Troubleshooting Tips
OVERLOAD
What Is Overload ?
The pump may be turned off by a “tripped” overload. An overload is the device typically located directly underneath the motor contactor, that monitors electrical current flowing to the pump motor. Detection is by means of a bimetallic metal strip that changes shape as current flows through the strip building up heat.
If the motor draws excessive current, this overload device will turn the motor off, but only after a time delay period depending upon how much excess current is being drawn. The so called “trip curve” is an inverse time delay because the higher the overload, the shorter the time delay.
Pump motors are almost always “induction” motors. This simply means that the rotor is magnetized by means of an induced current from the windings, rather than through brushes. Induction motors have one distinctive feature that is important to know about. The lower the voltage, the higher the current draw of the motor. This means that if a 230 volt motor is operated with 200 volts, the motor will draw more current than if it had been powered by 230 volts, and that motor will draw more power than it normally would, and that motor may be damaged by being operated at the lower voltage due to additional slippage causing additional heating that the motor overload protector cannot predict.
On Irrigation Craft pump stations the overload should be adjusted to match the motor nameplate FLA or FLC, which stands for Full Load Amps, and Full Load Current respectively. It is not necessary to adjust the overload to include the service factor. If however the station has been built by another manufacturer with overloading pumps, then it may be necessary to adjust the overload above the motor’s nameplate FLA. Underwriters Laboratories and the National Electric Code (NFPA 70) allows the overload adjustment to include the motor service factor, typically 15% or as stated on the motor nameplate 1.15, (FLA+15%). Some motors only have a 1.00 service rating meaning that they have no service factor, other motors can have 1.25 service factor meaning that they are rated by the manufacturer according to standard NEMA testing as being capable of handling 25% more power than the nameplate FLA.
An example of how service factor works is as follows:
A motor has an FLA of 100 amps and a service factor of 1.15. That motor can operate at 115 amps "legally", so the overload adjustment can therefore be set at 115 amps. It must be noted however that when a motor operates "in the service factor", at higher current, the motor will operate at a higher temperature. This higher temperature often results in a shortened motor life.
Irrigation Craft pump stations should be set at motor FLA without the service factor. If there are problems with the electrical service on site that are causing a high current draw by an Irrigation Craft pump station, then make every effort to solve the problem on site. If the problem cannot be solved on site, then you may set the overload to the service factor FLA, with the understanding that the motor may not last as long, therefore it is always better to correct the site power problem. This is part of the design intent by Irrigation Craft, to allow some site problems without immediate damage to the pump motor.
What Causes Overload:
Electrical – 1 & 3 PHASE
1. LOW VOLTAGE FROM POWER GRID - Low voltages can be due to “brown outs”, or “low voltage events”, which are system wide in the power grid. Low voltage events can occur when power grids are loaded to maximum, such as during severe cold spells, during the hottest days of the year, and during evening hours from 5:00 P.M. to 9:00 P.M.
2. LOW VOLTAGE FROM LOCAL CAUSES - Low voltages can also be caused by local system problems. Local system problems can be due to overloaded circuits in a building or on the site, undersized wire, or abnormal activity in the area overloading the power companies transformer or the feed wires to a site.
3. LOW VOLTAGE DUE TO POOR DESIGN OR INSTALLATION - Low voltage can be caused by improper design or installation of the power circuit. Examples of this type of problem would be: wire size too small, loose connections or wire nuts, faulty circuit breakers or contactor points. Low voltage problems can also occur if the pump drive motor is designed for 1 type of voltage say for example 230 volts, but is being fed power from a 200 volt power supply.
Electrical – 3 Phase Only
1. PHASE LOSS - IEC starters really have two trip curves, one for normal operation, and one for when a phase is lost. The “lost phase” curve is very short and designed to trip the motor off quickly to prevent “single phase” damage to the motor. This simply means that the motor overload protector can trip out, but the problem is not just overload, but a specific type of overload caused by phase loss. Check and make sure that all phases are in fact live.
2. VOLTAGE UNBALANCE (AKA - PHASE UNBALANCE) – Voltage Unbalance between phases in a three phase motor is calculated per NEMA MG1. Motors are rated to operate at their nameplate FLA if the voltage unbalance between all three phases is 1% or less according to NEMA MG1. MG1 also states that a motor should be capable of operating properly at a voltage unbalance not exceeding 2% between highest and lowest voltage on any two phases.
3. MALFUNCTIONING OVERLOAD - A third cause of overload could be a malfunctioning or inaccurate overload module. If the pump motor is monitored with an accurate amp meter, and the motor is drawing continuously on the highest measured leg, a current equal to or less than the set point of the overload, and the overload trips, then the overload is either set too low, or the overload is inaccurate. Replace the overload module if it is inaccurate.
Overload Caused by Excessive Demand (High Flow)
Most pumps and pump stations are overloading. This means that the pump impeller is capable loading the motor beyond the motors motor’s FLA rating into what is called the service factor. UL® and NEMA® allow this, but NEMA® states that the motor will have a shorter life expectancy if the motor runs in the service factor.
Irrigation Craft does not use or provide overloading pumps. Because Irrigation Craft does not use overloading pumps Irrigation Craft pump stations are not vulnerable to overload conditions caused by excessive water flow.
If the station is not an Irrigation Craft pump station, then OVERLOAD alarms may indicate high flow field problems. But if an Irrigation Craft station has an OVERLOAD alarm, the problem is more likely due to serious electrical problems, which is what the feature should be for.
If you have overloading pump(s) then high flow field problems (excessive demand) can be the cause of an overload problem. Examples would be:
1. Mainline break.
2. Zone valves sticking on.
3. Multiple valves operating simultaneously that should not be.
4. Multiple controllers operating simultaneously due to improper programming or time difference on controllers, or even irrigation controller malfunction.
5. Broken lateral lines (downstream of a valve) causing high flow on that zone.
Related Subjects on this Website:
Glossary - Service Factor (SF)
Glossary - Full Load Amps (FLA)
Non-Overloading Pumps and Motor Life Expectancy
See 2 Pump Graphs - Non-Overloading & Overloading
