April 1967 Electronics World
Table
of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
|
The April 1967 edition of Electronics World had a series of
articles on designing systems with electromechanical relays.
Even in today's high solid state relay world, there are still
lots of applications for electromechnical relays. Only a handful
of people actually design them, but the application tutorials
provided therein are as valuable to today's engineers and technicians
as they were 45 years ago.
Here are links to the other relay articles:
Operate and Release Times of Relays,
Reed Relays,
Time-Delay Relays,
Finding Relay Operate and Release Times,
Arc, Surge, and Noise Suppression
Operate and Release Times of Relays
Since graduating from the engineering school at the University
of Kentucky in 1937, the author has held supervisory positions
in just about every production and engineering department at
Guardian Electric. From 1960 to 1963 he served as Chief Design
Engineer and has been Assistant Chief Engineer since 1963. He
is a Registered Professional Engineer and holds many patents
on relay, switch, and stepper designs.
By Warren Wright / Asst. Chief Engineer, Guardian Electric
Mfg. Co.
Definitions of these important characteristics and
methods that are used to modify these parameters.

Since graduating from the engineering school
at the University of Kentucky in 1937, the author has held supervisory
positions in just about every production and engineering department
at Guardian Electric. From 1960 to 1963 he served as Chief Design
Engineer and has been Assistant Chief Engineer since 1963. He
is a Registered Professional Engineer and holds many patents
on relay, switch, and stepper designs.All too often relays are
placed in control and logic circuitry without enough consideration
being given as to whether or not the relay operational time
characteristics will assure proper functioning of the circuit
under various operating conditions. A working knowledge of which
factors affect relay operating time can give the circuit engineer
or technician confidence in his design.
In general, relays are electromechanically operated switches,
thus there are two items which must be evaluated when considering
the time elements of relay function. These are the electrical
characteristics and the mechanical characteristics.
But first, let's define the terms and then consider their relationship
to total relay function.
Definition of Terms
The operate time of a relay is the time interval from the
instant of coil-power application until completion of the last
contact function.
The release time is the time interval
from the instant of coil-power cut-off until the completion
of the last contact function. (See Fig 1.) Note that the operate
and release times do not include contact-bounce times.
When coil power is applied, coil energizing current increases
until the magnetic flux is sufficient to move the relay armature
and its contact-actuating members. Upon removal of the coil
power, magnetic flux does not collapse instantly, but decreases
for some period of time - depending on the circuit, the coil,
and the magnetic structure. When the magnetic flux drops below
the "hold-in" value for the particular relay, the armature and
its contact-actuating members return to the normal or de-energized
position.
With these fundamental characteristics in mind,
we can now consider the various relay designs and the effect
of circuit characteristics on operate and release times.
D.C. RelaysFor d.c. relays, the
operate time of a specific relay design may be reduced by three
methods. First, we can overdrive the relay. This is done by
increasing the control voltage, decreasing the coil resistance,
increasing the control voltage and adding a series resistance,
discharging a capacitor at an over-voltage charge into the coil,
pre-energizing at some value below pickup voltage (the lowest
voltage at which the relay always operates), using dual-wound
coils - one coil for overdrive, the other to hold the armature
in the operated position, using a series resistor shunted by
a capacitor, using a positive temperature coefficient resistor
in series with the coil, and using a series resistor shunted
by an N.C. switch - the switch being operated by the relay being
controlled.
Second, we can reduce the pickup voltage
of the relay by mechanical means, such as by reducing return
spring pressure, reducing the armature gap, or reducing contact
pressures and gaps.
Third, we can decrease the mechanical
inertia by reducing the mass of the moving elements such as
contacts, armature, and contact actuators.
For d.c. relays,
the inherent release time of a specific design may be increased
by using a parallel capacitor and series resistor, a parallel
shunt resistor or switch, parallel diode, or by reducing the
residual magnetic air gap.
Relay
manufacturers produce many varieties of relays with operate
and release times ranging from minimal values of less than one
millisecond to some of the more exotic solid-state relays with
maximum times of 30 minutes or more.
When specific time
characteristics are needed, the relay manufacturer can usually
provide relays to match the circuit requirements either from
"standard" relays or as "specials" designed for a specific function.
The National Association of Relay Manufacturers
(NARM) has not set standards with respect to fast or slow response.
In general, relays which have operate and release times under
3 milliseconds are considered fast-operate and/or fast-release.
Relays with function times of 50 milliseconds or more are usually
considered slow-operate or slow-release. Relays with function
times between 3 and 50 milliseconds are medium-operate and release
and this is the range into which most general-purpose relays
fall. Relays which are purposely designed for slow function
time are classified as time-delay relays. These are covered
in another article in this special section.
Relay manufacturers
produce a bewildering range of relays for use in over a hundred
"usage" classifications, with thousands of varieties and modifications
in each classification. It is obviously impractical to analyze
all of these types, therefore we will only cover some of the
most popular general-purpose relays.
The graphs of Figs.
2 and 3 show the effects of ambient temperature changes on the
attract and release times of some typical general-purpose, d.c.-powered
relays. From this we can see that the attract or operate time
increases with temperature and the release time decreases with
temperature rise (although circuit components may modify this).
When we consider that relays heat up either under extended
energization or repeated cycling, we can expect a relative shift
in operating time characteristics, depending upon the frequency
of operation.

Fig. 1. Typical d.c. operation of a relay
with a resistive d.c. load.
Relays from different manufacturers, but of the same type,
generally have similar operating-time characteristics. It will
be noted that some relay types change more with temperature
than others - this is inherent in the design of the relay but.
in general, the time change is within ±10% over an ambient temperature
range of 0°F to 160°F.
Other factors affect timing, such as changes in operating
voltage or current. in which the attract time varies inversely
with coil power and the release time varies directly with coil
power. Where operation time is critical, the relay should be
evaluated in the circuit under the most adverse combination
of variables.
Power-supply inductance will increase
the operate time of d.c, relays depending upon the L/C ratio
of the circuit. Resistance in the power-supply line will tend
to decrease the operate time. Relays which are wired in parallel
would aggravate these effects. Arc suppressors or coil shunts
will increase the release time of relays.
Basically,
d.c. relays are reasonably consistent in their timing characteristics
under the same operating conditions, but this is not the case
with a.c. relays.
A.C. Relays Other
variations in both attract and release times are due to the
added factor of the instantaneous (turn-on or turn-off) voltage
change. The source voltage varies from zero to peak 120 times
per second, thus any voltage from zero to peak may be applied
across the coil at the instant of either "turn-on" or "turn-off".
Examination of a typical 60-Hz sine wave will show that most
of the time the instantaneous voltage at "turn-on" or "turn-off"
will be higher than the pickup voltage of the relay, so that
the time characteristics are usually within a reasonable range.
Frequently, however, the probability of turning on or off at
lesser voltages catches up with us and the operating time suddenly
changes. This voltage variation is further complicated by the
magnetic flux distribution between two or more functional pole
faces. Remember that a.c. relays generally have one core face
shaded by a copper ring to cause a phase displacement in the
magnetic flux. This phase shift is necessary to provide relay
hold-in during current reversal. Thus, we have an operational
time range rather than a specific operation time. The manufacturer
generally specifies the average function time for a.c. relays.
Fig. 2. Variations in operate or attract
times at various ambient temperatures for six general-purpose
d.c. relays.

Fig. 3. Variations in the release times at
various ambient temperatures for the same six general-purpose
d.c. relays.
Modification of operational time for a.c. relays may be accomplished
in a manner similar to that for d.c. relays, but the actual
time of operation will vary because of the sine-wave voltage
applied. We must conclude, therefore, that straight a.c. relays
should not be used in circuits where precise repeatable narrow-range
operational times are required.
The mechanical characteristics
which affect operational time apply to both d.c. and a.c. relays
and what affects one type will usually affect the other type
in the same manner.
Increases in mass, whether it be
in contact actuators, the contacts, or armature will slow the
operational time. Release time will be increased by minor contact
welding or sticking, mechanical wear, and residual magnetism.
We have not attempted to give charts showing precise
operating characteristics of relays because the timing is subject
to so many variables from relay to relay type, depending upon
adjustment, type and number of contacts, voltage variation,
source impedance, line resistance, wear factors, etc. Information
on timing characteristics is available from the manufacturer
and should be used whenever operate and release timing is critical.
Posted 7/3/2012