1 vs. 3 Phase Contactors
Every 3 phase contactor has 3 poles, one for each phase, while every single phase contactor has either 1 or 2 poles (please see "Switch Terminology: Pole Count" for more information). To explain the difference in the number of poles, we need to first review how AC power works.
Remember that AC means alternating current and refers to the fact that the rotation of a conductor in the magnetic field of a generator produces a constantly changing voltage that, in turn, constantly changes the direction of the current in a circuit. The pattern of voltage change in an AC system can be described by a sine wave, which is why AC voltages are also described by their "waveforms."
Sine waves are used to describe AC power because:
• The movement of the rotor directly affects the AC power produced (e.g. frequency).
• Rotors in AC generators move in a circle.
• The sine function (and cosine) describes points on a circle.
The use of sine is preferred over cosine because we assume the generator produces 0 voltage at time t=0 and the sine graph starts at (0,0). Cosine starts at (0,1).
The difference in pole counts between a single phase and a 3 phase contactor has to do with the fact that there are three different AC voltage waveforms being supplied to a 3 phase motor, which requires 3 "hot" lines and 1 neutral (or ground), and there is one pole for each hot line. In contrast, single phase systems use voltage sources with the same AC voltage waveform, which requires only 2 hot lines and a ground.
Whether you use a single or double pole contactor for a single phase motor is up to you, but all 3 phase contactors come with 3 poles.
Since "phase" is a term that only applies to AC power systems, a 3 phase contactor should only be used for AC motors. However, you can still use DC power for the control circuit itself, since the control circuit (the circuit powering the contactor) is separate from the motor circuits.
Can I use a 3 phase contactor for a single phase motor?
Yes, but you will need to rewire the cables to make a series connection. For example, you could connect T1 on the overload relay to L2 on the 3 phase contactor (see the figure to the right).
Also, even though a 3 phase contactor can be used for single phase motors, contactors built especially for single phase motors are cheaper.