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There are numerous DC permanent motor controllers available for motors ranging
from 3 to 30 volts for the Radio Control Hobby industry. These controllers use a command
of a 1-2 ms positive Servo Command Pulse which is repeated every 20 ms., approximately.
One major class of R/C controllers is designed for R/C cars employing 7.2 VDC
Nicad battery packs. These model cars are driven for short periods in highly competitive
races. If there's a reverse, it's usually not proportional; after all you can't win going
backwards! Because winning is so important some manufacturers or importers of controllers
for the R/C car market feverishly out-advertise each other with outlandish claims, usually
s-t-r-e-t-c-h-ing the ampere capacity of their controller by an order of magnitude.
Hundreds of amps is not an uncommon claim. The connecting wire alone will not handle many
of the current claims.
"Continuous" current rating to these manufactures means the duration of
a 3 minute race. MOSFETransistors are the devices that carry the amperes of
currents. Sometimes claims are tied to the MOSFET peak ratings which apply to microsecond
surges and not practical motor start up surge times. Or claims from MOSTFET ratings that
are maxed out under ideal gate drive, perfect multiple MOSFET paralleling, or heat sink
conditions not realized in the R/C car controller design. Often there is no means
for heatsinking. The low on resistance of the MOSFETs is predicated upon keeping them
cool; they have a positive temperature coefficient. Ohms law tells us if the MOSFETs
increase resistance their voltage drop increases and volts-dropped times amperes of
current means more heat yet, a vicious cycle.
VANTEC manufacturers controllers in the U.S.A that accept the 1-2 ms positive
Servo Command Pulse, but that is where the simularity ends. All of our controllers
provide a mount for the MOSFETransistors which is electrically insulated but thermally
conductive to our aluminum chassis, plate or heatsink. The electrically neutral
chassis or plate can then be bolted to any additional heat conducting and radiating
structure, like your aluminum mounting bracket or your robot metal frame, or a commercial
heat sink with an aspect ratio that suits your vehicle. Our premium products use
thick machined aluminum extrusions to provide superior thermal conductivity.
VANTEC Continuous current ratings represent the amount of current
that the controller can safely conduct at 100% duty cycle for hours with a near
infinite heat sink. Amps as American as Apple Pie! Duty cycles between
10-99% slightly reduce the amount of current safely conducted because switching losses add
heat. Starting Surge current ratings are the amount of current
safely conducted for 5 seconds. For dual controllers these ratings are for one
controller at a time operating.
Todays trend in R/C model car controllers is "hi-rate" switching. To an
engineer versed in todays Pulse Width Modulated switch technology this means 200 KHz plus
rates. To R/C model car controllers this means 1500-6000 Hz which is VERY audible; the
motor laminations sing!
VANTEC controllers offer a selection of PWM chop rates from an almost sub-audible
170 HZ and 338 Hz, to a super-sonic 21 KHz. Some of our controllers have jumper selectable
PWM chop rates so you can choose what's best for your application. The 21 KHz yields
very smooth operation but with caveats; increased heat knocks 30% off the current ratings
and the faster rate generates more Radio Frequency Interference which can be a problem in
a Radio Controlled environment. Taming RFI problems can be done but it
requires more rigorous application engineering and additional fabrication
expense. The sub-audible rates provide reasonabley smooth operation and usually are
sonically quiet enough. |
