For many years, phase converters and four or six-inch submersible pump motors have been successfully applied to rural single phase lines. It is crucial to understand the unique characteristics of submersible pump motors and the specific considerations that arise when these motors are operated using a phase converter. These factors must be carefully evaluated to determine the appropriate size of the phase converter for the submersible pump motor.
The scarcity of water in rural America is a growing concern, and as these areas expand, the challenge will only intensify. One cost-effective solution for obtaining large quantities of water is through the use of submersible pumps. Submersible pump motors are typically available in single phase designs up to 7.5 HP. However, many rural installations require motors that exceed this size, which are only available in three phase designs. Unfortunately, three phase power lines are not widely available in rural areas, and the cost of extending these lines can exceed $30,000 per mile. For many, extending three phase power for a single submersible pump is not economically viable. If single phase power is available near the installation site, a static or rotary phase converter can be used to operate larger three phase submersible pump motors, providing a highly economical solution.
The Three Phase Submersible Pump Motor
Submersible pump motors are specialized units, typically manufactured in diameters of four or six inches. They must be sealed to protect against the surrounding fluid. Due to the diameter limitations, the motor must be lengthened to increase horsepower, resulting in a long, slender design that may encounter vibration issues not typically found in above-ground motors. However, the motor's placement in fluid ensures a constant ambient temperature and aids in cooling.
These motors do not conform to standard full-load current ratings like their above-ground counterparts of the same horsepower and speed. Submersible pump motors have a "maximum current rating" that includes the service factor amps, and the motor may operate near this maximum rating.
Examples
• Above ground motor
10 HP – 230 volt – 3 phase – 26.8 amps
15 HP – 230 volt – 3 phase – 39.2 amps
• Submersible pump motor
10 HP – 230 volt – 3 phase – 36.0 amps
15 HP – 230 volt – 3 phase – 51.0 amps
Given the increased full-load ampere and service factor ratings, submersible pump motor installations must be evaluated differently than above-ground motor installations of equivalent horsepower. Wire sizes will be larger, and the KVA requirements will be higher. Manufacturers provide charts specifying the correct wire size for each motor, and these recommendations should be followed precisely.
When applying these motors to solid three phase lines, installation procedures differ from those used for above-ground motors. Phase current balance in three phase submersible pump motors is critical; phase currents should typically be balanced within 5%.
Open Wye or Open Delta three phase services are generally not recommended for submersible pump applications. If they must be used, a larger horsepower motor and larger conductor size should be employed.
Submersible pump motors are often subjected to loads that cause the motor to operate near the service factor amps. These factors affect both the motor's installation on a three phase line and the application of a phase converter. The phase converter, whether static or rotary, must be properly sized to match the maximum ampere rating of the submersible pump motor. Some phase converters are specifically sized for submersible pumps, while others recommend using the next larger size converter for submersible pump motor applications.
Static Phase Converters
Static phase converters are typically manufactured in two styles: straight capacitor designs or autotransformer-capacitor designs. The straight capacitor design is not recommended for submersible pump motors; instead, the autotransformer-capacitor type should be used.
The autotransformer-capacitor design is the most commonly applied solution for submersible pump applications. This proven device is capable of operating a three phase motor at 100% rated horsepower on a single phase line. The unit consists of an autotransformer, running capacitors, starting capacitors, and a contactor to disconnect the starting capacitors once the motor reaches speed. Proper sizing of these units is essential to ensure satisfactory operation of the three phase submersible pump motor on a single phase line. The maximum ampere rating of the submersible pump motor should be compared to standard full-load ampere charts for three phase motors to determine the appropriate phase converter size. Some manufacturers offer specific phase converters designed for submersible pump motors.
The autotransformer-capacitor phase converter has been successfully applied to submersible pump motors throughout North America for over 40 years. Submersible pump motors typically have a high power factor, ranging from 80-90%, which allows for the use of higher autotransformer taps. Phase currents can be adjusted within the 5% balance required.
This type of converter offers excellent power line characteristics. The full-load single phase power factor, with the motor operating, will be near unity or slightly leading, depending on line characteristics, load, and conductor length between the converter and motor. The starting current will be 2.5-3 times the full-load rating of the converter on the single phase line, with a leading power factor during starting. Submersible pump motors usually start in less than one second with this type of converter. Continuous operation, such as open discharge, can be achieved with excellent motor phase current balance.
It is important to carefully check phase currents and take the necessary steps to achieve acceptable balance. This may involve adding or removing capacitors or adjusting the converter's transformer tap for optimal balance.
The main disadvantage of applying submersible pump motors to autotransformer-capacitor converters is that current balance can only be achieved under one load condition. Significant load fluctuations can cause phase currents to go out of balance.
The autotransformer-capacitor phase converter can be designed with a unique feature that allows it to operate a 460-volt three phase submersible pump motor from either a 230-volt or 460-volt single phase line. This feature can result in considerable installation savings, particularly if only 230-volt single phase power is available for a deep-set submersible pump motor. Using a 460-volt three phase motor on a 230-volt single phase system reduces wire size and pump control size compared to a 230-volt three phase installation.
Installation Considerations
The single phase electrical service to the autotransformer-capacitor converter should follow standard wiring practices for the single phase full-load ampere rating of the converter. This includes using a single phase safety switch or circuit breaker as the converter disconnect and selecting the appropriate wire size and type. Wiring from the phase converter to the submersible pump motor should follow the three phase recommendations provided by the submersible pump motor manufacturer. Specifications for motor starter size, overload protection, and conductor size for the pump's depth should be confirmed with the pump manufacturer.
The autotransformer-capacitor style converter has very low power consumption when the motor is not operating, with losses limited to core and copper losses in the autotransformer. For example, a 15 HP unit would dissipate about 60 watts. The starting capacitors in this converter provide approximately 150% starting torque to the submersible pump motor, which is typically sufficient for the application.
Rotary Phase Converters
Rotary phase converters are available in two types: non-adjustable rotary transformers and adjustable rotary transformers. Both have been successfully applied to three phase submersible pump motors on single phase lines. These converters consist of a rotary transformer (also known as a motor) and one or more capacitor banks.
The non-adjustable rotary converter offers limited control over motor phase currents. While phase currents can be adjusted by varying the capacitance in the circuit, achieving current balance within the 5% tolerance recommended for submersible pump motors is often challenging.
The adjustable rotary converter, however, enables installers to adjust phase currents within the 5% recommendation by adjusting the rotary transformer taps and varying the capacitance. This provides a distinct advantage, as phase currents remain better balanced over the load range of the submersible pump motor, minimizing mechanical motor vibrations caused by negative sequence currents.
Installation Considerations
Properly installed rotary converters for submersible pumps should include a single phase safety switch or circuit breaker and a magnetic contactor ahead of the converter. The converter's output should follow the submersible pump motor manufacturer's specifications regarding motor starter size, overload protection, and conductor size for the pump's depth.
Rotary converters typically develop very low starting torques. In applications where high starting torque is required, it may be necessary to add a starting panel to the installation. The starting panel should be connected to the load side of the magnetic starter. When wired this way, the starting panel will bring the motor up to speed in 2-3 seconds and automatically disconnect itself from the circuit when the motor reaches full speed.
Conclusions
It is possible to successfully operate a three phase submersible pump motor on a single phase line using either a static or rotary phase converter. However, proper installation can only be achieved by adhering to the recommendations and specifications of both the phase converter and submersible pump motor manufacturers.
Contact our Sales team for more information on the RONK line of PHASE CONVERTERS: 1-800-221-7665
