Electric Motor Types: Three-Phase and Single-Phase Models

Most electric motors used in industry and everyday life fall into two groups according to the supply they run on: three-phase and single-phase motors. From the outside they look very similar; both have a frame, fan, shaft and terminal box. The real difference is inside the motor: how the windings are arranged and how the motor starts turning.

In this article we compare the two motor types in terms of design, starting, connection and applications, explain the sub-types of single-phase motors, and describe in plain language which one to choose for which job. For a general overview of all motor families (AC-DC, synchronous, servo etc.), see our Electric Motor Types guide.

What is a three-phase electric motor?

A three-phase motor runs on a 380–400 V three-phase supply. Its stator (the stationary part) carries three separate winding groups spaced evenly from each other. When the three phases energise these windings in turn, a magnetic field that rotates by itself is created inside the motor, and the rotor turns by following this field.

That is why a three-phase motor starts on its own without any additional part. There is no capacitor, centrifugal switch or auxiliary winding. Fewer parts mean fewer points of failure.

• Power range: From small ratings up to hundreds of kilowatts. HEM Motor’s three-phase range covers 0.25 kW to 355 kW.
• Starting: High starting torque; easily turns machines that start under load.
• Applications: Factories, workshops, pumping stations, compressors, conveyors, cranes, mills.

What is a single-phase electric motor?

A single-phase motor runs on the 220–230 V single-phase supply found in homes and small businesses. A single phase does not create a rotating field inside the motor, only a field that “pulsates” back and forth. For this reason a single-phase motor cannot start turning on its own.

To solve this, a main winding and an auxiliary winding are placed in the stator. A capacitor is usually connected to the auxiliary winding. The capacitor shifts the current in the auxiliary winding in time, creating an effect similar to a rotating field and setting the motor in motion.

• Power range: Generally small ratings. HEM Motor’s single-phase range covers 0.18 kW to 5.5 kW.
• Starting: Depends on the capacitor arrangement; more limited than three-phase.
• Applications: Booster sets and small pumps, fans, small compressors, workshop benches, agricultural machines.

4 types of single-phase motor

Single-phase motors are divided into four types according to how they start. You can tell which one you have from the nameplate or by counting the capacitors on the terminal box.

1. Split-phase motor: Uses no capacitor; starting is achieved by winding the auxiliary coil differently. Low starting power, used for very light jobs.
2. Capacitor-start motor: A large capacitor is switched in at start, and a centrifugal switch disconnects it once the motor speeds up. High starting power.
3. Permanent split capacitor motor: A small capacitor stays in circuit all the time. Quiet and smooth running, but medium starting power. Common in fans and extractors.
4. Capacitor-start capacitor-run motor: Uses both capacitors together. It starts strongly and runs smoothly. The most suitable single-phase type for hard jobs such as compressors and pumps that start under load.

Differences between three-phase and single-phase motors

Which motor for which job?

The first and most important question is what supply is available where the motor will be installed. Everything else follows from that.

• If only 220 V single phase is available (home, garden, small shop, rural house): You must use a single-phase motor. Choose a permanent capacitor motor for light jobs, and a capacitor-start capacitor-run motor for compressors and pumps that start under load.
• If three phases (380–400 V) are available (factory, workshop, industrial estate): Prefer a three-phase motor whatever the power. You will do the same job with a smaller, more robust motor that needs less maintenance.
• Jobs above 3 kW: At these ratings a single-phase motor becomes large, draws a high current from the supply at start and strains the fuses. If possible, installing a three-phase line and using a three-phase motor is more economical in the long run.
• Machines with frequent starts and stops: The capacitor and switch wear with every start; in such jobs a three-phase motor lasts much longer.

Frame types: aluminium and cast iron

Three-phase motors are also divided by frame material. Aluminium frame motors are light and suitable for clean, enclosed environments. Cast iron frame motors are more resistant to impact, vibration, dust and humidity, and are preferred for heavy industrial conditions.

Conclusion

A three-phase motor is a powerful, low-maintenance motor that starts by itself on a three-phase supply. A single-phase motor runs on a single-phase supply with the help of a capacitor and is a practical solution for jobs at small power ratings. The right choice depends first on your supply, then on the starting load and the power.

To see three-phase motor prices and order right away, go to our Buy Electric Motors (online shop) section; the price and technical data of each of our 117 motors is on its own page. You can read how prices are formed on the Three-Phase Electric Motor Prices page. For the single-phase range, see the Single-Phase Electric Motor (220 V) page and our single-phase motor prices article. For questions, message us on WhatsApp or call +90 532 345 49 86.