The primary answer is response time. For instance, a typical overload relay 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 because of 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.
Voltage UnbalanceOperating a three-phase motor on unbalanced power also can cause serious overheating that will dramatically shorten its life. Figure 2 shows the effect of voltage unbalance on motor life.
Figure 3 (on p. 54) is a 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, detect faults like single phasing, voltage unbalance, and high and low voltage, electronically. Instead of taking up to 20 seconds to respond to a single-phase condition as with a mechanical overload relay, they can respond almost 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.
Voltage MonitorsThree-phase protection devices can be divided into three categories – phase monitors (also called voltage monitors), current monitors and multifunction devices.
Voltage monitors not only turn off pumps when the voltage is outside of preset, safe parameters, they prevent them from starting if voltage conditions are not right. In this regard, they offer better protection than current monitors, which require that the pump run for a few seconds before the current responds enough to the voltage problem to trip out the motor. In a single-phase or severe unbalance condition, this might damage the motor. In a reverse-phase condition, the pump could be damaged by running backward for a few seconds.
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 lineshaft turbine application to prevent the pump from restarting for a minute or so after a power interruption while the turbine 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 multifunction unit, for any motor larger than 15 HP.
Current MonitorsCurrent 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, through which a conductor runs, 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 mounted externally in the pump control panel.
Most current monitors use three CTs – one for each phase – and look for over-current, under-current, current unbalance, single-phase, and reverse-phase faults. Most are field-calibrateable. If you are not relying on the current monitor to detect unbalance, single- and reverse-phase faults because they are unlikely to occur in your local area, or because you already have a voltage monitor to perform these functions, a single coil current monitor can be used to monitor over-current and under-current, saving some money.
Multifunction DevicesMultifunction 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.
Finally, a word about voltage surges and lightning strikes. Solid-state motor protection devices are like any other piece of electronic equipment in that they are vulnerable to high voltage surges. Most have built-in surge protection to handle small, short duration surges and voltage spikes, but if you are in an area where large voltage spikes or lightning strikes are possible, the addition of lightning arrestors to protect your protector is recommended. If your protector does get taken out by a voltage surge, chances are, it will have saved your motor by interrupting the power on its way to the grave.
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 trouble-free service.
The following companies advertise three-phase pump protection devices in this and other ground water publications.
7120 W. 117th Ave. Unit B4
Broomfield, Colo. 80020
400 E. Spring Street
Bluffton, Ind. 46714
Load Controls Inc.
53 Technology Park Road
Sturbridge, Mass. 01566
222 Disk Drive
Rapid City, S.D. 57701