Re: Treadmill motor controller question. (Don Foreman, Wayne Cook?)

On Thu, 01 Sep 2005 12:48:25 -0700, Eric R Snow <etpm@xxxxxxxxxxx>
wrote:

>On Thu, 01 Sep 2005 12:46:36 -0500, Don Foreman
><dforeman@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>>On Thu, 01 Sep 2005 00:43:09 GMT, tesnow@whidbeyDOTcom (Eric R Snow)
>>wrote:
>>
>>>Back in 1998 several folks bought a 2 hp treadmill motor and control.
>>>I was one of them. A circuit mod was posted by, I think, Don Foreman.
>>>I believe it was Wayne Cook who figured out how to mod the circuit.
>>>Finally I'm going to actually use this motor to power a small lathe. I
>>>would like to have instant reverse and stop. Turning the power to the
>>>board on and off will work for starting the motor. A DPDT switch could
>>>be used between the control and the motor to reverse the connection
>>>and hence the rotation. But this scheme won't work for instant
>>>reverse. If the DPDT switch is used without first turning off the AC
>>>will this damage the control? If a center off switch is used could
>>>this be used instead of switching on and off the ACto the control?
>>>Since the control uses a tach for speed feedback will the control be
>>>damaged by trying to spin the motor to the speed set by the pot? I
>>>looked on the web and found both the motor control chip and the triac.
>>>If these did fry they can be replaced for about 10 bucks including
>>>shipping. But I'd rather not try anything without some guidance.
>>>Thanks,
>>>Eric R Snow
>>
>>In order to reverse it you must reverse the connection of the field
>>relative to the armature. Just reversing the connections to the motor
>>won't do it. The armature gets its juice thru the brushes, so that's
>>a good place to do your reversing: just reverse the brush connections
>>with a DPDT switch.
>>
>>"Instant reverse" by throwing this switch with power on would work,
>>but I wouldn't recommend it. It would result in a current spike that
>>would very likely blow something. Better to turn off and stop before
>>reversing.
>>
>>There was some stuff in the original controller schematic that must
>>have been particular to a treadmill. A basic controller could be
>>somewhat simpler. You can download a data*** for the TDA1085
>>chip from
>>http://www.datasheetcatalog.com/datasheets_pdf/T/D/A/1/TDA1085.shtml
>>
>>This has a schematic for a controller using this chip.
>>
>>The tacho is non-directional, doesn't matter which way the motor is
>>turning.
>>
>>I would use robust triacs, 25 or 35 amp 600 volt devices, well
>>heatsunk.
>Greetings Don,
>I looked up the devices a couple days ago and read the data sheets. I
>also have the schematic that was posted in 1998. I don't really
>understand the schematic very well but was able to determine that the
>TDA1085 controlled the triac. If a more robust triac was used would
>the rest of the components be still be at risk when instant reversing?

I don't think so, other than perhaps the current sense resistor.
However, if the triac does get zorched it may take the chip with it.

The TDA1085 has a "ramping" function to ramp speed up at a controlled
rate. That would be inoperative if you hot-reversed. However, it
also has a current limit function that serves to limit peak triac
current.

>If so, would using DPDT center off switch to reverse the field
>winding connections be OK to use if the center off position was held
>until the motor has stopped before changing rotation?
>Thanks,
>Eric

I think so. That's what I'd do. I'd probably add a 200-volt MOV
across the triac to soak up any voltage spikes.

I think I'd use an alternistor rather than a triac, or perhaps an SCR
inside a 35-amp fullwave bridge. Alternistor: Teccor, Q4040K7 or
Q6040K7, available from Digi-Key. These are 40-amp parts, 400 and
600 volt respectively, $6.35 and $7.02 respectively. These devices
are more tolerant of inductive loads than triacs, behave more like
back-to-back SCR's. Nice thing about "rolling yer own" is that you
don't have to shave the last nickel out of the design so you can
design conservatively for performance and robustness. These
alternistors are rated for a 1-cycle surge of 400 amps.

My experience with triacs is that they are most reliable when
conservatively rated (voltage and current) and well heatsunk.
Designs that operate triacs near their spec limits have high failure
rates in the field.



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