Direct on Line Starter (DOL)
Direct on Line Starter (DOL)
A
direct on line (DOL) motor starter is an electrical/electronic circuit composed
of electro-mechanical and electronic devices which are employed to start and
stop an electric motor. Regardless of the motor type (AC or DC), the types of
starters differ depending on the method of starting the motor. A DOL starter
connects the motor terminals directly to the power supply. Hence, the direct on
line motor is subjected to the full voltage of the power supply. Consequently,
high starting current flows through the motor. This type of starting is
suitable for small motors below 5 hp (3.75 kW). Reduced-voltage starters are
employed with motors ab
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A direct on
line starter, often abbreviated DOL starter, is a widely-used starting method
of electric motors. The term is used in electrical engineering and associated
with electric motors. There are many types of motor starters, the simplest of
which is the DOL starter.
Most motors are reversible or, in other words, they can be run clockwise and anti-clockwise. A reversing starter is an electrical or electronic circuit that reverses the speed of a motor automatically. Logically, the circuit is composed of two DOL circuits; one for clockwise operation and the other for anti-clockwise operation. A very well-known motor starter is the DOL Starter of a 3-Phase Squirrel-Cage Motor. This starter is sometimes used to start water pumps, compressors, fans and conveyor belts. With a 400V, 50 Hz, 3-phase supply, the power circuit connects the motor to 400V. Consequently, the starting current may reach 3-8 times the normal current. The control circuit is typically run at 24V with the aid of a 400V/24V transformer. An animation of the circuits of this starter is shown here. |
Induction motor
From Wikipedia, the free encyclopedia
Three-phase
induction motors
Animation of a
squirrel-cage AC motor
An induction motor (or asynchronous
motor or squirrel-cage motor) is a type of alternating
current motor where power is supplied to the rotor by means of electromagnetic induction.
An electric
motor converts electrical power to mechanical power in its rotor (rotating
part). There are several ways to supply power to the rotor. In a DC motor this
power is supplied to the armature directly from a DC
source, while in an induction motor this power is induced in the rotating
device. An induction motor is sometimes called a rotating transformer
because the stator
(stationary part) is essentially the primary side of the transformer
and the rotor (rotating part) is the secondary side. Unlike the normal transformer
which changes the current by using time varying flux, induction motor uses
rotating magnetic field to transform the voltage. The primary side's currents
evokes a magnetic field which interacts with the secondary side's emf to
produce a resultant torque, henceforth serving the purpose of producing
mechanical energy. Induction motors are widely used, especially polyphase
induction motors, which are frequently used in industrial drives.
Induction motors are now the
preferred choice for industrial motors due to their rugged construction,
absence of brushes (which are required in most DC motors) and — thanks to
modern power electronics — the ability to control the speed of the motor.
History
The induction motor was first
realized by Galileo Ferraris in 1885 in Italy. In 1888,
Ferraris published his research in a paper to the Royal Academy of Sciences in
Turin (later, in the same year, Tesla gained U.S. Patent 381,968) where he
exposed the theoretical foundations for understanding the way the motor
operates. The induction motor with a cage was invented by Mikhail Dolivo-Dobrovolsky about a year
later. Technological development in the field has improved to where a 100 hp (74.6 kW) motor from 1976 takes
the same volume as a 7.5 hp (5.5 kW) motor did in 1897. Currently,
the most common induction motor is the cage rotor motor
Principle of operation and comparison to synchronous
motors
A 3-phase
power supply provides a rotating magnetic field in an induction motor.
The basic difference between
an induction motor and a synchronous
AC motor is that in the latter a current is supplied into the rotor
(usually a DC current) which in turn creates a (circular uniform) magnetic
field around the rotor. The rotating magnetic field of the stator will impose
an electromagnetic torque on the still magnetic field of the rotor causing it
to move (about a shaft) and rotation of the rotor is produced. It is called
synchronous because at steady state the speed of the rotor is the same as the
speed of the rotating magnetic field in the stator.
By way of contrast, the
induction motor does not have any direct supply onto the rotor; instead, a
secondary current is induced in the rotor. To achieve this, stator windings are
arranged around the rotor so that when energised with a polyphase supply they
create a rotating magnetic field pattern which
sweeps past the rotor. This changing magnetic field pattern induces current in
the rotor conductors. These currents interact with the rotating magnetic field
created by the stator and in effect causes a rotational motion on the rotor.
However, for these currents to
be induced, the speed of the physical rotor must be less than the speed of the
rotating magnetic field in the stator, or else the magnetic field will not be
moving relative to the rotor conductors and no currents will be induced. If by
some chance this happens, the rotor typically slows slightly until a current is
re-induced and then the rotor continues as before. This difference between the
speed of the rotor and speed of the rotating magnetic field in the stator is
called slip. It is unitless and is the ratio between the relative speed
of the magnetic field as seen by the rotor (the slip speed) to the speed
of the rotating stator field. Due to this an induction motor is sometimes
referred to as an asynchronous machine.
AC Induction Motor
where
n = Revolutions per minute
(rpm)
f = AC power frequency (hertz)
p = Number of poles per phase
(an even number)
Slip is calculated using:
where s is the slip.
The rotor speed is:
Synchronous Motor
A synchronous motor always
runs at synchronous speed with 0% slip. The speed of a synchronous motor is
determined by the following formula:
where as p= no. of magnetic
poles
For example a 6 pole motor
operating on 60Hz power would have speed:
where v is the speed of
the rotor (in rpm), f
is the frequency of the AC supply (in Hz) and p is
the number of magnetic poles.
Note on the use of p:
Some texts refer to number of pole pairs per phase instead of number
of poles per phase. For example a 6 pole motor, operating on 60Hz power,
would have 3 pole pairs. The equation of synchronous speed then becomes: P=3 ,P
= no. of magnetic pole pairs.
Right
Construction
The stator consists of wound
'poles' that carry the supply current to induce a magnetic field that
penetrates the rotor. In a very simple motor, there would be a single
projecting piece of the stator (a salient pole) for each pole, with
windings around it; in fact, to optimize the distribution of the magnetic
field, the windings are distributed in many slots located around the stator,
but the magnetic field still has the same number of north-south alternations.
The number of 'poles' can vary between motor types but the poles are always in
pairs (i.e. 2, 4, 6, etc.).
Induction motors are most
commonly built to run on single-phase or three-phase power, but two-phase motors
also exist. In theory, two-phase and more than three phase induction motors are
possible; many single-phase motors having two windings and requiring a
capacitor can actually be viewed as two-phase motors, since the capacitor
generates a second power phase 90 degrees from the single-phase supply and
feeds it to a separate motor winding. Single-phase power is more widely
available in residential buildings, but cannot produce a rotating field in the
motor (the field merely oscillates back and forth), so single-phase induction
motors must incorporate some kind of starting mechanism to produce a rotating
field. They would, using the simplified analogy of salient poles, have one
salient pole per pole number; a four-pole motor would have four salient poles.
Three-phase motors have three salient poles per pole number, so a four-pole
motor would have twelve salient poles. This allows the motor to produce a
rotating field, allowing the motor to start with no extra equipment and run
more efficiently than a similar single-phase motor.
There are three types of
rotor:
The most common rotor is a
squirrel-cage rotor. It is made up of bars of either solid copper (most common)
or aluminum that span the length of the rotor, and those solid copper or
aluminium strips can be shorted or connected by a ring or some times not, i.e.
the rotor can be closed or semiclosed type. The rotor bars in squirrel-cage
induction motors are not straight, but have some skew to reduce noise and
harmonics.
- Slip ring
rotor
A slip ring rotor replaces the
bars of the squirrel-cage rotor with windings that are connected to slip rings.
When these slip rings are shorted, the rotor behaves similarly to a
squirrel-cage rotor; they can also be connected to resistors to produce a
high-resistance rotor circuit, which can be beneficial in starting
- Solid core rotor
A rotor can be made from a
solid mild steel. The induced current causes the rotation.
Speed control
The synchronous rotational
speed of the rotor (i.e. the theoretical unloaded speed with no slip) is
controlled by the number of pole pairs (number of windings in the stator) and
by the frequency of the supply voltage. Before the development of cheap power
electronics, it was difficult to vary the frequency to the motor and therefore
the uses for the induction motor were limited. As an induction motor has no
brushes and is easy to control, many older DC motors are being replaced with
induction motors and accompanying inverters in industrial applications.The
induction motor runs on induced current.speed of induction motor varies
according to the load supplied to the induction motor.As the load on the
induction motor is increased the speed of the motor gets decreased and vice
versa.
Starting of induction motors
Three
Phase
Direct-on-line starting
The simplest way to start a
three-phase induction motor is to connect its terminals to the line. This
method is often called "direct on line" and abbreviated DOL.
In an induction motor, the
magnitude of the induced emf in the rotor circuit is proportional to the
stator field and the slip speed (the difference between synchronous and rotor
speeds) of the motor, and the rotor current depends on this emf. When the motor
is started, the rotor speed is zero. The synchronous speed is constant, based
on the frequency of the supplied AC voltage. So the slip speed is equal to the
synchronous speed, the slip ratio is 1, and the induced emf in the rotor is
large. As a result, a very high current flows through the rotor. This is
similar to a transformer with the secondary coil short circuited, which causes
the primary coil to draw a high current from the mains.
When an induction motor starts
DOL, a very high current is drawn by the stator, in the order of 5 to 9 times
the full load current. This high current can, in some motors, damage the
windings; in addition, because it causes heavy line voltage drop, other
appliances connected to the same line may be affected by the voltage
fluctuation. To avoid such effects, several other strategies are employed for
starting motors.
Star-delta starters
An induction motor's windings
can be connected to a 3-phase AC line in two different ways:
- wye (star in Europe), where
the windings are connected from phases of the supply to the neutral;
- delta (sometimes mesh in
Europe), where the windings are connected between phases of the supply.
A delta connection of the
machine winding results in a higher voltage at each winding compared to a wye
connection (the factor is ). A star-delta starter initially connects the motor
in wye, which produces a lower starting current than delta, then switches to delta
when the motor has reached a set speed. Disadvantages of this method over DOL starting are:
- Lower starting torque, which may be a
serious issue with pumps or any devices with significant breakaway torque
- Increased complexity, as more contactors
and some sort of speed switch or timers are needed
- Two shocks to the motor (one for the initial
start and another when the motor switches from wye to delta)
Variable-frequency drives
Variable-frequency drives (VFD) can be of
considerable use in starting as well as running motors. A VFD can easily start
a motor at a lower frequency than the AC line, as well as a lower voltage, so
that the motor starts with full rated torque and with no inrush of current. The
rotor circuit's impedance increases with slip frequency, which is equal to
supply frequency for a stationary rotor, so running at a lower frequency
actually increases torque.
[edit]
Resistance starters
This method is used with slip
ring motors where the rotor poles can be accessed by way of the slip rings.
Using brushes, variable power resistors are connected in series with the poles.
During start-up the resistance is large and then reduced to zero at full speed.
At start-up the resistance
directly reduces the rotor current and so rotor heating is reduced. Another
important advantage is the start-up torque can be controlled. As well, the
resistors generate a phase shift in the field resulting in the magnetic force acting
on the rotor having a favorable angle[citation needed].
Autotransformer starters
such starters are called as
auto starters or compensators, consists of an auto-transformer.
Series
Reactor starters
In series reactor starter
technology, an impedance in the form of a reactor is introduced in series with
the motor terminals, which as a result reduces the motor terminal voltage
resulting in a reduction of the starting current; the impedance of the reactor,
a function of the current passing through it, gradually reduces as the motor
accelerates, and at 95 % speed the reactors are bypassed by a suitable
bypass method which enables the motor to run at full voltage and full speed.
Air core series reactor starters or a series reactor soft starter is the most
common and recommended method for fixed speed motor starting. The applicable
standards are [IEC 289] AND [IS 5553 (PART 3) ]
Single Phase
In a single phase induction
motor, it is necessary to provide a starting circuit to start rotation of the
rotor. If this is not done, rotation may be commenced by manually giving a
slight turn to the rotor. The single phase induction motor may rotate in either
direction and it is only the starting circuit which determines rotational
direction.
For small motors of a few
watts the start rotation is done by means of a single turn of heavy copper wire
around one corner of the pole. The current induced in the single turn is out of
phase with the supply current and so causes an out-of-phase component in the
magnetic field, which imparts to the field sufficient rotational character to
start the motor. Starting torque is very low and efficiency is also reduced.
Such shaded-pole motors are typically used in
low-power applications with low or zero starting torque requirements, such as
desk fans and record players.
Larger motors are provided
with a second stator winding which is fed with an out-of-phase current to
create a rotating magnetic field. The out-of-phase current may be derived by
feeding the winding through a capacitor, or it may derive from the winding
having different values of inductance and resistance from the main winding.
In some designs the second
winding is disconnected once the motor is up to speed, usually either by means
of a switch operated by centrifugal force acting on weights on the motor shaft,
or by a positive temperature coefficient thermistor which after a few seconds
of operation heats up and increases its resistance to a high value, reducing
the current through the second winding to an insignificant level. Other designs
keep the second winding continuously energised during running, which improves
torque.
Control of speed in induction
motor can be obtained in 3 ways:
1.scalar
control 2.vector control 3.direct torque control
Direct on line starter
From Wikipedia, the free encyclopedia
A direct on line
starter, often abbreviated DOL starter, is a widely-used method of starting electric
motors. The term is used in electrical engineering and associated with
electric motors. There are many types of motor starters, the simplest of which
is the DOL starter. The direct on line starter can be considered as a switch.
When the motor is required to run, the starter supplies the full supply voltage
(such as 400V, 50 Hz, 3-phase in the UK) to the motor. The motor will
start as quickly as it can. A DOL motor starter is distinct from a simple relay
in that it will also contain protection devices, and in some cases, condition
monitoring.
DOL starting is sometimes used
to start small water pumps, compressors,
fans
and conveyor
belts. In the case of an asynchronous motor, such as the 3-phase squirrel-cage motor, the motor will pull a high
starting current until it has run up to full speed. This starting current is
commonly around six times the full load current, but may as high as 12 times
the full load current. For this reason, larger motors will normally be soft
started or run on variable speed drives in order to minimise disruption to the
power supply.
Contents
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[edit]
DOL reversing starter
Most motors are reversible or,
in other words, they can be run clockwise and counter-clockwise. A reversing
starter is an electrical or electronic circuit that reverses the direction of a
motor automatically. Logically, the circuit is composed of two DOL circuits;
one for clockwise operation and the other for anti-clockwise operation. The
case of three phase motor inter changing of any two phases will do the same
[edit] Safety devices within the
starting circuit
These devices are used to
protect the motor as well as the user of the motor being operated:
1. Overload coil The overload coil, also known as a thermal overload, is designed to open the starting circuit and thus cut the power to the motor in the event of the motor drawing too much current from the supply. The overload coil is a normally closed device which opens due to heat generated by excessive current flowing through the circuit. Thermal overloads have a small heating device that increases in temperature as the motor running current increases. A bi-metallic strip located close to the heater deflects as the heater temperature rises until it mechanically causes the device to trip and open the circuit, cutting power to the motor should it become overloaded. A thermal overload is basically a circuit breaker that will accommodate the brief high starting current of a motor whilst being able to accurately protect it from a running current overload. This is because the heater coil and the action of the bi-metallic strip introduces a time delay that affords the motor time to start and settle into normal running current without the thermal overload tripping. Thermal overloads can be manually or automatically resettable depending on their application and have an adjuster that allows them to be accurately set to the motor run current.
2. KM1 No-Volt coil The
No-Volt coil serves a safety purpose of preventing a motor from restarting
automatically after a power failure. This coil is connected in parallel across
the start switch (which is a normally open switch) in the control circuit. The
no-volt coil is energized once the start switch is initiated and the energized
coil keeps current flowing through the control circuit. In the event that the
power being supplied to the motor is interrupted, the motor will stop, and the
No-Volt coil will become de-energized. In order for the motor to be restarted,
the start switch must be initiated and therefore restarting the motor, and
re-energize the energize the no-volt coil, re-starting the cycle. Only once the
no-volt coil is energized, will current remain flowing in the control circuit.
…………………………………………………………………………………………………………………………………………………………………
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What is a star delta starter and how does it work?
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[Improve]
The starting current of any heavy
electric motor can be more than 4 times the normal load current it draws when
it has gained speed and has reached its normal running output power and
temperature.
So, if it were started simply when connected in DELTA, the starting current would be huge and - just to be able to start the motor, not to run it normally - would require:
So, if it were started simply when connected in DELTA, the starting current would be huge and - just to be able to start the motor, not to run it normally - would require:
- large circuit breakers, big
enough to allow the start-up surge current to pass without immediately
shutting it off. (But the breakers would then be much too big to be
able to protect the motor from over-current faults whilst it is running
normally.)
- very thick 3-phase power
service cables. (But the cable would then be much bigger than is
necessary whilst the motor is running normally.)
- very large coils and contacts
on the relays or contactors used to control the motor. (But they would
then be much bigger than is necessary whilst the motor is running
normally.)
One solution to this problem is to start the motor in STAR and then, when the motor has gained sufficient speed, change its connections to DELTA to allow the motor to run at its full speed and torque from then on. It's a bit like using the gears of an automobile.
Update: Electronic motor-control systems, which offer soft-starts in DELTA configuration, are now replacing the use of manual or semi-automatic star-delta starters.
Technical explanation
When the windings of a 3-phase motor are connected in STAR:
- the voltage applied to each
winding is reduced to only (1 /.'/'3) [1 divided by
root three] of the voltage applied to the winding when it is connected
directly across two incoming power service line phases in DELTA.
- the current per winding is
reduced to only (1 /.'/'3) [1 divided by root three]
of the normal running current taken when it is connected in DELTA.
- so, because of the Power Law V
[in volts] x I [in amps] = P [in watts],
the total output power when the motor is connected in STAR is:
PS = [VL x (1/.'/'3)] x [ID x (1/.'/'3 )] = PD x (1/3) [one third of the power in DELTA]
where:
VL is the line-to-line voltage of the incoming 3-phase power service
ID is the line current drawn in DELTA
PS is the total power the motor can produce when running in STAR
PD is the total power it can produce when running in DELTA. - a further disadvantage when the
motor is connected in STAR is that the total output torque is only 1/3 of
the total torque it can produce when running in DELTA.
………………………………………………………………………………………………………
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Why use star-delta connections for a three phase
motor?
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Whichever way it is connected, no
matter whether it is in a star configuration or in a delta configuration, a
3-phase motor's start-up current can be more than 4 times its normal running
current.
If the star configuration is used when first switching-on power to a 3-phase motor, a much smaller "start-up surge" is forced onto the power lines than if it were switched-on directly in the delta configuration.
So "using star for start-up" achieves very worthwhile purchase cost savings because smaller circuit breakers and thinner 3-phase line wire sizes can be installed to supply power to the motor.
However, leaving it running in star has a major disadvantage: the motor can never deliver as much power and torque as when it is running in delta.
For that reason a 3-phase motor was usually started in star mode and then - after reaching a steady speed - switched over to run in delta mode to achieve its maximum power output.
The explanation for this is easier to understand if you draw a sketch of the wirings and their connections, but unfortunately we cannot use diagrams when giving an answer here! Anyway, if you draw the circuit diagram for the windings connected in a "star" or "Y" configuration, it should look like a three-pointed star, with a phase input power line attached to each point of the star.
Thus, when a 3-phase motor's three windings are connected in a star configuration, the current from each individual phase power input line goes directly into one winding and is then series-connected to both of the other two windings via the star's "center-point".
If you draw the circuit diagram for a delta configuration, it should look like a triangle with a phase power input line attached to each point of the triangle.
Thus, when a 3-phase motor's three windings are connected in a delta configuration, each winding is effectively connected directly to two phase supply lines. The third phase supply line is also connected to that winding, but indirectly via the other two windings. They are connected in series to one another, and that series pair is connected in parallel across the first winding, to form the "delta".
The much lower starting current is the main reason why a three-phase motor was usually started in star mode and then - after gaining a steady speed - was switched over to run in delta mode to achieve its maximum power output.
Update: Electronic motor-control systems, which offer soft-starts in DELTA configuration, are now replacing the use of manual or semi-automatic star-delta starters.
Technical explanation
When the windings of a 3-phase motor are connected in STAR:
If the star configuration is used when first switching-on power to a 3-phase motor, a much smaller "start-up surge" is forced onto the power lines than if it were switched-on directly in the delta configuration.
So "using star for start-up" achieves very worthwhile purchase cost savings because smaller circuit breakers and thinner 3-phase line wire sizes can be installed to supply power to the motor.
However, leaving it running in star has a major disadvantage: the motor can never deliver as much power and torque as when it is running in delta.
For that reason a 3-phase motor was usually started in star mode and then - after reaching a steady speed - switched over to run in delta mode to achieve its maximum power output.
The explanation for this is easier to understand if you draw a sketch of the wirings and their connections, but unfortunately we cannot use diagrams when giving an answer here! Anyway, if you draw the circuit diagram for the windings connected in a "star" or "Y" configuration, it should look like a three-pointed star, with a phase input power line attached to each point of the star.
Thus, when a 3-phase motor's three windings are connected in a star configuration, the current from each individual phase power input line goes directly into one winding and is then series-connected to both of the other two windings via the star's "center-point".
If you draw the circuit diagram for a delta configuration, it should look like a triangle with a phase power input line attached to each point of the triangle.
Thus, when a 3-phase motor's three windings are connected in a delta configuration, each winding is effectively connected directly to two phase supply lines. The third phase supply line is also connected to that winding, but indirectly via the other two windings. They are connected in series to one another, and that series pair is connected in parallel across the first winding, to form the "delta".
The much lower starting current is the main reason why a three-phase motor was usually started in star mode and then - after gaining a steady speed - was switched over to run in delta mode to achieve its maximum power output.
Update: Electronic motor-control systems, which offer soft-starts in DELTA configuration, are now replacing the use of manual or semi-automatic star-delta starters.
Technical explanation
When the windings of a 3-phase motor are connected in STAR:
- the voltage applied to each
winding is reduced to only (1 /.'/'3) [1 divided by
root three] of the voltage applied to the winding when it is connected
directly across two incoming power service lines in DELTA.
- the current per winding is
reduced to only (1 /.'/'3) [1 divided by root three]
of the normal running current taken when it is connected in DELTA.
- so, because of the Power Law V
[in volts] x I [in amps] = P [in watts],
the total output power when the motor is connected in STAR is:
PS = [VL x (1/.'/'3)] x [ID x (1/.'/'3 )] = PD x (1/3) [one third of the power in DELTA]
where:
VL is the line-to-line voltage of the incoming 3-phase power service
ID is the line current drawn in DELTA
PS is the total power the motor can produce when running in STAR
PD is the total power it can produce when running in DELTA. - a further disadvantage when the
motor is connected in STAR is that its total output torque is only 1/3 of
the total torque it can produce when running in DELTA.
For more information please see the answers to the Related Questions shown below.
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How does a DOL three phase motor starter work?
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A
DOL Starter means Direct On-Line starter. In this type of
starting a 3-phase motor, full voltage is applied to the motor through relays
and contactors. Its is the most common type of 3-phase motor starter used.
It has a "closing circuit" and an "opening circuit". The closing circuit is for applying the supply to the motor and the opening - or "tripping" - circuit protects the motor by cutting-off (or "tripping") power to the motor from the supply lines if there is any overload condition, a single phasing fault, etc.
For more information please see the answers to the Related Questions shown below.
It has a "closing circuit" and an "opening circuit". The closing circuit is for applying the supply to the motor and the opening - or "tripping" - circuit protects the motor by cutting-off (or "tripping") power to the motor from the supply lines if there is any overload condition, a single phasing fault, etc.
For more information please see the answers to the Related Questions shown below.
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What are the applications of a star delta starter?
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The Star/Delta starter is probably
the most commonly used reduced voltage starter, but in a large number of
applications, the performance achieved is less than ideal, and in some cases,
the damage and interference is much worse than that caused by a Direct On Line
starter.
The Star/Delta starter requires a six terminal motor that is delta connected at the supply voltage. The Star Delta starter employs three contactors to initially start the motor in a star connection, then after a period of time, to reconnect the motor to the supply in a delta connection. While in the star connection, the voltage across each winding is reduced by a factor of (1 /.'/'3) [1 divided by root three]. This results in a start-current reduction to (1 /.'/'3) [1 divided by root three] of the DOL start current and a start torque reduction to one third of the DOL start torque.
If there is insufficient torque available while connected in star, the motor can only accelerate to a partial speed compared to the full speed it would reach if connected in delta. When the timer operates (set normally from 5-10 seconds), the motor is disconnected from the supply and then reconnected in delta, resulting in full line voltage running currents and torque.
The transition from star connection to delta connection requires that the current flow through the motor is interrupted. This is termed "Open Transition Switching" and with an induction motor operating at a partial speed compared to full load speed, there is a large current and torque transient produced at the poi, unless proper protection methods are used, can cause severe damage to the supply service's infrastructire and to other connected equipment.
If there is insufficient torque produced by the motor when running in star, there is no way to accelerate the load to full speed without switching to delta and causing those severe current and torque transients. These must be allowed-for in the design of the motor and its starting system if they are to have an economic useful life.
Update: Electronic motor-control systems, which offer soft-starts in DELTA configuration, are now replacing the use of manual or semi-automatic star-delta starters.
Technical explanation
When the windings of a 3-phase motor are connected in STAR:
The Star/Delta starter requires a six terminal motor that is delta connected at the supply voltage. The Star Delta starter employs three contactors to initially start the motor in a star connection, then after a period of time, to reconnect the motor to the supply in a delta connection. While in the star connection, the voltage across each winding is reduced by a factor of (1 /.'/'3) [1 divided by root three]. This results in a start-current reduction to (1 /.'/'3) [1 divided by root three] of the DOL start current and a start torque reduction to one third of the DOL start torque.
If there is insufficient torque available while connected in star, the motor can only accelerate to a partial speed compared to the full speed it would reach if connected in delta. When the timer operates (set normally from 5-10 seconds), the motor is disconnected from the supply and then reconnected in delta, resulting in full line voltage running currents and torque.
The transition from star connection to delta connection requires that the current flow through the motor is interrupted. This is termed "Open Transition Switching" and with an induction motor operating at a partial speed compared to full load speed, there is a large current and torque transient produced at the poi, unless proper protection methods are used, can cause severe damage to the supply service's infrastructire and to other connected equipment.
If there is insufficient torque produced by the motor when running in star, there is no way to accelerate the load to full speed without switching to delta and causing those severe current and torque transients. These must be allowed-for in the design of the motor and its starting system if they are to have an economic useful life.
Update: Electronic motor-control systems, which offer soft-starts in DELTA configuration, are now replacing the use of manual or semi-automatic star-delta starters.
Technical explanation
When the windings of a 3-phase motor are connected in STAR:
- the voltage applied to each
winding is reduced to only (1 /.'/'3) [1 divided by
root three] of the voltage applied to the winding when it is connected
directly across two incoming power service lines in DELTA.
- the current per winding is
reduced to only (1 /.'/'3) [1 divided by root three]
of the normal running current taken when it is connected in DELTA.
- so, because of the Power Law V
[in volts] x I [in amps] = P [in watts],
the total output power when the motor is connected in STAR is:
PS = [VL x (1/.'/'3)] x [ID x (1/.'/'3)] = PD x (1/3) [one third of the power in DELTA]
where:
VL is the line-to-line voltage of the incoming 3-phase power service
ID is the line current drawn in DELTA
PS is the total power the motor can produce when running in STAR
PD is the total power it can produce when running in DELTA. - a further disadvantage when the
motor is connected in STAR is that its total output torque is only 1/3 of
the total torque it can produce when running in DELTA.
For more information please see the answers to the Related Questions shown below.
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Re: What
is the d/f between DOL & star delta starter & automatic star delta
starter?
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A DOL starter connects the motor terminals directly to the
power supply. Hence, the motor is subjected to the full
voltage of the power supply. Consequently, high starting
current flows through the motor. This type of starting is
suitable for small motors below 5 hp (3.75 kW). Reduced-
voltage starters are employed with motors above 5 hp.
Although DOL motor starters are available for motors less
than 150 kW on 400 V and for motors less than 1 MW on 6.6
kV. Supply reliability and reserve power generation
dictates the use of reduced voltage or not
To reduce the starting current of an induction motor the
voltage across the motor need to be reduced. This can be
done by autotransformer starter, star-delta starter or
resistor starter. Now-a-days VVVF drive used extensively
for speed control serves this purpose also.
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