[:en]Three phase motor connected to single phase power supply[:ro]Motor trifazat conectat la retea monofazata[:]

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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 $C_{s}$ and another capacitor, called run capacitor $C_{f}$ , this will be used to increase the motor efficiency. The values of these capacitors will be chosen as follows:

-for star connection:

$C_{f}=2800\cdot \frac{I_{n}}{U}[\mu F]$ $C_{s}=3\cdot C_{f}[\mu F]$

-for delta connection: $C_{f}=4800\cdot \frac{I_{n}}{U}[\mu F]$ $C_{s}=3\cdot C_{f}[\mu F]$

$C_{f}$ – run capacitor

$C_{s}$ – start capacitor

$I_{n}$ – motor rated current

$U$ – 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 $C_{s}$ 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 $C_{s}$ 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.

Components calculation

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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fig.1 Conexiunea triunghi

Se intampla destul de des ca motoare trifazate sa fie conectate la tensiune monofazata. Aceasta nu prezinta o foarte mare problema de conexiune, singura problema cu adevarat importanata este cea a consumului de energie, deoarece aceste motoare o sa isi piarda din randament. Daca este un motor de putere mica si este utilizat mai rar asta nu ar fi o problema, insa daca este un motor, sa zicem de 4 kW si il utilizati zilnic cateva ore, poate ar trebui sa va ganditi serios sa comandati unul monofazat. In principiu un motor trifazat conectat la tensiune monofazata se poate realiza in diferite moduri, dar cele mai des intalnite sunt conexiunile stea si triunghi. Acum, dupa cum am spus randamentul motorului trifazat conectat la tensiune monofazata scade si asta indiferent ce mod de conexiune alegi, dar va fi si mai ineficienta daca il vei conecta in stea, prin urmare ori de cate ori vei alege o conexiune monofazata aceasta sa fie in triunghi, cum se spune din doua lucruri rele alegem raul cel mai mic. Din punct de vedere tehnic motorul nu o sa porneasca asa simplu, deorece nu prea mai exista defazaj, asa ca o sa il ajutam ori cu o sfoara ori o sa punem ceva condensatori, unul de start, reprezentat in scheme ca $C_{s}$ si unul de functionare $C_{f}$ . Valorile acestor condensatoare vor fi alese dupa cum urmeaza:

-pentru conexiune stea: $C_{f}=2800\cdot \frac{I_{n}}{U}[\mu F]$ $C_{s}=3\cdot C_{f}[\mu F]$

-pentru conexiunea triunghi: $C_{f}=4800\cdot \frac{I_{n}}{U}[\mu F]$ $C_{s}=3\cdot C_{f}[\mu F]$

$C_{f}$ – condensator functionare

$C_{s}$ – condensator start

$I_{n}$ – curent nominal al motorului

$U$ – tensiune de alimentare motor

Dupa cum am spus exista mai multe metode de conectare din trifazat in monofazat, eu am desenat doar doua scheme pentru stea si triunghi, care evident sunt aproape identice singura diferenta este la placuta cu borne a motorului unde veti conecta in stea sau triunghi.

Modul de operare: Prin acţionarea butonului Start se energizează bobina contactorului Q2, motorul v-a fi conectat și se automenţine după eliberarea butonului Start prin contactul auxiliar al Q2, butonul Stop si butonul de urgenta Emergency Stop(care nu este obligatoriu). Butonul Start este un buton cu revenire la actionare si are 2 contacte, contactul 3-4 l-am reprezentat in ambele scheme si cea de comanda si cea de forta, pentru a intelege mai bine cum trebuie conectat la condensatorul de pornire $C_{s}$ , astfel contactul 3-4 v-a fi conectat numai cat timp este tinut apasat butonul Start. Prin urmare in momentul cand vei porni motorul vei tine apasat butonul Start pentru a conecta si condensatorul de start $C_{s}$ , pana motorul v-a ajunge la turatia nominala, dupa care eliberezi butonul si motorul o sa functioneze in conditii normale . La suprasarcină se deschide disjunctorul magneto-termic Q1 astfel ca motorul este oprit instantaneu.

Dimensionarea componentelor

Valorile pentru Q1, Q2 trebuie alese in functie de puterea nominala a motorului in conformitate cu datele de pe placuta cu informatii data de producator . Atentie cand cumparati componentele trebuie sa tineti cont ca motorul il conectati la tensiune monofazata, spre exemplu un disjunctor magneto-termic (Q1 in schema) pentru un motor trifazat de 4 kW va putea fi utilizat de un motor monofazat de doar 2,2 kW, idem si pentru Q2. Sigurantele fuzibile F1, pentru partea de comanda pot avea valori de pana la max 10 A.

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[:en]Three phase motor connected to single phase power supply[:ro]Motor trifazat conectat la retea monofazata[:] Leave a comment