How to Choose a Replacement Swamp Cooler Motor: HP, RPM, Frame, Shaft and Amps

The fastest way to pick a replacement swamp cooler motor is to copy the old motor's nameplate: horsepower, voltage, speed (almost always 1,725 rpm), single or two speed, frame size and shaft diameter, rotation, mounting, full-load amps and thermal protection type. Match each of those and the new motor will bolt in, accept your existing pulley, and move the same air. Where the nameplate is gone or the old motor was wrong to begin with, this page shows you how to work out each spec from the cooler itself.

If you are still deciding whether the motor is actually bad, start with swamp cooler motor not running. For an overview of how cooler motors work and fail, see the swamp cooler motor guide.

Read the old nameplate first

Pull the old motor (power off at the disconnect, belt off, wires labeled) and clean the nameplate with a damp rag. Write down or photograph every line. The fields that matter, and what they mean:

HP

Rated output horsepower, typically 1/3, 1/2, 3/4 or 1 on residential coolers.

V

Voltage: 115 (120) or 230 (240). Some motors are dual-voltage with a jumper or lead change.

RPM

Full-load speed. For two-speed motors, both speeds may be listed, such as 1725/1140.

FLA or A

Full-load amps at each speed and voltage.

FR

NEMA frame, usually 48 or 56, sometimes with a suffix such as Y or Z indicating special mounting or shaft.

Rotation

CW, CCW or reversible, viewed from the shaft end or lead end as the label states.

SF

Service factor, such as 1.15 or 1.35. The motor can run safely at rated HP times SF.

Thermal protection

"Auto" (automatic reset), "Manual" (red reset button), or "None".

Enclosure

ODP (open drip-proof) or TEAO (totally enclosed air over). Both are used on coolers.

Horsepower and speed

Horsepower

Cooler manufacturers size the motor to the blower, the pulley ratio and the external static pressure of a typical duct system. As a rough guide for belt-drive residential units:

These ranges overlap because duct design, pulley ratio and blower size differ between models. If the cooler's data plate lists a motor size, use it. If you are replacing a motor that someone already "upgraded," check that the cooler's circuit and switch are rated for the larger motor's amps.

Speed: why 1,725 rpm matters

Belt-drive cooler motors are 4-pole induction motors with a synchronous speed of 1,800 rpm at 60 Hz, which run about 1,725 rpm under load. The blower's speed depends on that number and the pulley ratio:

A 1,140 rpm (6-pole) motor would slow the blower by about a third and cut airflow accordingly. A 3,450 rpm (2-pole) motor would roughly double blower speed and overload everything. Direct-drive coolers (common on window and some side-draft units) are different: they often use 1,075 to 1,140 rpm multi-speed motors with the blower wheel mounted right on the shaft. Never swap between belt-drive and direct-drive motor types.

Frame size, shaft and mounting

NEMA frame numbers fix the shaft height and shaft diameter, so they are what make a motor fit the cradle and the pulley.

Measure the shaft with calipers on a clean section. A worn shaft can read a few thousandths small, but you will not confuse 0.500 with 0.625 inch. Also note:

• Mounting. Cooler motors commonly use a resilient (rubber-isolated) cradle base or a rigid base bolted to an adjustable slide. Some cooler designs clamp the motor with a band around the shell instead. Match the base so your existing slide and belt adjuster work.
• Shaft length and flat. The shaft must be long enough for the pulley to align with the blower pulley. Most shafts have a flat for the pulley set screw.
• Lead location and length. Leads must reach your junction box without strain.

Single speed vs two speed, voltage and electrical type

Match the number of speeds to your control. If your switch or thermostat has high and low positions, buy a two-speed motor; installing a single-speed motor means low does nothing. If you want to add a low speed to a cooler that never had one, you also need a two-speed switch and possibly new wiring, which is covered in two-speed motor wiring.

Voltage must match the circuit exactly. Many coolers in the Southwest run on 120 volts; larger units and some installations use 240 volts for lower current. A dual-voltage replacement motor gives flexibility, but you must set it to the right voltage per the label before powering it.

Electrically, cooler motors are usually split-phase (internal centrifugal start switch, no capacitor) or permanent split capacitor (external run capacitor). Either works on a belt-drive cooler if horsepower, speed and frame match. If your replacement is a PSC motor, mount the capacitor somewhere dry and secure.

Thermal protection and bearings

A thermally protected motor shuts itself off when windings overheat, which saves the motor when a belt is too tight or a bearing drags.

Automatic reset is the norm on residential cooler motors. For bearings, choose ball bearings for low maintenance, or sleeve bearings if you prefer a quieter motor and will oil it. See blower wheel and bearings for the same tradeoff on the blower side.

Matching the motor pulley

The motor pulley (often an adjustable or variable-pitch sheave) sets blower speed. When you replace a motor you can reuse the old pulley if the bore matches, or buy a new one. Two decisions matter:

• Bore. 1/2 inch for 48 frame, 5/8 inch for 56 frame.
• Pitch diameter and groove. Keep the same belt section (commonly 4L or A for cooler drives, sometimes 3L on small units) and a similar pitch diameter so the belt length and blower speed stay the same.

That example is the reason to measure amps after any motor or pulley change. Set the belt tension, close all access panels (open panels reduce load and give a falsely low reading), make sure the house has open windows for relief air, and clamp an ammeter on one supply wire with the motor on high. The reading should be at or below the nameplate FLA. If it is over, reduce the motor pulley pitch diameter slightly. Belt tension and sheave adjustment are covered in belt and pulley adjustment, and relief air in venting and relief air.

Replacement checklist and common mistakes

1. Record the nameplate or, if missing, measure the shaft and frame height and count the speeds on your switch.
2. Confirm voltage at the cooler disconnect with a meter.
3. Buy matching HP, rpm, speeds, frame, voltage and rotation. A rotation-reversible motor covers either case.
4. Check the pulley bore and buy a pulley if needed.
5. Mount, align and tension. Sight down the belt to align pulleys; set tension to manufacturer spec.
6. Wire per the new motor's diagram, not the old color scheme.
7. Test both speeds and measure amps on high with panels closed.

Mistakes technicians commonly find in the field: a 1,140 rpm direct-drive motor installed on a belt-drive cooler, a two-speed motor wired so both settings run high, an adjustable pulley opened all the way to "get more air," and a new motor installed while the real problem, a seized blower bearing, was left in place. If the blower does not spin freely by hand with the belt off, fix that before the new motor goes in. For sourcing universal motors and other parts, see the universal replacement parts guide, and to see whether your airflow target is right for your house, try the cooler size calculator.