Three-phase Pump Control Panels - Part 2
NEMA vs. IEC - There are two commonly used component standards for pump control panels - National Electrical Manufacturers Association (NEMA) and International Electrotechnical Commission (IEC). What are commonly called IEC panels in this country actually are UL panels using IEC-style components. For discussion purposes in this article, we will refer to them as IEC panels.
NEMA components are physically larger, which, according to NEMA supporters, make them better. IEC supporters say the technology built into IEC components allows them to be smaller and still hold up as well as NEMA components. One thing is for sure - it is hard to convince a NEMA guy that IEC is better, and vice versa.
NEMA panels are rated by an arbitrary numbering system starting with 00 for a 2-HP, 460-volt motor to 7 for a 600-HP, 460-volt motor. By changing the heaters or adjusting the overloads, a NEMA panel can be used for several size motors. For instance, a size 1 panel covers the range from a 5-HP, 230-volt motor up to a 10-HP, 460-volt motor.
IEC panels are rated by horsepower, so there is a specific IEC panel for each motor size. IEC supporters argue that a NEMA 1 panel is overkill for a 71⁄2- and 10-HP pump, and more expensive than it has to be. NEMA supporters point out that since a one-size panel will work for several motor sizes, you can carry fewer panels in your inventory. Both have a point. As I have said many times before, go with the product that is popular in your area because it will be more readily available, as will be parts and technical support.
The starter (contactor) is a relay, which uses the magnetic energy of an energized coil to close a set of contacts to start a motor (see Figure 1). The coil is connected to the control circuit, along with whatever other control devices are required in your particular application, such as pressure switches, liquid level controls, time clocks, etc. Your panel could have any or all of these control devices.
Another portion of the starter/control circuit often overlooked but required by the NEC are fuses in the control circuit. Without fuses in the control circuit, a short circuit will fry the weakest conductor, which is the coil in the contactor. Always use fuses in the control circuit.
The overload device typically is attached to the starter (see Figure 2). Inside the overload housing is a separate overload element or heater for each of the three legs of the three-phase power. An overload fault is defined as any over-current condition above normal operating current. Normal operating current includes the service factor rating of the motor defined as the percentage over nameplate full load amps that the motor can be run continuously without damage to the motor - typically 15 percent above the full load amps. A 10-HP motor with a 15-percent service factor really is a 11.5-HP motor, and most pump manufacturers design their pumps to use that additional horsepower to make themselves look good against the competition. Always take service factor into consideration when working with pumps, conductors (pump cable) and control panels.
The overload trip point depends on the size, service factor rating and application of the motor. It is recommended that the trip point be set no higher than the service factor amp rating of the motor unless the motor manufacturer gives you specific permission to set it higher.
Some panels use heaters for overload protection, others use adjustable electronic overload relays. Heaters are non-adjustable and are sized to a motor. Overload relays (overloads) can be adjusted to accommodate a range of motors. Both function by heating internally in an overload condition, which opens the starter control circuit, turning off the motor. Whether you use heaters or adjustable overloads, make sure they are ambient-compensated. This means that their sensitivity will not change as the temperature inside the control box changes, which particularly is important for submersible pump applications where the panel and pump are in different temperature environments (see Figure 3). Franklin's Submersible Motor Book has a good section on heater selection and adjustable overload relay settings; call 800-348-2420 for a copy.
Single-phase and current unbalance protection - In addition to straight overload protection, both heaters and overload relays provide single-phase protection. If one of the three legs in a three-phase system fails, you will lose two of the phases, leaving you in a single-phase condition. Motors designed for three-phase power will fail in a short time in a single-phase condition, so it is very important to protect them from this possibility. Because the remaining leg will be in a severe overload condition, its overload or heater will trip in 15 seconds to 20 seconds, turning off the motor. Unfortunately, a motor operating in a single-phase condition for 15 seconds to 20 seconds can sustain damage, which is why many pump contractors add phase monitors to their three-phase pump control panels as added insurance against motor damage due to single-phase conditions.
Most adjustable overload relays provide a degree of protection against current unbalance. Because the three internal overloads are connected together, if they sense a dissimilar load, as in an unbalanced condition, they will shut down the motor. Here again, the addition of a phase monitor will provide much quicker response to an unbalance condition. We will talk more about three-phase motor protection devices in the coming months.
Next month, we will look at soft-start panels that start the three-phase motor, gradually reducing the large in-rush of current that occurs when a motor is started at full voltage. 'Til then ….