DC Motors in Remote Control and Power Switching Applications

Most power- or control-switching applications provide both manual and remote operator control capabilities. For these remote switching functions, D.C. motors serve a vital role as remote motor operators. Two remote control applications that employ DC motors are circuit switchers in electrical transmission and distribution substations in the utilities industry and rail track switches in the transportation industry.

Electrical Power Circuit Switchers

An electrical utilities substation is a high-voltage electric facility that’s used to switch, connect, or disconnect generators, equipment and other circuits in or out of service throughout a power grid. There are four types of electrical substations: Step-Up, Step-Down, Distribution and Underground Distribution Transmission Substations. 1 In these applications, circuit switchers are used to switch A.C. voltages up to 1100kV, and D.C. voltages up to 500kv. 2

Circuit switchers are electro-mechanical assemblies typically consisting of an interrupter sub-assembly, disconnect switch, fault sensing & protection, and a motor operator. The motor operator is used for remote switching or when the “disconnect switch’s function is integrated into a comprehensive system monitor and performance scheme such as a supervisor control and data acquisition system (SCADA).” 3

The motor operator is commonly a permanent magnet DC motor (brushed or brushless) that has a high torque, torsional output. The motor is typically a NEMA-rated, 48 VDC or 125 VDC, D.C. motor with features that can include permanently greased ball bearings, overload protection and dynamic braking. The motor operator “can be powered either via a substation battery source or via the input from an auxiliary AC source.” 4 For multiple switching operations, some motor operators “have their own internal batteries that can be fed from an auxiliary A.C. source via an A.C. to D.C. trickle charger for emergency operations in the event of loss of auxiliary AC power supply.”

Railroad Track Switches

Railroad switching is integral to the safe and normal operations of railroad transportation. It entails the manual or remote movement of switch rails, which are laterally moved from one position to another in order to change a railway junction point or spur so railway traffic can progress on its scheduled course.

The rail switching function can be accomplished manually or remotely. Manual, rail switching is performed by an operator at the railroad switch who moves a lever, rail operator or a hand pump to change the rail track’s position. Remote operation is performed via a track switch machine that is computer controlled remotely. This computer controller can sense when a train is on a specific length of rail track, provide warning signals, and initiate a rail switching operation by energizing an electric motor that drives the track from one position to another. 5 The track switch, drive motor is commonly a brushless, permanent magnet DC motor. “The controller has control circuitry for energizing and de-energizing field coils of the [brushless D.C.] motor sequentially as the armature rotates.” 6

Another type of rail switch machine is the electro-hydraulic rail switch. This type uses an electro-hydraulic power pack to provide the force to move the switched rail. The hydraulic power pack includes a manifold, control valves, pressure switch, pump and DC electric motor. The moving force is provided by a hydraulic cylinder, including a position transducer that provides feedback to control the movement of the cylinder or actuator. This rail switching type can be powered from a D.C source, a battery bank or solar cells for rail switch operations even in remote areas. In case of a loss of electrical power, a hand pump is available to manually switch the rail. 7

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