Voltage & Current vs Speed and Turque

[Just]

Hi All,

As for the BLDC motors does the Voltage means Speed and Current means Torque?

Will increasing the continuous current limit in the Controller increase its RPM?

Thank you!

[Lollandster]

I tried this on my smartpie, I change it from 15 amp to 20 amp and while I can now go up hills I previously couldn't without pedaling, it didn't change the top speed.

If you have a wattmeter you'll probably see that the power goes down as the speed increases and if you meet a hill the speed will go down until you reach about continuous amp limit. This makes me think there is little to nothing in terms of top speed to gain from increasing the current limit.

As for the maximum unloaded RPM, this is a constant on BLDC motors that is dependent on the supplied voltage as RPM/voltage aka K_v. So that wont change with current. This has something to do with the back EMF and Lenz's law and is way outside of my basic understanding.

[Lollandster]

During the short time I've been on this forum you have asked a lot of very interesting questions, so I have to ask: Are you building something very cool?

Also I just realized that the GM site is only calling their inrunners for BLDC so the SmartPie would fall outside that as it is an outrunner, but they are all Brushless DC motors and works the same way. The biggest difference is that inrunners are high RPM low torque while outrunners are high torque low RPM.

[MAGICPIE3FOCUSPOWER]

Brushless motor = more V = more RPM.

[Just]

Lollandster, thanks for your response and compliments!    Actually I don't think that I'm working on something cool... I'm just trying to learn the basics 

Anyway, nowadays BLDC motors consume too much current (IMHO). It seems that an optimal configuration for a bicycle is having a high-end mid-motor, which would be able to absorb up to 50A in continuous and up to 100A in peak modes. Having a good Controller is also very important (programmable, should accept wide range of input voltages). Of course, good batteries are also important.
As for the Controller settings, I prefer to set it to 15A limit for the continuous current (just to reduce the current consumption) and 100Amps for peaks (just to provide a good acceleration on the hills for up to 10 sec). Having a 21-speeds bicycle would also help on hills and reduce the current consumption. On the flat, 48V 15A system (mid-motor 500RPM) should be good enough for reaching 50-60 km/h speed on the 26" bicycle.

So, these are my thoughts... You comments are more than welcomed!

[Just]

BTW, increasing the current limit would also help in increasing the speed if you are on up-hills :-)

[Andrew]

Phase wire current setting should also be taken into consideration as this will help how quickly the motor accelerates (not just battery current ). 

[Just]

Phase wire current setting

Hm... What Controller are you about? As for the Current settings, most programmable Controllers usually allow to set only Continuous and Peak current limits.

Kelly Controllers allow to define two current settings - the first one is for the Motor and second one is for Battery. Honestly, I don't understand why these two limits are necessary. Actually the current, which is drawn from the Battery, consumed by the Motor. So, why two separate definitions are required?

[Andrew]

Sorry, I should have said, I use infineon based controllers that have their own programming software.  You can choose your battery current limit and your phase current limt which is 2 or 3 times higher.
For instance, 30amp limit and 75amp phase limit.  From what I understand, the higher you put the phase current the faster response/acceleration  you get. Though this is limited by phase wire thickness and by controller  and motor temperatures.

[Just]

the higher you put the phase current the faster response/acceleration  you get

Your battery should have a good response time in order to support this. Usually are accelerations done with peak currents... Please check whether your controller has another setting for a peak current (besides the phase and battery limits).