Siren Motors

This document explains the details of different types of siren motors, and answers the question "Why are there so few single-phase sirens these days?" This section is an original work copyright Adam Smith, BSEE ©2001. Permission is required for republishing any of this text verbatim.

Permanent Magnet DC Motors
Used in: Federal 2001 (1/3HP), only for rotating the siren

The permanent magnet motor is a DC-only commutated motor that uses permanent magnets to provide the stationary field poles. Power is applied only to the DC rotor (armature) through a set of carbon brushes.

Series-Wound DC Motors
Used in: Federal 2001 (6HP) and other DC sirens, including all fire engine sirens.

The series-wound DC motor is a commutated motor which consists of wound field coils in series with a wound rotor called the armature. Electrical connections to the armature's commutator are made through replaceable carbon brushes. Since these motors are indentical in principle to salient-pole universal (AC-DC) series motors, details of operation are discussed in the following section on universal motors. Most series-wound DC motors operate very well on a low-frequency AC source of the same voltage (RMS) as the intended DC supply, but generally produce less starting torque.

12V DC series motor from
a fire truck siren

Series-Wound Universal Motors
Used in: Federal Model 2 (2HP), Thunderbolt chopper, Models L and D (1/2HP), A and B9 (1/3HP) plus others

The universal (AC-DC) motor is a single-phase commutated motor with wound field coils in series with a DC type armature (wound rotor). This type of motor is indentical in principle to the DC series motor, but a few modifications have been made to optimize the motor for AC use: The cores of the field poles are made from stacks of laminated sheet metal punchings like you find in transformers, instead of solid iron. This is to reduce the eddy-current losses in the core. In addition, the slots of the armature are slanted slightly to reduce AC buzzing and give the motor uniform starting characteristics regardless of the armature's initial orientation relative to the field coils. Shown here are the armature and field coils of a typical universal motor. This motor happens to be from a vacuum cleaner, but the design is common to siren motors as well.

Repulsion-Induction Motors
Used in: Federal Models 3, 5, 7, STH-10, STL-10, SD-10 and Federal's other old single-phase sirens

The repulsion-induction motor is a combination of a repulsion motor and a squirrel-cage induction motor. In the case of siren duty, this motor is always a 2-pole configuration. The stator winding is identical to the run winding of a 2-pole split-phase or capacitor-start motor. The rotor is nearly identical to a universal series motor armature, with the exception of having a greater number of windings (in most cases) and no connection to a power source. The brushes are connected to each other directly, in order that they may complete a circuit through windings within the rotor.

The closed-loop circuits in the rotor are effectivly the short-circuited secondaries of a transformer, where the motor's field windings are the primary coil. The currents induced in the rotor create a magnetic field which repels that of the field winding (Lenz's law). This repulsion is what gives the motor it's torque. Rotation happens because the brushes are offset 15 or so degrees from the field poles, so that the repulsive forces are pushing on the rotor somewhat tangentially to it's rotation axis (see the schematics below).

In addition to this repulsion motor setup, the rotor also has buried within it a squirrel cage winding. As the repulsion-induction motor comes up near synchronous speed (3600 RPM on 60Hz), the squirrel-cage winding is responsible for most of the torque, and the repulsion effect diminishes.

Capacitor-start Single-phase Induction Motors
Used in: Sentry and ASC single-phase sirens

No text yet for this motor type.

Polyphase (3-phase) Squirrel-cage Induction Motors
Used in: Many 3-phase sirens

No text yet for this motor type.