To maintain plant operation and process continuity in power plants and industrial facilities, motor buses may require transfer from a present (old) source to a new source. Motor Bus Transfer (MBT) schemes and systems are employed to maintain process continuity in processes served by large motors or aggregates of smaller and large motors. Larger motors, of both the synchronous and induction variety, may require comprehensive, integrated source transfer strategies in order to avoid mechanical damage.

The coast down period and resultant voltage and frequency decay may take seconds, and unsupervised source transfer may cause damage. During improper transfer, mechanical damage may occur in the motor, the coupling to the load or the load itself, and is primarily caused by excessive shaft torque.

The total mission of a MBT system is—

• Maintain process continuity
• Effect source transfers so as not to cause damage to motors and connected loads

Match the Conditions at the Instant of Transfer—

• The M-4172 and M-4272 Digital High Speed Motor Bus Transfer System addresses fast (high-speed sync-check), in-phase (high-speed synchronizing), residual (undervoltage), fixed time transfer and manual transfer applications. With Beckwith Electric’s MBT System, all methods are running in parallel, to ensure the best method is used.
• You can use loose supplied relays or the Motor Bus Transfer System cabinet for specific automatic and manual transfer types.

Bus Transfer Zones

Enhanced Power Delivery and Safety—

• Dynamically adapts to source conditions, composite motor bus inertia and voltage decay characteristics to automatically determine the best method for transfer— even under adverse fault conditions.
• Mitigate the effects of overvoltage, high volts-per-hertz (overexcitation) and hard loading (mechanical shock) to your motors by assuring source transfer with a minimum of electrical and mechanical stress.
• Avoid needless power plant shutdown due to unavailability of auxiliary services after faulty source transfer.

Main-Backup Application

Main-Tie-Main Application

Motor Bus Transfer System Cabinet

The utility-grade Motor Bus Transfer System Cabinet consists of the M-4272 Digital High Speed Motor Bus Transfer Systems, test switches, control switches, terminal blocks and optional M-3919A Graphic Display Unit / Remote HMI. The system performs all of the necessary logic and control functions for motor bus transfers in two directions by automatically selecting either fast, in-phase or residual voltage transfer methods, according to the bus voltage decay characteristics. The system provides a fast transfer by supervising the delta phase angle and delta frequency between the motor bus and the new source, and blocks a fast transfer if the delta phase angle or delta frequency is excessive. If fast transfer is impossible because of excessive phase angle or delta frequency, the system performs a high-speed synchronizing transfer to close the motor bus in-phase with the new source at the first phase coincidence (first zero crossing). If bus voltage conditions prevent transfer by either method, the system contains a circuit that measures bus voltage, regardless of the frequency, and allows a residual voltage transfer to occur when the motor bus voltage has decayed to a safe level.

In addition to automatic transfer, the system provides manual hot parallel transfer (closed transition / make-before-break transfer) or manual open transition (break-before-make) transfer.

M-4272 Digital Motor Bus Transfer System

M-4172 Digital Motor Bus Transfer System

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