Stator

This article is about a component of electromotive devices. For the component of a compressor, see Axial compressor.

Rotor (lower left) and stator (upper right) of an electric motor

Stator of a 3-phase AC-motor

Stator of a brushless DC motor from computer cooler fan.

The stator is the stationary part of a rotary (electric) system or a Civil defense siren, found in an electric generator, electric motor, siren, or biological rotors.

In motors

Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the armature, receiving its influence from moving field coils on the rotor. The first DC generators (known as dynamos) and DC motors put the field coils on the stator, and the power generation or motive reaction coils on the rotor. This was necessary because a continuously moving power switch known as the commutator is needed to keep the field correctly aligned across the spinning rotor. The commutator must become larger and more robust as the current increases.

Stator winding of a generator at a Hydroelectric power station.

The stator of these devices may be either a permanent magnet or an electromagnet. Where the stator is an electromagnet, the coil which energizes it is known as the field coil or field winding.

The coil can be either copper or aluminum. To reduce load losses in motors, manufacturers invariably use copper as the conducting material in windings. (For more information, see: Copper in energy efficient motors). Aluminum, because of its lower electrical conductivity, may be an alternate material in fractional horsepower motors, especially when the motors are used for very short durations.

An AC alternator is able to produce power across multiple high-current power generation coils connected in parallel, eliminating the need for the commutator. Placing the field coils on the rotor allows for an inexpensive slip ring mechanism to transfer high-voltage, low current power to the rotating field coil.

It consists of a steel frame enclosing a hollow cylindrical core (made up of laminations of silicon steel). The laminations are to reduce hysteresis and eddy current losses.

In sirens

In a siren, the stator is the crucial part which actually cuts off and reopens air flow pushed by the blades of the chopper as the blades move past holes in the stator known as ports, generating the siren's tone(s). One can determine the port ratio of a siren by looking at the number of ports in each row on a stator. A siren with only one row of ports is called a single tone siren. A siren with two rows of ports is known as a dual tone siren.

Tone variations

• By placing a second stator over the main stator and attaching a solenoid to it, one can repeatedly close and open all of the stator ports thus creating a tone called a pulse. If this is done while the siren is wailing (rather than sounding a steady tone) then it is called a pulse wail.
• By doing this separately over each row of ports on a dual tone siren, one can alternately sound each of the two tones back and forth, creating a tone known as Hi/Lo. If this is done while the siren is wailing, it is called a Hi/Lo wail. This equipment can also do pulse or pulse wail.
• Using the same equipment as Hi/Lo or Pulse, one can manually open the stator ports for short dits and long dahs as in Morse code. A siren which can do both pulse and Morse code is known as a code siren.