Should a 3 phase motor run on star or delta?

Answer 1

It does not matter to the motor, just as long as the three phase voltage is what the motor specification requires.

HOWEVER..... While the motor will run in either star or delta, it will draw 3 times as much current in delta as it does in star, and will develop 3 times more torque (and power) in delta compared to star.

3 phase motors are often started in star, as it draws less current and is less likely to blow fuses, damage contacts and upset the supply system due to voltage drops. When the motor is near full speed, the motor is switched to delta connection for full power. This is called star-delta starting, and is suitable for light, low inertia loads such as fans or pumps. Because of the motor's reduced startup torque, it shouldn't be used for compressors, machines with heavy flywheels, or machines that come onto load before they reach full speed. Motors designed for star-delta starting will have 6 live terminals in their connection box.

Let's get a little bit technical. In star, each winding has L-N voltage across it (about 58% of the L-L voltage applied when in delta) - in Australia, our voltage from the active (live wire) to the neutral (return wire) is 230 volts, and the voltage from one active to another active is 400 volts. (230 / 400 is about 0.58, or 58%. and 400 / 230 is 1.73 or 173%). Connecting a motor in star puts 230 volts across each winding, and connects ONE winding to each supply phase. when running, it draws (say) 10 amps, and develops about 6 kW / 8HP. In delta, each winding has 400 volts across it, and each supply line powers 2 windings, so the current increases by 1.73 (400 volts compared to 230 volts) and again by 1.73 (2 windings on a 3 phase system draw 1.73 times more current than one winding.) 1.73 squared is 3 - so in delta, the motor draws 3 times more current (30 amps), and delivers 3 times more power (18kW / 24HP).

Some motors are designed to be operated ONLY in star - they may have a label advising you of this, or they may have only one value of voltage and current on the nameplate. They will also usually have only 3 terminals inside the connection box. If you suspect that the motor is 'star only', but want to make sure, connect the motor in delta, and check the running current as soon as the motor is up to speed; if it is drawing more than the nameplate current, or the motor is noisy in delta, but not in star, stop the motor immediately and reconnect it in star, or the motor WILL burn out very quickly.

Answer 2

Not really because the voltage across the winding coils are at a lower value than when running. The voltage is reduced by the inverse of the square root of 3 or 57.7% of full voltage. On a 480 volt motor the wye voltage will be 277 volts. The amperage will be reduced by 57.7 percent and the starting torque will be reduced by 33 percent while in the star configuration.

The timer should be set to change to the delta configuration as soon as the motor's RPM is going as fast as it can in the star configuration. You have to remember, the connected load is based on the motor operating in the delta configuration to develop it rated horsepower.

With the new technology today many of these types of starters are being replaced with soft start controllers.