The Siren Board

All the STH-10s I've seen in my time are "A"s. I have never seen a "B" in real life. I have seen pictures of an STH-10"B", but it looks slightly different than the STH-10"A." How come "A"s seem to be SO much more popular. Also, all the STHs I've heard sound like "A"s.

the B-series STH was the single phase version. The motor on the STH10B was a little taller. The A series is more common because it is more common for a city to have 3 phase available for their warning system.

The vast majority of motors greater than a few HP are 3 phase, regardless of whether they are powering a siren or other equipment. The three phases are 120 electrical degrees apart and are connected to separate motor windings that are 120 mechanical degrees apart. This results in a "rotating" magnetic field in the motor, so called because it increases in strength in a radial fashion around the circumfrence of the stator. The rotational speed is proportional to the line frequency and number of stator pole windings.

When a short circuited rotor winding (ofter called a squirrel cage) is placed in the stator, a current is induced. The corresponding magnetic field in the rotor opposes the stator field, casing a torque which makes the rotor winding spin. As the rotor spins, it keeps up with the rotating stator field, minus a small amount of "slip" which keeps a rotor current induced that is sufficient to equal the load torque on the motor. This is called a polyphase induction motor, and is the industry standard in high power constant speed AC machines.

A single phase motor has an alternating magnetic field, as opposed to a rotating magnetic field, which requires some way to make the rotor move radially instead of just oscillate. This is often accomplished with a commutator, which can be thought of as a "mechanical rectifier". It keeps the rotor current aligned with the stator current, but at the cost of additional mechanical parts (commutator segments and brushes). It can also be accomplished with capacitive start windings or shaded poles to simulate a rotating magnetic field. The added machanical parts and higher per watt phase current of single phase motors are the main reasons that 3 phase motors are preferred for higher power applications.