What it is

A shaded-pole motor is the simplest, cheapest AC motor in the trade. It has no start capacitor, no start switch, and no run capacitor — just a main winding and a clever trick (a "shading coil") that gives it just enough phase shift to start in one direction. You'll find shaded-pole motors driving low-torque, light loads: small condenser/evaporator fans, draft inducers and combustion-air blowers on older furnaces, small exhaust fans, and refrigeration evaporator fans.

It's a low-efficiency, low-torque motor, but it's bulletproof and dirt cheap, which is why it survives on light-duty jobs. This article covers identifying it, picking a replacement by HP/voltage/RPM/rotation, and its failure modes.

How it works

The motor has salient (sticking-out) poles wound with the main coil. On one corner of each pole face is a heavy copper loop — the shading coil. As the main field builds and collapses, it induces a current in that copper loop, and the loop's field lags the main field in time. That tiny lagging field on one side of the pole face nudges the rotating field to sweep from the unshaded side toward the shaded side. That sweep is enough to start the rotor turning in one fixed direction.

Because the phase shift comes from the shading coil, no capacitor is needed — but the trade-off is weak starting torque and poor efficiency. That's why shaded-pole motors are limited to loads that start easy and don't demand much.

The fixed-direction part is key: a shaded-pole motor turns toward its shaded poles, and you cannot reverse it by swapping leads. Rotation is built into the construction.

In the field

Identify it. Small motor, two line leads (sometimes with a tapped speed lead), NO capacitor, and you can often see the copper shading loops on the pole faces if you look at the stator. Common on furnace draft inducers (older), small fan-coil and refrigeration evaporator fans, and bath/exhaust fans.

Match horsepower — but note it's often rated in watts or fractional HP. These are small: think 1/100, 1/70, 1/50, 1/40, 1/30, 1/20 HP, or rated directly in watts. Match or slightly exceed; they have so little torque that undersizing means it won't start the load.

Match voltage and RPM. Usually 115V (also 208/230V). Speeds commonly 1550, 1050, or 3000 RPM depending on poles. Match the speed — airflow depends on it.

Match rotation — this is the big one. You can't reverse a shaded-pole motor electrically. Order it for the rotation you need (CW or CCW, and note from which end). Get this wrong and the part is useless; the fan will run backward and you can't flip it.

Match the frame, shaft, and mount. These motors mount in lots of ways — studs, belly bands, brackets. Shaft diameter and length must fit the fan/blower wheel.

Normal values & targets

  • Horsepower: very small — roughly 1/100 to 1/20 HP, or rated in watts (a few watts to ~50W is common).
  • Voltage: 115V most common; 208/230V on some.
  • RPM: 1050, 1550, or 3000 are typical; some 825/900 RPM units exist.
  • Run amps: low — fractions of an amp to a couple amps depending on size.
  • Winding resistance: main winding reads a sensible low-to-moderate resistance lead-to-lead; should read open (infinite) to the case. A reading to the case means a grounded winding.
  • No capacitor: if a small fan motor has a cap, it's a PSC, not a shaded-pole. Absence of a cap is a quick identifier.

Common faults & what they mean

  • Motor hums but won't start, hard to turn by hand → seized or dragging sleeve bearings; shaded-pole motors have so little torque that even mild bearing drag stops them.
  • Bearing noise, wobble, end-play → worn sleeve bearings, end of life. They're not usually serviceable; replace the motor.
  • Open winding (reads OL lead-to-lead) → burnt-out winding, replace.
  • Grounded winding (reads continuity to case) → replace; this can trip protection.
  • Runs backward after replacement → wrong-rotation motor ordered. You cannot fix this with wiring — get the correct rotation part.
  • Weak airflow / fan barely turning → low voltage, dragging bearing, or an undersized replacement that can't pull the load.

Tech tips & gotchas

Rotation is everything on a shaded-pole motor. Unlike a PSC, you can't swap two leads to reverse it — the direction is physically built in. Verify CW vs CCW and from which end before you order. A backward inducer or evap fan is a guaranteed callback.

Bearings are the usual killer. Because the motor makes so little torque, a bearing that another motor would shrug off will stall a shaded-pole. If it hums and won't turn freely by hand, it's the bearing, not the winding — but either way the fix is a new motor.

Many shaded-pole inducers on older furnaces have been replaced by PSC or ECM inducers on newer equipment. If you're matching an old part, confirm whether the modern replacement is still shaded-pole or an upgraded type.

Don't oil a sealed sleeve bearing as a "repair." Some have oil ports and respond to a few drops, but a dry, worn bearing is past saving — you'll be back. Replace it.

Because they're cheap, keep a couple of common sizes/rotations on the truck for fan-coil and refrigeration evap fan work — it turns a return trip into a same-visit fix.

Safety / code notes

Even though there's no capacitor to bleed, de-energize and lock out before working — the leads are line voltage. Match the replacement's horsepower, voltage, RPM, and especially rotation to the equipment so the fan moves air the right direction and the combustion/draft sequence works as listed. On a draft inducer, a wrong or weak motor can compromise venting and the pressure-switch proof — never leave a combustion appliance with a marginal inducer. A grounded motor that trips protection should be replaced, not reset.