Castair Automatic Two Compressor Alternator Relay 230 Volt ALT230

The Castair ALT230 is a 230-volt automatic alternator relay that manages lead/lag operation between two compressors sharing a common air system. It automatically switches which compressor runs as the lead unit each cycle, so both machines accumulate equal operating hours over time. If you are running two compressors on a shared tank or header and manually deciding which one starts, this relay eliminates that entirely — and it does something manual rotation never can: it tracks the swap automatically, every time, without depending on anyone remembering.

This is not a luxury item for large facilities. Any shop running two compressors that share a load has a real reason to buy an alternator relay: equal wear, predictable maintenance, continuous uptime, and in many cases, manufacturer warranty compliance. The ALT230 is the 230-volt version for single-phase or 230V three-phase service. If your compressors run on 460V, see the ALT460 or ALT460M instead.

Why Shops Buy Alternator Relays

The honest reason most shops end up with two compressors is that the first one was not enough, so they added a second. Now they have two machines that both need maintenance, both have duty cycle ratings, and both need to be kept ready. If one compressor does 90% of the runtime, that machine will fail years ahead of schedule while the second sits nearly new. When the worn machine finally goes down, you discover the second one has not been exercised in months and the oil has degraded. The alternator relay prevents all of this.

Three specific reasons shops buy this relay:

• Even Wear Across Both Units: Compressor manufacturer warranties commonly specify maximum duty cycles — how many minutes per hour a reciprocating or rotary unit should run continuously. The ALT230 keeps both machines inside their rated duty cycles by sharing the load. It also prevents one unit from becoming the permanent lead and wearing out while the second ages sitting idle.
• Continuous Uptime: When one compressor needs service, the second takes over without manual rewiring. For a shop that cannot afford downtime — collision centers, dealerships, production facilities — this redundancy is not optional. The alternator relay is what makes the backup compressor actually useful instead of just theoretical backup.
• Predictable Maintenance: Equal hours across both units means both machines need service at the same intervals. One oil change schedule, one filter schedule, one belt inspection schedule. Shops that track maintenance on two compressors with wildly different hours are always guessing. Equal hours fixes that.

How the ALT230 Works

• Lead/Lag Designation: One compressor is designated lead, the other lag. The lead compressor starts first when system pressure drops below the cut-in setpoint. If pressure continues to drop (demand exceeds lead capacity), the lag unit starts to assist.
• Automatic Alternation: After each pressure cycle or at configurable intervals, the ALT230 swaps the lead and lag designations. The machine that just ran as lead becomes lag on the next cycle. Hours distribute evenly without any manual input.
• Lag Start on High Demand: When the lead compressor cannot keep up with demand alone, the relay brings the second unit online automatically. Both run until pressure recovers, then the lag unit shuts down first.
• 230 Volt Operation: Rated for 230V electrical service. Verify your compressor motor nameplates and panel voltage before ordering. For 460V systems, use the ALT460 or ALT460M.

Features

• Automatic Lead/Lag Swap: No manual intervention required — the relay handles alternation every cycle
• Dual Compressor Start Coordination: Manages start and stop sequencing for both units to prevent simultaneous starts and power surges
• 230V Rated: Correct for 230V single-phase or 230V three-phase compressor installations
• Relay-Only Format: The ALT230 is the relay without an enclosure — install in your own panel, junction box, or control cabinet. For a pre-mounted panel assembly, see the ALT230M.
• Compatible with Castair System Accessories: Works alongside Castair hour meters (HRM75, HRM50), time delays (TD100), low oil controls (6304, 6304B), and pressure controls for a complete duplex compressor control system
• Castair Manufacturing: Designed and built by Cast Air, a Minnesota manufacturer specializing in duplex and industrial compressor systems and controls since their founding

What's in the Box

• Castair ALT230 automatic alternator relay (relay unit only, no enclosure)
• Manufacturer documentation and wiring diagram

Note: The ALT230 is a relay-only unit. If you need the relay pre-mounted in an enclosure, order the ALT230M instead. An electrician familiar with compressor control wiring should handle the installation.

Available Configurations — Full Matrix

Castair makes this alternator relay in four configurations to match your voltage and installation preference:

The voltage choice (230V vs. 460V) is determined by your compressor motor nameplates and your electrical panel service voltage. Do not mix voltages. The format choice (relay-only vs. mounted) depends on your installation setup. Call us if you are unsure — this is a common question and we can help you confirm the right unit before you order.

When Does a Shop Need This vs. Stepping Up or Down

You need the ALT230 if:

• You are running two compressors on 230V service sharing a common air system or storage tank
• You want to equalize wear between both machines without manual tracking or rotation schedules
• Your compressor manufacturer's warranty specifies a duty cycle limit that one compressor alone cannot reliably meet under your shop's actual demand
• You are building a new duplex compressor installation and want proper lead/lag control from day one
• One of your compressors does all the work and you want to know which one before it fails

The ALT230M is the better choice if:

• You want a finished installation — relay in an enclosure, ready to mount and connect — without building your own panel
• Your installation environment requires enclosed electrical components (dust, moisture, physical contact risk)
• You want a cleaner, more permanent-looking installation

You need the 460V version (ALT460 or ALT460M) if:

• Your compressor motors are rated for 460V three-phase service
• Your electrical panel supplies 460V (also commonly labeled 440V or 480V — confirm with your relay spec)
• Mixing 230V and 460V components is a wiring error that will damage equipment — verify before ordering

You do not need an alternator relay if:

• You only have one compressor — alternator relays are specifically for two-compressor systems
• Your two compressors are on completely independent systems with no shared tank or header — though equal wear is still a maintenance best practice

Technical Specifications

• Model: ALT230
• Part Number: ALT230
• Manufacturer: Cast Air (Castair), Minnesota
• Voltage: 230 Volt
• Application: Two-compressor automatic alternation (lead/lag control)
• Format: Relay only (no enclosure included)
• Compatible Accessories: HRM75 (hour meter 24-277V), HRM50 (hour meter for gas engines), TD100 (time delay 18-240V, 1-1023 seconds), 6304 (low oil control), 6304B (float switch for low oil)
• Price: $398.86
• Dimensions/Weight: Contact us to confirm shipping dimensions

Why Buy From Air Compressor Factory Direct?

Air Compressor Factory Direct has specialized in compressed air systems and components since 2016. We carry the full Castair control accessory line — alternator relays, hour meters, time delays, low oil controls — and we know how these components fit together in a real duplex compressor installation. When you call us, you talk to someone who understands the difference between the ALT230 and ALT230M, knows when to use the 460V version, and can help you confirm you are ordering the right unit for your electrical service before the order ships.

• Free Shipping: On qualifying orders — confirm at checkout or by phone
• Free Technical Support: Call 866-217-0063 before or after your purchase with installation questions
• Full Manufacturer Warranty: Castair parts covered under manufacturer warranty — we handle any warranty issues directly
• Honest Guidance: If the ALT230 is not the right unit for your installation, we will tell you that before you order, not after
• Specialist Focus: We do not sell everything to everyone. Compressed air systems are our focus, and Castair controls are a core part of that inventory.

Call 866-217-0063 for quick answers and help!