You are watching: What is the difference between a motor starter and a contactor
Only the simplest and also smallest draft — usually through single-phase motors 5 hp or smaller sized or three-phase motors 15 hp or smaller sized — expropriate direct-on-line (also called across-the-line) connection to the electric power source without hazard of a motor overdraw and also a line undervoltage condition. Three-phase motors driven this means can have windings associated in a straightforward wye (also referred to as star) or delta construction … and dual-voltage electric motors (convenient for just how they can accept 230 V or 460 V input) have twin coil set that have the right to run in parallel or (for the greater voltage) in series.This Siemens SIRIUS 3RV2011-1HA10 size S00 circuit breaker is a current-limiting breaker for load feeders to 3 kW at three-phase 400 Vac. A short-circuit security of 104 A and an flexible overload security of 5.5 to 8 A reliably protects electric motors. Picture courtesy Automation24 Inc.
Everywhere else, across-the-line motor starts existing too plenty of problems because that the engine itself and systems attached come the motor — consisting of detrimental electrical effects and also excessive wear on mechanical power-transmission components. The design objectives of safety, productivity, and also precision normally necessitate the usage of much more advanced motor-driving approaches.Starting present is vital parameter in the ideal sizing and also pairing that motors and also motor starters. Beginning current indigenous the motor starter need to be sufficient to aid the motor accomplish torque and also acceleration requirements yet mustn’t reason excessive voltage drop on the electrical supply line.
Terminology groundwork: The difference in between contactors and also motor starters
In a previous section of this style Guide, we in-depth how contactors and relays space distinct contents — in spite of occasional industry intake of the terms that argues otherwise. Contactors and motor starters are additionally distinct components. Below the state are offered interchangeably due to the fact that their core is the very same exact technology — a switch capable of managing high voltages.This direct-on-line motor starter is a SIRIUS 3RM1001-1AA04 indigenous Siemens with a control voltage of 24 Vdc and an flexible pickup current overload release of 0.1 to 0.5 A. The imparts solid-state motor protection and is perfect for equipment with little motors up to 0.12 kW an industry-standard 22.5 mm width consumes minimal space inside regulate cabinets. Picture courtesy Automation24 Inc.
The difference is that motor starters have one extra device or systems not found in contactors — an overload relay that some kind to cut voltage input need to that relay sense a motor-overload or thermally compromising condition due an extensive running overcurrent. Those designated as self-protecting engine starters also include short-circuit protection. Below again, specific use of terminology is key: quite than utilizing short circuit to refer to any type of electrical malfunction, it’s only suitable to usage the term when stating a suddenly overcurrent emerging from the flow of electric power that has discovered some unintended path of travel. Short-circuit protection acts instantaneously to reduced off the device from the strength source.This is an instance of a strength contactor. That is a Siemens SIRIUS 3RT2015-1BB41 for providing power to three-phase motors and also electrical furnaces up to 3 hp at 480 Vac. The strength contactor offers a control voltage the 24 Vdc functions a NO contact and screw-type cable receptacles.In fact, there are plenty of sizes and versions of this strength contactor for load feeders with circuit breakers — and various SIRIUS switching gadgets for the safe and also functional convert of electrical loads.• 3RT2 contactors come in sizes S00 come S3. 3RT1 contactors come in size S6 come S12• 3RT.0 power contactors and also 3RT12 vacuum contactors are for switching motorized loads• Four-pole 3RT23 contactors (and three-pole 3RT24 / 3RT14 contactors) move resistive loads• Four-pole 3RT25 contactors space for transforming the polarity of hoisting equipment motors• 3RH2 contactor relays switch in the manage circuit• 3RT26 capacitor contactors switch capacitive lots (AC-6b)• 3RT1 / 3RT2 / 3RH2 contactors have expanded operating arrays … 3RT10 / 3RT20 / 3RH21 contactors space for rail applications …and 3RT20 / 3RH21 coupling relays space for system-compliant interaction with digital controllers• 3RT1…-.S.36 contactors have actually failsafe control inputs because that safety-related applicationsReversing contactor assemblies are also easily accessible — as space contactors for beginning three-phase electric motors with diminished starting-current peaks ( in to kind of contactor assemblies for star-delta arrangements.
Another difference between contactors and motor starters is related to just how the two components are rated and also specified. Contactors are usually classified by their voltage capacity. In contrast, engine starters are typically rated by their existing capacity and also the speech of the motors for which they’re compatible … even while accommodating inrush present upon startup there is no nuisance tripping. That is typically done v a slight delay in relay tripping — as numerous motors (especially smaller sized motors) have the right to reach complete operating speed in just a couple of seconds.Circuit diagrams the sample variations of contactors, full-voltage engine starters, and also soft starters display their differences and similarities. Click come enlarge.
Motor beginning at that is most simple level is share as hands-on or automatic.
Manual starting contains hand-turned on-off switches that just make or rest the engine input circuit as soon as activated by tree personnel. Some versions that qualify as true engine starters (as designated above) combine a thermal-overload relay to de-energize the motor if it becomes overheated.
In contrast, automatically triggered motor starting is sometimes referred to as magnetic starting for the electromechanical contactors that are core to this design.
As with any type of electromechanical relay technology, these have actually stationary electromagnetic coils that (upon a command native a pushbutton, limit switch, timer, rise switch, or various other relay) force together 2 circuits. This circuits encompass input strength contacts and a adjustment carrier the (once close up door together) enable current circulation into the motor windings. One variation on this design is a mix starter, which contains the magnetic action as well together some means of disconnecting electrical power when required … either through a fuse, breaker, or motor circuit switch.
Wye-delta motor starting (one form of reduced-inrush system) sends full-line voltage the motor’s wye windings throughout startup — though voltage throughout each engine winding is diminished by the station of the square root of 3 (57.7%) i m sorry is why this plan is occasionally (rather inexactly) referred to as reduced-voltage starting. Climate a circuit (usually through a contactor for each phase, overload relay, timer, and mechanical interlock) switches the motor input to it is provided it full-line voltage into its delta windings.
Part-winding motor starting — offered in conjunction v the specialty dual-voltage electric motors mentioned above — uses across-the-line voltage to just one component (half or two thirds) the the engine windings (typically ripe or twelve) ~ above start. Then once a set time has actually passed or set voltage is detected, a relay or timer springs into activity and commands that the rest of the windings be added and provided power together well. Acceleration might be irregular, but part-winding motor beginning impedance has no impact on starting torque … and enables low-torque starts that are beneficial for pumps, fans, and also blowers. Like wye-delta starting, part-winding beginning is a kind of reduced-inrush system and also delivers a decreased full-line voltage upon engine startup — however doesn’t technically qualify as reduced-voltage starting.
Full-voltage reversible starting leverages just how induction motors change rotation direction upon reversal of any type of two power leads. Reversing starter systems merely incorporate a pair of copy contactors complemented by interlocking subcomponents to allow run-forward and run-reverse conditions. Much faster rotational-direction reversals deserve to be made v plugging, i m sorry is the momentary powering the both circuits.
More controllable: Reduced-voltage motor starters
Besides the family members of full-voltage motor-starting options are reduced-voltage starters. Where device axes require smooth non-jarring acceleration to complete speed (to defend attached machine equipment or part attached load) reduced-voltage motor starters are essential. In fact, they’re likewise useful in settings regulated by neighborhood power utilities that limit voltage fluctuations and current surges on strength supplies during motor starting.
Reduced-voltage motor starters encompass four usual subtypes.Primary resistor motor starters
Primary resistor engine starters are a cost-effective alternative that uses resistors and also some variety of contactors — through the last dictating the number of starting voltage steps. This steps deserve to be somewhat abrupt as result of the circuit’s low inductance. Though the resistors deserve to be bulky and introduce inefficiency, this starter type delivers trustworthy motor-starting torque.Primary reactor engine starters
Primary reactor motor starters space most typical on large high-voltage motors. They rental a reactor (inductor) activity in a circuit favor that the a main resistor engine starter. Reasonably long smooth accelerations are feasible (even come a dozen secs or more) though added system inductance deserve to degrads in its entirety efficiency — and a negative power aspect degrades torque-generating present components and also motor flux.Autotransformer engine starters
Primary reactor engine starters are fairly costly but helpful where adjustable start torque is required. Autotransformer motor starters use a single-winding electric transformer — v the last being a passive electrical device for transferring electrical energy indigenous one circuit come another. Much more specifically, autotransformer starters employ a trio of electrical contactors on one autotransformer having actually selectable taps. The imparts stepped voltage starting for long smooth acceleration upon startup — even to a few dozen seconds. Begin voltage have the right to be 50% to 80% of heat voltage because that high start torques in applications where that (and not efficiency) is a leading architecture objective.Soft starters
Soft starters use solid-state semiconductor modern technology are capable of the many controllability the end of all motor-starter options. They’re also the gentlest top top motors’ inner subcomponents and also attached power-transmission mechanisms. At your core, soft starters consist of various thyristor or SCR species … so for example, some designs have actually a pair of thyristors on each of the three lines into the motor. Testimonial this style Guide’s section on solid-state relays because that the basics that this technology. These switching tools work to manage electrical power right into the motor windings (as depicted by the soft-starter diagram mirroring firing angles) when leveraging just how motor voltage in addition to current and torque space low upon initial startup. Then they slowly raise voltage and also torque according to a preset routine.
Motor soft-starter programming dictates the precise parameters the the rise to collection voltage. Take into consideration the operation of a representative SCR-based soft starter: right here a conducting (gated) SCR has a movable gate point … and adjusting back this rate value (called ramp time) causes an increase in voltage buildup before the SCR switches on. Then when the engine windings reach complete voltage, the SCR switches off.
One caveat: too much ramp time can make existing exceed the motor’s safety boundaries or note a current-limit safety and security cutoff.
Besides the benefits currently mentioned, soft starters impart engine protection (even throughout phase imbalances throughout electric-utility brownouts) and also the capacity to soft stop. The latter is helpful where motors drive designs such together conveyors the involve inertias qualified of changing or breaking throughout transport.
Of course, variable frequency cd driver (VFDs) are an additional option because that soft-start functionality. They provide the exact same controlled starting and stopping features of a soft starter, albeit in a different means — by differing motor-input voltage frequency quite than voltage magnitude. Various other VFDs benefits over soft starters incorporate the capacity to control motor rate over the whole operating range. VFDs can additionally deliver strength for holding torque (full torque at zero speed) i m sorry is an essential in motor-driven applications such as cranes and elevators.
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However, for some designs VFDs space overly costly and also complicated. Reduced-voltage engine starters tend to be much more suitable 보다 VFDs where there’s no obtain in effectiveness to be had actually from running the attached motor listed below its peak speed rating.