Star Delta Circuit Diagramme

7.4. Star-delta start of three-phase motors

When switching on a motor a current will flow through the stator’s winding being many times stronger than the rated motor current. This current is called starting current. It will highly stress the electrical network. On increasing rotational speed the starting current will decrease until the rated current is reached at the rated rotational speed.

By star-delta starting a reduction of the starting current will be achieved. This starting connection is the most frequently applied starting method for asynchronous squirrel-cage motors of greater power.

The star-delta starting operation is practically executed via

- automatic star-delta switches,
- star-delta contactor combinations.

The stardelta hand switch (cam switch) is hardly to be met any more in actual operation. Functioning of the star-delta starting necessitates that the motor is suitable for this method of starting, i.e. with a three-phase four-wire system of 380/220 V every winding of the motor must be designed for 380 V.

Table 7.3. Connection of three-phase squirrel cage motors to mains

Voltage stated on rating plate	voltage of feeding mains	motor connection
380/220 V	3 x 220 V	delta connection
	380/220 V	star connection
380 V		delta connection
(660/380 V)	380/220 V	star-delta starting
660 V	660 V	delta connection
		star-delta starting

A star-delta contactor combination consists of the following electrotechnical units:

- 3 contactors (network, star and delta contactors),
- 1 time relay (pull-in delayed),
- 1 three-pole thermal overcurrent release,
- 3 fuse elements or automatic cut-outs for the main circuit,
- 1 fuse element or automatic cut-out for the control circuit,
- 4 signal lamps, e.g. 24 V
- 1 motor 380 V

The mode of function of a star-delta starting circuit is shown as an all-pole representation (circuit diagram) in fig. 7.13. and 7.14. as well as a general wiring diagram in fig. 7.15.

Fig. 7.13. Star-delta connection, all-pole representation of power part (circuit diagram)

F1...F3 fuses, F5 thermal overcurrent release, Q1 network contactor, Q2 delta contactor, Q3 star contactor, M1 three-phase motor

Fig. 7.14. Circuit diagram of an automatic star-delta contactor circuit with indication

a control, b indication (e.g. 24 V~)

S1 OFF key, S2 ON Key, Q1 network contactor in current path 4, Q₄1 normally open contact of Q1; Q2 delta contactor in current path 3, Q₃2 normally open and closed contacts of Q2, Q3 star contactor in current path 1, Q₁3 normally open and closed contacts of Q3, K1 pull-in-delayed time relay in current path 2, K₂1 normally closed contact of K1, F5 thermal overcurrent protection, H1 ... H4 signal lamps, 1 ... 8 numbering of current paths,

normally closed contacts,
normally open contacts (to be shown for each contactor in the current paths concerned)

Fig. 7.15. Star-delta connection (general wiring diagram of power part)

Switching sequence for fig. 7.14.

Key S2 is actuated!

Time stage 1

The circuit for Q3 is closed via F5, S1, S2, Q₃2, K₂1 and star contactor Q3 in current path 1 responds, closes the normally open contact Q₁3 and opens the normally closed contact Q₁3.

Thereby the circuit to the delta contactor Q2 in current path 3 is interrupted.

Simultaneously the time relay K1 in current path 2 starts operating. Further by the normally open contact Q₁3 in current path 3 the network contactor Q1 in current path 4 is pulled up. In current path 4 the normally open contact Q₄1 is closed and via F5, S1 of the line connection and the normally open contact Q₄1 the network contactor Q1 in current path 4 and the star contactor Q3 in current path 1 are pulled up, and the motor starts in star connection. By switching on star contactor Q3 in current path 1 and the network contactor Q1 in current path 4 the normally open contact Q₄1 in current path 5 and the normally open contact Q₁3 in current path 7 will be closed.

Signal lamp H1 will indicate the network contactor and signal lamp H3 the star contactor to be switched on.

At the same time signal lamp H4 will go out which indicates the star-delta circuit not to be operating.

Time stage 2

When the time set at time relay K1 in current path 2 has elapsed, in current path 1 the normally closed contact K₂1 will open and the circuit to the star contactor Q3 will be interrupted. The normally closed contact Q₁3 in current path 3 will be re-closed and current is flowing via holding contact Q₄1 of the network contactor in current path 4 and the conductor connection to current path 3 as well as normally closed contact Q₁3, so that the delta contactor Q2 in current path 3 pulls up. The motor runs in delta connection. Simultaneously the normally closed contact Q₃2 in current path 1 will be opened and star contactor Q3 in current path 1 cannot be switched on via S2.

At the same time of opening of contacts Q₃2 and K₂1 in current path 1 and drop out of star contactor Q3 the normally open contact Q₁3 will be reopened and signal lamp H3 will go out. By switching on delta contactor Q2 in current path 3, however, the normally open contact Q₃2 in current path 6 will be closed and signal lamp H2 (for the delta connection) will light. The wiring to the star-delta connection is made according to available drawings resp. the instructions of the maker. In case of star-delta starting of the motor the bridges on the motor terminal board will have to be removed. The motor supply line from network contactor Q1 is to be connected to the beginnings of windings U, V, W. The lines for the star-delta change-over from contactors Q2 and Q3 are to be connected to the ends of winding X, Y, Z. The protective conductor is to be clamped to the point provided and marked for this purpose. The coordination of the motor’s accessories is to be taken from table 7.4. After the motor is wired, it will have to be checked whether it runs in correct rotational direction. If the motor runs opposite to the desired direction of rotation, two phase-conductors will have to be exchanged in the motor supply line. In case the direction of the rotating field cannot be ascertained by test run of the motor, this can be found out also by connecting a phase-sequence indicator to the motor supply line. For switching capacities of more than 10 kW application of an auxiliary contactor for extinguishing the arc of the star contactor will be appropriate. Further examples of switching arrangements of motors are to be taken from the textbook “Circuits, Formulas and Tables of Electrical Engineering”, section 4.

Fig. 7.16. Phase-sequence indicator

Table 7.4. Choice of accessories for three-phase motors with 380 V rated voltage

Motor Pn	In	fuse	contactor	temperature relay/type setting range	motor protection switch	flexible connection to motor
(kW)	(A)	(A)	(A)	(A)	(A)	(A)
0,16	0,42 - 0,68	2	16	0,4 ... 0,68	0,4 - 0,6	4 x 1,5
0,25	0,7 - 1,0	4	16	0,65 ... 1,1	0,6 - 1,0	4 x 1,5
0,4	1,0 - 1,6	4	16	1,0 ... 1,7	1,0 - 1,6	4 x 1,5
0,6	1,5 - 2,0	6	16	1,0 ... 1,7	1,6 - 2,5	4 x 1,5
				1,6 ... 2,8
0,8	1,9 - 2,4	6	16	1,6 ... 2,8	1,6 - 2,5	4 x 1,5
1,1	2,4 - 3,0	6	16	1,6 ... 2,8	1,6 - 2,5	4 x 1,5
				2,6 ... 4,5	2,5 - 4
1,5	3,4 - 4,2	10	16	2,6 ... 4,5	2,5 - 4	4 x 1,5
					4 - 6
2,2	4,7 - 5,6	10	16	4,3 ... 7,5	4 - 6	4 x 1,5
3,0	6,5 - 7,5	16	16	4,3 ... 7,5	6 - 8	4 x 1,5
4,0	8,4 - 9,6	20	16	7,2 ... 12,5	8 - 10	4 x 1,5
5,5	11,2 - 12,6	25	16	7,2 ... 12,5	10 - 16	4 x 1,5
7,5	15 - 17	35	25	10-16/16-25	10 - 16	4 x 1,5
					16 - 25
10	19,8 - 21,4	50	25	16 ... 25	16 - 25	4 x 2,5
13	25,6 - 27	50	40	25 ... 40	-	4 x 4
17	33 - 35	63	40	25 ... 40	-	4 x 6
22	43 - 45	80	63	40 ... 60	-	4 x 6
30	58 - 61	100	63	40 ... 64	-	4 x 10
40	78 - 82	125	100	64 ... 100	-	4 x 16
55	116 - 114	160	200	100 ... 140	-	4 x 25
75	145 - 155	200	200	140 ... 200	-	4 x 50
100	193 - 207	260	250	160 ... 280	-	4 x 70
125	240 - 255	300	250	160 ... 280	-	4 x 95
160	304 - 328	350	400	280 ... 400	-	4 x 150
200	370 - 400	430	400	250 ... 420	-	4 x 185
250	460 - 500	500	630	420 ... 600	-	-
320	590 - 610	600	630	-	-	-