Autotransformer (AIT) starting - Reduced voltage starting

Starting of Squirrel Cage Induction

4.2 Reduced voltage starting

4.2.2 Autotransformer (AIT) starting

For smoother acceleration and to achieve a still lower starting current than above, this type of switching, although more expensive, may be employed. In this case also the starting current and the torque are reduced in a square proportion of the tapping of the autotransformer. The normal tappings of an autotransformer are 40%, 60%

and 80%. At 40% tapping the starting current and the starting torque will be only 16% that of DOL values. At 40% tapping, therefore, the switching becomes highly

m I

8

O/COn Off

I

trip

Motor windings in A

(b) Power and control circuit diagram

vulnerable as a result of greatly reduced torque and necessitates a proper selection of motor.

To determine the tapping of the autotransformer Consider an autotransformer with a tapping at 40%. Then by equating the powers of the primary and the secondary sides of the autotransformer (Figure 4.4)

or I,, ⁼(0.4)' . V, I Z while the starting current on DOL:

I,,, = 2:3 . V,IZ

...

^IAT= (0.4)'

.

^{I D O L}

i.e. proportional to the square of the tapping, where IDOL = starting current on a DOL switching

IAT = starting current on an autotransformer switching Z = impedance of the motor windings referred to the Generalizing the above equation, autotransformer tapping for a particular starting current, IAT is

stator side per phase

4/74 Industrial Power Engineering and Applications Handbook

R Y B N

i- ta

h l

1 1 1 1 1

oic

Trip Notes

(1) C, and C, switches or contactors are interlocked among themselves, so that only one is ON at time.

(2) C, is essential to isolate the transformer after performing the starting duty, to eliminate avoidable transformer

u

0.4K losses. Moreover the transformer is only short-time rated.

Tapping =

4 5

^{. 100%}

Figure 4.4 Auto-transformer starting (or 40% tapping)

(4.1) From this equation, the desired tapping of the autotransformer, to limit the starting current to a desired value can be determined.

Example 4.1

A squirrel cage motor has its lSi on DOL as six times its rated current. Find the required tapping on an autotransformer to limit the starting current to 1.5 times.

or 50%

Rating of autotransformer

Since an autotransformer is in the circuit for only a short period (during the start only), it can be short-time rated.

The rating of the autotransformer can be calculated from kVA (CMR, =

43

. kV ^'I,,

where

kV = applied voltage, and

(4.2)

kVA(,,,, = continuous rating of the autotransformer.

Since the transformer will be in the circuit for only 15 to 20 seconds, the approximate short-time rating of the transformer can be considered to be 10-15% of its continuous rating. T h e manufacturer of t h e auto transformer would be a better judge to suggest the most appropriate rating of the transformer, based on the tapping and starting period of the motor.

Rating of contactors

Star contactor C , and AIT contactor C2 must be rated for the square of the percentage tapping. For a tapping of 80%, for instance, the rating of the contactors Cl and C2 will be (0.Q2 or 64% of the full-load current of the motor.

The main contactor C,, however, will be rated for the full-load current.

For power and control circuit diagrams refer to Figure 4.4.

Example 4.2

For a 3.3 kV 450 kW motor, with a full-load current of 100 A and starting current on DOL as six times the rated current, the kVA rating of the transformer for 50% tapping will be

Starting of squirrel cage induction motors 4/75

transients, in addition to causing current transients. The current transients may far exceed even 14-20 times the rated current of the motor, as illustrated in Figure 4.5, depending upon the transient recovery voltage (TRV) (Section 17.6.2). We will describe the effect of an open transient condition on an LT and HT machine separately.

/AT = 0.!j2 x 100 x 6 = 150 A from equation (4.1)

:.

kVA(CMR) = 4 3

-

x 3.3 x 150

= 860 kVA

An autotransformer of nearly 100 kVA continuously-rated should be sufficient for this application.

Note The above example is only for a general reference.

The CDF of the transformer, for the short-time rating, should be increased with the starting time and the number of starts per hour. Refer to the transformer manufacturer for a more appropriate selection.

4.2.2(a) Open transient condition during a Whenever a changeover of a switching device (contactors generally) from one condition to another takes place, as discussed above, in changing over from one tapping to another, as in an autotransformer switching, or from star to delta as in a Y/A switching, there appears a small time gap of, say, 20-80 ms before the next contactor closes, after the first has dropped. During this period, while the machine will drop its speed only very marginally, and which may not influence the load, the power from across the motor terminals will cease for this duration (except its own induced e.m.f.). This time gap, when switching HT motors particularly, may cause switching transients and prove disastrous for the motor windings, as discussed in Section 17.7.2. This is termed an open transient condition. The situation is aggravated further because of the motor's own induced e.m.f., which may fall phase apart with the applied voltage and magnify the voltage

reduced-voltage switching sequence

ISt(A)

T

a E

lst(Y)

1 LT motors In LT motors, such a situation may not be a matter of concern as no switching surges would generally occur. The voltage would be too low to cause a re-strike between the interrupting contacts of the contactors (Section 17.7.6) and cause surges. The motor's own induced e.m.f. may, however, fall phase apart with the applied voltage and the voltage across the motor windings may double. In all likelihood the windings of a motor would be suitable to withstand effect of the same (Table 1 1.4). In locations, however, that are humid or chemically contaminated, or where the motor is likely to be switched on after long gaps, it is possible that the windings may have attained a low dielectric strength to withstand a voltage up to twice the rated one. Open transient conditions must be avoided in all such cases.

2 HT Motors Y / A switching in HT motors is rare, while an A / T switching may become necessary when the capacity of the feeding transformer is not adequate to withstand the start-up inrush of DOL switching, or when the drive calls for a frequent switching, such as in a large compressor or pump house, and the feeding transformer is not adequate for such a duty, or when a number of large drives are to be switched in quick succession and the feeding transformer may not be adequate to sustain such heavy inrush currents.

Speed

-

⁰

$3 \

Note

ob = Peak up to twice (= 14/J the actual /(A) current ob, = No current transient during a closed transient c'c = 1 to 2 ms. (opening + closing time of Y and 3 Explanation:

A. (i) ob is the current transient during changeover from Y to A in an open transient condition.

(ii) Voltage transient across the windings: 2.45 to 4.1 V,

od (it may even exceed 14-201,) switching

contactors respectively)

B. Sequence of a closed transient changeover, (i) When the bridging resistor is introduced in

parallel to Y windings at point, a, the total impedance of the windings gets reduced and current has small overshoot to ob compared to its normal current, oa.

(ii)There is no voltage transient now and the voltage across the windings remains at V,.

(iii) The Y contactor drops at bl, the impedance of the windings becomes high, current drops to

(iv) The bridging resistor drops at b3 and the motor current traces back its normal A current curve

~1 d.

Figure 4.5 Open and closed transient conditions in YlA switching

4/76 Industrial Power Engineering and Applications Handbook

However, unlike an LT distribution system, which may impose a limitation while switching large motors on DOL, the HT feeding lines in all probability may not pose any such limitation, as it may be feeding many more loads and may already be of a sufficient capacity. In HT systems an open transient condition may lead to severe voltage transients, which may prove disastrous for the motor windings. All motors that are switched A/T are therefore recommended to have a surge suppressor on each interrupting pole as noted in Section 18.8 which will take care of these surges.

Precautions such as adopting a closed transient switching method noted below will be essential where surge suppressors are not provided.

4.2.2(b) Closed transient switching

1 In a Y/A starter When desired, the above situation can be averted by inserting a bridging resistor in the motor windings through an additional contactor, which can be called a transition contactor, C. A typical power and control scheme is shown in Figure 4.6 for a Y f A switching. This contactor is energized through a timer, T I , just before the desired time of changeover (before the Y contactor opens) and energizes the auxiliary

contactor, d, which de-energizes the Y contactor S through its NC contact and energizes the A timer, T2.

Timer ^T2energizes the A contactor D, thus bridging the time of the second contactor and eliminating the condition of an open transient. In fact, the use of timer T2 becomes redundant with the introduction of the auxiliary contactor, d , which introduces the required delay (by its closing time) to close the A contactor

D.

It is, however, provided to allow only for an additional delay. The time of this timer, when provided, may be set low to account only for the transient time. As soon as the changeover is complete, the resistor contactor, C, drops through the NC contact of D .

The scheme is termed a closed transient switching.

A comparison of the two methods in terms of voltage transients and current overshoots is given in Table 4.1.

2 In an A/T starter The same logic can be applied as discussed above. The star point of the AfT is opened and connected through the main contactor C, to provide a near replica to a Y / A switching. Figure 4.7 illustrates the revised scheme.

Pressing the start P.B. will energize the auxiliary contactor d and timer T. The star contactor C , is switched on and energizes the AIT contactor C2 at the desired tapping. The motor starts at the required

Power circuit

1

OCR Reset

1'"

Control circuit I

Trip

-

I

M = Main contactor D = Delta contactor S = Star contactor C = Resistor contactor

T1 = Resistor ('ON delay timer)

R = Delta ('OFF' delay timer) d = Auxiliary contactor Figure 4.6 Circuit diagram for a closed transient Y / A switching

Starting of squirrel cage induction motors 4/77 Table 4.1 Comparison between an open and a closed transient switching in terms of voltage and current

Serial no. Condition Open transient Closed transient

._ _-_____

There is no voltage transient. The voltage across the motor windings remain at ‘V;

The current curve becomes ah,h,h,c,d (Figure 4.5). There is no current overshoot beyond the normal current in A

Voltage transient across the Up to 3 to 5 p.u. i.e. 3 to 5

(+)

motor windings Current overshoot during changeover from Y to A, point, a , on Y current curve

(Figure 4.5) may exceed 14-201,

or 2.45 to 4.1Vr Section 17.7.2 The current curve becomes a b c d (Figure 4.5) and current overshoots from oa to ob momentarily, which 2

. .-

__ _. .- _. ^~

Note Since the surge impedance of a circuit is normally very high, as noted in Section 17.8, it is the voltage transient that is the cause of concern in the above case than the current transient.

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Power circuit

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Control circuit c

Trip c1 = Star contactor

C2 = Transformer contactor C3 = Main contactor

d = Auxiliary contactor

T = Star connection-ON delay timer

h3 f

Figure 4.7 Circuit diagram for a closed transition ATswitching

reduced voltage. After the preset time of timer T the star contactor C1 falls out. The motor is still energized through the transformer winding without any inter- ruption during the changeover. The main contactor C, now energizes and the motor runs at full voltage.

The AIT contactor C2 also falls out thus achieving a closed transient switching sequence.

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