watts) = (volts * current) in general with all other conditions being equal. I've also heard (amps affects torque and voltage affects speed). I understand the voltage affecting speed but the amps affects torque sort of has me a bit confused... I would think that (power affects torque and volts affect speed) is more accurate... But maybe I'm missing something. So, as a hypothetical question, all else bring equal, will a motor perform (pull) the same under these two scenarios... 1. (5 amps at 72v @500 rpm) 2. (10 amps at 36v @500 rpm). The reason I added rpm is I wanted to ensure speed was not a limit in either case.
I'd be curious to what others think and please try to keep the topic as simple to understand as possible. Thank you.

 

[lnanek]

Grin has a motor simulator you can use.

All the power and torque in the world is useless for achieving a top speed, however, if you don't have the voltage needed to overcome the back EMF generated by that speed. So it's not like an IC dirt bike or something. So voltage gets picked based on the speed you want.

That only leaves amps as the rating you need to select after for getting enough torque. Since people care about top speed and they are forced to select a certain voltage for a certain KV rated motor for that.

 

[Chalo]

Motors have an RPM per volt constant, and a Nm per amp constant. There's are sometimes referred to as kV and kT. You need volts to push amps against resistance, but the constant values apply.

One confounding factor is the ability of most brushless motor controllers to trade volts for more amps when the motor RPM allows it. That's why raising voltage raises low speed torque. The controller is multiplying current by 2-3X as long as the motor back EMF is low enough that the incoming battery power can be converted to higher current.

 

[E-HP]

All the power and torque in the world is useless for achieving a top speed, however, if you don't have the voltage...

Can't have power without voltage.

I've also heard (amps affects torque and voltage affects speed). I understand the voltage affecting speed but the amps affects torque sort of has me a bit confused... I would think that (power affects torque and volts affect speed) is more accurate... But maybe I'm missing something. So, as a hypothetical question, all else bring equal, will a motor perform (pull) the same under these two scenarios... 1. (5 amps at 72v @500 rpm) 2. (10 amps at 36v @500 rpm). The reason I added rpm is I wanted to ensure speed was not a limit in either case.
I'd be curious to what others think and please try to keep the topic as simple to understand as possible. Thank you.

In a world with no losses (wind resistance, rolling resistance, etc.), motor RPM and vehicle speed are directly proportional to voltage. In the real world, power is required in order to overcome losses, in order to get a motor and vehicle closer to it's "no load" speed.

Torque requires power, therefore increasing either current or voltage will increase torque, but will affect torque differently, so depending on where you want the torque advantage, you may opt to increase current or voltage to obtain it.

These are two examples; the first show the effect of doubling current has on the blue torque curve. The second show the effect of doubling voltage. Both double the power. My personal preference is to gain as much higher RPM torque as possible, since the torque curve on an electric motor is descending. I want to be able to accelerate out of dicey situations when needed. Too much torque off the line tends to be unusable anyway, since past a point, it becomes uncontrollable. I prefer higher voltage to achieve higher torque, but I don't need the higher speed, so for normal riding, I just use less throttle (same efficiency as riding full throttle on a system with half the battery voltage), but the power/torque needed to accelerate is always there at any speed I normally ride at. Going to the higher current system route, the torque advantage starts petering out at half top speed, and goes away completely even before hitting top speed.

 

[amberwolf]

I moved this thread to the motor technology subforum, where this has been discussed before.

This thread

How Motors Work - some educational links

Here's a very detailed and well illustrated tutorial on the basics of AC induction motors and brushed DC motors. You can learn a lot from this. If you're not the techie type, ignore the mathematical formulas and just look at the pictures. http://www.reliance.com/mtr/mtrthrmn.htm Here's a...

endless-sphere.com

has some info, and there are various other threads you can find in this subforum discussing various aspects of all this in as much detail as you can stand. Some of them are about specific motors or systems, some the topic in general.

There are also threads around the forum such as some of the ones in this list

Search results for query: speed torque

endless-sphere.com

and this list

Search results for query: volts amps

endless-sphere.com

other threads can be found with variations on the terms (like voltage current instead of volts amps, etc).

 

[ZeroEm]

Hi all. This might sound like a strange question but here it goes... First off, I understand power (watts) = (volts * current) in general with all other conditions being equal. I've also heard (amps affects torque and voltage affects speed). I understand the voltage affecting speed but the amps affects torque sort of has me a bit confused... I would think that (power affects torque and volts affect speed) is more accurate... But maybe I'm missing something. So, as a hypothetical question, all else bring equal, will a motor perform (pull) the same under these two scenarios... 1. (5 amps at 72v @500 rpm) 2. (10 amps at 36v @500 rpm). The reason I added rpm is I wanted to ensure speed was not a limit in either case.
I'd be curious to what others think and please try to keep the topic as simple to understand as possible. Thank you.

Here is one thing to think about. You need fat wires to push amps, to get more watts thru smaller wires you up the Voltage.

 

[TRS300]

Can't have power without voltage.

In a world with no losses (wind resistance, rolling resistance, etc.), motor RPM and vehicle speed are directly proportional to voltage. In the real world, power is required in order to overcome losses, in order to get a motor and vehicle closer to it's "no load" speed.

Torque requires power, therefore increasing either current or voltage will increase torque, but will affect torque differently, so depending on where you want the torque advantage, you may opt to increase current or voltage to obtain it.

These are two examples; the first show the effect of doubling current has on the blue torque curve. The second show the effect of doubling voltage. Both double the power. My personal preference is to gain as much higher RPM torque as possible, since the torque curve on an electric motor is descending. I want to be able to accelerate out of dicey situations when needed. Too much torque off the line tends to be unusable anyway, since past a point, it becomes uncontrollable. I prefer higher voltage to achieve higher torque, but I don't need the higher speed, so for normal riding, I just use less throttle (same efficiency as riding full throttle on a system with half the battery voltage), but the power/torque needed to accelerate is always there at any speed I normally ride at. Going to the higher current system route, the torque advantage starts petering out at half top speed, and goes away completely even before hitting top speed.

View attachment 371381
View attachment 371382

This was extremely helpful and to be candid answered really what I was trying to better understand. Thank you.

 

[TRS300]

Thank you everyone for the insight. Much appreciated!

 

[TRS300]

I moved this thread to the motor technology subforum, where this has been discussed before.

This thread

How Motors Work - some educational links

Here's a very detailed and well illustrated tutorial on the basics of AC induction motors and brushed DC motors. You can learn a lot from this. If you're not the techie type, ignore the mathematical formulas and just look at the pictures. http://www.reliance.com/mtr/mtrthrmn.htm Here's a...

endless-sphere.com

has some info, and there are various other threads you can find in this subforum discussing various aspects of all this in as much detail as you can stand. Some of them are about specific motors or systems, some the topic in general.

There are also threads around the forum such as some of the ones in this list

Search results for query: speed torque

endless-sphere.com

and this list

Search results for query: volts amps

endless-sphere.com

other threads can be found with variations on the terms (like voltage current instead of volts amps, etc).

Thank you for the additional links and for moving the post. Still learning my way around the site with so much information it's not always obvious where to look. Best regards.

 

[amberwolf]

Thank you for the additional links and for moving the post. Still learning my way around the site with so much information it's not always obvious where to look. Best regards.

We;re always looking for organizers, so if you find info of various types that's useful you can gaterh links and quotes and place it in a thread aobut that, and pass that on to any of the knowledgebase editors, or volunteer to become a KBE:

ES Knowledgebase

endless-sphere.com

Knowledgebase

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