A variable-frequency drive (VFD) (also termed adjustable-frequency drive, variable-speed drive, AC drive, microdrive or inverter drive) is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage


SCRs are unidirectional devices (i.e., can conduct current only in one direction) as opposed to TRIACs which are bidirectional (i.e., current can flow through them in either direction). SCRs can be triggered normally only by currents going into the gate as opposed to TRIACs which can normally be triggered  by either a positive or a negative current applied to its gate electrode.


A motor–generator (an M–G set or a dynamotor for dynamo–motor) is a device for converting electrical power to another form. Motor–generator sets are used to convert frequency, voltage, or phase of power. They may also be used to isolate electrical loads from the electrical power supply line. Large motor–generators were widely used to convert industrial amounts of power while smaller motor–generators were used to convert battery power to higher DC voltages.

Door Edge

Door Edges or Light curtains are optoelectronic devices that are used to safeguard persons in the vicinity of moving elevator doors with the potential to cause harm. Light curtains are supplied as a pair with a transmitter and receiver. The transmitter projects an array of parallel infrared light beams to the receiver which consists of a number of photoelectric cells. When an object breaks one or more of the beams a stop signal is sent to the door operator.

Battery Lowering

When the power goes off, Battery Powered Lowering comes on while all elevator safety features remain operative. Immediately the functions of control and door operation are given their proper voltage by the use of a battery operated transistorized power supply. The elevator then descends to the lowest landing and opens its doors to discharge passengers who might otherwise become trapped.

ADA Phone

The 2006 International Building Code, most likely adopted by your local building inspectors, states that all elevators must comply with ASME A17.1. That engineering document requires a two-way means of emergency communications in passenger elevators.  This means of communication must connect with emergency or authorized personnel and not an automated answering system. The communication system must be push-button activated. The activation button must be permanently identified with the word “HELP”.  A visual indication acknowledging the establishment of a communications link to authorized personnel must be provided.  The visual indication must remain remain on until authorized personnel terminates the call.  The building location, the elevator car number, and the need for assistance must be provided to authorized personnel answering the emergency call.  The use of a handset by the communications system is prohibited.  Only the authorized personnel answering the call can terminate the call.  Operation instructions for the communications system must be provided in the elevator car.