BLDC speed control

[Tony]

Hello all,

I am interested in driving an industrial-type brushless DC motor with
a hobby-type electronic speed controller. There are industrial speed
controllers but they are larger, heavier, and much more expensive.

Are there any issues with doing this? What is different between the
two? I did see a warning in a manual for an RC ESC which said that it
is not designed to be operated with a power supply. It did not say
why but I am guessing it is because there is no schottky diode?

Any input would be appreciated.

thanks!

Tony

 

[Ron van Sommeren]

Hello all,

I am interested in driving an industrial-type brushless DC motor with
a hobby-type electronic speed controller. ... ...

A switching power supply may give problems. A brushless controller does
a lot of PWM switching/chopping too (not at full throttle though).
You could also anwer in the DIY Electronics subforum at RGGroups:
http://www.rcgroups.com/diy-electronics-199/

Workings of a brushless controller:
http://www.torcman.de/peterslrk/SPEEDY-BL_eng.html

Several diy designs:
<http://www.rcgroups.com/forums/showthread.php?t=140454>

Excellent active development thread:
<http://www.rcgroups.com/forums/showthread.php?t=200567>

Development discussion group:
http://www.yahoogroups.com/group/osmc

Vriendelijke groeten Ron van Sommeren
near Nijmegen, Netherlands
int. electric fly-in http://home.hetnet.nl/~ronvans/

 

[Morgans]

A switching power supply may give problems. A brushless controller does a
lot of PWM switching/chopping too (not at full throttle though).

Probably right on, as to why it says not to use a power supply.

For the OP, one work around is to use the power supply to charge a battery,
and hook the speed controller to the battery. The battery conditions the
power to make it truely DC, and the power supply keeps the battery charged.

 

[Phil Allison]

"Tony"

I am interested in driving an industrial-type brushless DC motor with
a hobby-type electronic speed controller. There are industrial speed
controllers but they are larger, heavier, and much more expensive.

Are there any issues with doing this? What is different between the
two?

** There are many differences.

Hobby speed controllers are designed for the remote control of battery
powered vehicles without need for precise rpm regulation or interfacing with
anything other than a standard R/C receiver.

They must be lightweight and cheap to make too = no need to be repairable.

The direct opposite applies, in each case, to an industrial BLDC speed
control.



...... Phil

 

[Tony]

Thanks all for the comments. I guess I am still unclear on why the RC
controller manufacturer would say that it cannot be driven from a
power supply. Most of the industrial controllers are switchers as
well, as that is the only way to obtain reasonable efficiency, and
they have no such restriction.

The only things I can think of is that without any diodes on the
output, and with the relatively high source impedance of a power
supply (compared to a battery) the RC controller will be introducing
large voltage spikes across the power supply terminals. The supply
would have to be designed to drive an inductive load. A battery
however will not be damaged by (brief) spikes across its terminals.

Also, the RC controllers all appear to be sensorless, meaning that
they use back-emf to sense motor speed. This works ok for lightly
loaded motors, but there are issues for startup/low speed operation
and with high torques.

-Tony

 

[Adam S]

Thanks all for the comments. I guess I am still unclear on why the RC
controller manufacturer would say that it cannot be driven from a
power supply. Most of the industrial controllers are switchers as
well, as that is the only way to obtain reasonable efficiency, and
they have no such restriction.

The only things I can think of is that without any diodes on the
output, and with the relatively high source impedance of a power
supply (compared to a battery) the RC controller will be introducing
large voltage spikes across the power supply terminals. The supply
would have to be designed to drive an inductive load. A battery
however will not be damaged by (brief) spikes across its terminals.

Also, the RC controllers all appear to be sensorless, meaning that
they use back-emf to sense motor speed. This works ok for lightly
loaded motors, but there are issues for startup/low speed operation
and with high torques.

-Tony

From what I've seen in these RC speed controllers, is they have limited
input bulk capacitance, just a minimum number of large ceramics to
handle the high frequency input ripple current needed with all half
bridge drivers. The cost conscious designers would take advantage of
battery's low impedance ranging from DC to many 10's of kHz. This is why
battery leads are short and close together to minimise inductance.

So to run your controller off a power supply you need to provide a very
low impedance voltage source for the ripple current demand. This is
easily achieved by connecting a few large low ESR electrolytic
capacitors in parallel. Solder an array of capacitors in a row with +ve
and -ve wires running in parallel geometry. To minimise lead inductance,
minimise wire distances between the capacitors and the controller
(comparable to battery lead length and separation). Now your power
supply only has to be capable of the DC current drain, and you can use
wires as long as you like.

I think worst case input ripple current for a 3 phase half bridge is 1/6
the average input (battery) current. Your capacitor bank must be rated
for this amount of ripple. If they start getting warm then the ESR is
too high or my calculations are wrong.

regards,

Adam Seychell

 

[Phil Allison]

"Tony"

Thanks all for the comments. I guess I am still unclear on why the RC
controller manufacturer would say that it cannot be driven from a
power supply.

** Likely because, unlike a battery, PSUs cannot absorb current supplied
from the load.

The output voltage will rise up and BAD things happen !!

The only things I can think of is that without any diodes on the
output, and with the relatively high source impedance of a power
supply (compared to a battery) ...

** Drivel.



...... Phil