(ET) AC-DC Relay dif and hi-current mower

[Rhett George <rtg ee duke edu>]

 - Greetings -

Two good questions recently.  Someone, maybe Mr. Landis, asked about AC vs 
DC
relay construction.  If one will first study the iron post on a typical DC 
relay, the one which support the coil, you will see that it is solid across
the surface to which the armature is pulled.  Not surprising.  Now examine 
the post of an AC relay.  The top is split for 1/16" to 3/16" right across
the center.  A D-shaped washer is pressed into the gap in the middle and 
fits
around one-half of the post.  This washer is a fat, shorted turn.  It 
creates 
a delay in the magnetic flux in that half of the pole due to the eddy 
current 
flowing in the D-shaped washer.  When, due to the alternating current, the 
flux in the free half of the pole goes to zero, flux still exists in the 
half
enclosed by the washer.  Therefore the flux does not go to zero across the 
face of the post and there is little or no tendency for the armature to 
buzz.

This washer in a different shape on a small fan motor of the shaded-pole 
design is called a shading coil.  It performs the identical task of 
providing
a delayed flux.

Regarding the mower which may or may not put the ammeter in the red zone, 
it 
may be due to a bad plug or cable (or possibly mower motor).  It may be a 
short between plus and ground or minus and ground somewhere in the mower 
side.
It may be in conflict with a ground leakage on the tractor which manifests 
iself only when the mower is plugged in and only when an intermittent short
occurs on the mower unit.

Hope this helps.

                                                Rhett George