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Brushless three phase controller bypassed on full throttle?

Started by John Doe September 10, 2015
As far as I know, bypassing the controller when using two power wires on
a brushed cordless drill just means connecting the battery directly to
the motor through a physical switch built into the trigger. 

Is it likely that they bypass the controller when the trigger is fully 
depressed when using a brushless motor with a three-phase motor 
connection thingy? 

They have the standard warning in the manual about not running the tool 
for long on partial power, but I don't understand how bypassing the 
controller is implemented in the brushless three-phase models. 

Thanks. 



https://www.flickr.com/photos/27532210@N04/? 
A brushless dc motor is basically the same internally as a 3 phase ac motor, 
the controllers job is to convert the dc power into correctly timed pulses 
on the three motor phase windings to run the motor at the desired speed 
under the current load.  You cannot "bypass" a brushless motor controller 
because if you feed dc directly into a brushless dc motor it won't run.

-----
Regards,
Carl Ijames

"John Doe"  wrote in message news:mst519$k8r$1@dont-email.me...

As far as I know, bypassing the controller when using two power wires on
a brushed cordless drill just means connecting the battery directly to
the motor through a physical switch built into the trigger.

Is it likely that they bypass the controller when the trigger is fully
depressed when using a brushless motor with a three-phase motor
connection thingy?

They have the standard warning in the manual about not running the tool
for long on partial power, but I don't understand how bypassing the
controller is implemented in the brushless three-phase models.

Thanks.



https://www.flickr.com/photos/27532210@N04/? 


John Doe  <always.look@message.header> wrote:
>As far as I know, bypassing the controller when using two power wires on >a brushed cordless drill just means connecting the battery directly to >the motor through a physical switch built into the trigger. > >Is it likely that they bypass the controller when the trigger is fully >depressed when using a brushless motor with a three-phase motor >connection thingy?
Nope. With a brush motor, the commutation (that is, reversing the direction of current flow through the windings, as the shaft rotates) is done mechanically, through the brushes. No controller is needed to do this. With a brushless motor, there are (obviously) no brushes. Reversing the current going to any given coil segment requires switching the two ends of the coil segment back and forth between the two power supply voltages (e.g. positive, and ground) electronically - that is, turning big MOSFETs on and off. That's the job of the controller. No controller, no switching of the MOSFETs, and thus no torque once the shaft has rotated slightly.
>They have the standard warning in the manual about not running the tool >for long on partial power, but I don't understand how bypassing the >controller is implemented in the brushless three-phase models.
It isn't. I suspect that the "partial power" warning is due to the fact that when you're running the motor at lower speed, the MOSFETs are actually being pulse-width-modulated on and off to feed the coils a lower (average) voltage. This causes the MOSFETs to spend more time in the transition region (when switching on and off) and they dissipate more power and get hotter. Then, if you're slowing the drill down to try to "power through" a difficult load, and the drill is still being called upon to try to deliver lots of torque at the low speed, the motor is going to try to draw a higher amount of current (the lower speed means that the "back EMF" from the coils will be lower), and this will cause the MOSFETs to heat up even more.
On 09/11/2015 01:51 AM, John Doe wrote:
> As far as I know, bypassing the controller when using two power wires on > a brushed cordless drill just means connecting the battery directly to > the motor through a physical switch built into the trigger. > > Is it likely that they bypass the controller when the trigger is fully > depressed when using a brushless motor with a three-phase motor > connection thingy? > > They have the standard warning in the manual about not running the tool > for long on partial power, but I don't understand how bypassing the > controller is implemented in the brushless three-phase models. > > Thanks. > > > > https://www.flickr.com/photos/27532210@N04/? >
While they can't physically bypass the controller, the controller may be able to turn off phase vector modulation and switch to block commutation. (full on or full off instead of sine wave PWM). Noise and power would both go up a little.