NetNews Usenet Archive 1992 #19/NN_1992_19.iso/spool/comp/sys/dec/4794

Internet Message Format | 1992-08-30 | 2.0 KB

Path: sparky!uunet!gatech!rutgers!spcvxb.spc.edu!terry
From: [e-mail] (Terry Kennedy, Operations Mgr.)
Newsgroups: comp.sys.dec
Subject: Re: WANTED old PDP-11?
Message-ID: <[e-mail]>
Date: 31 Aug 92 01:54:09 GMT
References: <[e-mail]>  <[e-mail]>
Organization: St. Peter's College, US
Lines: 29

In article <[e-mail]>, [e-mail] (Paul Repacholi) writes:
> I think we are crossing terminology some-where in this. Could some-one
> conversant with both systems explain.

  There are two possible 3 phase configurations, one called "Delta" and one
called "Wye". In Wye the center point of the Y is neutral/earth, while in
Delta there may not be any relationship between any of the leads and neutral/
earth (depending on which country's power system you're talking about.

  Additionally, there's the "nominal" voltage of each country's system to
consider. Here in the US, regular consumer outlets are 120V nominal, and
Wye power is 208V. 240V single phase is derived from a single phase line,
with the center tap of the transformer being neutral. Thus, there's 120 V
on either side of the neutral, 180 degrees out of phase. The 208 in 3-phase
is the difference due to the 120 degree (vs. 180 degree) phase difference.

  The RM05 wants 208V, normally found between two 208 Wye legs, but can be
jumpered for 240V as in residential single phase service. The RP05/06/07 all
require 3 phase power (as opposed to the 11/70 and VAX-11/78x, which have 3-
phase plugs but only have 120V receptacles inside, between the 3 legs and
neutral.

  And yes, 3 phase motors will reverse if any two phase legs are swapped. If
you drop one leg, the motor will be far more likely to stall. It will also
turn into a nice 208 V resistance heater 8-).

    Terry Kennedy        Operations Manager, Academic Computing
    [e-mail]    St. Peter's College, Jersey City, NJ USA
    [e-mail]    [phone]