Electric motors are absolutely crucial to automating innumerable applications about the world. In most cases, the driving of electric motors — the offering of electric power to them — calls for some engineered device that must also be compatible v the motor’s winding arrangement. Since these motor-powering systems space often discovered employing or accompanying the other electrical control and also connectivity devices currently covered in this design Guide, we’ll now review their most common permutations. Extr information on motor drives having actually functions past motor starters deserve to be discovered at this charline-picon.com article.

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Only the simplest and smallest draft — usually with single-phase electric motors 5 hp or smaller or three-phase electric motors 15 hp or smaller — expropriate direct-on-line (also called across-the-line) link to the electrical power source without hazard of a engine overdraw and a line undervoltage condition. Three-phase motors driven this way can have windings connected in a simple wye (also called star) or delta configuration … and also dual-voltage electric motors (convenient for exactly how they have the right to accept 230 V or 460 V input) have actually twin coil set that deserve to run in parallel or (for the greater voltage) in series.

This Siemens SIRIUS 3RV2011-1HA10 dimension S00 circuit breaker is a current-limiting breaker for pack feeders come 3 kW at three-phase 400 Vac. A short-circuit security of 104 A and an adjustable overload defense of 5.5 come 8 A reliably protects electrical motors. Image courtesy Automation24 Inc.

Everywhere else, across-the-line motor starts present too many problems because that the engine itself and also systems attached come the motor — consisting of detrimental electrical effects and also excessive wear on mechanical power-transmission components. The architecture objectives of safety, productivity, and also precision normally necessitate the use of an ext advanced motor-driving approaches.

Starting current is an important parameter in the appropriate sizing and pairing of motors and motor starters. Beginning current native the motor starter must be adequate to help the motor meet torque and acceleration requirements yet mustn’t cause excessive voltage autumn on the electrical supply line.

Terminology groundwork: The difference in between contactors and also motor starters

In a previous section of this architecture Guide, we thorough how contactors and also relays space distinct materials — in spite of occasional industry intake of the state that argues otherwise. Contactors and motor starters are likewise distinct components. Here the terms are used interchangeably due to the fact that their core is the exact same exact modern technology — a switch capable of dealing with high voltages.

This direct-on-line motor starter is a SIRIUS 3RM1001-1AA04 native Siemens through a regulate voltage that 24 Vdc and an flexible pickup present overload relax of 0.1 to 0.5 A. It imparts solid-state engine protection and is an ideal for solution with small motors approximately 0.12 kW one industry-standard 22.5 mm broad consumes minimal space inside manage cabinets. Photo courtesy Automation24 Inc.

The difference is that motor starters have actually one extra device or equipment not discovered in contactors — an overload relay the some form to reduced voltage input have to that relay sense a motor-overload or thermally compromising condition due prolonged running overcurrent. Those designated as self-protecting engine starters additionally include short-circuit protection. Here again, an exact use of hatchet is key: quite than utilizing short circuit come refer to any kind of electrical malfunction, that only proper to use the ax when pointing out a suddenly overcurrent occurring from the flow of electric power the has found some unintended route of travel. Short-circuit protection acts instantaneously to reduced off the system from the strength source.

This is an example of a strength contactor. It is a Siemens SIRIUS 3RT2015-1BB41 for giving power to three-phase motors and also electrical furnaces up to 3 hp at 480 Vac. The strength contactor supplies a manage voltage the 24 Vdc features a NO contact and also screw-type cable receptacles.In fact, there are numerous sizes and also versions that this power contactor for load feeders v circuit breakers — and various SIRIUS switching tools for the safe and functional convert of electrical loads.• 3RT2 contactors come in size S00 to S3. 3RT1 contactors come in size S6 come S12• 3RT.0 power contactors and 3RT12 vacuum contactors room for switching motorized loads• Four-pole 3RT23 contactors (and three-pole 3RT24 / 3RT14 contactors) switch resistive loads• Four-pole 3RT25 contactors room for an altering the polarity of hoisting gear motors• 3RH2 contactor relays move in the control circuit• 3RT26 capacitor contactors switch capacitive tons (AC-6b)• 3RT1 / 3RT2 / 3RH2 contactors have expanded operating arrays … 3RT10 / 3RT20 / 3RH21 contactors are for rail applications …and 3RT20 / 3RH21 coupling relays space for system-compliant interaction with electronic controllers• 3RT1…-.S.36 contactors have actually failsafe control inputs because that safety-related applicationsReversing contactor assemblies space also obtainable — as room contactors for starting 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 motor starters is connected to just how the two materials are rated and also specified. Contactors are typically classified by your voltage capacity. In contrast, motor starters are commonly rated through their existing capacity and the horsepower of the motors for which they’re compatible … also while accommodating inrush present upon startup without nuisance tripping. The is generally done through a slight hold-up in relay tripping — as countless motors (especially smaller sized motors) can reach complete operating rate in simply a few seconds.

Circuit diagrams that sample sport of contactors, full-voltage motor starters, and also soft starters show their differences and similarities. Click come enlarge.

Motor beginning at the most basic level is divide as hands-on or automatic.

Manual starting consists of hand-turned on-off switches that merely make or break the engine input circuit once activated by tree personnel. Part versions the qualify as true motor starters (as designated above) combine a thermal-overload relay come de-energize the engine if it i do not care overheated.

In contrast, automatically triggered motor beginning is sometimes called magnetic starting for the electromechanical contactors that room core come this design.

As with any kind of electromechanical relay technology, these have actually stationary electromagnetic coils that (upon a command indigenous a pushbutton, border switch, timer, rise switch, or other relay) pressure together 2 circuits. These circuits include input power contacts and a adjustment carrier that (once closed together) permit current flow into the motor windings. One sport on this style is a combination starter, which consists of the magnetic activity as well as some method of disconnecting electrical power when essential … either v a fuse, breaker, or engine circuit switch.

Wye-delta engine starting (one form of reduced-inrush system) sends full-line voltage the motor’s wye windings during startup — though voltage throughout each motor winding is reduced by the inverse of the square source of 3 (57.7%) i m sorry is why this arrangement is occasionally (rather inexactly) called reduced-voltage starting. Climate a circuit (usually v a contactor because that each phase, overload relay, timer, and also mechanical interlock) switches the motor input to it is provided it full-line voltage into its delta windings.

Part-winding motor starting — used in conjunction with the specialty dual-voltage motors mentioned above — applies across-the-line voltage to only one part (half or two thirds) the the engine windings (typically ripe or twelve) top top start. Then as soon as a collection time has passed or set voltage is detected, a relay or timer springs into activity and commands that the remainder of the windings be added and provided power as well. Acceleration may be irregular, yet part-winding motor starting impedance has no effect on beginning torque … and permits low-torque starts that are helpful for pumps, fans, and blowers. Favor wye-delta starting, part-winding starting is a form of reduced-inrush system and also delivers a decreased full-line voltage upon engine startup — yet doesn’t technically qualify as reduced-voltage starting.

Full-voltage reversible starting leverages just how induction motors adjust rotation direction top top reversal of any type of two strength leads. Reversing starter systems just incorporate a pair of copy contactors additional by interlocking subcomponents to enable run-forward and also run-reverse conditions. Quicker rotational-direction reversals deserve to be made with plugging, i beg your pardon is the temporary powering the both circuits.

More controllable: Reduced-voltage motor starters

Besides the household of full-voltage motor-starting options are reduced-voltage starters. Where maker axes require smooth non-jarring acceleration to full speed (to protect attached device equipment or some attached load) reduced-voltage engine starters room essential. In fact, they’re additionally useful in settings regulated by regional power utilities the limit voltage fluctuations and also current surges on strength supplies during motor starting.

Reduced-voltage engine starters incorporate four typical subtypes.

Primary resistor motor starters

Primary resistor engine starters room a cost-effective alternative that offers resistors and some variety of contactors — v the last dictating the variety of starting voltage steps. This steps have the right to be rather abrupt because of the circuit’s low inductance. Despite the resistors deserve to be bulky and also introduce inefficiency, this starter kind delivers reputable motor-starting torque.

Primary reactor engine starters

Primary reactor engine starters room most common on big high-voltage motors. They employ a reactor (inductor) action in a circuit choose that the a main resistor motor starter. Reasonably long smooth accelerations are feasible (even to a dozen seconds or more) though extr system inductance can degrads all at once efficiency — and also a negative power aspect degrades torque-generating present components and motor flux.

Autotransformer engine starters

Primary reactor engine starters are fairly costly but advantageous where flexible start speak is required. Autotransformer motor starters usage a single-winding electrical transformer — v the last being a passive electrical device for transferring electrical energy from one circuit come another. An ext specifically, autotransformer starters rental a trio of electrical contactors on an autotransformer having selectable taps. That imparts stepped voltage beginning for long smooth acceleration ~ above startup — also to a couple of dozen seconds. Start voltage can be 50% come 80% of line voltage for high begin torques in applications wherein that (and not efficiency) is a leading design objective.

Soft starters

Soft starters use solid-state semiconductor technology are capable of the most controllability the end of all motor-starter options. They’re also the gentlest top top motors’ interior subcomponents and also attached power-transmission mechanisms. At your core, soft starters consist of of miscellaneous thyristor or SCR kinds … so for example, some designs have actually a pair the thyristors on every of the 3 lines into the motor. Evaluation this design Guide’s section on solid-state relays for the basics the this technology. These switching tools work to control electrical power into the engine windings (as illustrated by the soft-starter diagram reflecting firing angles) if leveraging how motor voltage together with current and also torque space low top top initial startup. Climate they progressively raise voltage and torque follow to a preset routine.

Motor soft-starter programming dictates the precise parameters that the boost to set voltage. Take into consideration the operation of a representative SCR-based soft starter: below a conducting (gated) SCR has a movable gate point … and also adjusting back this speed value (called ramp time) causes rise in voltage buildup before the SCR switches on. Then when the engine windings reach complete voltage, the SCR switches off.

One caveat: excessive ramp time can make current exceed the motor’s safety limits or prompt a current-limit safety cutoff.

Besides the benefits currently mentioned, soft starters impart engine protection (even throughout phase imbalances throughout electric-utility brownouts) and the capability to soft stop. The latter is beneficial where motors drive design such as conveyors that involve inertias capable of changing or breaking throughout transport.

Of course, variable frequency drives (VFDs) are another option because that soft-start functionality. They administer the exact 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. Other VFDs benefits over soft starters incorporate the capability to manage motor speed over the whole operating range. VFDs can additionally deliver strength for holding speak (full torque at zero speed) which is key in motor-driven applications such together cranes and elevators.

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However, for some designs VFDs space overly costly and complicated. Reduced-voltage motor starters have tendency to be an ext suitable than VFDs whereby there’s no acquire in effectiveness to be had from running the attached motor below its peak speed rating.