Motor starting methods: DOL, soft starter, and VFD

What a motor starting method is and why there is more than one

A motor starting method is how you bring a three-phase induction motor from rest up to running speed without wrecking the supply, the motor, or the thing it drives. The simplest method is to throw a contactor and connect the motor straight to the line. Every other method exists to soften what that first connection does.

The reason there is more than one comes down to a single, stubborn fact. A motor at rest looks almost like a short circuit to the supply. Connect it across the line and it pulls a large surge of current and a jolt of torque, and both of those have consequences. The surge sags the voltage and stresses the supply. The torque jolt slams the driven load. On a small motor on a stiff supply, nobody cares. On a large motor, on a weak supply, or on a load that cannot take the jolt, you reach for something gentler.

So the choice is never abstract. You are matching three things: the motor and how big its surge is, the supply and how much sag it can take, and the load and how much starting torque it needs and how hard a jolt it can survive. The rest of this guide is how each method fits, or fails to fit, those three.

Why does a motor draw so much current when it starts?

A three-phase induction motor started across the line draws roughly 6 to 8 times its full-load current for the first moments of starting, and depending on the motor's NEMA code letter it can run from about 4 times to over 10 times. This is the locked-rotor or inrush current, sometimes written LRA on the nameplate, and it sits on the conductors for a few seconds while the rotor accelerates, then collapses to the running value.

The mechanism is back-EMF, or rather the lack of it. A spinning motor generates a voltage that opposes the supply and holds the current down. At the instant of starting the rotor is not turning, there is no back-EMF, and the stator windings look like a low-impedance load close to a short circuit. As the rotor comes up to speed the back-EMF builds and the current falls back into its normal band.

Two things ride along with that current. There is a torque surge, a mechanical kick into the driven load at the moment of connection. And there is a voltage dip, because that large current pulled through the supply impedance drops voltage that everything else on the bus feels. The current is what trips and heats. The torque surge is what breaks couplings and belts. The dip is what makes the lights flicker and drops out other equipment. Every starting method is aimed at one or more of those three.

The current-versus-torque tradeoff at the center of everything

Here is the rule that decides every starting decision, and it is not negotiable: in a reduced-voltage method, torque falls with the square of the voltage. Cut the voltage to the motor and you cut the inrush, but you cut the starting torque harder. You cannot have low inrush and full torque from the same trick. That is the whole game.

Drop the motor to 58 percent voltage and the current drops to roughly a third, but the torque drops to roughly a third as well, because torque follows voltage squared and 0.58 squared is about 0.33. Drop to 50 percent voltage and the torque falls to about a quarter. This is why a reduced-voltage start is only an option when the load will actually accelerate on the reduced torque you have left. Starve a loaded conveyor of torque and it sits there drawing locked-rotor current the whole time the relay lets it, which is worse than a clean DOL start.

The one method that escapes the tradeoff is the VFD, and it escapes it by changing frequency instead of just chopping voltage. At low frequency the motor makes full torque on low current, because the volts-per-hertz ratio stays right. That is why the VFD is in a different class from everything else here. Every other method trades torque for inrush. The VFD does not.

What is across-the-line (DOL) starting?

Across-the-line starting, also called direct-on-line or full-voltage starting, connects the motor straight to full line voltage through a contactor. It is the simplest, cheapest, and most common method: a contactor, an overload relay, and the short-circuit protection ahead of it, nothing more. The motor gets the full surge of inrush and the full slug of starting torque, which is exactly what most loads want.

Full torque is the advantage people forget. A DOL start gives the highest breakaway torque of any method, so it starts heavy and stuck loads that a reduced-voltage start would stall. It is the default, and it should stay the default unless something specific forbids it. The reasons to move off DOL are short and concrete: the motor is large enough that its inrush sags the supply past what the utility or the other equipment will tolerate, the load cannot take the mechanical jolt, or you need controlled starting or stopping for the process.

On small motors on a stiff supply, none of that applies, and a reduced-voltage starter on a 5 hp fan is money and complexity spent on a problem that does not exist. Start across the line, set the overload to the nameplate, and move on. The skill is knowing when the motor and the supply have grown big enough that DOL stops being free.

Reduced-voltage starting, the idea behind all of it

Reduced-voltage starting means you feed the motor less than full line voltage at the moment of starting to hold the inrush down, then bring it up to full voltage once it is moving. Every classic reduced-voltage method is a different mechanical or electrical way of doing that one thing, and every one of them pays for the lower current with lower starting torque, per the voltage-squared rule.

The methods sort by how they make the lower voltage and how they transition to full voltage. Wye-delta reconfigures the windings. Autotransformer taps the voltage down through a transformer. Primary resistor or reactor drops voltage across an impedance in series. The soft starter chops the voltage electronically with SCRs. They differ in cost, in how much torque you keep for a given current cut, and in how smooth or how violent the changeover to full voltage is.

One number frames the whole family. To get a meaningful inrush reduction you are usually down around 50 to 65 percent voltage at start, which leaves you somewhere between a quarter and 40 percent of full starting torque. If the load needs more breakaway torque than that, reduced voltage is the wrong tool no matter how badly you need to cut the inrush, and you are looking at a VFD or a bigger supply instead.

What is wye-delta starting?

Wye-delta starting, also called star-delta, starts the motor with its windings connected in wye, then switches them to delta to run. In wye the phase windings see line voltage divided by the square root of three, about 58 percent, so the motor draws roughly a third of the line current it would pull across the line and makes roughly a third of the torque. Once it is near speed, the contactors reconfigure the same windings into delta for full-voltage running.

It needs a motor built for it: a six-lead motor whose windings are rated to run in delta at line voltage, with all six ends brought out to the starter. A standard three-lead motor cannot do it. The starter itself is three contactors and a timer, which is why wye-delta was for decades the cheap way to start a large pump or fan.

The catch is the transition. In an open-transition wye-delta the motor is briefly disconnected during the changeover from wye to delta, and when it reconnects in delta out of phase with the line, the current can spike hard, in the worst case well above the locked-rotor current it was supposed to avoid. That transient stresses the windings and the couplings and can be worse than the DOL start you were trying to soften. Closed-transition wye-delta adds resistors and a fourth contactor to keep the motor energized through the changeover and kill that spike, at more cost and panel space. If you specify wye-delta, specify closed transition unless someone has a real reason not to.

And the torque is the real limit. A third of starting torque is fine for a centrifugal pump or fan that unloads at low speed. It is not enough for a loaded conveyor, a positive-displacement pump, or a crusher, which will sit and stall on the wye step. Wye-delta is a low-torque starter, full stop.

Part-winding starting

Part-winding starting energizes only part of the motor's windings first, then connects the rest a moment later. It needs a special motor, typically nine or twelve leads, with the stator built as two or more parallel winding circuits. On start, one circuit is connected across the line; after a short delay the second comes in parallel with it for running.

Because only part of the winding is energized at start, the inrush is reduced, but the reduction is modest compared to wye-delta or autotransformer, and the starting torque is reduced along with it. The transition is closed by nature, since the first winding stays energized when the second comes in, so there is no open-transition spike to worry about.

Part-winding shows up where a motor was already wound for dual-voltage or for this purpose, and it is mechanically simple: two contactors and a timer, no transformer, no resistors. It is a light-duty reduced-voltage method, suited to loads that need only a small break on the inrush and start easily. The limitation is the motor. If the nameplate does not say the windings are arranged for part-winding start, you do not have the method, and forcing it on the wrong motor overheats the energized half.

Autotransformer starting

An autotransformer starter feeds the motor through a tapped transformer that drops the voltage at start, then switches the motor to full voltage to run. The taps are commonly 50, 65, and 80 percent, so you can pick the start voltage to fit the load instead of being stuck with the fixed ratio wye-delta hands you. Pick the lowest tap the load will still accelerate on.

The reason to choose it over wye-delta is torque per line amp. With an autotransformer the line current drops with the square of the tap, while the motor still gets transformer action, so for a given inrush limit at the utility you keep more starting torque than any other classic reduced-voltage method. On the 65 percent tap you pull far less line current than a DOL start while keeping useful torque, which is why autotransformer starting was the workhorse for large fans, pumps, and compressors on supplies that could not take a full DOL inrush.

It is closed transition in the common designs, so there is no out-of-phase reconnection spike like open-transition wye-delta. The cost is the transformer itself: it is large, heavy, expensive, and the windings have a limited number of starts per hour before they overheat, so it is built for occasional starting, not for a load that cycles constantly. On a frequently started load the autotransformer is the wrong choice and a soft starter or VFD fits better.

Primary resistor and reactor starting

Primary resistor and primary reactor starters put an impedance in series with the motor at start, drop voltage across it, then short it out to bring the motor to full voltage. The resistor version uses resistors; the reactor version uses inductors. Both are closed-transition and give a smooth, stepless acceleration as the motor speeds up, because the current falls and the voltage across the motor rises on its own as it comes up to speed.

These are the older methods, and they have largely given way to the soft starter, which does the same job of a smooth voltage ramp electronically and in a smaller box. The resistor type burns the dropped voltage as heat in the resistor bank, which wastes energy at every start and needs space and ventilation for the bank. The reactor type is more efficient on energy but adds inductance that pulls the power factor down during starting and is usually picked for larger motors.

You still find primary resistor and reactor starters on existing installations and in some heavy or high-voltage applications, so it is worth knowing how they work when you walk up to one. For a new low-voltage install they rarely win against a solid-state soft starter that ramps voltage with no resistor bank to heat the room.

The solid-state soft starter

A solid-state soft starter uses SCRs, silicon-controlled rectifiers, to ramp the voltage to the motor up smoothly over a set time, controlling the acceleration and holding the inrush to a programmed current limit. It is the modern reduced-voltage starter, and it replaced most autotransformer and primary-resistor starting on new low-voltage work because it does the same job in a fraction of the space with a lot more control.

What you get over the classic methods is adjustability. You set the ramp time and the current limit, commonly somewhere in the range of 150 to 350 percent of full-load amps, and the starter holds the motor to it. You also get soft stop, which ramps the voltage back down on shutdown instead of dropping the motor instantly. On a pump that soft stop is the feature that matters most, because it kills the water hammer that a hard stop sends through the piping when the check valve slams.

Most soft starters include a bypass contactor that closes once the motor is near full speed, around 90 percent, to short out the SCRs and let the motor run on the line directly. That keeps the SCRs from running hot and dissipating power during normal operation, so the heat sinks only have to handle the start. When you size and lay out the panel, account for whether the bypass is internal or external, because an external bypass is more wiring and more space.

The limit you cannot design around: a soft starter does not give speed control. It ramps the voltage at the fixed line frequency during the start, then bypasses to full line power, and after that the motor runs at full speed on the line like any other. It controls how the motor starts and stops, nothing about how it runs. If you need the motor to run at a variable speed, a soft starter is the wrong device and you want a VFD. Compare the two in the next section, and see the VFD install guide for the speed-control side.

The VFD as a motor starter

A variable frequency drive starts the motor by ramping the frequency up from near zero, and because it holds the volts-per-hertz ratio right as it does, the motor makes full torque on almost no inrush. This is the best start there is. A VFD can bring up a heavy load with full breakaway torque while pulling little more than running current from the line, which no reduced-voltage method can do, because every reduced-voltage method trades torque for current and the VFD does not.

And the VFD does not stop at starting. It runs the motor at whatever speed the process wants, which is where the energy savings live. On a centrifugal pump or fan, slowing the motor cuts power roughly with the cube of speed, so running at 80 percent speed instead of throttling a valve at full speed saves a large fraction of the energy. On a variable load that runs most of the time at part load, that saving usually pays for the drive.

The cost is real. A VFD is the most expensive and most complex of these options, and it brings its own install requirements: the reflected-wave and cable-length issues, the bearing currents and shaft grounding, the inverter-duty motor, the input and output protection. Those are the subject of the VFD install guide, and they are not trivial. Putting in a VFD only to soften the start, on a constant-speed load that will never need variable speed, is usually paying for capability you will never use. If the load truly runs at one speed and just needs a gentle start, a soft starter starts it for less money and less to go wrong. The VFD earns its keep when you need the speed control and the energy savings, and the soft start comes along for free.

What is the difference between a soft starter and a VFD?

The difference is speed control. A soft starter only starts and stops the motor gently, by ramping voltage at the fixed line frequency, and once the motor is up it runs at full line speed. A VFD varies the frequency, so it starts the motor with full torque and almost no inrush and then runs it at any speed you set, which also saves energy on variable loads. Both soften the start; only the VFD controls the run.

That difference drives the cost and the choice. A soft starter is cheaper, smaller, simpler, and has nothing on its output to worry about, because after bypass the motor runs on a clean sine wave from the line. A VFD costs more, takes more panel space, generates heat that needs cooling, and puts a chopped waveform on the motor that brings the reflected-wave, cable-length, and bearing-current problems with it.

The decision rule is short. If the load runs at one speed and only needs a soft start, or a soft stop to kill water hammer, use a soft starter. If the load benefits from running slower part of the time, on a pump, a fan, or a process that varies, use a VFD and take the soft start as part of the package. Do not buy a VFD just for the start when a soft starter would do, and do not try to make a soft starter do variable speed, because it cannot. The VFD install guide covers the run side in detail.

How do you choose a motor starting method?

You choose by working through five things in order: the motor size, the stiffness of the supply, what the driven load needs, whether you need controlled stopping or speed control, and the cost. The supply and the load usually make the decision before cost ever enters, because they are constraints, not preferences. A load that needs full starting torque rules out every low-torque method no matter how cheap, and a supply that cannot take the inrush rules out DOL no matter how simple.

Start at DOL and only move off it for a reason. If the motor is small and the supply is stiff, start across the line. If the inrush sags the supply or the utility limits it, go to a reduced-voltage method, and pick the one whose torque matches the load: autotransformer when you need the most torque per line amp, wye-delta when the load starts light and you want it cheap, soft starter when you want a controlled ramp and a soft stop in a small package. If the load needs full torque at low current, or it wants variable speed, go to the VFD and accept the cost and the install complexity.

The table below is the shape of that decision. Read inrush and starting torque as relative to a DOL start, which is the reference.

The driven load decides as much as the motor

The motor's inrush tells you what the supply has to take, but the load tells you how much starting torque you have to deliver, and that is what kills most reduced-voltage applications when nobody checked it. Match the method to the load's torque demand and its inertia, not just to the motor's horsepower.

Centrifugal pumps and fans are the easy case. They start nearly unloaded, because a centrifugal load needs little torque at low speed and the demand rises with speed, so a low-torque method like wye-delta or a soft starter brings them up fine. A pump is also the textbook case for soft stop, because slamming a check valve on a hard stop sends water hammer back through the piping, and a soft starter ramping the voltage down on stopping prevents it.

The hard cases are loaded conveyors, positive-displacement pumps, crushers, and anything with high breakaway torque. They need real torque from a standstill, and a method that gives a third of starting torque leaves them stalled and drawing locked-rotor current until something trips. High-inertia loads like large fans and centrifuges are their own problem: they accelerate slowly, so the inrush sits on the conductors longer, and a soft starter or VFD that controls the ramp keeps the motor from overheating during the long pull-up. Get the load's torque curve and its inertia before you pick the method, not after the motor sits there humming and not turning.

Why do large motors need reduced-voltage starting?

Large motors need reduced-voltage starting because their inrush is large enough to sag the supply voltage across the whole bus, and the voltage dip trips other equipment, dims lights, and in extreme cases stalls the motor's own start. A small motor's surge disappears into a stiff supply. A large motor on a weaker supply pulls enough starting current that the drop across the supply impedance is felt by everything sharing the source.

The utility is often the deciding voice. Power companies limit how much inrush they will allow on a service above a certain motor size, because the dip a large across-the-line start causes shows up as flicker on the neighbors' lights, and they enforce a starting-current limit to keep it off their system. Past a size that varies by utility and by how stiff the local distribution is, the utility simply requires a reduced-voltage or soft start, and the across-the-line option is off the table whether you like it or not. Find that limit early, because it changes the gear and the panel.

The same dip matters inside the building. Sensitive electronics, control circuits, drives, and contactors have a voltage tolerance, and a deep enough start dip can drop out a contactor or reset a controller elsewhere on the bus. On a data center or a hospital, where the loads on the shared bus cannot take a dip, the starting method for a big chiller or pump motor is chosen around the dip the rest of the load will tolerate, not around the motor alone.

The starter is not the protection

A starting method controls how the motor comes up to speed. It does not protect the motor or the circuit, and conflating the two is a mistake that leaves a motor unprotected. Motor protection is two separate jobs, handled by two separate devices, and the NEC keeps them separate in Article 430.

Overload protection guards the motor and its conductors against a sustained overcurrent that overheats the windings, the kind that comes from a stalled rotor, a jammed load, or single-phasing. It is the overload relay, sized to the motor's nameplate full-load current, and it is set to let the inrush pass on a normal start while tripping on a real running overload. Short-circuit and ground-fault protection is a different device, the branch-circuit protective device ahead of the starter, sized to let the inrush through but clear a fault fast. The NEC treats these as separate requirements, with the branch-circuit short-circuit and ground-fault protection commonly addressed at 430.52; confirm the section against the adopted edition.

A combination motor controller packages the disconnect, the short-circuit protection, the contactor, and the overload into one assembly, which is what you find in an MCC bucket. Whatever the starting method, the overload still has to be set to the motor and the short-circuit device still has to be selected for the available fault current. A soft starter or VFD does not remove the need for either; it sits between them. The MCC commissioning guide covers setting the overload to the nameplate and proving the protection bucket by bucket.

NEMA and IEC starters

Starters come in two rating systems, NEMA and IEC, and on a North American job you will see both, sometimes in the same lineup. The difference matters when you size a replacement or read a spec, because the two systems pick the device by different logic.

NEMA starters are sized by discrete size numbers, Size 00, 0, 1, 2, 3, and up, each rated for a horsepower and a current band with margin built in. A NEMA starter is rugged, oversized for its rating, and forgiving: you pick the size for the horsepower and it has headroom. IEC starters are sized more tightly to the actual motor current and the application's utilization category, so they are smaller and cheaper for the same motor but have less margin and depend on selecting the right overload and the right coordination.

The practical read is this. NEMA gear costs more and takes more space but tolerates a looser selection and hard service. IEC gear is compact and economical but unforgiving of a sloppy size, because there is no spare margin to cover an error. When you replace an IEC contactor, match it to the motor current and the utilization category, not just the horsepower, or you will undersize it. The project spec usually dictates which system, and mixing them in one panel without understanding the difference is how a too-small replacement ends up on a hard-starting load.

Starts per hour and the heat that limits them

Every start dumps heat into the motor, because the inrush is large current flowing while the rotor is slow and the motor is doing little useful work, so most of that energy becomes heat in the windings and the rotor bars. Start a motor too often and that heat does not have time to shed between starts, and the motor cooks even though no single start was abnormal. The nameplate or the manufacturer's data gives a starts-per-hour limit, and it is a real limit, not a suggestion.

Across-the-line starting is the hardest on the motor per start, because it delivers the full inrush and the full torque jolt every time. A high-inertia load makes it worse, because the long acceleration keeps the inrush on the windings longer. If a load cycles frequently, the cumulative start heating, not any single start, is what sets the method, and a controlled start that limits the current and the duration is easier on the motor across many cycles.

This is a quiet reason the soft starter and the VFD win on frequently cycled loads. The soft starter limits the starting current and the VFD nearly eliminates it, so each start puts less heat into the motor, and the starts-per-hour ceiling moves up. The autotransformer is the opposite case: its own transformer windings have a starts-per-hour limit that can be tighter than the motor's, so it is built for occasional starting and the wrong choice for a load that cycles all shift. Count the real duty cycle before you pick, because the motor that survives one start can still die from twenty an hour.

Reversing and jogging

Reversing a three-phase motor means swapping any two of the three line leads, which reverses the rotation, and a reversing starter does it with two contactors mechanically and electrically interlocked so both can never close at once. Closing both at once puts a phase-to-phase short across the swapped leads, so the interlock is not optional and you verify it on commissioning.

The thing that makes reversing hard on the gear is plugging, reversing direction while the motor is still turning the old way. The motor sees nearly twice line voltage relative to its rotation, the current spikes well past a normal locked-rotor start, and the torque reversal slams the load. If the application reverses under motion, the contactors and the overload have to be rated for it, and a soft starter or VFD that can decelerate the motor before reversing is easier on everything than slamming two contactors.

Jogging, or inching, is momentary running to position a load, and it bypasses the normal seal-in so the motor runs only while the button is held. Each jog is a start, with a start's inrush and a start's heat, so a load that gets jogged repeatedly to position runs into the same starts-per-hour heating as any frequently started motor. Where jogging is heavy, a VFD doing it at low speed is far easier on the motor than repeated across-the-line bumps.

Big pump, fan, and chiller motors and the data center

The large constant-speed motors that run buildings and plants are where the starting method gets specified hard, because they are big enough to matter to the supply and they often sit on a bus that cannot take a dip. The big centrifugal chiller, the cooling-tower fans, the condenser-water and chilled-water pumps: these are the motors where the start dip and the utility limit drive the choice.

On a centrifugal chiller compressor, the motor is large and the load is a near-unloaded centrifugal at start, so a soft starter or reduced-voltage method brings it up without the full DOL dip, and many large chillers come with a starter matched to the compressor. On the pumps, soft start plus soft stop earns its place, because the soft stop prevents the water hammer that repeated hard stops would beat into the piping over years. On the fans, the inertia is the issue, and a controlled ramp keeps the long acceleration from overheating the motor.

In a data center the rule tightens, because the critical load on the shared bus cannot take the dip a big across-the-line start would cause, and the reliability requirement pushes toward starting methods that are gentle and predictable. Large mechanical motors on a critical bus are commonly started on soft starters or VFDs, the VFD also giving the part-load energy savings that matter when the cooling runs every hour of every day. The starting method there is part of the power-quality design, not an afterthought bolted to the motor.

What to document

The starting method is a decision someone has to defend later, when the motor trips on start, the supply dips, or a replacement starter has to match what was specified. A short record of why the method was chosen and how it was set saves the next person from guessing. Capture the choice, the reason, and the settings, because the settings are what nobody can reconstruct from looking at the box.

Record the motor data that drove it, the method and why, and the actual settings programmed into a soft starter or VFD, since a ramp time and a current limit left at the factory default are not a commissioned start. Note the transition type on a wye-delta, the tap on an autotransformer, and the overload setting against the nameplate, because those are the numbers an inspector or the next tech checks first.

Common mistakes

• Starting a motor too large for the supply across the line, sagging the bus and tripping or dimming everything else on it.
• Putting wye-delta or another low-torque starter on a loaded conveyor or high-breakaway load that then stalls on the start step.
• Specifying open-transition wye-delta where the reconnection spike is worse than the DOL start it was meant to soften.
• Using a soft starter on a load that needs full breakaway torque the reduced voltage cannot deliver.
• Buying a VFD only to soften the start on a constant-speed load, paying for speed control that is never used.
• Ignoring the starts-per-hour limit, especially on autotransformer windings, and cooking the motor or the starter on a frequently cycled load.
• Leaving a soft starter or VFD at its factory ramp and current-limit defaults instead of setting them to the load.
• Treating the starter as the protection and never setting the overload to the motor nameplate.

Field checklist

Standards and references

The NEC, NFPA 70, Article 430, governs motors, motor circuits, and controllers, and it is where the protection framework around any starting method lives. Overload protection and short-circuit and ground-fault protection are treated as separate requirements within Article 430, with branch-circuit short-circuit and ground-fault protection commonly addressed at 430.52 and motor controllers and overloads in their own parts of the article. The exact section numbers shift between code cycles, so confirm them against the edition the jurisdiction has adopted and any local amendments before citing them.

Starter ratings come in two systems. NEMA ICS standards define the NEMA starter sizes and the discrete horsepower and current bands; the IEC 60947 series defines the IEC contactor and starter ratings and the utilization categories that size them to the application. Match the replacement to the system the panel was built to, by current and utilization category for IEC and by size for NEMA.

For the equipment itself, the manufacturer's instructions and the project specification control. A soft starter's current-limit range and starts-per-hour, an autotransformer's tap and duty, a motor's locked-rotor code and starts-per-hour, and a VFD's application data all come from the nameplate and the manufacturer, and where a listing or a spec imposes a tighter limit than the rule of thumb, the listing governs. The utility's starting-current limit for the service is its own constraint, set by the local distribution, so confirm it with the power company for any large motor.

Units and terms

Motor starting carries a handful of terms that show up across nameplates, drawings, and starter spec sheets, and the same idea can read a few different ways depending on the source.

Inrush is the surge of current at start, also called locked-rotor current and often stamped as LRA, locked-rotor amps, on the nameplate, while the running current is the full-load amps, FLA. Starting torque is the torque the motor makes at the moment of starting, sometimes called breakaway or locked-rotor torque. DOL means direct-on-line, the same thing as across-the-line or full-voltage starting. Reduced-voltage starting, RVS, is the family that lowers the start voltage, and a solid-state version is an RVSS, a reduced-voltage solid-state starter, which is another name for the soft starter.

Inrush / LRA

Locked-rotor current, the surge a motor draws at start, commonly 6 to 8 times full-load amps

FLA

Full-load amps, the running current the motor draws at rated load

Starting torque

Breakaway or locked-rotor torque, the torque made at the instant of starting

DOL / across-the-line

Direct-on-line, full-voltage starting through a contactor with no voltage reduction

RVS / RVSS

Reduced-voltage starting, and the solid-state soft starter version of it

Open vs closed transition

Whether the motor is briefly disconnected during a wye-delta changeover (open) or kept energized (closed)

Combination starter

Disconnect, short-circuit protection, contactor, and overload in one assembly, as in an MCC bucket

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