X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Sat, 16 Nov 2013 18:28:27 -0500 Message-ID: X-Original-Return-Path: Received: from rc5-smtp.comporium.net ([208.104.2.19] verified) by logan.com (CommuniGate Pro SMTP 6.0.7) with ESMTP id 6602217 for lml@lancaironline.net; Sat, 16 Nov 2013 11:37:46 -0500 Received-SPF: pass receiver=logan.com; client-ip=208.104.2.19; envelope-from=snopercod@comporium.net Received: from rg20.comporium.net ([208.104.244.60]) by rc5-smtp.comporium.net ({548c1f54-a96b-4a18-a171-d2818e6ccf4d}) via TCP (outbound) with ESMTP id 20131116163710865 for ; Sat, 16 Nov 2013 16:37:10 +0000 X-RC-FROM: X-RC-RCPT: Received: from 94.245.235.68.dsl.brvdnc.dynamic.citcom.Net (EHLO _127.0.0.1_) ([68.235.245.94]) by rg20.comporium.net (MOS 4.3.4-GA FastPath queued) with ESMTP id CDI47925 (AUTH snopercod); Sat, 16 Nov 2013 11:37:09 -0500 (EST) X-Original-Message-ID: <52879F2B.2010305@comporium.net> X-Original-Date: Sat, 16 Nov 2013 11:36:59 -0500 From: John Cooper User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.1.0 MIME-Version: 1.0 X-Original-To: lml@lancaironline.net Subject: Re: N181AM nose gear collapse Content-Type: multipart/alternative; boundary="------------020206040806010102070008" X-MAG-OUTBOUND: comporium.redcondor.net@208.104.244.48/28 This is a multi-part message in MIME format. --------------020206040806010102070008 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit The ZNRs I installed on my hydraulic pump were Panasonic ERZ-V20D220 with a varistor voltage of 20V and a Maximum Allowable Voltage of 18V. These will take a 3,000A surge which is probably overkill but size, weight, and cost weren't really a consideration for me; Longevity was. I chose not to use the part number with the next lower (14V) Maximum Allowable Voltage because the Panasonic ERZ-V20D180 could possibly have conducted at normal battery voltage when taking the 10% tolerance into account. In reading further about protecting switch contacts, some companies recommend using a resistor and capacitor _in addition to_ the MOV. You can read about that at Littelfuse Varistor Design Examples "CONTACT ARCING DUE TO INDUCTIVE LOAD". > Each time the current in the inductive load > is interrupted by the mechanical contacts, > the voltage across the contacts builds up as - > L di/dt. When the contacts arc, the voltage > across the arc decreases and the current in > the coil can increase somewhat.The extin- > guishing of the arc causes an additional > voltage transient which can again cause the > contacts to arc. It is not unusual for the > restriking to occur several times with the > total energy in the arc several times that > which was originally stored in the inductive > load. It is this repetitive arcing that is so > destructive to the contacts. Littelfuse recommends installing _both _an MOV and a series resistor and capacitor in series across the contacts. I really don't have the proper test equipment to determine the proper values of the R and C, though. It would be a worthy project for someone to do some testing to determine the best protection scheme for the hydraulic pump relays, but I don't really have the time or the equipment at the moment. I'm also wondering what the inrush current is on the hydraulic pump motor. Inrush can weld relay contacts as well as arcing. I think I can use my Fluke in the "peak hold" mode to measure the current across the battery shunt as the hydraulic pump kicks on. I also installed two of those same ZNRs across my starter coil contactor - one from the input terminal to the disc and another from the output terminal to the disc. Those protect the contactor from arcing, but I need to add two more from the input and output terminals to ground to protect the starter switch on the panel. *Safety tip*: Be aware that when these MOVs fail due to excessive load, they can fail _shorted_. In that case they can either catch fire or explode. In consumer electronic equipment, I have seen them covered with a fiberglass "fire sleeve" in order to meet UL requirements. It's something to think about... --------------020206040806010102070008 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit The ZNRs I installed on my hydraulic pump were Panasonic ERZ-V20D220 with a varistor voltage of 20V and a Maximum Allowable Voltage of 18V. These will take a 3,000A surge which is probably overkill but size, weight, and cost weren't really a consideration for me; Longevity was. I chose not to use the part number with the next lower (14V) Maximum Allowable Voltage because the Panasonic ERZ-V20D180 could possibly have conducted at normal battery voltage when taking the 10% tolerance into account.

In reading further about protecting switch contacts, some companies recommend using a resistor and capacitor in addition to the MOV. You can read about that at Littelfuse Varistor Design Examples "CONTACT ARCING DUE TO INDUCTIVE LOAD".
Each time the current in the inductive load
is interrupted by the mechanical contacts,
the voltage across the contacts builds up as -
L di/dt. When the contacts arc, the voltage
across the arc decreases and the current in
the coil can increase somewhat.The extin-
guishing of the arc causes an additional
voltage transient which can again cause the
contacts to arc. It is not unusual for the
restriking to occur several times with the
total energy in the arc several times that
which was originally stored in the inductive
load. It is this repetitive arcing that is so
destructive to the contacts.
Littelfuse recommends installing both an MOV and a series resistor and capacitor in series across the contacts. I really don't have the proper test equipment to determine the proper values of the R and C, though. It would be a worthy project for someone to do some testing to determine the best protection scheme for the hydraulic pump relays, but I don't really have the time or the equipment at the moment.

I'm also wondering what the inrush current is on the hydraulic pump motor. Inrush can weld relay contacts as well as arcing. I think I can use my Fluke in the "peak hold" mode to measure the current across the battery shunt as the hydraulic pump kicks on.

I also installed two of those same ZNRs across my starter coil contactor - one from the input terminal to the disc and another from the output terminal to the disc. Those protect the contactor from arcing, but I need to add two more from the input and output terminals to ground to protect the starter switch on the panel.

Safety tip: Be aware that when these MOVs fail due to excessive load, they can fail shorted. In that case they can either catch fire or explode. In consumer electronic equipment, I have seen them covered with a fiberglass "fire sleeve" in order to meet UL requirements. It's something to think about...
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