The primary answer is response time. For instance, a typical overload relay (Figure 1) can take up to 20 seconds to respond to a single-phase condition. Imagine the heat build-up in the one remaining winding in a single-phased motor having to carry the entire load by itself. If it doesn't fail immediately, its life will have been substantially shortened by the damage to the winding insulation caused by overheating. A conventional overload relay responds slowly to single-phase conditions because it is reacting to the effect of the single-phase condition, which is heat, not to the actual occurrence of the single-phase condition.
Figure 3 is the horsepower derating curve related to voltage unbalance. For instance, if you had a 71⁄2 HP load and 5-percent voltage unbalance, it would be necessary to use a 10 HP motor to prevent premature motor failure.
Solid-state phase monitors, as opposed to overload relays, electronically detect faults like single phasing, voltage unbalance, and high and low voltage. Instead of taking up to 20 seconds to respond to a single-phase condition as with a mechanical overload relay, they can respond immediately to these conditions. A single-phase condition occurs, and click - in less than a second, the motor is turned off. Or if the single-phase condition occurs while the motor is off, the phase monitor will prevent the motor from coming on until the condition is corrected. The same holds true for reverse-phase conditions and severe voltage unbalance as well.
Three-phase protection devices can be divided into three basic categories: phase monitors (also called voltage monitors), current monitors and multifunction devices.
Voltage monitors are relatively inexpensive and provide protection for single-phase, reverse-phase and phase-unbalance faults, plus over- and under-voltage. Some also include a power-up delay timer, which is necessary in a line shaft turbine application to prevent the pump from restarting after a power interruption while the turbine still is back-spinning.
Their drawback is that they ignore well-related problems because they don't monitor current. Their best use in a pump application is to supplement a current monitor that does not have built-in voltage protection. I always recommend both voltage and current monitors, or a combination unit, for any motor larger than 15 HP.
Current monitors compare the current going to the pump with preset, safe levels, and turn the pump on or off accordingly. They read the current indirectly using a device called a current transformer (CT). A CT is a coil, about the size of a donut, which installs over a conductor, producing a secondary current proportional to the current flowing through the conductor. This secondary current then is read by the current monitor. The CTs can be built into the current monitor or can be external to it.
Multifunction devices combine several functions in one package. For instance, Coyote's three-phase units are available with a built-in contactor that will handle up to 40 HP at 460 volts. Symcom's Model 777 combines a voltage monitor, current monitor, power monitor and ground fault monitor into one unit. The 777 is available in several configurations, including one that monitors the motor winding temperature for applications with motors equipped with a temperature sensor. Franklin's SubMonitor also has this feature.
To summarize, the motor protection provided as standard in a pump control panel will not always protect the motor from being damaged by electrical and load-related anomalies. The use of additional protection, such as electronic voltage monitors, current monitors and multifunction devices, is cheap insurance to protect your pump motor and help it give you years of solid, trouble-free service.
The following companies advertise three-phase pump protection devices in National Driller and other ground water publications.
7120 W. 117th Ave. Unit B4
Broomfield, Colo. 80020
400 E. Spring St.
Bluffton, Ind. 46714
1460 Cavan Ct.
Boulder, Colo. 80303
Load Controls Inc.
10 Picker Road
Sturbridge, Mass. 01566
P. O. Box 2578
Rapid City, S.D. 57709