All clutches mechanically join two separate driveshafts for the transfer of energy. Typically the use of clutches are made so the driver, an electric motor or a combustion engine can be connected or disconnected as needed. Perhaps the most common use of an electric clutch is an automobile's air conditioning system compressor. Since the compressor must always be connected to the combustion engine's movement with a fan belt, there must be a method for engaging the compressor when the air conditioning system is turned on. The simplicity of the electric clutch fits this operational criteria.
The electric clutch is manufactured in such a way that all of the mechanical and electrical parts are enclosed within the rotating clutch assembly. This allows for a small profile. In other words, the electric clutch can be used in areas with very little room for operation. All electric clutches utilise a set of wire coils that will produce a magnetic field when electrical power is switched on. When this magnetic field is energised, it pulls or actuates a set of metal pieces called clutch dogs. These clutch dogs ride inside a metal cup. The metal cup is mechanically attached to the driven device. In an automobile air conditioning system, the rotating dogs will physically engage the inside of the metal cup when activated by the electric coils. This causes the transfer of mechanical energy to drive the air conditioner's compressor.
The electric brakes that are used on trailers act exactly like the electric clutch. In fact it is the same mechanism, but the action of the electric coils mechanically engages brake pads to stop the moving trailer. Enclosed within the rotating wheel cylinder drum is a set of curved brake pads. Theses brake pads mimic the action of the clutch dogs for an electric clutch. When power is sent to the brakes' electric coils, they push the brake pads outward to grab the moving wheel drum. Since the trailer wheels are physically attached to the drum, the brake pads slow and then stop the movement when they come in contact with the smooth steel interior. The brake pads are rigidly attached to the fixed portion of the trailer frame. This fixed position allows the brakes to act as a stopping mechanism. They still transfer energy, but in this case the brakes pads or "dogs" transfer this energy to the fixed trailer frame and cause the trailer to slow down and eventually stop.