Three phase motor connected to single phase power supply

fig.1 Delta connection

              It happens quite often that three phase motors to be connected to single-phase voltage. This does not present a great connection problem, only problem really important is the energy consumption, because these motors efficiency will decrease. If is a low-power motor and is rarely used that would not be a problem, but if it is a motor, let's say 4 kW and you use it daily a few hours, maybe you should seriously consider to order a single phase one . In principle a three-phase motor connected to single-phase voltage can be made in different ways, but the most common are star and delta connections. Now, as I said three-phase motor efficiency connected to single phase voltage decrease whatever connection mode you choose, but will be even more inefficient if you will connect it in star mode, therefore whenever you choose a single phase connection this must be in delta mode. From a technical standpoint the motor will not start so simple, because there is not not any phase shift, so we must install some capacitor to help the motor to start, called start capacitor  and another capacitor, called run capacitor  , this will be used to increase the motor efficiency. The values of these capacitors will be chosen as follows:

-for star connection:

-for delta connection:

 - run capacitor

 - start capacitor

 - motor rated current

 - motor rated voltage

As I said there are several connection methods from three phase to single phase, I drew only two schematics star and delta, which are almost identical only difference is on motor terminal box where you connect it in star or delta.

Operating mode:By pressing  Start pushbutton the contactor coil Q2 is energized, the motor will be connected and maintains itself after the button is released by means of auxiliary contact of Q2 , Stop button and Emergency Stop(not mandatory). The Start button is a momentary push button and has two contacts, the contact 3-4 I represented it in both control and power circuits to better understand how start capacitor  should be connected with Start button, so the contact 3 -4 will be connected only while the Start button is pressed. Therefore when you start the motor you must hold the Start button to connect start capacitor until the motor reaches to rated speed, then when you release the button the start capacitor  will be disconnected and the motor will operate under normal conditions. When overload occur thermal-magnetic circuit breaker Q1 opens, so that the motor is stopped instantly.

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Components calculation

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The values for Q1, Q2 should be chosen according to the rated motor power in accordance with data on the motor plate. Be careful when buying components must keep in mind that you connect the motor to single phase supply, for exemple a thermal-magnetic circuit breaker (Q1 in the drawig) for a 4 kW three-phase motor can be used by a single phase motor of only 2.2 kW, same situation for Q2. Fuses F1 for the control circuit may have values up to max 10 A.

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