Download Electrical Motor Starters

Motor Starters
When the stator windings of an induction motor are connected directly to its 3phase supply, a very large current (5-8 times full load current) flows initially. This
surge current reduces as the motor accelerates up to its running speed.
Induction motors can be Direct-on Line (DOL) started in this way. The starting
current will not cause damage to the motor unless the motor is repeatedly
started and stopped in a short space of time. This is called ‘fast cycling’. When
very large motors are started direct-on-line they cause a disturbance of voltage
(voltage dip) on the supply lines due to the large starting current surge. This
voltage disturbance may result in the malfunction of other electrical equipment
connected to the supply.
To limit the starting current some large induction motors are started at reduced
voltage and then have the full supply voltage reconnected when they have run
up to near rated speed.
Direct-on-line starting is the method most commonly used, the most usual
consideration being whether the generator and the distribution system can
withstand the starting current without excessive voltage dips. In the case of
loads involving considerable inertia, such as centrifugal oil separators, the
starting time may also be a factor. In case of doubt, the motor manufacturer
should be consulted.
The starting current as we have already seen may be five to eight times the full
load current, and the heating of the windings is proportional to the square of the
current. At starting it will therefore be 25-64 times normal. Furthermore, at the
instant of start there is no windage and no radiation. Therefore a very long
starting period may result in overheating.
For these reasons it is also undesirable to make repeated successive starts
without intervening periods for cooling.
The contactor coil is connected in series with a start button, stop button and
overload trip contacts. This is called the control circuit and is energised from two
lines of the 3-phase supply – usually via a step-down transformer. When the
start button is pressed the control supply is connected to the contactor coil. The
contactor closes and then starts the motor. When the start button is released its
contacts spring open. An auxiliary contact on the contactor keeps the contactor
coil energised after the start button is released.
Pressing the stop button breaks the control circuit to the contactor coil; the
contactor trips and the motor stops.
If the motor takes too much current because it is mechanically overloaded or
stalled, the overload coils will either magnetically or thermally open the overload
trip contacts which will stop the motor and prevent overheating. Note, the
correct term is ‘overcurrent’ rather than the commonly used ‘overload’.
Reduced voltage starting is used for large motors driving loads like cargo pumps
and bow thrusters. Two methods of reduced voltage starting are star-delta
starting and autotransformer starting.
After DOL starting, the next most common method is the star-delta method.
Both ends of each phase of the motor starter windings must be brought out and
connected to starter. In the start position the windings are connected in star; in
the running position they are reconnected in delta. The voltage across each
phase winding in the start position is 58% (1/Ö3) of line voltage, with
consequent reduction of starting current. The starting torque is also reduced to
one-third of that which would obtain with d.o.l. starting. With a single-cage or
double-cage rotor of average performance, this represent about 80% of full-load
torque, assuming normal line voltage, but if there is appreciable line drop the
torque will be proportionately lower. These factors must be taken into account
when deciding whether star-delta starting is acceptable for the driven machine.
It will be acceptable for centrifugal fans and pumps if, in the latter case, the
friction at starting is not excessive.
When the operating handle is placed in the ‘start’ position the motor stator
windings are connected in star across the supply. As the motor approaches
normal running speed the operator must quickly change the handle to run
position which changes the motor connection from star to delta. If the operator
does not move the handle quickly from start to run the motor may be
disconnected from the supply long enough for the motor speed to fall
considerably. When the handle is eventually put into the run position the motor
will take a large current may be large current and accelerate up to speed again.
This surge current may be large enough to cause appreciable voltage dip. The
prevent this, a mechanical interlock is fitted to the operating handle. The handle
must be moved quickly from start to run otherwise the interlock jams the handle
in the start position.
An automatic change over is preferable and this is achieved by using contactors.
Star-Delta starter sequence:
Operator closes motor isolator IS then presses start button.
Start button connects the supply to contactor coil S.
Contactor contacts S close and auxiliary contacts S1 close.
L close, motor windings are star connected to 3-phase supply, motor starts.
Auxiliary contacts L1 close at the same time as contactor contacts L. The
operator may now release the start button since supply to L is maintained
through L1. After a time interval which allows the motor to run up to speed,
auxiliary contacts L2 and L3 close.
Contactor coil S is de-energised and its contacts S open; so do the auxiliary
contact S1. Contactor coil D is energised and the motor is now delta connected
to the 3-phase supply. In some cases a mechanical interlock is fitted between
the contactor contact S and D so that both cannot be closed at the same time.
The auto-transformer starter is more expensive than two type so far described
and is generally used only for the larger types of motor. It is suitable for motors
in which each end of the three phases is not brought out, and which would
therefore be unsuitable for star-delta starting. The starting conditions depend on
the position of the tapping on the transformer winding, i.e. on the secondary
voltage. Usually three or more tappings are provided so that there is a choice of
starting conditions such as 40, 60 or 75% of line voltage. The starting torques on
these different tappings can be estimated as they are proportional to the square
of the voltage. On the 60% tapping the torque will b approximately the same as
with star-delta starting, and on the 40 and 75% tapping it will be proportionately
lower and higher respectively.
Starting large motors with long-run up periods demands a very high current
surge from the supply generator. This causes a severe voltage dip which affects
every load on the system. Reduced voltage starting will limit the starting surge
current.
One way to reduce the initial voltage supplied to the motor is to step it down
using a transformer. Then, when the motor has accelerated up to almost full
speed, the reduced voltage is replaced by the full mains voltage. The transformer
used in this starter is not the usual type with separate primary and secondary
windings. It is an autotransformer which uses only one winding for both input
and output. This arrangement is cheaper, smaller and lighter than an equivalent
double-wound transformer. For induction motor starting, the autotransformer is
a 3-phase unit, and because of expense, this method is only used with large
motor drives, e.g. electric cargo pumps.
The autotransformer with its range of tapping points gives a set range of starting
voltages to limit the motor starting surge current to a reasonable value.
As with the star-delta starter, the autotransformer may use what is called an
open transition switching sequence or a closed transition switching sequence
between the start and run conditions. In the former, the reduced voltage is
rapidly reconnected to the motor.
The circuit diagram below shows a manually operated open transition,
autotransformer starter.
The problem with open transition is that a very large surge current can flow after
the transition from reduced to full voltage.
An arrangement which overcomes the transition switching problem is the closed
transition ‘Korndorfer’ starting method. A typical circuit is shown below.
When the start button is pressed the first stage contactor coil is energised which
closed the main 1st contacts and also 1st/1 and 1st/2. The timer relay coil and the
second stage contactor coil are also energised. The main 2st contacts close,
which applies a reduced voltage from the autotransformer to the motor
windings. The motor starts. After a preset time interval the times relay opens
tr/2 which drops out the second stage contactor. The star point of the
transformer is opened by 2st so that transformer action no longer takes place.
The transformer winding no just acts as an inductive voltage dropping
impedance in the supply lines to the motor. The voltage applied to the motor is
now higher than before but is still less than the full supply voltage. After a
further time interval the timer relay closes tr/3, which energises the changeover
relay. The changeover relay closes the run contactor which puts full voltage on
to the motor. Auxiliary contacts rn/2 and rc/1 on this contactor drop out the first
stage contactor and maintain the supply to the run contactor coil, respectively.
The stop button or overcurrent relay trips out the run contactor to stop the
motor.
The windings of the autotransformer are short-time rated and the starting period
must not be unduly prolonged. The rating for ‘ordinary’ during is usually suitable
for not more than two starts per hour. After two consecutive starts a minimum
subsequent cooling period of 60 minutes is necessary. If more frequency starting
is required, starters rated for intermittent duty (40 starts per hour) should be
specified.
Electronic starters often referred to as ‘soft start’, are finding acceptance in the
marine industry. Solid-state technology is employed to provide a method of
starting without the current and torque surges mentioned previously. Thyristors
or a combination of thyristors and diodes are used to control the current flow
during motor starting. The basic circuit diagrams for these two alternatives are
shown below.
The electronics for controlling the firing of the thyristors is normally
accommodated on a small printed circuit board within the motor controller.
Although the thyristor/diode configuration is cheaper it has the disadvantage that
it generates third and even harmonic currents in the motor windings, whereas
the all-thyristor arrangement restricts the even harmonics.
With this type of starter there are normally three adjustments that have to be set
to suit the drive machinery:
1. Voltage ramp - This sets the time for the starter to achieve full voltage
output. It should be noted that the ramp time is the time taken for the output
voltage to reach its maximum and not for the motor to reach its full speed.
If a motor is lightly loaded it may well achieve full speed before full voltage is
applied.
2. Current limit - This adjustment is used to prevent the starting current
exceeding a preset value. Because torque is proportional to the square of the
current it must be set sufficiently high that adequate torque is developed to
accelerate the load from rest.
3. Initial firing angle - It is often important that a drive should start as soon
as voltage is applied, e.g. if the drive is standby to a duty unit.
If the initial firing angel is set too small there will be a delay in starting the drive
until the voltage has been ramped to a value permitting sufficient torque to be
developed to accelerate it from the rest. If the initial firing angle is set too large
the load may be suddenly grabbed rather than accelerated smoothly.
Triacs can also be used for electronic starters. However, since they have
relatively low current ratings and breakdown voltages they are generally suitable
only for low-current low-voltage applications