Bagaimana prilaku motor induksi ketika di start

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METHODS OF STARTING CAGE MOTORS

- Direct Starting – Problems- Star/delta (wye/mesh) starter- Autotransformer starter- Resistance or reactance starter- Solid-state soft starting- Starting using a variable-frequency inverter

Equivalent circuit of supply system

Phasor diagrams showing the effect of supply-system impedance on the outputvoltage with (a) inductive load and (b) resistive load

Direct Starting – Problems

V = Vs - IZs Zs = Xs

Masalah start langsung motor induksi :

Arus starting 5 – 7 kali arus ratingcos ø rendahDrop tegangan cukup signifikan

INDUCTION MOTOR CURRENT, TORQUE AND POWER FACTOR/SPEED CURVES

Example:Q A 6.6kV, 400hp motor has a full-load power factor of 0.8 and efficiency of 90%. Its standstill current, at smarting is 5 times its full-load current and at 0.2 pf. What are its starting current and its active and reactive starting loads?A 4, 400 hp is equivalent to 0.75 x 400 = 300kWm

If efficiency is 90%, full-load input power is 333 kWe If full-load power factor is 0.8, full-load input = = 416 kVA. At full-load, kVA = 3kV x /, where kV and / are line voltage and current.

In this case kV = 6.6 and kVA has been calculated 416. 416 = 3 x 6.6 x / or / = = 36.4A (at full-load)

Starting current is 5 x full-load current = 5 x 36.4 = 182A (at standstill) ............ .(i)

Starting power factor is given as 0.2 (= cos)............ (ii)

sin = 0.98 (from tables) Active power = 3kV..cos = 3x 6.6x1 82x0.2 = 416kW ........ ..(iii) Reactive power = 3kV.sin =3 x 6.6 x 182x 0.98 = 2040kvar ........... .(iv)

Summarizing the figures (i), (ii), (iii) and (iv):Starting current = 182A

(36A) Starting power factor = 0.2 (0.8) Starting active power = 416kW (333kW) Starting reactive power = 2040kvar

(250kvar)

If these four figures are compared with the corresponding quantities in the top line of the table of Chapter 4 for the motor running on full-load (repeated above in brackets alongside each), it will be seen that, when starting:(a) The starting current is very much greater (it was given as 5 times). (b) The starting power factor is very much lower (it was given as 0.2).(c) The starting active power is of the same order as the full-load running active power (in this case a little higher).(d) The starting reactive power is vastly greater than the full-load running reactive power (in this case eight times).

Star/delta (wye/mesh) starter

This is the simplest and most widely used method of starting. It provides for the windings of the motor to be connected in star (wye) to begin with, thereby reducing the voltage applied to each phase to 58% (1/V3)of its DOL value.

Then, when the motor speed approaches its running value, the windings are switched to delta (mesh) connection.

Autotransformer starter

The motor is first connected to the reduced voltage output, and when the current has fallen to the running value, the motor leads are switched over to the full voltage.

If the reduced voltage is chosen so that a fraction a of the line voltage is used to start the motor, the starting torque is reduced to approximately α2 times its DOL value, and the current drawn from the mains is also reduced to α2 times its direct value.

Resistance or reactance starter

By inserting three resistors or inductors of appropriate value in series with the motor, the starting current can be reduced by any desired extent, but only at the expense of a disproportionate reduction in starting torque.

For example, if the current is reduced to half its DOL value, the motorvoltage will be halved, so the torque (which is proportional to the squareof the voltage – see later) will be reduced to only 25% of its DOL value.

This approach is thus less attractive in terms of torque per ampere ofsupply current than the star/delta method. One attractive feature, however,is that as the motor speed increases and its eVective impedancerises, the volt drop across the extra impedance reduces, so the motorvoltage rises progressively with the speed, thereby giving more torque.

When the motor is up to speed, the added impedance is shorted-out bymeans of a contactor. Variable-resistance starters (manually or motoroperated) are sometimes used with small motors.

Solid-state soft starting

(a) Thyristor soft-starter, (b) typical motor current waveforms

The most widely used arrangement comprises three pairs of backto-back thyristors connected in series with three supply lines, as shown in Figure (a).

Each thyristor is fired once per half-cycle, the firing being synchronised with the mains and the Wring angle being variable so that each pair conducts for a varying proportion of a cycle. Typical current waveforms are shown in Figure (b): they are clearly not sinusoidal but the motorwill tolerate them quite happily.

Starting using a variable-frequency inverter

General arrangement of inverter-fed variable-frequency induction motorspeed-controlled drive

The majority of inverters used in motor drives are voltage sourceinverters (VSI), in which the output voltage to the motor is controlled to suit the operating conditions of the motor.