Braddudya said:
So I have decent electrical knowledge but I am realizing that I may be thinking about some topics too simply.
How does one go about determining the best voltage for a project?
Is it simply speed calculations for your motor? Like what voltage is required to attain a certain speed, or avoid the motor tearing itself apart.
I see people discuss how increased voltage doesn't produce as much heat as increased amperage but aren't the two tied by ohm's law? Can you increase voltage without increasing current for a motor with a fixed resistance?
I guess I am hoping to learn more about everyone's strategy. In my mind it seems like "more voltage is better, but more expensive" but I am guessing it isn't that simple. Maybe it is haha.
It's not simple, but it's easier if you know what your requirements are for speed, torque, rideablity and budget. Here are some basics.
Motor Kv (rpm per volt) and battery voltage are the parameters needed for the unloaded speed of the motor. Actual speed while riding might by 3/4 of that. So for speed, you can change voltage, or use a faster or slower wind motor; so two parameters to consider.
For torque, higher power provides higher torque; higher current or higher voltaage provide higher power, so higher torque. So for torque, you can change current or voltage, which will change the motor power at a given speed, and the torque at that speed; so two parameters to consider. Motor kV will come into play for the torque at a particular speed, so also a factor.
For rideablity, consider that increased voltage still needs to be controlled via PAS or throttle. Twisiting the throttle results in anything between 0V and battery voltage flowing to the motor. The difference in twisting the throttle fully at 36V, is represented by very little throttle twist at 72V, especially with the non-linear functioning of hall based throttles. With enough available battery, barely touching the throttle will land you on your back in a split second. So as you go up in voltage you need other electronics to tame the throttle, which either means a high end pot throttle or a programable controller that controls the throttle ramp rates.
For budget, you need to consider that at anything over 52V, the list of options for controllers, etc., begins to narrow and costs go up, and over 72V it narrows significantly and cost spikes up to something only avid hobbyests will likely be OK with.
I suggest playing with the Grin simulator to get a feel for the relationships between voltage and current vs speed, torque, and load. Read the page to understand how the simulator works, and choose a motor or motors that have the motor temps modeled so you can see how the temps rise when you increase the grade, and how long it takes for the motor to overheat, etc. When you play with the throttle, or slide the vertical lines left or right, note throttle auto checkbox (unchecked assumes full throttle, checked assumes applying more or less throttle).
https://ebikes.ca/tools/simulator.html?motor=MGRIN2706_SA&batt=cust_72_0.05_24&cont=cust_70_200_0.03_V&hp=0&axis=mph&frame=mountain&autothrot=false&throt=100&grade=0&bopen=true&cont_b=cust_70_200_0.03_V&motor_b=MGRIN2706_SA&batt_b=B5220_GA&hp_b=0
I've found that torque greater than 100Nm is generally unusable down low, and needs to be controlled, but you can see that the higher voltage also provides greater torque throughout the range, so personally I deal with the downsides of the higher voltage in order to capture the torque (acceleration) benefits at the higher rpms.
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