Three-phase Pump Control Panels
A three-phase pump control panel is the interface between the incoming three-phase power and the pump. A pump control panel has four major components that provide four basic functions in a three-phase system. First, the disconnect is the means of disconnecting the pump and its control equipment from the incoming power. Second, the contactor switches the pump on and off as directed by a control device (pressure switch, float switch, timer, etc.). Third, the fuses or circuit breaker are the primary protection for short circuit electrical faults, and fourth, overload devices protect the equipment from being damaged by overload faults. Figure 1 shows a full-voltage start pump control panel. Figure 2 is a schematic for the same panel.
The enclosure - The components providing the four functions listed above are housed in a protective enclosure. One of the primary functions of a pump control panel is to protect you, your customer and the general citizenry from being injured or killed by the high voltage inside of the panel. The safety features of a pump control panel that provide the necessary shock protection are the metal or fiberglass enclosure, the lockable door and the interlock function of the disconnect switch, which keeps the door from being opened when the panel is turned on. Also, most pump control panels manufactured today use “finger safe” components.
Beyond safety, the enclosure protects the electrical components from the panel's environment, dust, dirt and the weather. A NEMA 3R enclosure rating is standard in most pump control panels. Figure 3 is a chart we printed several months ago on the various types of environmental and safety protection offered by the most common NEMA ratings offered in pump control panels.
Some of the other things to look for in the enclosure are:
1. The paint finish should be suitable for the environment in which it will serve.
2. It should have mounting brackets, hubs, etc. - the hardware necessary to mount the panel and bring in the power, either from the bottom or from the top.
3. Good quality latches and door gaskets are important to provide the necessary seal for the life of the panel.
The Disconnect - This is the component that turns the electricity to the pump on and off, much like how the main disconnect (just below your electric meter) turns the electricity on and off to your house. Often, the disconnect incorporates the short circuit protection like fuses or a motor circuit protector.
The disconnect should be lockable in the off-position so the service person can work on the pump equipment without fear of someone unexpectedly turning on the power. The disconnect handle itself should be interlocked so that the panel door only can be opened with the disconnect switch in the off-position.
Short circuit protection is provided by either fuses or a motor circuit protector (MCP or circuit breaker as it often is called). Each varies greatly in its ability to respond to short circuit faults. For partial shorts, just larger than an overload, MCPs are faster to respond than fuses, but for a dead short, fuses are faster. The severity of a dead short will vary depending on how close the short is to the transformer, but could be in the range of 10,000 to 30,000 amps. In this case, an MCP would respond in about one cycle, or 16 milliseconds. A Class R fuse would respond in about 1⁄2 cycle, or 8 milliseconds, and a Class J fuse in about 1⁄4 cycle, or 4 milliseconds.
The J Class fuses are so fast that the fuse manufacturers offer a replacement guarantee for the components down line from the fuses. If any component is damaged by a short for the life of the panel, they will replace the damaged component at no charge. Of course, there is some fine print, but the guarantee says something about their confidence in the J Class fuses.
Overload faults vs. short circuit faults - There sometimes is confusion between the differences in protection needed for overloads vs. short circuits. Why don't fuses protect for overloads, and conversely, why don't the overload devices protect against shorts? Let's review a couple of concepts we talked about in previous articles. First, a motor will draw close to the nameplate current under normal operating conditions. But during the short time when a motor is starting and accelerating up to speed, the in-rush current is approximately six times the nameplate current. The acceleration time varies depending on the size of the motor, from a fraction of a second for a small motor, to several seconds for a large motor.
Secondly, the wiring in the system from the power pole to the pump is sized to carry the amount of current drawn by the pump during normal operation, plus a small safety margin. Any excessive current draw will cause the wiring to heat up, increasing its resistance, which makes it heat up more. Even the in-rush current at start-up causes some heat build-up. The function of fuses and motor circuit protectors is to provide a weak link in the circuit that will fail before the wiring heats up to the point of melting the insulation, and causing any damage.
So, to provide good short circuit protection, the fuses or MCP must be sized small enough to trip before any damage occurs, and large enough to not trip during start up. Or, they must have a time delay built into them to allow the motor to fully accelerate before tripping. Fuses used in pump control panels are the dual element, time-delay type, so they can be sized at around 125 percent of the nameplate amperage.
MCPs used in the pumping industry usually are the magnetic-only, instantaneous type, so they must be sized at greater than the 600 percent in-rush value. They usually are set at 700 percent of nameplate current, but if they trip on start up, the code allows for up to 1,300 percent of the full load current. Most MCPs are adjustable, and obviously, the lower the setting, the better the protection.
It is true that if a system does not have separate overload protection, and the overload fault is larger than the rating of the fuses or the MCP setting, they will open up and provide overload protection. However, in a three-phase system, an overload can occur in any one of the three legs of the three-phase power, which is why the National Electric Code requires a separate overload device in each of the three legs in a three-phase system.
Overload devices are designed to trip relatively quickly for large overloads and more slowly for small overloads. They are rated by how quickly they respond to a 600 percent overload in terms of seconds. For instance, a class 10 overload will respond in 10 seconds to a 600 percent overload. In other words, if you have a motor that operates at 100 amps and have the overload protection properly set up, it will trip in 10 seconds if the load reaches 600 amps. This gives ample time for the motor to accelerate.
Next month, we will continue this section on pump control panels by explaining the difference between NEMA and IEC components, and the relative merits of each. We also will describe some of the accessories often added to a pump control panel and explain when and why they are used. 'Til then ... .