Method for Converting Three-Phase Asynchronous Motor to Single-Phase 220V Operation

Core Premise: Applicable only to squirrel-cage three-phase asynchronous motors (the commonly used type in industry); modification of wound-rotor three-phase asynchronous motors is not recommended. After conversion, the motor’s output power will drop to 60%~70% of the original power. It is recommended to modify only small-power motors of 1.5kW or less (high-power motors will suffer low efficiency, difficult starting and severe heating after conversion). This conversion is only suitable for light-load/no-load starting scenarios (예를 들어, light-load working conditions of fans, water pumps and small machine tools), and not applicable for heavy-load starting.

Key Accessories: Non-polar AC capacitors (running capacitor + starting capacitor, with a withstand voltage of ≥450V; CBB60/CBB61 types are preferred, electrolytic capacitors are prohibited), centrifugal switch (or normally open button), single-phase 220V power supply, fuse/thermal relay (for motor protection).

1. Core Principle

Based on the capacitor phase-shifting principle, single-phase 220V AC is split into two AC currents with a phase difference of approximately 90°, which supply power to two phase windings of the three-phase motor, and the third phase winding serves as a common terminal to form a rotating magnetic field that drives the motor to operate. The starting capacitor provides a larger phase-shifting current for a short time to boost the starting torque, and is disconnected after starting (to avoid overheating and burning of the capacitor), leaving only the running capacitor to work for a long time.

1. Capacitor Selection (Core Parameter, Must Follow the Formula)

Insufficient capacitor capacity will lead to difficult starting and insufficient torque; excessive capacity will cause high motor current and severe heating. The withstand voltage must be ≥450V (the peak value of 220V single-phase electricity is about 311V, leaving a sufficient safety margin).

1. Capacity Calculation Formula

Running capacitor (long-term operation): $C_{Running} ≈ 10 × P$ (where $P$ is the original three-phase power of the motor in kW), unit in μF

Starting capacitor (short-term starting): $C_{Starting} = 2~3 × C_{Running}$, unit in μF (must be disconnected immediately after starting, with a working time of ≤3 seconds)

2. Capacitor Selection Reference for Common Power Ratings (Direct Application)

Example: For a 0.75kW three-phase motor, select an 8μF/450V running capacitor plus a 20μF/450V starting capacitor.

III. Two Basic Conversion Schemes for Three-Phase Motor Winding Connections

The stator windings of a three-phase motor have two connection methods: star (Y) and delta (△), with different wiring methods for conversion. First, confirm the motor’s original connection method (marked Y/△ in the motor junction box, or by the connection piece of terminal posts: Y-type is 3 pieces connected up and down, △-type is 3 pieces connected left and right).

Premise: All 6 terminal posts (U1/U2, V1/V2, W1/W2) of the motor junction box must be led out (if there are only 3 terminal posts, the winding taps need to be disassembled, which is not recommended for non-professionals).

Scheme 1: Conversion of Motor with Original Star (Y) Connection (Simplest, No Winding Modification Required)

1. Retain the internal Y-type connection of the motor (U2/V2/W2 shorted as the common terminal);
2. Connect U1 to the single-phase live wire (엘), and V1 to the live wire (엘) in series with the running capacitor $C_{Running}$;
3. Connect W1 to the single-phase neutral wire (N) to form the main and auxiliary winding circuit;
4. Connect the starting capacitor $C_{Starting}$ in parallel with the running capacitor, then in series with a centrifugal switch/normally open button (closed during starting, disconnected when the motor speed reaches 70%~80% of the rated speed).

Simple Wiring Mnemonic: Y-type common terminal to N, one phase to L, one phase to L in series with running cap, starting cap in parallel with running cap and in series with switch.

Scheme 2: Conversion of Motor with Original Delta (△) Connection (Higher Efficiency, Recommended)

The winding voltage of a △-connected motor is the line voltage (original 380V). After conversion to single-phase, the winding bears 220V, resulting in higher winding utilization and a slightly higher torque than the Y-type conversion, making it the preferred scheme.

1. Retain the internal △-type connection of the motor (U1-W2, V1-U2, W1-V2 connected);
2. Connect U1 to the single-phase live wire (엘), and V1 to the live wire (엘) in series with the running capacitor $C_{Running}$;
3. Connect W1 to the single-phase neutral wire (N);
4. Connect the starting capacitor $C_{Starting}$ in parallel with the running capacitor and in series with a centrifugal switch/normally open button (same as Y-type conversion).

Supplementary Note: If the original motor is 380V Y-type and you want to increase the torque after conversion, you can first convert it to △-type (reconnect the terminal post connection pieces), then wire it according to the △-type conversion scheme (메모: After converting to △, the winding bears 220V, much lower than the original 380V, with no risk of burning).

1. Practical Wiring Steps (Universal Version)
2. Power Off and Disassemble: Disconnect all power supplies of the motor, open the junction box, clean the oxide layer on the terminal posts, and confirm that the marks of the 6 terminal posts (U1/U2, V1/V2, W1/W2) are clear;
3. Confirm/Adjust Connection Method: Retain the original connection method for light-load working conditions; convert Y-type to △-type to increase torque (only applicable to 380V Y-type motors);
4. Connect the Running Capacitor: According to the corresponding scheme, connect one end of the running capacitor to V1 and the other end to the live wire L;
5. Connect the Starting Circuit: Connect the starting capacitor in parallel with the running capacitor, then in series with a centrifugal switch (or normally open button). Fix the centrifugal switch inside the motor end cover (linked with the rotor, automatically disconnected when the speed reaches the standard), and install the button in the control cabinet (release the button within 3 seconds after manual starting);
6. Connect Power Supply and Protection Devices: Connect the live wire L to U1 and the neutral wire N to W1. Connect a fuse (rated current ≈ 1.2 times the motor’s rated current) 그리고 열 릴레이 (overload protection, setting current = motor’s rated current) in series at the live wire end;
7. Test Run: Conduct a no-load test run, observe the motor’s rotation direction and speed. If the rotation direction is reversed, swap the power wiring of any two phase windings (예를 들어, swap the wiring of V1 and W1);
8. Load Test: After the test run is normal, apply the load gradually (not exceeding 60% of the original power), run for 5~10 minutes, and touch the motor housing. The conversion is qualified if there is no obvious overheating (≤70℃) and no abnormal noise.
9. Key Accessories and Alternative Schemes
10. Substitute for Centrifugal Switch (When No Centrifugal Switch is Available)

If there is no centrifugal switch, a normally open button (예를 들어, inching button) can be used as a substitute. Starting Operation: Press the button to start the motor, and release the button to disconnect the starting capacitor when the motor speed increases significantly (about 3 초). This method requires manual operation; do not press the button for a long time.

2. Taboos for Capacitor Selection

Polarized electrolytic capacitors are prohibited (only applicable to DC, and will be broken down quickly under AC);

Metallized polypropylene capacitors (CBB60/CBB61) are preferred, which are temperature-resistant, impact-resistant and suitable for motor working conditions;

The capacitor must be fixed in a ventilated place outside the motor, and avoid close contact with the motor housing (to prevent the capacitor from being damaged by high temperature baking).

1. Important Notes (Key to Avoiding Mistakes, Must Read)
2. Load Restriction: The motor torque drops significantly after conversion. Full-load/heavy-load starting is strictly prohibited (예를 들어, air compressors and pulverizers with load starting). It is only applicable to light-load/no-load starting equipment such as fans, water pumps and small conveyors;
3. Heating and Heat Dissipation: The operating current of the motor after conversion is higher than that in the original three-phase operation, resulting in increased heating. Ensure good ventilation of the motor, prohibit sealed installation, and clean the heat sink regularly;
4. Protection Measures: A fuse (단락 보호) 그리고 열 릴레이 (overload protection) must be installed. Do not connect directly to the power supply (the motor is easy to burn without protection);
5. Power Upper Limit: Modification of three-phase motors over 1.5kW is not recommended (such motors will have difficult starting and extremely low efficiency after conversion, and are easy to burn during long-term operation; it is better to directly purchase a single-phase motor);
6. Rotation Direction Adjustment: If the motor’s rotation direction after conversion is opposite to that of the original three-phase connection, swap the wiring between the winding terminal connected to the capacitor and the neutral wire (예를 들어, swap the wiring of V1 and W1), without adjusting the capacitor wiring;
7. Power Supply Requirements: The single-phase 220V power supply must have sufficient capacity to avoid a sudden voltage drop during motor starting (예를 들어, too thin lines, insufficient total power);
8. Grounding Protection: The motor housing must be reliably grounded to prevent electric shock due to leakage.

Ⅶ. 문제 해결 (Common Problems After Conversion)

VIII. Conversion Summary

The core of converting a three-phase motor to single-phase is capacitor phase shifting + light-load operation. Squirrel-cage motors below 1.5kW can meet the requirements of light-load working conditions after conversion with low modification cost (the total cost of capacitors and switches is about 20~50 yuan). However, the power and torque decrease and heating increases after conversion, so this is only an emergency solution. For long-term use, it is recommended to directly purchase a matching single-phase motor (with higher efficiency and more stable operation).

Modification of three-phase variable frequency motors, brake motors and explosion-proof motors is prohibited (their winding structure is special, and modification may easily damage the motor or cause safety accidents).