CHAPTER 5 STARTING OF INDUCTION MOTORS

                                                                                                     

5.1       GENERAL

The a.c. motors throughout all offshore and onshore installations are almost exclusively of the induction type, with squirrel-cage rotors.  They may be of high voltage supplied from HV busbars, or they may be of low voltage supplied from one or other of the LV busbars; the latter form the great majority of offshore and industrial motors.

The range of powers, extends from about 1 2000 hp (9 000kW mechanical) down to 1 hp or even less.  Motors below about 300 hp (220kW_m) run from the 440V or 415V system, but those of higher power are usually of the high-voltage type.

When a motor is started direct-on-line (‘DOL’ starting), at the instant of switch-on the rotor is stationary, and the motor behaves as a short-circuited transformer.  The initial starting current at standstill is therefore very large, though at low power factor.  Typically it may be four to six times the normal full-load current, with a power factor of the order of 0.3 to 0.2.

FIGURE 5.1

INDUCTION MOTOR CURRENT, TORQUE AND POWER FACTOR/SPEED CURVES

Figure 5.1 shows the varying conditions as a motor runs up from standstill to full speed.  As the motor begins to move and to generate back-emf, the low power factor reactive current steadily falls off, but the motor begins to draw more active power as it accelerates.  The net effect is that the initial starting current persists fairly constant through most of the run-up period, but the power factor steadily improves.  Finally the speed settles down to just below synchronous, that is with a small ‘slip’, and the current falls sharply to whatever the load demands.  This is indicated as current/speed (blue) and power factor/speed (green) curves in Figure 5.1, which also shows the characteristic ‘Rock of Gibraltar’ torque/speed (black) curve.

With very few exceptions all offshore motors are direct-on-line started, whether of the high-voltage or low-voltage type.  The 440V or 415V motors, which form the great majority, are usually remotely started through contactors in separate panels of LV switchboards (also called ‘Motor Control Centres’ or ‘MCCs’), which are described in greater detail in the manual ‘Electrical Distribution Equipment’, Part ‘A’.  HV motors are remotely started through circuit-breakers or vacuum or air-break contactors backed by High Rupturing Capacity (HRC) fuses in HV switchboards.

The run-up time of a direct-on-line started motor depends on:

·                                      The starting torque of the motor

·                                      The inertia of the driven load

·                                      Whether or not the driven end is loaded.

Motors are designed for different starting torques.  The 60% of Figure 5.1 is only typical, but the range is much wider, depending on the application.  In general, a high starting torque gives a lower efficiency.

Clearly the inertia of, say, a centrifugal fan is far less than that of a motor generator set, and the run-up time would be much shorter.  Loading the driven end (as distinct from starting light) will clearly extend the run-up time.

Torque depends on the square of the applied voltage.  Therefore an induction motor is very sensitive to voltage drop at its terminals, however caused.  For example, a 20% drop in applied voltage (i.e. to 0.8 nominal) will result in only 64% (0.8²) of the designed torque. In extreme cases this may prevent a motor from even starting at all, especially if there is much breakaway friction (‘stiction’), as with reciprocating drives.  This is further discussed in Chapter 6.

5.2       REDUCED VOLTAGE STARTING

If it is desired to reduce the heavy starting current to a level well below that which would be experienced with direct-on-line starting, various methods are available which reduce the voltage applied to the motor at the instant of starting, and restore it to full value when the motor has run up to speed.  Two of the principal methods are described below.  It should be noted however that reduced voltage starting also means reduced starting torque, so that, where high starting torques are needed, these methods may not always be suitable.

5.2.1    Star/Delta Starting

Figure 5.2(a) shows the conventional method of starting a motor direct-on-line.

Figure 5.2(b) is known as ‘star/delta’ starting.  All six ends of the motor windings are brought out, and a 3-pole changeover hand-switch or contactor connects them alternatively in star or delta.  (The changeover switch is not, of course, inside the motor as indicated in Figure 5.2(b), but it is shown there for electrical clearness.)  It is drawn in the ‘star’ position; delta is the running condition, where full line voltage is placed across each phase winding.  When starting, the motor is temporarily connected in star, so that the voltage appearing across any one phase winding is line voltage divided by √ 3 (see the inserts).  Thus a 440V machine would have 254V applied to each phase - this is the ‘reduced voltage’ - and the starting line current taken at that voltage is reduced to one-third of what it would have been with a delta connection.  When the motor is up to speed and the starting current has fallen to its running value, the motor is reconnected into delta and takes its full voltage across each phase.  As it is then running at nearly synchronous speed, the changeover will cause only a small increase of current.

FIGURE 5.2

STAR/DELTA STARTING

The changeover from star to delta may be by hand changeover switch; it must be provided with an interlock to prevent it operating until it has been returned to the star position.  The star and delta positions are usually tallied START and RUN respectively.  Alternatively the reconnection may be by contactor, especially with an auto-started motor.  Interlocks will ensure the correct sequence of operating.

Delta-connected motors are less easy to protect than star-connected ones and are little used on offshore installations.  Consequently star/delta starting is seldom found offshore but is very common in industrial installations, in which case the machines run as delta-connected motors.

5.2.2    Auto-transformer Starting

Figure 5.3 is a different form of reduced voltage starting which uses an auto-transformer to provide the lower starting voltages.  (An ordinary double-wound transformer could be used, but, as the ratio is close, an auto-transformer is just as effective and is smaller and cheaper.)

FIGURE 5.3

AUTO-TRANSFORMER STARTING

Here the motor is started on the auto-transformer tap (typically 50 to 70%).  It runs up on the reduced voltage, taking an appreciably lower starting current.  When it is up to speed the auto-transformer and its tap are disconnected by a hand-switch or contactors and the motor is connected directly across the full-voltage mains.  As in the case of star/delta starting, the motor will be running at nearly synchronous speed at the end of the first auto-transformer stage, so that the changeover to full voltage will cause only a small increase of current.  This method, unlike star/delta, allows a star-connected motor to be used.

As with direct-on-line starting, the run-up time depends on the motor design, on the inertia of the driven end and on whether the machine starts loaded or light.  In the case of auto-transformer starting it will also depend on the voltage ratio of the auto-transformer and hence on the voltage applied during the first stage.

Certain features of auto-transformer starting should, however, be noticed:

(a)     The ‘start’ and ‘run’ switches, if of the contactor type, must be positively interlocked so that both cannot be closed at the same time, otherwise part of the auto-transformer primary will be shorted out.

(b)     The changeover from ‘start’ to ‘run’ must be time-delayed. When power is taken off a large running motor, it remains magnetised for a time by its rotor current until it has been damped out.  While it continues, the motor will be generating back-emf.  If the full voltage is applied while this is happening, it might come in anti-phase, with consequent currents of the order of a short-circuit.  Time delays of about two seconds are usual with large motors.

FIGURE 5.4

KORNDORFFER METHOD

5.2.3    Korndorffer Method

The disadvantages are overcome by a modification to the basic auto-transformer method, known as ‘Korndorffer’ starting, shown in Figure 5.4.

Here the auto-transformer remains constantly connected, but it is not actually energised - that is, it does not carry any current - until its star-point is completed by a circuit-breaker or contactor.  As soon as the main breaker and star-point breakers are closed, the motor, which is permanently connected to the auto-transformer taps, receives its reduced voltage and starts its first stage run-up.

When it is up to near synchronous speed, the star-point breaker opens, leaving the motor still connected to the mains through the auto-transformer primary section acting as a choke.  Immediately after the star-point breaker has opened, and without special time delay, the ‘run’ breaker closes, connecting the motor direct to the full-voltage mains to complete its second stage and shunting out the choke.  At no time during transition from first to second stage has the motor been totally disconnected from the mains; therefore its back-emf can never become out-of-phase with the mains voltage.

In large motors the sequence of switch closing and opening is automatic and is tied in with other mechanical sequences such as pre-start purging of the motor casing, running up of lubricating pumps and correct opening of valves on the driven load side.  The completion of each stage is monitored by timing relays, and, unless it is completed within a certain preset time, the main breaker is tripped and the start aborted, with appropriate alarm lamps lit on the main control panel.

On most installations the only motors large enough to require auto-transformer starting are typically those driving the Gas Compression Pipeline Boosters and Re-injection Compressors.  On Brent ‘A’, ‘B’, ‘C’ and ‘D’ these motors are rated between 7 160kW_m and 9 250kW_m (9 600 hp and 12 400 hp) and are started using Avon (18MVA) generators as supply source and in each case the Korndorffer method is used

5.3       EFFECT ON SYSTEM OF STARTING A LARGE MOTOR

One very important consequence of starting these very large motors must be realised.  Even with reduced voltage starting and the consequent limiting of the starting current, the call on generator capacity is considerable.  Because the starting power factor is low, most of the starting current is reactive, at least at first, and it is shown in Chapter 3 of the manual ‘Electrical Generation Equipment’ that it is the reactive load current which causes voltage drop in generators.  In the case of the 9 250kW_m re-injection compressor motor the reactive starting load is about 23Mvar, which is more than one generator, even of Avon size (18MVA), can reasonably cope with.  At least two generators should be on line to enable such a motor to be started, and even then their AVRs will be extended to the limit to maintain voltage.  The situation will be aggravated if the generators already have an appreciable standing load, and load-shedding may be found necessary to enable a start to be made.

In the extreme case the voltage dip even with full AVR action may be so great, and the starting torque so heavily reduced, that the motor may not be able to break away against its ‘stiction’ and the motor may fail to start at all.

In any case a severe voltage dip on the whole offshore installation system may be expected until the motor is up to speed, and that very dip may itself cause the motor to be sluggish in running up even if it does not prevent it.  If the dip is large or prolonged, other motor contactors may trip on undervoltage, and operators must be prepared to find this.  Those installations with only two Avons will be more susceptible than those with four.

FIGURE 5.5

STARTING OF LARGE MOTOR

Figure 5.5(a) is an example of such a situation, and Figure 5.5(b) shows the voltage situation at the motor terminals when a large motor is direct-on-line (i.e. single-stage) started while another is running. It is assumed that the AVR is already compensating for all other running loads, including that of the large motor which is already running.

In the example shown, at the instant of start and before the AVR can react, there is a sudden voltage drop of 30% due partly to the reactive starting current flowing through the generator reactance and partly to voltage drop in the feeder cable. This is followed by a short period of partial recovery of about half a second while the AVR reacts and increases excitation to the limit; the voltage rises somewhat but stays at 15% dip during most of the run-up period because of continuing voltage drop in the cable. When the motor is up to speed and its current falls to its ‘run’ value, the voltage rises further as the cable drop is reduced until it settles at its controlled value near 6.6kV. The voltage at the motor terminals, however, will still be a little down (typically 2¼%) because of the voltage drop in the feeder cable due to the motor’s running current.

Note that the intermediate drop level (15% in Figure 5.5(b)) to which the voltage partially recovers under AVR action will depend on the standing load and the extent to which the AVR is already committed. The amount and speed of recovery will depend on the margin left in the AVR.

It must be stressed that it is the reactive loading on the generators due to the motor starting which is the critical factor. The active (MW) load demanded by the gas compression motors during starting is well within the capacity of one Avon unless it is already heavily loaded. When shedding loads, therefore, those with large reactive components (e.g. motors) should be selected rather than, say, heating. The Mvar meter on the switchboard is the one to watch.

The whole question of how far a generator may be loaded by running motors, or by motors which are about to be started, is discussed more fully in Chapter 2 of the manual ‘Electrical System Control’ under the heading ‘Capability Diagram’.

Starting a large motor in an extensive onshore installation, which is backed up by a big supply network, is less of a problem; the principles set out above remain, although their effect may be less noticeable.

5.4       REPEATED STARTING

If, when a start signal is given, a motor does not start, there is an understandable urge to ‘try again’.

Each time an attempt is made to start a motor and the contactor actually closes but the motor does not move, a severe starting current of the order of five to six times full-load current flows continuously through the contactor contacts and the windings of the stationary motor. This gives rise to heating (I²R) at a rate 25 to 36 times normal for the motor, and without ventilation. When in due course the overcurrent protection operates and trips the contactor, not only has a large amount of heat been generated inside the motor, but also the contactor has to break some 500% of the normal current, and at a low power factor - a most severe condition.

If another attempt is made immediately, the overcurrent protection resets. It has the same time delay as before, and therefore an equal amount of heat is released to be added to the first. Each starting attempt will therefore raise the motor winding temperature still further. Too many attempts will raise it to such a temperature that there is severe risk of damage to the insulation and of an immediate or early breakdown, even though the protection is working properly. Cables and motor cable boxes may also be thermally overstressed, with similar risk of breakdown.

The repeated opening of the contactor under such severe conditions is likely not only to overheat it but to burn, and possibly weld, the contacts.  Many examples of this have been reported.  If this happens and the contactor cannot open because of welding, all protection of the motor is lost, and a burnout is almost inevitable, and possibly fire.

Some motors are now provided with thermistor or resistance thermal detector elements embedded in their windings.  They detect the temperature rise from whatever cause and, if it exceeds a certain preset level, they trip and lock out the motor until it has cooled (always assuming, of course, that the contactor has not welded-in).  The operator can seldom know whether any particular motor is so protected, and he should observe a general rule that if, after two attempts, a motor does not start, he should make no further try but call in Maintenance.

Indeed some operators have laid down a general instruction that motors over 50kW are only suitable for two successive starts when cold.  Another one start attempt is allowable after a cooling period of 30 minutes at standstill.  No more than three starts may be attempted in any one hour.

For certain very large motors the manufacturers may make special stipulations not only on the number of starting attempts, but also on the maximum permitted number of successful starts over a given time.  For example, for the large gas compression motors (7.1MW to 9.2MW) they stipulate not more than two attempted starts in succession, not more than one successful start in any 30 minutes and not more than 300 starts in one year.

5.5       EXAMPLE OF A MOTOR STARTING CALCULATION

At the end of Chapter 4 a calculated example was given on how to estimate the current and power taken by a motor when running at various loads.  Now an example is given of the calculation needed to estimate the current and power (active and reactive) of a motor when starting.  The same motor as was assumed for the Chapter 4 example is used, but certain additional starting information is given, such as would be found on the motor’s rating plate.

Summarising 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           = 2 040kvar     (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).

As the motor runs up and approaches synchronous speed, these figures gradually change until, at whatever load the motor is to run, they settle at the figures in the table of
Chapter 4.

Since the voltage drop in a generation and distribution system is due principally to the reactive loading, it becomes clear that a voltage dip will be experienced whenever a motor, particularly a large motor, is started. The sudden demand for considerable reactive power will cause the voltage to drop before the AVR is able to correct it. This appears on an installation as a momentary dip of the lights.