It is used where frequent opening and closing ON — OFF operation needed in electrical equipment such as motors, light and heaters etc. According to NEMA, The main function of the contactor is to repeatedly establish and interrupt and electrical power circuit i.
A Contactor depends on information from the motor starter control system and energizes and de-energizes the motor circuit. A Motor starter gets information from contactor and systems of contactors to energize and de-energize the motor. A Contactor function is same as a circuit breaker or switch but the working principle is different. This is not the case with the contactor i. This way, contactor protects the motor and operation process of the motor circuit.
Motor Starter can be a single circuit breaker or contactor or a system of motor starters, autotransformer to reduce the voltage for motor starting operation or a solid state device like VFD variable frequency drive which control the waveform sent to the motor to control motor starting operation.
Starter is rated in amperes or related to motor HP Horse Power rating and protect the motor circuit from overload surges and prevent overheating. But a starter itself can be another internal-combustion engine in the case of very large engines.
An internal-combustion engine is a feedback system that relies on the inertia from each cycle to initiate the next one. In a four-stroke engine, the first two strokes are not powered by the engine itself, but by the starter. Once the engine starts running, the starter is not required anymore as the feedback loop becomes self-sustaining.
A few features of a starter are:. This is a very often asked question. And the confusion is also an understandable one. Since contactors and starters both control electric motors, people usually use the two terms interchangeably.
Autotransformer motor starters use a single-winding electrical transformer — with the latter being a passive electrical device for transferring electrical energy from one circuit to another. More specifically, autotransformer starters employ a trio of electrical contactors on an autotransformer having selectable taps.
That imparts stepped voltage starting for long smooth acceleration upon startup — even to a few dozen seconds. Soft starters employing solid-state semiconductor technology are capable of the most controllability out of all motor-starter options. At their core, soft starters consist of various thyristor or SCR arrangements … so for example, some designs have a pair of thyristors on each of the three lines into the motor.
These switching devices work to control electrical power into the motor windings as illustrated by the soft-starter diagram showing firing angles while leveraging how motor voltage along with current and torque are low upon initial startup. Then they gradually raise voltage and torque according to a preset routine.
Motor soft-starter programming dictates the exact parameters of the increase to set voltage. Consider the operation of a representative SCR-based soft starter: Here a conducting gated SCR has a movable gate point … and adjusting back this speed value called ramp time causes an increase in voltage accumulation before the SCR switches on. Then once the motor windings reach full voltage, the SCR switches off. Besides the benefits already mentioned, soft starters impart motor protection even during phase imbalances during electric-utility brownouts as well as the ability to soft stop.
The latter is helpful where motors drive designs such as conveyors that involve inertias capable of shifting or breaking during transport.
Of course, variable frequency drives VFDs are another option for soft-start functionality. Contactor coils have a number of insulated turns of wire designed to give the necessary ampere-turns to operate on small currents. As contactors are used to control different line voltages, the voltage used to control the coil may vary. Therefore, when selecting coils you must choose one that matches the available control voltage.
The reason for this is that higher voltages will increase the speed of the electromagnet at closing. Lower voltages will decrease the speed at closing. Both these factors can lead to a higher level of contact bounce at closing, which can be a major cause of wear and erosion.
Magnetic coil voltage specifications include rated volt age, pickup voltage, hold-in voltage, and dropout voltage. Rated voltage refers to the coil supply voltage and must match that of the control circuit power source.
Pickup voltage is the amount of voltage required to overcome the mechanical forces, like gravity and spring tension, trying to keep the contacts from closing. Hold-in voltage is the amount of voltage needed to maintain the contacts in their closed position after pickup voltage is reached hold-in voltage is normally less than pickup voltage.
All contactors that are electrically held in are sensitive to voltage dips occurring in the electrical supply. The dropout volt age is the amount of voltage below which the magnetic field becomes too weak to maintain the contacts in their closed position. AC and DC contactor coils with the same voltage ratings are not normally interchangeable, the reason being that with a DC coil only the wire ohmic resistance limits the current flow, whereas with AC coils both resistance and reactance impedance limit the current flow.
Direct cur rent contactor coils have a large number of turns and a high ohmic resistance compared to their AC counterparts. For a DC-operated coil, since current is limited by resistance only, the current flow through the coil upon closing is the same as the normal energized current flow.
However, this isn't the case when the coil is AC operated. With a deenergized AC coil, part of the magnetic path has an air gap because the armature isn't pulled in Ill. When the contactor closes, the armature closes the magnetic path, causing the inductive reactance of the coil to increase and the current to decrease. This results in a high current to close the contactor and low current to hold it.
The inrush current for an AC coil may range from 5 to 20 times that of the sealed current. When current in an inductive load, such as a contactor coil, is turned off, a very high voltage spike is generated. If not suppressed, these voltage spikes can reach several thou sand volts and produce surges of damaging currents.
This is especially true for applications requiring interface with solid-state components such as PLC modules. The resistor and capacitor connected in series slows the rate of rise of the transient voltage. Contactor open; Air gap; Coil deenergized. Suppression module; Contactor; Contactor coil; Suppression module. Magnet coil current; Shading coil current. Contactor coils operated from an AC power source experience changes in the magnetic field surrounding them.
The attraction of an electromagnet operating on alternating current is pulsating and equals zero twice during each cycle. As the current goes through zero, the magnetic force decreases and tends to drop the armature out. When magnetism and force build up again, the armature is pulled back in. This motion of the armature, in and out, makes the contactor buzz or chatter, creating a humming noise and wear on the contactor's moving parts. The noise and wear of AC contactor assemblies can be prevented by the use of shading coils or rings, as illustrated in Ill.
Unlike the contactor coil, shading coils are not electrically connected to the power source, but mounted to inductively couple with the contactor coil. The shading coil consists of a single turn of conducting material generally copper or aluminum mounted on the face of the magnetic assembly. It sets up an auxiliary magnetic attraction that's out of phase with the main field and of sufficient strength to hold the armature tight to the core even though the main magnetic field has reached zero on the sine wave.
With well-designed shading coils, AC contactors can be made to operate very quietly. A broken or open shading coil will make its presence known; the contactor will immediately become extremely noisy. The core and armature of an AC contactor assembly are made of laminated steel, whereas DC assemblies are solid. This is due to the fact that there are no eddy currents generated with continuous direct current applied.
Eddy currents are small amounts of current flow induced in the core and armature material by the varying magnetic field produced by AC current flow through the contactor coil. Using a solid iron core would result in greater circulating currents and for this reason the core of AC coils is made up of a stack of thin insulated laminations.
Misalignment or obstruction of the armature's ability to properly seat when energized causes increased current flow in an AC coil. This could occur as a result of pivot wear or binding, corrosion, or dirt buildup, or pole face damage from impact over a long period of time.
Depending on the amount of increased current, the coil may merely run hot, or it may burn out if the current increase is large enough and remains for a sufficient length of time. Improper alignment will create a slight hum coming from the contactor in the closed position. A louder hum will occur if the shading coil is broken because the electromagnet will cause the contactor to chatter. Today, most contactor contacts are made of a low resistance silver alloy Ill.
Silver contacts are used because they ensure a lower contact resistance than other less expensive materials. Depending on the size of the contactor, the main power contacts can be rated to control several hundred amperes. Most often silver inserts are brazed or welded on copper contacts on the heel , so silver carries the current and copper carries the arc on interruption. Most manufacturers recommend that silver contacts never be filed.
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