You are watching: What is the difference between a contactor and a magnetic starter?
Only the simplest and also smallest draft — usually v single-phase motors 5 hp or smaller sized or three-phase electric motors 15 hp or smaller sized — accept direct-on-line (also dubbed across-the-line) connection to the electrical power source without danger of a engine overdraw and a line undervoltage condition. Three-phase electric motors driven this method can have windings linked in a straightforward wye (also called star) or delta configuration … and also dual-voltage electric motors (convenient for how they deserve to accept 230 V or 460 V input) have actually twin coil to adjust that have the right to run in parallel or (for the higher voltage) in series.This Siemens SIRIUS 3RV2011-1HA10 size S00 circuit breaker is a current-limiting breaker for pack feeders come 3 kW in ~ three-phase 400 Vac. A short-circuit defense of 104 A and an flexible overload protection of 5.5 to 8 A reliably protects electrical motors. Image courtesy Automation24 Inc.
Everywhere else, across-the-line engine starts present too plenty of problems because that the motor itself as well as systems attached to the motor — including detrimental electrical effects and also excessive undertake on mechanically power-transmission components. The architecture objectives the safety, productivity, and also precision generally necessitate the usage of more advanced motor-driving approaches.Starting existing is crucial parameter in the suitable sizing and pairing of motors and motor starters. Beginning current from the engine starter should be enough to help the motor fulfill torque and also acceleration requirements but mustn’t reason excessive voltage fall on the electrical supply line.
Terminology groundwork: The difference between contactors and motor starters
In a previous ar of this design Guide, we detailed how contactors and also relays are distinct contents — despite occasional industry consumption of the terms that suggests otherwise. Contactors and motor starters are also distinct components. Here the terms are used interchangeably because their main point is the exact same exact an innovation — a switch qualified of taking care of high voltages.This direct-on-line engine starter is a SIRIUS 3RM1001-1AA04 from Siemens through a control voltage the 24 Vdc and also an adjustable pickup existing overload release of 0.1 to 0.5 A. That imparts solid-state motor protection and is suitable for systems with tiny motors approximately 0.12 kW one industry-standard 22.5 mm width consumes minimal room inside regulate cabinets. Image courtesy Automation24 Inc.
The distinction is the motor starters have one extra device or solution not discovered in contactors — an overload relay of some type to cut voltage input need to that relay feeling a motor-overload or thermally compromising condition due lengthy running overcurrent. Those designated together self-protecting engine starters additionally include short-circuit protection. Here again, specific use of ax is key: rather than utilizing short circuit come refer to any electrical malfunction, it’s only proper to usage the hatchet when mentioning a suddenly overcurrent emerging from the flow of electric power the has uncovered some unintended route of travel. Short-circuit defense acts instantaneously to reduced off the device from the power source.This is an instance of a power contactor. It is a Siemens SIRIUS 3RT2015-1BB41 for giving power come three-phase motors and also electrical heaters up to 3 hp in ~ 480 Vac. The strength contactor uses a regulate voltage the 24 Vdc functions a NO contact and also screw-type cable receptacles.In fact, there are plenty of sizes and also versions of this power contactor for load feeders v circuit breakers — and also various SIRIUS switching devices for the safe and functional convert of electrical loads.• 3RT2 contactors come in sizes S00 to S3. 3RT1 contactors come in sizes S6 come S12• 3RT.0 power contactors and also 3RT12 vacuum contactors are for convert motorized loads• Four-pole 3RT23 contactors (and three-pole 3RT24 / 3RT14 contactors) move resistive loads• Four-pole 3RT25 contactors space for changing the polarity of hoisting equipment motors• 3RH2 contactor relays move in the regulate circuit• 3RT26 capacitor contactors switch capacitive loads (AC-6b)• 3RT1 / 3RT2 / 3RH2 contactors have extended operating varieties … 3RT10 / 3RT20 / 3RH21 contactors space for rail applications …and 3RT20 / 3RH21 coupling relays room for system-compliant communication with digital controllers• 3RT1…-.S.36 contactors have actually failsafe control inputs because that safety-related applicationsReversing contactor assemblies space also obtainable — as are contactors for beginning three-phase electric motors with reduced starting-current peaks ( in to form of contactor assemblies because that star-delta arrangements.
Another difference between contactors and also motor starters is associated to how the two contents are rated and specified. Contactors are generally classified by their voltage capacity. In contrast, engine starters are generally rated by their present capacity and also the horsepower of the motors for which they’re compatible … even while accommodating inrush existing upon startup without nuisance tripping. The is generally done through a slight hold-up in relay tripping — as countless motors (especially smaller motors) deserve to reach full operating rate in just a couple of seconds.Circuit diagrams that sample sports of contactors, full-voltage motor starters, and soft starters present their differences and also similarities. Click come enlarge.
Motor starting at the most an easy level is divide as hand-operated or automatic.
Manual starting has hand-turned on-off switches that simply make or break the motor input circuit when activated by tree personnel. Part versions that qualify together true motor starters (as designated above) combine a thermal-overload relay to de-energize the engine if it i do not care overheated.
In contrast, immediately triggered motor beginning is sometimes referred to as magnetic starting for the electromechanical contactors that room core to this design.
As with any electromechanical relay technology, these have actually stationary electromagnetic coils that (upon a command from a pushbutton, limit switch, timer, float switch, or other relay) force together two circuits. These circuits encompass input strength contacts and also a mating carrier that (once closeup of the door together) enable current circulation into the motor windings. One sport on this design is a combination starter, which contains the magnetic action as well as some way of disconnecting electric power when required … either through a fuse, breaker, or engine circuit switch.
Wye-delta motor starting (one kind of reduced-inrush system) sends full-line voltage the motor’s wye windings throughout startup — despite voltage throughout each motor winding is diminished by the train station of the square source of 3 (57.7%) i m sorry is why this plan is sometimes (rather inexactly) called reduced-voltage starting. Then a circuit (usually through a contactor because that each phase, overload relay, timer, and mechanical interlock) switches the engine input to supply it full-line voltage right into its delta windings.
Part-winding motor starting — provided in conjunction with the specialty dual-voltage motors mentioned above — uses across-the-line voltage to only one component (half or 2 thirds) that the motor windings (typically ripe or twelve) top top start. Then as soon as a collection time has actually passed or collection voltage is detected, a relay or timer springs into activity and commands that the rest of the windings be added and supplied power together well. Acceleration might be irregular, but part-winding motor beginning impedance has no effect on beginning torque … and allows low-torque starts that are valuable for pumps, fans, and also blowers. Prefer wye-delta starting, part-winding starting is a form of reduced-inrush system and also delivers a lessened full-line voltage upon motor startup — yet doesn’t technically qualify together reduced-voltage starting.
Full-voltage reversible starting leverages exactly how induction motors change rotation direction top top reversal of any type of two strength leads. Reversing starter systems merely incorporate a pair of copy contactors complemented by interlocking subcomponents to allow run-forward and also run-reverse conditions. Faster rotational-direction reversals have the right to be made with plugging, i m sorry is the short-term powering that both circuits.
More controllable: Reduced-voltage motor starters
Besides the household of full-voltage motor-starting options are reduced-voltage starters. Where device axes need smooth non-jarring acceleration to full speed (to safeguard attached maker equipment or part attached load) reduced-voltage engine starters room essential. In fact, they’re likewise useful in setups regulated by neighborhood power utilities the limit voltage fluctuations and current surges on power supplies throughout motor starting.
Reduced-voltage motor starters include four usual subtypes.Primary resistor engine starters
Primary resistor engine starters space a cost-effective choice that uses resistors and also some variety of contactors — with the last dictating the number of starting voltage steps. This steps can be somewhat abrupt because of the circuit’s low inductance. Though the resistors have the right to be bulky and introduce inefficiency, this starter type delivers reliable motor-starting torque.Primary reactor motor starters
Primary reactor engine starters space most usual on big high-voltage motors. They employ a reactor (inductor) activity in a circuit prefer that the a primary resistor motor starter. Reasonably long smooth accelerations are feasible (even come a dozen seconds or more) though additional system inductance can degrads overall efficiency — and also a bad power element degrades torque-generating existing components and motor flux.Autotransformer engine starters
Primary reactor engine starters are reasonably costly but advantageous where flexible start talk is required. Autotransformer engine starters usage a single-winding electrical transformer — through the latter being a passive electrical maker for transferring electric energy indigenous one circuit come another. More specifically, autotransformer starters employ a trio of electric contactors on one autotransformer having actually selectable taps. That imparts stepped voltage starting for long smooth acceleration ~ above startup — also to a couple of dozen seconds. Begin voltage can be 50% come 80% of line voltage for high begin torques in applications wherein that (and not efficiency) is a leading architecture objective.Soft starters
Soft starters employing solid-state semiconductor modern technology are capable of the many controllability out of every motor-starter options. They’re also the gentlest ~ above motors’ inner subcomponents and also attached power-transmission mechanisms. At their core, soft starters consist of miscellaneous thyristor or SCR arrangements … so because that example, part designs have actually a pair of thyristors on each of the 3 lines right into the motor. Testimonial this design Guide’s ar on solid-state relays for the basics the this technology. These switching gadgets work to regulate electrical power right into the engine windings (as depicted by the soft-starter diagram reflecting firing angles) while leveraging just how motor voltage along with current and torque are low ~ above initial startup. Climate they slowly raise voltage and also torque follow to a preset routine.
Motor soft-starter programming dictates the exact parameters that the increase to set voltage. Take into consideration the operation of a representative SCR-based soft starter: here a conducting (gated) SCR has a movable gate allude … and adjusting ago this rate value (called ramp time) causes rise in voltage build-up before the SCR switches on. Then when the motor windings reach complete voltage, the SCR switches off.
One caveat: extreme ramp time deserve to make existing exceed the motor’s safety boundaries or note a current-limit security cutoff.
Besides the benefits already mentioned, soft starters impart motor protection (even during phase imbalances during electric-utility brownouts) and the capacity to soft stop. The latter is useful where motors drive design such as conveyors that involve inertias qualified of moving or breaking throughout transport.
Of course, variable frequency cd driver (VFDs) are an additional option for soft-start functionality. They administer the very same controlled beginning and stopping functions of a soft starter, albeit in a different way — by differing motor-input voltage frequency rather than voltage magnitude. Various other VFDs benefits over soft starters incorporate the capability to manage motor rate over the whole operating range. VFDs can additionally deliver power for holding speak (full torque in ~ zero speed) i beg your pardon is key in motor-driven applications such as cranes and also elevators.
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However, for part designs VFDs space overly costly and complicated. Reduced-voltage motor starters often tend to be much more suitable 보다 VFDs where there’s no gain in effectiveness to be had actually from running the enclosed motor below its height speed rating.