Patent EP-0877392-A1 (EP)
Inventor: BLACK CHARLES EMORY
IPC: H01C1/084
Filed: 1998-05-08
Abstract: A sandwich-like electrical resistor is disclosed,
comprising a first thermally conductive sheet metal outer
plate, a first thin flat outer electrical insulator stacked
against the plate, a first thin flat electrically resistive
sheet metal resistor element stacked against the first
insulator, a first thin flat inner electrical insulator
stacked against the first resistor element, an electrically
and thermally conductive sheet metal midplate stacked
against the inner insulator, a second thin flat inner
electrical insulator stacked against the midplate, a second
thin flat electrically resistive sheet metal resistor
element stacked against the second inner insulator, a
second thin flat outer electrical insulator stacked against
the second resistor element, a second thermally conductive
sheet metal outer plate stacked against the second outer
insulator, and means connecting and compressing the
previously mentioned components into a stacked unit. Each
of the resistor elements is an electrically resistive sheet
metal stamping having at least first and second terminals
and ribbons interconnected in one piece to form a
continuous electrically resistive path between the
terminals. A thermal fuse or other circuit breaker is
thermally engaged with a seat on one edge of the midplate
and is initially conductive but becomes nonconductive if
heated above a limiting temperature to prevent overheating
of the resistor. Each resistor element comprises at least
one structural tie bar extending in one piece between one
ribbon and one terminal for initially imparting enhanced
structural integrity to the resistor element. The tie bar
is severable prior to assembly of the resistor element with
the other resistor components. Each resistor element may
also include at least one resistance adjusting bypass tie
bar extending in one piece between two of the ribbons for
bypassing portions thereof to reduce the electrical
resistance thereof. Each bypass tie bar is severable to
increase the resistance. Each terminal of the resistor 
elements comprises a tab which is initially flat but is
folded twice upon itself to form a three-layer wire-like
prong thereon. The midplate comprises sheet metal loops
sheared from the midplate for receiving a terminal prong on
one of the resistor elements and a terminal wire on the
thermal fuse. The loops are clenched against the prong and
the wire. The resistor is received and supported by
channels extending from an electrically insulating terminal
head having conductive terminals thereon with sheet metal
loops formed thereon for receiving other prongs of the
resistor elements, such loops also being clenched against
the prongs. Some of the ribbons are connected together in
a serpentine series array, while other ribbons are
connected in a parallel array. 
     The outer insulators are thicker than the inner
insulators so that the thermal conductivity between the
resistor elements and the midplate is greater than the
thermal conductivity between the resistor elements and the
outer plates. Thus, the midplate is heated more rapidly
than the outer plates during a fault condition caused by a
locked rotor in the blower motor. The thermal fuse is in
contact with the midplate and is also heated more rapidly.
Consequently, the fuse is heated to its circuit-opening
temperature before the outer plates are heated to an
unacceptably high temperature.