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Three-phase auto-transformers for the start-up of motor

to avoid strain on the network and on the motor at start-up

The start-up of a motor with a three-phase auto-transformer consists in reducing (via an appropriate connection and only for a few seconds in the starting phase) the supply voltage to the motor.
Usually it is adopted for high-power motors because, as voltage is never lacking during start-up, it eliminates current spikes.
The rotor can be either of the wound type or the cage type.

It also enables a more gradual start-up as various intermediate taps can be used for a three-phase auto-transformer.
Considering the dependence of the torque on the square of the voltage and the low value usually adopted for initial voltage, the motor must never start at full load.

autotrasformatore-avviamento-motoriThe figure on the right shows the diagram of a start-up by means of an auto-transformer with a single intermediate tap. Note that u, v and w are the start terminals of the motor phases. The protection devices are not indicated.

The sequence of operations has to be carried out to start the motor is as follows:
First of all switch T1 is closed so that the auto-transformer is set up for star-type operation. T2 is then closed (keeping T1 closed) and the motor is powered via the auto-transformer at a voltage of 0.7Vn. Then, T1 is opened so that the motor is fed through the auto-transformer reactors which are in series with the line. Finally, the T3 is closed and the motor is powered at full voltage as the reactors are short-circuited.

In the case we have considered of an auto-transformer with a single intermediate tap, the starting torque is reduced to 1/2 (as the voltage is 70% of the nominal value) and current consumption is also reduced to 1/2 of the nominal level (provided the motor is taken as a constant impedance load; in fact, 70% of its nominal voltage is applied to the motor and it will absorb 70% of the normal starting current, and this current is then reduced by a further 70% as the auto-transformer voltage ratio is 1 : 0.7).
As they are intended for a time-limited service (10-20 sec. approx.), the auto-transformers for this type of use operate with high values of magnetic induction and with high current density values. For this reason they are much more compact and less expensive than auto-transformers for continuous service.

The table below indicates the dimensions (mm)  of some auto-transformers that we have standardized with a line voltage of 400V and an output of 280V at 50-60 Hz.

POWER

 

dimensions in mm.

weight

KW

HP

code

A

B

C

D

E

G

Kg.

7

10

AP010

180

100

175

150

75

120

8,4

11

15

AP015

180

110

175

150

86

130

10,5

15

20

AP020

240

110

242

200

81

122

14

22

30

AP030

240

120

245

200

92

135

17,5

30

40

AP040

240

130

245

200

100

142

21

36

50

AP050

240

145

245

200

120

160

25

55

75

AP075

300

140

290

250

102

150

33

74

100

AP100

300

160

290

250

122

200

44

92

125

AP125

300

180

290

250

142

220

53

110

150

AP150

360

170

345

325

135

190

53

132

180

AP180

360

190

345

325

155

210

70

162

220

AP220

420

195

410

375

152

250

87

205

280

AP280

420

215

415

375

172

270

104

250

340

AP340

480

200

480

425

160

250

112

330

450

AP450

480

210

480

425

170

290

150

autotrasformatore avviamento motori

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    THE MAIN CHARACTERISTICS OF AUTO-TRANSFORMERS MOTOR START-UP

    • 2 consecutive starts and 3 starts/hr of 12 sec.
    • A 30% drop in voltage at no load and 35-45% with respect to the nominal value at motor start-up – The torque of the motor at the start amounts to less than half the nominal value.
    • The torque of the motor at the start amounts to less than half the nominal value
    • Insulation materials of Class F (155°C) and of class H (180°C) and a Ta of 45° C.
    • Degree of protection IP00 and protection class ‘I’
    • Impregnation with tropicalizing paints and resins with oven-firing.
    • Routine or type-based testing on each piece.
    • All metal components are galvanized. 
    • Electrolytic copper conductors in class H or higher.
    • Yn 0 vector group with a neutral wire that can be disconnected.
    • Standards IEC EN 61558-2 BS 13 171.
    • Thermostatic sensors inserted between the windings if requested 

    Carbognin srl

    We are suppliers of each type of industry which requires the use of electrical and electronic equipment.

    Where we are

    V. Crispi, 9 (z.a).
    36075 - Montecchio Maggiore
    Vicenza - Italy

    Contacts

    Phone: +39 0444 698877
    Fax: +39 0444 698759
    carbognin@carbognin.it

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