Water Systems
Franklin AID

Vol. 16, No. 1, January / February 1998

Franklin AID Home

Contractor Coil Characteristics


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 stops.

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.

Figure 2.

In the next issue of Franklin Aid we will continue our discussion on "Contactor Coil Characteristics"