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Volts from controller - volts from back EMF = working voltage.

At low RPM the voltage from the controller is high and volts from back EMF is low so you have lots of working voltage.  As RPM increases the working voltage approaches zero.

Zero working voltage = zero torque

Basically, the harder you push the more it pushes back. But field weakening limits the amount it pushes back.

Field weakening basically allows you to keep positive working voltage at high RPM.  Field weakening can be applied to SPM motors but requires LOTS of current and you need to have a better cooling solution because of the amount of heat generated.

IPM motors are perfect candidates for field weakening and it can be done at the power levels required at lower RPM.  As a result IPM motors can hit much higher RPM without additional cooling required

Great info Mrwilsn, thanks!

Can you point to some books and/or papers online about field weakning?

IPM = internal mounted PM rotor and SPM = surface mounted PM rotor?

What about oil cooling? read ˜4 posts after this one:
http://electricmotorcycleforum.com/boards/index.php?topic=4424.msg29049#msg29049

"simply filling the motor slightly less than halfway with ATF almost doubled the continuous power output. "

Frodus, this version is compatible with the older version of the DVT, are you sure?

Send me a PM if you have the DVT!!!!

Thank you for the heads up, Doc.

Enviado do meu K014 através de Tapatalk

RNM, at low switching speeds, the DC resistance of the copper windings dominates the inductance in limiting how much current you can drive through the motor; at some transition speed, inductance becomes a factor and you can no longer get maximum current through the windings (or, stated equivalently, BEMF starts to rise), which reduces the torque the motor can put out. That's called the "field weakening" point; IIRC it occurs around 3500 rpm on my bike (a 2014 SR), which is somewhere around 60 mph. BEMF is not a factor at low speeds, DC resistance of the copper wire is.

DC winding resistance is another component of the motor time constant (L/R), which let's us calc the current rise and fall.
See the spreadsheet in this link:
http://www.endless-sphere.com/forums/viewtopic.php?f=30&t=29852&start=15#p433095

2 notes for the spreadsheet:
- Time constant varies a lot from motor to motor
- "Applied Voltage" on the top-left is the V (motor) = V (battery) - V(BEMF)

If you play a bit with it, you can see that having 0 BEMF or 50V BEMF, with a 100V battery , you get twice the current in the same time interval.

in fact the Sevcon Gen4 does provide full current at 0RPM,

If people are complaining that the bike is slow at very low speeds, I think the ESC is not providing full torque at 0rpm.
Can the Sevcon do that? Perhaps.

Another place to look for answers is the MBB settings (Main Bike Board):
http://electricmotorcycleforum.com/boards/index.php?topic=2925.0
http://electricmotorcycleforum.com/boards/index.php?topic=4357.0

Good luck guys! 

The peak current is 360A for 10 secs. That's what it should deliver when accelerating from standstill

Lecram, it's a limitation of the ESC. Full current (full torque) at 0 rpm would induce great current ripple because the BEMF is very low. To have full current, you would have to increase ESC switching freq a LOT, but that means a lot of additional heat (=burned MOSFETs)

Note the 10mph minimum on the chart