The Castair ALT230M is the mounted-panel version of the 230-volt two-compressor alternator relay. It performs the same function as the ALT230 — automatically alternating the lead/lag designation between two compressors to equalize running hours — but it ships with the relay already installed in a panel enclosure. Wire to the terminals, mount it to the wall, and the installation is done. No enclosure fabrication, no guesswork about component spacing or mounting hardware.
The M in ALT230M stands for mounted. That distinction matters for how you plan the installation. If you are dropping this relay into an existing control cabinet or custom panel, order the ALT230. If you want a finished, self-contained assembly ready for permanent wall installation, the ALT230M is the correct unit. Both control two compressors on 230V service. Both do the same job. The difference is the enclosure.
Why Lead/Lag Control Matters for Two-Compressor Systems
Every two-compressor system eventually develops the same problem without an alternator relay: one machine does most of the work and wears out early, while the second sits mostly idle. The idle machine ages without exercise, its oil degrades between uses, and when the worn primary finally fails, the backup is not in the condition you expected. The ALT230M prevents this by automatically rotating which compressor is the lead unit each cycle, so hours accumulate evenly across both machines.
Beyond maintenance, there are three operating reasons shops specify alternator relays:
- Duty Cycle Compliance: Reciprocating compressors have rated maximum duty cycles — typically expressed as minutes of run time per hour. Running one compressor continuously in a high-demand shop exceeds those ratings and voids the warranty. With the ALT230M cycling both units, each compressor gets recovery time between run cycles, staying inside manufacturer-specified duty cycle limits.
- Redundancy That Actually Works: Having a second compressor is only useful as backup if that compressor is maintained, exercised regularly, and ready to carry load. The ALT230M keeps the second unit active so when the primary needs service, the shop does not discover the backup has been sitting cold for six months.
- Predictable Maintenance Scheduling: When both compressors run equal hours, they need service at the same intervals. One oil change schedule. One filter schedule. One belt inspection schedule. Shops tracking two compressors with unequal hours are always guessing which one needs service and when.
Why the Mounted Version (ALT230M) vs. the Relay-Only (ALT230)
The ALT230 and ALT230M perform identically. The choice is about your installation setup:
- Choose ALT230M if: You want a complete, self-contained assembly. The enclosure is done, the relay is mounted, and you wire to labeled terminals. Cleaner installation, less field work, and the relay is protected from the shop environment by the enclosure.
- Choose ALT230 if: You are installing the relay inside an existing control cabinet, junction box, or custom panel assembly. You supply the enclosure and do the mounting. Lower price, more flexibility on placement and panel layout.
- In either case: Have a licensed electrician perform the installation. Compressor control wiring involves line voltage and must be done correctly.
Features
- Pre-Mounted Panel Assembly: Relay arrives installed in enclosure — no separate panel fabrication required. Mount to the wall, wire to terminals, done.
- 230V Rated: Correct for 230V single-phase or 230V three-phase compressor systems. Verify your compressor motor nameplates and panel voltage before ordering.
- Automatic Lead/Lag Alternation: Switches which compressor runs as lead unit each cycle without manual intervention. Hours distribute evenly across both machines automatically.
- Lag Compressor Start: Brings the second unit online automatically when demand exceeds the lead compressor's capacity. Both run until pressure recovers, then lag shuts down first.
- Enclosed Format: Panel enclosure protects relay from shop dust, moisture, and incidental contact — appropriate for permanent installation in a compressor room or mechanical space.
- Compatible with Castair Control Accessories: Works with Castair hour meters (HRM75, HRM50), time delays (TD100), and low oil controls (6304, 6304B) for a complete duplex compressor control system.
- Castair Manufacturing: Built by Cast Air in Minnesota, a manufacturer focused on duplex and industrial compressor controls and systems.
What's in the Box
- Castair ALT230M automatic alternator relay, pre-mounted in panel enclosure
- Manufacturer documentation and wiring diagram
Installation by a licensed electrician is recommended. The ALT230M is a line-voltage control component for compressor systems.
Available Configurations — Full Matrix
Castair offers four alternator relay configurations to match your voltage and installation preference:
| Model | Voltage | Format | Price | Best For |
|---|---|---|---|---|
| ALT230 | 230V | Relay only — you supply enclosure | $398.86 | Installing into an existing panel or custom control cabinet on 230V service |
| ALT230M (this unit) | 230V | Relay pre-mounted in panel enclosure | Call for pricing | Clean permanent wall-mount installation on 230V service — no enclosure fabrication |
| ALT460 | 460V | Relay only — you supply enclosure | $398.86 | 460V three-phase systems, installing into existing panel |
| ALT460M | 460V | Relay pre-mounted in panel enclosure | Call for pricing | 460V three-phase systems, clean permanent wall-mount installation |
The voltage choice is determined by your compressor motor voltage rating and your panel service. The format choice depends on whether you are integrating the relay into an existing panel (ALT230) or want a standalone mounted assembly (ALT230M). Call 866-217-0063 to confirm the right unit for your setup before ordering.
When Does a Shop Need This vs. Stepping Up or Down
The ALT230M is the right choice if:
- You are running two compressors on 230V service and want a finished panel assembly rather than a bare relay
- Your installation environment requires enclosed components — dusty compressor rooms, areas with moisture, or anywhere exposed relay components are not appropriate
- You want a professional, permanent-looking installation without building a custom enclosure
- You are replacing a failed alternator relay and want a clean drop-in replacement that does not require panel modification
The ALT230 (relay-only) is better if:
- You have an existing control cabinet or panel where you are integrating this relay alongside other components
- Your electrician prefers to supply and mount the enclosure to match existing panel hardware on site
- You want to minimize the purchase cost and handle enclosure yourself
You need the 460V version if:
- Your compressor motors are rated for 460V three-phase — confirm on the motor nameplate
- Installing a 230V relay on a 460V circuit is a wiring error that will damage the relay and potentially the compressor. Verify voltage before ordering.
Technical Specifications
- Model: ALT230M
- Part Number: ALT230M
- Manufacturer: Cast Air (Castair), Minnesota
- Voltage: 230 Volt
- Application: Two-compressor automatic alternation (lead/lag control)
- Format: Relay pre-mounted in panel enclosure
- 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: Call for current pricing and availability
- 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 stock the full Castair control accessory line and understand how these components fit together in a real duplex compressor installation. We can help you choose between the ALT230 and ALT230M, confirm your voltage requirement, and make sure the unit you order matches your actual installation before it ships.
- Free Shipping: On qualifying orders — confirm at checkout or by calling us
- Free Technical Support: Call 866-217-0063 with installation questions before or after your purchase
- Full Manufacturer Warranty: All Castair parts covered under manufacturer warranty — we handle warranty issues directly
- Honest Guidance: If the ALT230M is not the right unit for your installation, we will tell you that before the order ships
- Specialist Focus: Compressed air systems and components are what we do. Castair controls are a core part of our inventory and we know the product line.
Call 866-217-0063 for quick answers and help!
