The water well industry often debates the preferences and features of a single-phase 2-wire motor vs. a single-phase 3-wire motor to operate their submersible pumps. While some features differ significantly, many of the basics are the same.
These motors are better characterized by their general motor type.
3-wire submersible motors are of two basic types:
• Capacitor Start - Induction Run
• Capacitor Start - Capacitor Run
2-wire submersible motors are of types:
• Permanent Split Capacitor
• Split Phase
All single-phase motors have a “Start or Auxiliary Winding” and a “Main Winding”. It is a matter of how these windings are powered and connected that makes a difference. Here are the differences.
In all single-phase motors, the primary work is done by the “main” winding which provides an alternating electro-magnetic field to attract the rotor in the same direction. The problem with a single-phase 3450 rpm motor is that it only can provide 2 magnetic poles. If the rotor is not turning to begin with, it doesn’t know whether to turn left or right, so it just sits still and hums.
The “start or auxiliary” winding is placed 90 degrees apart from the main winding and a capacitor is placed in series with this winding that shifts the magnetic field by ¼ of the turn or 90 degrees. This extra field causes the rotor to rotate, and the motor is referred to as a “Capacitor Start - Induction Run” motor.
When the motor gets to speed, a switch disconnects the capacitor and auxiliary winding, while the main winding continues doing the work. The capacitor used to start the motor delivers a large charge to the winding but can only do so for a short time without heating and melting down. A motor of this type struggles against heavy loads to maintain this speed.
With larger 3-wire motors a “Run Capacitor”, which is capable of continuous operation but delivers a smaller charge, is added to the start capacitor. It bypasses the switch so that it can continue to energize the auxiliary winding and assist the main winding. This is referred to as a “Capacitor Start - Capacitor Run” motor. A submersible motor of this type would use a control box with both types of capacitors. Both types use a third wire to connect the capacitor to the windings.
2-wire motors have the same basic windings as the 3-wire motors. One type includes a large run capacitor inside the motor that acts as a start and run capacitor and does not need to be removed from the windings with a switch. This motor requires only two motor leads as the capacitor is inside the motor and requires no control box since a switch is not necessary. The motor may have a little less starting torque due to less capacitance, but it also runs smoother because the auxiliary winding is always energized. This motor is referred to as a “Permanent Split Capacitor” motor.
Another type of 2-wire motor is the “Split Phase” motor. This motor also has a main and auxiliary winding. In this type of motor, the auxiliary winding is pulled out of the main winding which causes the magnetic field to be shifted much like the capacitor causes a shifted field of ¼ turn.
When the motor is started, a switch in the motor opens the auxiliary winding and the motor continues to run on the main winding in a way similar to the Capacitor Start - Induction Run motor. Only 2 conductors are needed to power this motor as well. Current reading on a Permanent Split capacitor will usually be lower than on a Split Phase because some current is diverted to the auxiliary winding. This has little to no effect on motor wattage, however and actual power requirements remain about the same.
Both types of 2-wire motors are limited in horsepower rating to about 1 ½ hp. This is due to the inability to power the motors with the needed capacitance that larger horsepower motors require.
All these motor types are proven to be reliable and failure rates are very similar. The 3-wire motor requires extra components, but these are located outside of the well and are easily accessible should replacements be needed. 3-wire motors can be built for much larger loads than the 2-wire because of the ability to add more components. Most manufacturers say there is little difference in failure rates between the different types.
The advantages and disadvantages of using a 2-wire over a 3-wire are minimal. The choice should be the preference of the pump installer who can apply the features that best fit his given area.