HAVELLS Motor Starter Price List w.e.f 5th September 2013 (VIEW) HAVELLS Customer Care No:+91-120-4771000
L&T Motor Starter Price List w.e.f 1st November 2013 (VIEW) L&T Customer Care No:18002094545
Introduction of Motor Starter
Motor soft starters can consist of mechanical or electrical devices, or a combination of both. Mechanical soft starters include clutches and several types of couplings using a fluid, magnetic forces, or steel shot to transmit torque, similar to other forms of torque limiter.
AC (alternating current) motor starters are used on electric motors that utilize a start and stop button or switch for the operation. Safety switches can also be employed in the low-voltage circuit that controls the power to the AC motor starter. AC motor starters are also used on large motors in which the electrical power requirements are so large that it would be unsafe to operate a single switch to turn the motor on. The motor starter can also be located at a great distance from the electric motor, so remote or automatic operation of the motor is made possible. The AC motor starter generally has three main components, the pull-in coil, the electrical contacts and the over current protection.
Different starting methods are employed for starting induction motors because Induction Motor draws more starting current during starting. To prevent damage to the windings due to the high starting current flow, we employ different types of starters.
The simplest form of motor starter for the induction motor is the Direct On Line starter. The Direct On Line Motor Starter (DOL) consist a MCCB or Circuit Breaker, Contactor and an overload relay for protection. Electromagnetic contactor which can be opened by the thermal overload relay under fault conditions.Typically, the contactor will be controlled by separate start and stop buttons, and an auxiliary contact on the contactor is used, across the start button, as a hold in contact. I.e. the contactor is electrically latched closed while the motor is operating.
Principle of Direct On Line Starter (DOL)
To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush current for a very short time, the magnetic field in the iron, and then the current will be limited to the Locked Rotor Current of the motor. The motor will develop Locked Rotor Torque and begin to accelerate towards full speed.As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 85% of synchronous speed. The actual starting current curve is a function of the motor design, and the terminal voltage, and is totally independent of the motor load.
The motor load will affect the time taken for the motor to accelerate to full speed and therefore the duration of the high starting current, but not the magnitude of the starting current.
Provided the torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque delivered by the motor is less than the torque of the load at any speed during the start cycle, the motor will stops accelerating. If the starting torque with a DOL starter is insufficient for the load, the motor must be replaced with a motor which can develop a higher starting torque.
The acceleration torque is the torque developed by the motor minus the load torque, and will change as the motor accelerates due to the motor speed torque curve and the load speed torque curve. The start time is dependent on the acceleration torque and the load inertia.
This may cause an electrical problem with the supply, or it may cause a mechanical problem with the driven load. So this will be inconvenient for the users of the supply line, always experience a voltage drop when starting a motor. But if this motor is not a high power one it does not affect much.
STAR-DELTA MOTOR STARTER
A device called a "star-delta motor starter" is used to make three-phase motors start and run efficiently. A longer answer 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 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.
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