Three-Phase Pump Control Panels - Part 2
Last month, we talked about full voltage start pump control panels. We described the various features to look for in the enclosure, the disconnect, and the short circuit protection, be it fuses or a motor circuit protector. This month we will cover the other two main components in a standard panel, the contactor or starter, and the overload protection devices (the heaters or adjustable overloads). In addition, we will go over the single-phase and current unbalance protection offered by panels.
NEMA vs IECCurrently, there are two commonly used component standards for pump control panels, NEMA (National Electrical Manufacturers Association), and IEC (International Electro-technical Commission). What are commonly called IEC panels in this country are actually 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 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 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 an 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 to a 10 HP 460-volt motor.
IEC panels are rated by horsepower, so there is a specific IEC panel for each motor. IEC supporters argue a NEMA 1 panel is overkill for a 7-1/2 and 10 HP pump, and more expensive than it has to be. NEMA supporters point out since 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 and technical support will be better.
The starter (contactor) is a relay, which uses magnetic energy of an energized coil to close a set of contacts to start a motor. 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 these control devices.
For the pump to run, all control devices must be in the "on" position for current to flow to the coil to close the starter contacts. The control circuit can operate at a different voltage than the pump, as is often the case in higher voltage installations. In such a case, a transformer is used to step down the voltage. Remember when ordering a panel to specify the control voltage and the pump voltage.
Another portion of the starter/control circuit often overlooked but required by the NEC is 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 is typically attached to the starter for convenience. There is a separate overload element or heater for each of the legs of the 3-phase power. An overload fault is defined as any over current condition above normal operating current. That is, above the service factor rating of the motor, typically 15% above the full load amps, up to, and including the locked rotor current (about 600% of normal running amperage). The service factor rating of a motor is defined as the percentage over nameplate full load amps that the motor can be run continuously without damaging the motor. A 10 HP motor with a 15% service factor is really a 11.5 HP motor and most pump manufacturers design their pumps to use that additional HP to make themselves look good against the competition. Always take service factor into consideration when working with pumps, conductors (pump cable) and control circuits.
Overloads are there to protect the elements of the motor circuit, including motor, conductors, and motor controls, against damage due to excessive heating caused by motor overloads. Remember from last month that overload faults, unless they are very large, do not trip fuses or motor circuit protectors, which are there to guard against short circuits.
The overload trip point depends on size, service factor rating, and application of the motor. For motors with a service factor of not less than 1.15, which includes most pump motors, the trip point can be set up to 125% of the full load amp (FLA) rating of the motor. If the service factor is lower than 1.15, the trip point must be set no higher than 115% of FLA.
Some panels use heaters for overload protection, others use adjustable electronic overload relays. Heaters are non-adjustable, and are sized for a particular 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. Franklin's Submersible Motor Book (call 800-348-2420 for a copy) has a good section on heater selection and adjustable overload relay settings.
Single-Phase and Current Unbalance ProtectionIn addition to straight overload protection, 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, it's overload or heater will trip in 15 to 20 seconds, turning off the motor. Unfortunately, a motor operating in a single-phase condition for 15 to 20 seconds can sustain major 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.
In addition, most adjustable overload relays provide a degree of protection against current unbalance, because the three overloads are connected internally, and if they sense a dissimilar load, as in an unbalanced condition, they will shut down the motor. Here again, addition of a phase monitor will provide much quicker response to an unbalance condition. More on three-phase motor protection devices in the coming months.
Next month we will talk about three-phase panels that start the motor gradually, reducing the large inrush of current that occurs when a motor is started at full voltage including autotransformer panels, part winding panels, soft start panels among others. Till then...