The alternating relay is the control-logic device that balances duty between two (or more) compressors or pumps in a duplex or triplex room, it swaps which machine leads on each cycle so run hours stay even and the standby unit actually runs instead of sitting cold until it seizes. It lives in the Control & Logic layer, wired between the system pressure switch (or float/level control) and the two motor starters. On demand it starts the lead machine; on the next demand it makes the other machine lead. The result is automatic backup and balanced wear, neither machine carries the whole load, and the one that would otherwise sit idle for months gets exercised on a schedule. Any room with 2+ compressors should have one; without it, the lead machine runs every cycle, wears out first, and the "backup" is a machine nobody has confirmed will start.
When a duplex room needs an alternator, what it actually buys you, and where a bare relay falls short. Review the points below.
Without an alternator the lead machine runs every single cycle and the other sits idle. The lead wears out first, and the "backup" is a machine nobody has confirmed will start. The alternator swaps lead on each cycle so run hours stay even across both.
A compressor that sits cold for months seizes, weeps oil, and won''t start when you need it. Alternating exercises the standby on a schedule, so the day the lead drops, the backup is already proven and warm. Automatic backup is the real product here.
It''s a small part with an outsized job, it doubles effective machine life by halving cycles on each unit and turns a paper backup into a real one. If a plant has a duplex room and no alternator, that''s an open quote every time.
Quote the alternate / lead-1 / lead-2 selector so the customer can lock a known-good machine while the other is serviced. And at commissioning, cycle demand twice and confirm the lead actually swaps: a relay that doesn''t alternate looks fine and quietly runs one machine into the ground.
A bare alternator runs one machine at a time. If a heavy call needs both, the room falls behind and pressure sags. → Add the simultaneous-demand contact (a second demand input) so both come on together under load. Confirm peak vs single-machine output before quoting.
Electromechanical alternators with no delay can pull both contactors near-simultaneously on a fast cycle, hammering the building''s electrical service with stacked inrush. → On larger motors quote the electronic duplexer with lead/lag delay so the lag machine waits a few seconds.
The alternator switches the starter coils (pilot duty), not motor current, but undersize the contacts to the coil inrush and they weld or burn. → Size the contact rating to the starter coil VA from the starter data, with margin, never to motor HP.
Same job: keep two machines sharing the load, but the build type and the extra contacts decide how much control and protection you get. Match these to the room, along with the voltage.
A DPDT relay or alternator wired to transfer lead on each cycle. Simplest and cheapest, plug-in octal base for fast swap. Alternates strictly on cycle with no run-time logic. The right call for a basic duplex room that just needs even wear and automatic backup.
Adds adjustable lead/lag delay, run-time equalization, and status/alarm outputs. Holds the lag machine off a few seconds so two compressors don''t slam the building on the same start, and can equalize hours rather than strictly alternate. Quote it where the room needs sequencing finesse or remote status.
A second demand input (a second pressure or level switch) that brings BOTH machines on together when one can''t keep up. Without it the duplex runs one machine at a time and falls behind on a heavy call; with it the room has surge capacity. Specify when peak demand can exceed a single machine''s output.
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Each industry below uses this product across the listed areas. Open an industry to see how it fits the rest of its system.
Also applies to Duplex compressor rooms · Duplex pump and booster systems · Triplex and larger rooms (as part of the system) · Compressor-package retrofits · Distributor counter & RFQ work
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The alternator is a single point that can idle both machines if it fails, so multi-room plants reward holding a matched plug-in spare on the shelf, same coil voltage and base as the installed units turns a both-machines-down event into a minutes-long swap. Pair the spare alternator with a spare pressure switch and starter coil so the whole control circuit can be restored in one visit.
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