The following issue of Franklin Aid is an
expanded and updated version of our November/December 1988 issue.
This information is repeated in response to several inquires made
to our Hotline personnel.
Single-phase deluxe control boxes and
three-phase pump panels typically have two distinct electrical
circuits as shown in Figure 1. First is the main line or motor
power circuit. This circuit normally runs from the main power
disconnect to the motor, through a contactor or starter. These
systems also use a second circuit typically known as a control loop
or control circuit.
The control circuit will include the
magnetic coil of the contactor, power supply for the control
circuit and any number of control switches. These control switches
can be pressure switches, float switches, timers, or any type of
on/ off control device.
The magnetic coil of a contactor or starter
is similar to the coil in a potential or voltage relay. This
magnetic or "mag" coil consists of a series of wire wrapped around
a bobbin. When electricity is passed through this wire, it creates
an electromagnet. When properly energized, a moveable contact or
"point" is pulled together, closing a pump system's non-nay open
contacts. As the contacts close, main line or motor supply current
flows through the contacts to power the motor. When the control
circuit voltage is interrupted, the coil de-energizes and the motor
Magnetic coils, whether in a single-phase
contactor or in a three-phase pump panel, are single-phase devices.
Control circuit coil voltage can be supplied from either inside the
control box/pump panel or from a source outside the box. In either
case the control circuit supply voltage must match the coil's rated
voltage. Note: control circuit coil voltage does not have
to equal main line power voltage. Step-down transformers are
commonly used in control circuits.
Magnetic coil specifications typically use
terminology like rated voltage, pick-up voltage, sealing voltage
(a.k.a. hold-in voltage), and drop-out voltage. Rated voltage is
coil supply voltage and must match the control circuit power
source. Coils are found in common supply voltages (i.e. 480, 240,
120 volts) and can also be found using low voltage (i.e. 24 volts).
Pick-up voltage is the amount of voltage required to overcome the
mechanical forces, like gravity and spring tension, trying to keep
the contacts from moving together or closing. Sealing voltage or
hold-in voltage is the amount of voltage needed to maintain the
contacts in their new, typically closed, position after pick-up
voltage is reached. Sealing voltage is normally less than pick-up
voltage. Drop-out voltage is the amount of voltage below which the
magnetic field becomes too weak to overcome the mechanical forces
trying to return the contacts to their at-rest position. An
important point about contactor coils, is that most of them will
overheat and can burnout if subjected to either abnormally
high or low voltage. Figure 2 shows operating
characteristics of a typical contactor. The National Electric
Manufacturers Association (NEMA) requires that standard magnetic
control devices operate satisfactorily between 85% and 110% of
rated voltage, and many contactors have lower sealing voltages.
However, most coils will begin to overheat and may burnout when the
current (amps) exceeds 110% of rated current at rated voltage. If
the contacts are open, this current can be exceeded at a voltage
well below either pick-up or sealing voltage.