From: wwebster@bnr.ca (Bill Webster)
Subject: Re: Question About Speed Controls.
Followup-To: rec.models.rc
Organization: Bell Northern Research
Date: Mon, 23 Nov 1992 23:40:29 GMT

In article <92325.123603IO10046@MAINE.MAINE.EDU>, <IO10046@MAINE.MAINE.EDU>
wrote:
> 
>     I have a question about the operation of speed controllers.
> What is ment by high frequency speed controls?  I known that they
> are supossed to be better for the motor and its brushes but why?
> Does it reduce brush arching?  What does the output of a controller
> look like?  Maybe:
> 
>     ----  ----  ----    7.2V
>     |  |  |  |  |  |
>     |  |  |  |  |  |
>  ----  ----  ----  ---  0.0V
> 
>     What is the frequency?  In the KHz?
> 
> 
> Aaron Brasslett
> University of Maine

I just got e-mail access and don't know how far this reply will get.

I couldn't help but notice the _quality_ reply which was posted earlier.
So, here's an attempt to better answer your questions.

The older ESC's (before high-freqency hype) pulsed current thru the motor
at the same frame rate as the radio receiver signal.  This works out to
20ms
or 50Hz.  The duty cycle of the 20Hz current was varied in proportion to
the 
received signal from the receiver, and this gave you proportional control
over the forward speed.  Newer ESC's which claim to be high-frequency,
pulse
the current thru the motor at a frequency greater than 20Hz.  In the case
of Tekin and Novak, this frequency is between 2300-2500Hz.  Other speed
control manufacturers use frequencies much higher with better overall
results.
The higher frequencies I've seen are 5800Hz, 11,000Hz and 21000Hz.

The higher frequency provides many benefits.  The ESC MOSFETs run cooler
since they are switched on/off rapidly.  This allows the MOSFET to operate 
in its most efficient operating region.  When a MOSFET is fully turned on,
it is at its lowest resistance state.  But, if left on too long, it begins
to
heat up and becomes less efficient.  The ESC also responds better to the
signal from the receiver as it's more linear in design.  If you hold the
motor
in your hand and use an older _frame-rate_ controller, you'll notice how
jerky the motor feels.  Do the same experiment with a _high-rate_
controller
and you'll notice how smoothly the motor responds.

Is the high-rate controller _better_ for your motor?  This depends on how
long you want it to last.  The instantaneous power is quite high and this
does strain the motor windings/brushes/armature and inevitably the bearing
surfaces.  This type of high rate controller was used many years ago in
model trains.  But, since it kept burning out the train motors, it wasn't
very popular.  Yes, you can say that todays RC car motors are designed to
take the high currents.  But, eventually something has to give.  And as I
see it, if you want to be competitive in any sport, you have to push the
limits.

Does it reduce arcing?  Yes.  But not for the reasons you'd expect.  The
high-rate ESC manufacturers found they had to put a fast diode across the
motor in order to contain the newly generated high frequency noise.  This
fast diode should have been incorporated into the older frame-rate ESC's,
but the manufacturers were too cheap.  They relied on the inherent body
diode
of the MOSFET used for the brake.  I don't expect you to understand all
this,
but there will be some out there who will.  Also,  this fast diode was 
necessary to protect the MOSFETs from the huge negative voltage pulse 
created when the forward MOSFETs are switched off.  This same protection
is also necessary when the brushes cross the armature gap.  One last factor
which helped reduce arcing is what I call the hit ratio.  If the ESC is
pulsing current thru the motor, there will be times when the current is
switched off just as the brush is crossing the gap.  If there's no current,
there's no arc.  So, the higher the ESC frequency, the fewer arc hits.  By
reducing the arcing, you reduce brush wear and armature damage. 

As a sidebar, there are _no_ manufacturers of high-rate ESC's designed
with the chips from a servo.  All high-rate speed controls use custom
designed
integrated circuits except Jomar speed controls.  Jomar had high-rate ESC's
in the mid 70's and it took nearly 15 years for the RC car industry to
catch up.
Jomar uses off-the-shelf components, none of which come from a servo.  The
manufacturers using custom IC's I've seen are Tekin, Novak, JEM, SCI, Zeta,
Victor, Corally.  There's lots to choose from. 

Hope this has shed some light on the topic.


Bill Webster (WWEBSTER@BNR.CA)
Newsgroups: rec.models.rc,rec.models.rockets,rec.models.scale,rec.models.railroad