0
A device called a "star-delta motor starter" is used to make three-phase motors start and run efficiently.
Some electric motors are designed to run on three-phase services, which means they have three incoming service lines. (Normal household electrical services are of a type called "single phase", which means they have just one service line and a neutral wire.) The three windings inside a three phase motor can be connected in one of two different ways: either in "star" or in "delta".
During motor start-up a way must be found to make the connections change over from star to delta at the right time. One way is to use a special circuit called a "star-delta motor starter".
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:
- 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.
More Information:
Star and Delta are two different configurations of the connections of a three phase AC electrical circuit , typically applicable for connections of a three phase motor.
Three phase supply is where the Alternating power supply voltage in the three live wires is identical in magnitude when referenced to a common supply point ( also called the neutral.) However there is a phase difference of 120 degrees with respect to each of the phase , resulting in a higher magnitude of voltage when it is measured between any two phases. This implies that if the circuit is configured to allow current to pass identically as measured with the common terminal ( star configuration ) , its value will be lesser as compared to when the circuit is configured to pass current between each pair of the three phases ( delta configuration )
The motor when its starts from rest ( or a low speed ) it will always draw a very large current as compared to its normal working ( rated ) current. This was a practical limitation which was unavoidable ( at least till the advent of Electronic Inverter Motor Controllers ) and can be damaging if not corectly limited in magnitude & duration.
Thus the Star configuration was used to start the motor with a lower resultant magnitude of the starting current. However this configuration does not allow the motor to operate at its full mechanical load driving capacity due to the lower final working currents. To overcome this , the delta configuration is used to enable full power motor operation.
Since this involves a change of motor terminal supply connection wiring pattern,
a system using relay/contactoris used, which allows changing of multiple electrical connections simultaneously, which is named as Star-Delta starter.
The star to delta changeover can be done manually after verifiying that the motor has achieved a minimium desired speed, or it can be done automatically by using a delay timer which is set for an appropriate duration, so that the system switches over from star to delta with requiring any manual operation.
Modern Electronic inverter based motor controllers need not use this changeover as other techniques are used to monitor / provide motor operation within the specified limits.
Wiki User
∙ 9y ago
Add your answer:
Earn +20 pts
What is the difference between a Y - delta starter and a star - delta starter?
No difference. Y is used as a symbol to indicate the star connection.
What are the different types of starter?
DOL, Star Delta, Soft starter, VFD
How many contactors are used in a star-delta starter?
3 contactors are used: one for the main incoming service lines, one to make the star connection and one for the delta connection. Auxiliary to the contactors, a delay timer, adjustable from 500-1000 seconds, is used to shift from star to delta configuration.
Why use link in star contactor?
The link in the star contactor is used to form the star point of three of the motor leads. On most star wound motors the star point is inside the motor windings. On a motor that can be used for star delta starting it is a six wire motor. The star point winding depends on the contactor to tie the coil ends together externally in the control panel of the star delta starter.
What is a timer in a star delta starter?
A timer in star delta starter for a three phase motor is an electronic device designed to do the change over or transition from star connection - using which the motor suns on a reduced voltage and current and produces less torque - to the delta connection necessary for running the motor at its full power, using high voltage and current to produce a high torque. The timing interval that is set is dependent how long the motor needs to start up from standstill to normal speed. Some motors require 15 seconds for the initial startup in star connection and after that given value of time the timer signals to the star contactor to go to the open position and the main delta contactor to go to the closed position, thus forming the delta circuit.
What is the difference between a Y - delta starter and a star - delta starter?
No difference. Y is used as a symbol to indicate the star connection.
What is a manual star-delta starter?
In manual star delta starter, the change over from star to delta does not happen automatically. The operator or user puts on the starter, and after few seconds he pushes a lever or handle to change over to Delta. In automatic method, change over happens from star to delta using an automatic preset timer.
What is a Manual starter?
In manual star delta starter, the change over from star to delta does not happen automatically. The operator or user puts on the starter, and after few seconds he pushes a lever or handle to change over to Delta. In automatic method, change over happens from star to delta using an automatic preset timer.
What are the different types of starter?
DOL, Star Delta, Soft starter, VFD
What are several types of starters?
Types of starters: 1. Direct on line starter 2. Autotransformer starter 3. Star Delta starter 4. Resistance starter 5. Fully automatic star delta starter 6. Series reactor starter
Why timer can be used in star delta starter?
delay timer
When a rewinded 60hp motor is tested on empty with star delta starter it picks up speed in star and falls its rpm suddenly when the starter is in on condition reasons?
Check the star - delta wiring. It does not seem to be correctly wired.
What device may be wired into the starting circuit of a motor to improve the starting torque?
I assume a 3 phase, AC induction motor. A star delta starter or a soft starter can be used. A soft starter is better compared to a star delta starter, but it is more expensive.
Is there any dual star delta starter?
Nope .. Never heard of it .
Type of starters in three phase circuit?
There is many type of starter on three phase circuit but the most common are 1: D.O.L (Direct on line). 2: Manual Star Delta Starter. 3: Automatic Star Delta Starter.
How much KW range can be use star-delta starter?
The use of a star delta starter is for motor control only and not resistive heating loads. This type of starter is used to dampen the inrush current by using a lower voltage to start the motor.
What will happen if induction motor is in star-star connected?
If a motor is connected in a star configuration it will operate at a slower speed. This is the working process that a star delta starter operates on. At first through the starter the motor starts in the star connection. When the motor gets up to its highest speed in that connection it times a timer out , the star starter drops out and the delta starter pulls in and then connects the motor in the delta connection. This allows more voltage to be applied to the motor and it then speeds up to its nameplate rated RPM.
