3.12 Heating Considerations
The primary heating considerations in the evaluation and application of relays
are the effects of winding temperature on relay performance under normal
circuit conditions and under trouble conditions.
Normal heating is the condition imposed on a coil with respect to duration of
energization and wattage dissipation when the circuit is functioning in a
normal manner. Relay coils should be capable of withstanding normal heating
for their required life.
"Trouble" heating is a condition imposed on a coil when the circuit ceases to
function normally, resulting in a dissipation of power greater than that for
which the coil was designed. Circuit design should not impose a trouble
heating condition that could create a fire hazard; i.e., the coil and circuit
design should assure that the coil will be self-protecting.
Three factors should be considered in establishing normal operating
temperature limits and corresponding maximum safe normal operating voltages,
current, and power dissipation:
1) Ability of the coil to withstand the cumulative hours of heating likely to
be imposed during its required life.
2) Ability of other parts of the relay structure-such as insulation,
actuators, springs, and contacts-to withstand the temperatures imposed without
impairing performance of the relay.
3) The possibility of contamination of contacts from volatile substances in
the coil and relay structure and the effect on adjacent components.
Design criteria for trouble temperature limits may vary with the application.
In many fields, it is imperative that fire hazards be avoided. Trouble
heating is, therefore, limited to a temperature that the coil will be capable
of withstanding for a period of time well in excess of the likely duration of
the trouble. The safe trouble temperature limit is set at a value below the
temperature at which progressive short circuited turns may develop as a result
of deterioration of the wire or coil insulation. Relays experiencing a
trouble condition that causes heating above the normal operating temperature
limit are not relied on to function satisfactorily thereafter. In fact, such
relays should be removed from service even though they may still function
adequately because the extent of deterioration and its affect on future
performance cannot be determined.
Methods of evaluating heat resisting properties of coil and wire insulations
will not be discussed in this section. Note, however, that finer gauge wires
have a shorter life than coarser gauges. Safe operating limits, therefore,
should be based on the heat resisting properties of the finest wire or should
be determined for ranges of wire size.
In many systems, curves showing the relation between mean winding temperature
and power dissipation are required for the application of relay. These curves
are obtained at one or more ambient temperatures depending on the application
data required.
Typical curves showing final mean winding temperature (temperature at thermal
equilibrium) related to power dissipation are plotted in Fig. 3.10. On the
initial wattage curves Wo = Eo^2/Ro in which
Eo is the applied voltage and Ro is the resistance
at ambient temperature To. On the final wattage curves,
W1 = EoI1 and I1 is the current at
thermal equilibrium. Since the applied
voltage remains constant, power dissipation is inversely proportional to
absolute temperature, i.e.
in which To and T1 are in degrees C.
It will be noted that over a temperature rise range of +37.78°C to +65.56°C
the relationship between final wattage and temperature is essentially linear.
This linearity makes it possible to calculate, with a reasonable degree of
accuracy, the temperature rise O, and final mean winding temperature for a
variety of circuit conditions. This is done by determining the thermal
conductance (p) of the coil, expressed in watts/degree rise. To illustrate,
thermal conductance of the coil of Fig. 3.10, obtained at the +121.11°C point
on the +37.78° C ambient temperature curve, is 10.3 watts divided by 65.56
or 0.1571 watts/°C.
Substitution of p in the applicable formulas of Table 3.11 permits calculation
of limiting circuit conditions that will provide margins to assure reliable
operation and to prevent overheating.
To avoid errors in calculation temperatures and circuit constants where the
final watts/mean winding temperature curve departs significantly from a
straight line, it may be necessary to choose a series of values for p, each
over a limited range.
When a relay is energized under pulsing conditions, the allowable power
dissipation is greater than for continuous energization. Experiments have
shown that for pulse up to one second duration at any duty cycle, allowable
wattage dissipation is obtained from the expression
P2 = P1 a+b/a in which a is the on time
b the off time, and P1 the power that may be dissipated
continuously. For other pulsing conditions, a more comprehensive treatment in
involving the load factor, thermal conductance, and thermal capacitance is
required. Unless such information is needed for a large variety of circuit
functions, it would be more logical, for isolated cases, to conduct studies of
several representative samples under the worst possible circuit conditions to
determine the winding temperature and circuit margins for the limiting
condition. The worst circuit conditions for heating are maximum voltage and
minimum winding resistance, or maximum current through a maximum resistance
winding. For determining pickup margin, the limiting case is excitation of
the maximum resistance winding at maximum voltage or current followed by
application of minimum voltage or current when thermal equilibrium has been
reached.
Some contactors have energy saving windings tied to the contacts. A low
resistance winding is in series aiding with the high resistance holding
winding. When the coil circuit in energized, a normally closed (back) contact
shunts the high resistance winding so that a high inrush current picks up the
armature, and opens the holding high resistance "economizer" winding shunt,
thereby conserving power during the hold period.
Fig. 3.11 General temperature rise formulas for coils of electromagnets.
E= applied volts
I= current in amperes
R= main winding resistance at initial temp. (Oo) in ohms
Rs=extemal or internal shunt resistance (zero temp. coefficient), in ohms
r=extemal or internal series resistance (zero temp. coefficient), in ohms
Wo=initial watts 0 = temperature in deg F
=EI watts (constant power) 0o initial temp. in 0F
—I2R watts (constant current) K = thermal conductance in watts per F
=E2R watts (constant) 8=temperature rise above o, in 0F
a=temperature coefficient of resistance of
copper (based on inferred absolute zero
resistivity of copper at —390F)
1
=________
390 + 0o
