A split-phase motor is a type of single-phase electric motor that uses two separate stator windings to start itself from a standstill. It’s one of the most common small motors found in household appliances and light-duty equipment, typically powering loads up to 1/3 horsepower (about 250 watts). The “split phase” name refers to how the motor artificially creates a second phase from a standard single-phase power supply, giving it just enough of a rotating magnetic field to begin spinning.
Why a Single-Phase Motor Needs Help Starting
A standard wall outlet delivers single-phase alternating current. If you feed that current into a single winding wrapped around a motor’s stator, the magnetic field doesn’t rotate. It simply flips back and forth, north to south, in sync with the AC cycle. A rotor sitting inside that pulsating field has no reason to turn in either direction, so it stays still. This is the fundamental problem every single-phase induction motor has to solve: how to create the illusion of a rotating magnetic field from a power source that only provides one phase.
Three-phase industrial motors don’t have this problem because each of their three windings is already offset in time, naturally producing rotation. A split-phase motor mimics that offset using a clever wiring trick with two windings that carry current at slightly different times.
How the Two Windings Create Rotation
The stator of a split-phase motor contains two separate windings connected in parallel to the same single-phase supply. The main winding (also called the run winding) is made from thicker wire with lower electrical resistance and higher reactance, which causes its current to lag behind the supply voltage. The auxiliary winding (the start winding) uses thinner, higher-resistance wire with fewer turns, so its current stays closer in step with the voltage.
This difference in wire size and resistance means current peaks in the start winding slightly before it peaks in the main winding. The offset is roughly 30 degrees, enough to make the magnetic field appear to sweep around the stator rather than just pulse in place. The rotor, which is a simple cage of aluminum bars, picks up induced current from this sweeping field and begins to follow it. That interaction between the stator’s rotating field and the rotor’s induced current is what produces torque and gets the shaft spinning.
The Centrifugal Switch
The start winding is only needed to get the motor moving. Once the rotor reaches about 75% of its full operating speed, a centrifugal switch mounted on the shaft flips open and disconnects the start winding from the circuit entirely. From that point on, the motor runs on the main winding alone.
This matters for two reasons. First, the start winding is built from thinner wire and isn’t designed for continuous duty. Leaving it connected would cause it to overheat. Second, the motor is more efficient running on just the main winding once it’s up to speed. The rotor’s own momentum and the pulsating field from the main winding are sufficient to keep it turning. If you’ve ever heard a small motor make a distinct click a second or two after starting, that’s often the centrifugal switch opening.
Starting Torque and Power Range
Split-phase motors produce starting torque in the range of 100% to 125% of their full-load torque. That’s enough to get a fan blade or a pump impeller spinning under light load, but it’s modest compared to other single-phase motor designs. They also draw relatively high current during startup, which is another reason the start winding disconnects quickly.
These motors are generally built for applications up to about 1/3 horsepower. Beyond that, the 30-degree phase shift between the windings simply can’t generate enough starting torque for heavier loads. For bigger jobs, you’d step up to a capacitor-start motor, which adds a capacitor in series with the start winding to push the phase difference much wider and produce significantly more torque at startup.
Split-Phase vs. Capacitor-Start Motors
The easiest way to understand where split-phase motors fit is to compare them with capacitor-start motors, since both are single-phase designs that use a start winding and a centrifugal switch.
- Starting torque: Split-phase motors deliver 100% to 125% of full-load torque. Moderate capacitor-start motors reach up to 175%, and high-torque versions exceed 300%.
- Cost and simplicity: Split-phase motors have no capacitor, which makes them cheaper to build and maintain. Capacitor-start motors add a start capacitor (and sometimes a run capacitor), increasing cost and adding a component that can fail.
- Starting current: Split-phase motors draw high inrush current relative to their size. Capacitor-start motors produce better torque with more moderate current draw.
- Applications: Split-phase motors handle easy-start loads like belt-driven fans, blowers, and light pumps. Capacitor-start motors power compressors, heavy-duty pumps, and farm or industrial equipment where the load resists rotation from the start.
If the load is light and the motor is small, a split-phase design keeps things simple and inexpensive. If the motor needs to push through significant resistance the moment it turns on, a capacitor-start motor is the better choice.
Common Applications
You’ll find split-phase motors in household and light commercial equipment where the load is easy to start and the power demand is low. Typical examples include small fans, furnace blowers, washing machine agitators, belt-driven shop tools like bench grinders, and centrifugal pumps that start unloaded. These are all situations where the motor doesn’t have to fight heavy resistance at startup, so the moderate starting torque of a split-phase design is perfectly adequate.
Reversing the Direction of Rotation
To reverse a split-phase motor, you swap the connections on either the start winding or the run winding, but not both. Reversing one winding changes the direction of the rotating magnetic field during startup, which sends the rotor the other way. If you reverse both windings at the same time, the phase relationship stays the same and the motor spins in its original direction. On motors designed for field-reversible installation, the lead wires for one winding are brought out separately so you can swap them without opening the motor housing.