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Re: (ET) E15 alltrax death

At 09:49 AM 12/4/2007 -0700, Tim Humphrey wrote:

>> -----Original Message-----
>> From: elec-trak-bounces cosmos phy tufts edu
>> [mailto:elec-trak-bounces cosmos phy tufts edu] On Behalf Of David Roden
>> On 4 Dec 2007 at 7:42, Konstanty, Walter (GE Indust, ConsInd) wrote:
>>
>>> The motor can't generate a spike as
>>> it's a resistive element.
>>
>> What kind of motor is pure resistance?  All the motors with which I'm
>> acquainted, being electromagnetic devices, have some amount of
>> inductance.
>> Some types have more than others, of course.
>>
>> That said, my gut reaction is that the PTO contactor is the likely
>> culprit,
>> mainly because I've seen HUGE spikes from contactors without suppression
>>
>> diodes.
>>
>>> if the voltage drops low enough it might "shut off" the Alltrax SCR's.
>>
>> I'm pretty sure the Alltrax is a MOSFET design.
I'm thinking two MOV's, one across the contacts and the other across the coil of the PTO contactor may be cheap insurance. Or maybe better to put one high power one across he Alltrax terminals, then the tractor wiring can help absorb some of the induced energy.
MOV's would not be a good choice. The ratio between their suppression voltage and maximum working voltage is quite large and not very precise so in this application they either end up conducting all the time or providing no protection. 

In addition, unlike diodes they wear out during use.
The last thing you want to do is allow the energy into the wiring. If it gets there it's going to affect the controller. You need to suppress at the source. 

Robert

http://www.aeolusdevelopment.com/

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