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Volts vs. Wattage, Speed vs. Torque

I have a Razor Ground Force that is currently 24v, 250w. I am going to upgrade it for more torque and higher top speed. I am trying to understand how changing the voltage (upgrade to 36v or 48v) compares with upgrading the motor wattage (500w, 750w, or 1000w). I understand the basic of voltage times amperage equals watts. But how does this affect a scooter or go kart?


What if you replace a 250w 24v (running at 24v) system with a 250w 48v (running at 48v) system? Will a higher voltage motor with the same wattage have more torque? Longer battery life?


What if you replace a 250w 24v (running at 24v) system with a 500w 24v (running at 24v) system? Will a higher wattage motor with the same voltage have more torque? Longer battery life?


So I am trying to decide if I should upgrade the big three (battery, controller, motor) to 36 or 48v. I don't plan on over-volting but rather use gearing to do it right. I am also trying to decide if I should upgrade the motor to a higher wattage? But I don't know how these factors interact. 

It's already geared at 11 teeth motor side. I like the speed where it's at and wouldn't like to lose any. I was hoping to avoid having to get a whole new motor. 


I have a different project (scooter) that is running a currie 24v 350w motor, 36v SLA batteries, and a 36v 1000W 30A controller that is plenty fast and has the torque needed. Thanks for the reply!

Hi Justin, that is a great question and a subject which I have been thinking about recently and wanting to talk about. 


Electric vehicle controllers and motors have the same properties as amplifiers and speakers in an audio system. In an audio system, the controller outputs a certain amount of Watts to the speakers and the speakers will run at the Watts level that the controller provides to them no matter how many Watts they are rated for. And in an electric vehicle, the exact same relationship applies between the controller and motor. 


A 500 Watt controller could be used with a 350 Watt motor and the motor would then have 500 Watts of power available to consume if and when it demanded that much. However, the motor is rated to run at 350 Watts of continuous power consumption without overheating, so if it ran under a 500 Watt load for an extended period of time then it might overheat and burn out. 


If you upgrade from a 350 Watt controller to a 500 Watt controller then you should feel increased power from the motor when accelerating and going uphill.  


The controller is what is limiting the power consumption and power output of the motor. Under the right conditions, the motor can and will demand more Amps than it is rated for and if provided with more Amps than it is rated for then use the extra Amps and convert them into output power. 

Thank you for the perfect explanation. The simile really helped to put it into perspective. I think this topic specifically is one of the most informative about diy electrical propulsion systems out there.

You amp draw of the motor is determined based on the design of the motor and load. You want your controller to at least have the minimum watts or amps rating to be able to handle that amp draw without burning up. At WOT the motor determines the amp draw and the controller rating is to make sure it can handle it. So always get a controller with the same voltage rating of your battery and either the same amp/wattage draw or larger. (volts x amps = watts)  


The only other limiting factor you may have is your lipo battery. If the C rating is too low it may not be able to output the amps the motor wants so to speak.  


Your scooter is overvolted so the motor is spinning faster than it is rated for. You will have more torque and rpm (speed) at the sacrifice of the motor. You could overvolt your scooter but who knows how long the motor may last. Maybe one time, maybe forever.  


I don't recommend overvolting motors as they burn up pretty easy. Mine lasted all of one day. Better to buy a higher voltage and wattage motor/controller and regear to get your speed/torque you want. 

"Electric vehicle controllers and motors have the same properties as amplifiers and speakers in an audio system. In an audio system, the controller outputs a certain amount of Watts to the speakers and the speakers will run at the Watts level that the controller provides to them no matter how many Watts they are rated for. And in an electric vehicle, the exact same relationship applies between the controller and motor. 


A 500 Watt controller could be used with a 350 Watt motor and the motor would then have 500 Watts of power available to consume if and when it demanded that much. However, the motor is rated to run at 350 Watts of continuous power consumption without overheating, so if it ran under a 500 Watt load for an extended period of time then it might overheat and burn out. 


If you upgrade from a 350 Watt controller to a 500 Watt controller then you should feel increased power from the motor when accelerating and going uphill.  


The controller is what is limiting the power consumption and power output of the motor. Under the right conditions, the motor can and will demand more Amps than it is rated for and if provided with more Amps than it is rated for then use the extra Amps and convert them into output power."


Kind of but not really... The maximum amperage a motor can draw is based upon the design of the motor and the load that is placed on it. Here is a simple idea of it. Remove the controller completely. Hook the motor up to directly to the battery. Will the motor instantly burn up because there is suddenly hundreds of amps that could flow to it. No, the motor just turns and runs. The motor determines the amp draw. The controller limits the voltage to control the speed of the motor. The amp rating of the controller is the maximum amount of current that can flow through the controller without it overheating and failing. If you go with a higher rating on the controller the battery will still only draw so many amps. (The manufacturers rate it as watts, I am calling it amps, but it is actually current) Remove the controller completely and the motor will still draw the same amount of amps it was designed for.


The only time you would need to upgrade controllers is if you switch to a different voltage battery or a larger motor that will draw more amps then the controller can handle. Otherwise the design of the motor is the limiting factor (presuming your battery and wired are sized correctly to keep up with the motor demands). Going to a controller that is rated higher than what the motor will draw will only ensure a longer lifespan of the controller and will not increase the current flowing to the motor.    

Another easy test you can do is hook up an ammeter between the motor and battery and measure the current where you have high load (ie sit your scooter uphill, remove controller, install a heavy duty on/off switch rated for the amps, turn on and measure the draw while going uphill). Now install your controller and hook up the ammeter between the controller and battery. Do same test. If the current (amps) are lower than your controller is not rated sufficiently for the motor and you need a higher wattage controller. If the amps are the same then your controller is not limiting your performance.   

Hi Brett, thanks for your replies. You started this topic and we do not want to walk on your feet, however, in the interest of our forum providing correct information to our readers we have to respectfully disagree with your comments regarding speed controllers and we will explain why below, with references. 


Modern light electric vehicle speed controllers are engineered with internal shunts which are utilized by their microprocessor to detect the controller's output current and then actively limit the maximum current output through lower pulse-width modulation duty cycles. This active current limiting function senses the demand that the motor places on the controller and if the motor places more demand on the controller than the controller's current limit is set to then the controller will limit its output current to be within specifications for the controller regardless of how much current the motor demands. This is a necessary function of the controller in order for it to not demand too much current from the battery pack which could damage and or overheat it, and to help prevent overheating of the motor under normal operating conditions. References: Current Sensing Circuit Concepts and Fundamentals


The heat dissipation capability of most light electric vehicle controllers does not exceed the amount of heat that is produced when the controller is at or near its current limit so unfortunately in most cases the controller's current limiting function will only protect the battery pack and will not protect the controller itself, which is why many controllers overheat and fail under extremely demanding conditions.

 

Shunt used by the microprocessor to actively limit the controller's output current.


Controllers do not limit or change the Voltage that they output to the motor, they chop the Voltage on and off thousands of times per second in different patterns to control the amount of time that the motor receives the full Voltage of the battery pack. The lower the pulse-width modulation duty cycle is the lower the average Voltage will be that the motor receives which is what causes the motor's speed to be adjusted up and down. Here is a drawing of a pulse-width modulation signal which shows that the controller always outputs full Voltage, and chops the voltage on and off in different duty cycles to control the speed of the motor. References: Pulse Width Modulation


Pulse-Width Modulation Voltage Levels and Duty Cycles


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