A squirrel-cage induction motor is the simplest machine in the plant and the hardest thing on the supply the moment it is switched on. For a second or two it draws six to eight times its running current, at a poor power factor, and it does so whether the shaft is loaded or bare. Everything a starter does is a way of managing that second: paying for it in full and getting the whole torque (direct-on-line), cutting it to a third and losing two thirds of the torque (star-delta), capping it wherever you like and accepting the torque that goes with the cap (soft starter), or removing it altogether with a device that then runs the motor for the rest of its life (a drive). Which of the four is right depends on the motor, the load, the supply and the number of starts, and the calculator that goes with this guide puts numbers on all of them.
What happens when an induction motor is switched on
At standstill an induction motor is a transformer whose secondary, the rotor cage, is short-circuited. The stator sees the supply voltage across nothing but its own leakage reactance and resistance and the rotor's, both small by design, so the current is large: the locked-rotor current, which IEC 60034-1 requires on the nameplate as the ratio IA/IN. For modern IEC design N motors (IEC 60034-12) it is typically 6 to 8 times the full-load current, and high-efficiency motors sit at the high end of that range because the low rotor resistance that makes them efficient also lets more current flow at standstill. The power factor at locked rotor is poor, roughly 0.2 to 0.4, because the current is mostly reactive.
As the rotor accelerates the slip falls, the frequency of the currents induced in the cage falls with it, and the rotor impedance changes character from mostly reactive to mostly resistive. The current stays close to its locked-rotor value for most of the run-up and drops to the running value only over the last ten or fifteen percent of speed. The torque follows its own curve: the locked-rotor torque is typically 1.5 to 2.5 times rated torque for small and medium motors, it may dip slightly on the way up (the pull-up torque), it peaks at the breakdown torque of two to three times rated at around 80 % speed, and it falls steeply to meet the load torque at the running slip of one or two percent.
The run-up time is set by the difference between the motor's torque curve and the load's, acting on the inertia of the rotor and the driven machine. A pump or a small fan reaches speed in half a second to two seconds; a large fan or a centrifuge with high inertia can take ten seconds or more, during which the motor carries near locked-rotor current and heats quickly. That is what limits the number of starts an hour a motor can make, and why the starter's overload relay must be chosen with a trip class (10, 20 or 30 seconds at 7.2 times its setting) that lets the run-up finish.
Direct-on-line: cheap, hard and usually right
A direct-on-line (DOL) starter is a contactor, an overload relay and a short-circuit device, and nothing else. Press start, the contactor coil pulls in, the contacts close and the motor gets full voltage. It develops its full locked-rotor torque, so it will start anything it can run, and it draws its full locked-rotor current, so the supply and everything on it feel the step. The parts are the cheapest of any method, the panel is the smallest, there is nothing to set but the overload relay, and the motor needs only three leads.
DOL is the right choice for most motors up to about 7.5 to 11 kW on a stiff supply, and for much larger ones where the supply can take it: an industrial plant with its own distribution transformer starts 45 kW motors direct-on-line without a second thought. What rules it out is a weak supply, where the voltage dip during the start upsets other loads or the motor cannot develop enough torque to accelerate; a machine that must not be snatched, such as a belt conveyor or a gearbox with backlash; a supply from a generator; and some public networks, where the utility limits the size of motor that may be started DOL on a shared low-voltage feeder.
Star-delta: a third of the current, a third of the torque
A star-delta starter needs a motor whose delta connection suits the supply, which for a 400 V supply means a motor with a 400/690 V nameplate, with all six ends of its three windings brought out to the terminal box. Three contactors and a timer do the work. For the start, the main and star contactors close, connecting the windings in star: each winding sees 400 ÷ √3 = 230 V, its current is 1/√3 of what it would be in delta, and the line current, which in delta is √3 times the winding current, ends up one third of the DOL value. Torque goes with the square of the voltage, so it is also one third. After a set time, typically five to fifteen seconds, the star contactor drops out, the delta contactor pulls in, and the motor runs on full voltage.
That changeover is the weak point. In the ordinary open-transition starter the motor is disconnected for a few tens of milliseconds between star opening and delta closing. Its rotor keeps turning and its residual flux keeps generating a voltage, which by the time delta closes is no longer in phase with the supply; the reconnection draws a current peak that can be as large as the DOL inrush, with a torque jolt to match. Closed-transition starters bridge the gap through resistors and cost more; most of the star-delta starters in service are the open kind, which is why the contactors and the short-circuit device must be chosen for the transition peak, not for the star current.
Star-delta fails when the load needs more than a third of the locked-rotor torque to accelerate. A fan, a centrifugal pump started against a closed valve, a machine tool or an unloaded compressor accelerate on a third; a loaded conveyor, a reciprocating compressor with pressure on it, a crusher or a positive-displacement pump do not, and the motor either stalls in star or is still far below speed when the timer switches to delta, at which point the starter has delivered a DOL start after a wasted ten seconds of heating. The six-lead cable is also easy to get wrong: a swapped pair at the terminal box reverses one winding, and the motor runs in delta with a current far above rated.
Soft starters: a ramp on the voltage
A soft starter is a pair of thyristors in each phase, fired late in each half-cycle at the start and earlier and earlier as the ramp proceeds, so the motor sees a voltage that rises smoothly from a set initial value (commonly 30 to 60 %) to full over a set time. Most starters also close a current limit loop around the ramp: set it to 3 × FLC and the starter holds the current there, adjusting the voltage as the motor accelerates, for as long as the run-up takes. Torque is proportional to the square of the voltage, and the current is roughly proportional to the voltage, so at a current limit of 3 × on a motor with a locked-rotor current of 7 × the torque is (3 ÷ 7)² = 18 % of the locked-rotor torque. That number is why soft starters suit loads whose torque is low at standstill and rises with speed: pumps, fans and centrifugal machines. A load that needs full torque from rest is not a soft-starter load.
Once the motor is at speed the thyristors either stay in conduction, dropping a volt or so and dissipating a few watts per amp, or a bypass contactor closes around them; almost every starter above a few kilowatts is bypassed, and many have the bypass built in. The starter itself is rated by motor current and by the severity of the start (the current limit and the run-up time, and the number of starts an hour), and it still needs an overload relay, its own or an external one, and a short-circuit device. What it buys is a start with no step: no torque jolt on the coupling, no belt slip, and for a pump the soft stop that closes the check valve gently instead of slamming it, which is the cure for water hammer in a rising main. Conveyors, fans with long belts and machines with gearboxes are the other natural customers, and so is any supply that would rather see 3 × than 7 ×.
Variable-frequency drives: the starter that also runs the motor
A variable-frequency drive rectifies the supply to a DC bus and inverts it back into a three-phase output whose voltage and frequency it controls together. At the start it applies a low frequency and a proportionally low voltage, so the motor runs at a small slip from the first cycle, develops rated torque at about rated current, and accelerates along a ramp the drive sets. The supply never sees an inrush; the drive limits its output current to 110 to 150 % of rated, and its input current at the start is smaller still. Where the load needs full torque from rest, or where the number of starts an hour would wear out any contactor, the drive is the only starter that does the job without stress on anything.
Its real case is usually not the start but the running. A fan or a centrifugal pump obeys the affinity laws, so at 80 % speed it draws about half the power, and if the process is throttled or dampered for much of the day the drive returns the throttling loss as energy saved; the pump motor guide works through that. The price is complexity. A drive draws harmonic currents from the supply and may need a line reactor or an active front end to meet the limits the utility or IEC 61000-3-12 impose; its output is a train of fast pulses that stress the motor insulation and, over a long cable, reflect to double the voltage at the motor terminals, so makers quote a maximum unfiltered motor cable length of the order of 50 to 100 m and sell output reactors and dv/dt filters for longer runs; the motor cable should be screened and the screen earthed at both ends; bearings on larger motors can suffer from circulating currents; and a standard motor cooled by its own fan loses cooling at low speed. A motor built for drive duty (IEC 60034-25) and a screened cable are cheap insurance; a drive bought as a mere starter for a motor that then runs at full speed all day is usually money that a soft starter would have saved.
Protection and coordination
Two faults are protected against separately. An overload, a current a little above rated for a long time, comes from a jammed machine, a lost phase, low voltage or a wrong setting, and it cooks the winding over minutes; the overload relay, thermal or electronic, models that heating and trips before it does harm, and it is set to the motor's full-load current (or 0.58 × FLC where it sits in the winding leads of a star-delta starter). A short circuit is thousands of amps for milliseconds, and the relay is useless against it; a motor protection circuit breaker (MPCB), a motor-rated MCB or an MCCB clears it. The short-circuit device is rated at or above about 1.25 × FLC so it does not trip on running current, and its instantaneous trip must sit above the starting current with margin, which for a DOL start of a 7 × motor means above 8.4 × FLC: a type C MCB, whose instantaneous trip may lie anywhere from 5 to 10 × its rating, can open on the inrush, and that is why motor circuits use type D or an MPCB with its magnetic trip at 12 to 13 × the setting. An MPCB combines both functions in one device, which is the common choice up to about 30 kW.
The contactor is chosen by its AC-3 rating (IEC 60947-4-1), which is the current it can make and break for the life of the device when starting a cage motor and switching it off while running. AC-4 duty, inching, plugging and reversing under load, derates the same contactor heavily. And the three devices must be coordinated: under a short circuit the breaker must clear the fault before the contactor and relay are destroyed. IEC 60947-4-1 defines type 1 coordination, where the starter may be damaged but the enclosure stays safe, and type 2, where the contactor and relay are fit for further use apart from possible light contact welding. Which combinations achieve type 2 at a given prospective fault current is tested by the maker and published as a table, and the right way to select a starter is to read the ratings off the calculator and then find the tested combination in that table.
The cable, finally, is sized on the full-load current with the cable sizing calculator, with its voltage drop checked at the starting current as well as the running current: a long cable that is fine at 28 A may drop enough at 196 A to stall the start.
The example in numbers
A 15 kW four-pole motor on a chilled-water pump in a Karachi office building: 400 V three-phase, nameplate efficiency 91 %, power factor 0.85, locked-rotor current 7 × FLC.
- Full-load current = 15 000 ÷ (√3 × 400 × 0.91 × 0.85) = 28.0 A.
- Starting current: 196 A direct-on-line; 65 A in star (2.3 × FLC); 84 A on a soft starter set to 3 × FLC; 42 A on a drive limited to 1.5 × FLC.
- Starting torque: 100 % of locked-rotor torque for DOL; 33 % for star-delta; (3 ÷ 7)² = 18 % on the soft starter; rated torque from standstill on the drive.
- Direct-on-line: a 32 A AC-3 contactor (for 28.0 A), the overload relay set to 28.0 A, a 40 A MPCB or motor-rated MCCB (1.25 × 28.0 = 35.0 A, next rating up) whose instantaneous trip must exceed 1.2 × 196 = 235 A, and a cable designed for 28.0 A.
- Star-delta instead: main and delta contactors of 18 A (each carries 16.2 A), a 12 A star contactor (9.3 A), the overload relay set to 16.2 A in the winding leads, and six motor leads each designed for 16.2 A, with the supply cable to the starter still at 28.0 A.
For a pump, the sensible answer is the soft starter: it removes the water hammer at stop, cuts the supply step from 196 A to 84 A, and a pump does not need more than 18 % of locked-rotor torque to leave standstill. A drive is the answer if the chilled-water flow should follow the load, which in an office it usually should. The motor starter sizing calculator reproduces these figures and lets you change the motor, the locked-rotor multiple and the method.
Starting on a generator
The starter choice matters most where the supply is weakest, and no supply is weaker than a standby set. A utility transformer of a few hundred kVA barely notices a 196 A step; a generator sees it as a demand of √3 × 400 × 196 = 136 kVA at a power factor of 0.3, and it responds with a voltage dip set by its alternator's transient reactance and an engine that must pick up the real power as the motor accelerates. A set that can carry the building's running load with ease may be unable to start its 15 kW pump direct-on-line without the lights dimming and the drives tripping on undervoltage, which is why the generator sizing guide treats the largest motor start as a separate check and often finds it governs the set size.
The same motor started on a soft starter at 3 × FLC demands 58 kVA at the start; on a drive at 1.5 × FLC, 29 kVA. The choice of starter can therefore halve the generator a plant needs, or let an existing set start a motor it otherwise could not. Ahmedonics designs motor control and standby power together for that reason: the starter, the set and the load sequence are one problem. Where a site runs on generator through Pakistan's load-shedding hours, the generator sizing calculator takes the starting multiple as an input, so you can see in kVA what each starting method costs.
References
- IEC 60947-4-1, Low-voltage switchgear and controlgear — Part 4-1: Contactors and motor-starters — Electromechanical contactors and motor-starters — utilisation categories, trip classes, type 1 and type 2 coordination
- IEC 60034-12, Rotating electrical machines — Part 12: Starting performance of single-speed three-phase cage induction motors — design N and H starting characteristics
- IEC 60034-1, Rotating electrical machines — Part 1: Rating and performance — nameplate content, including the locked-rotor current ratio
- IEC 60034-25, Rotating electrical machines — Part 25: AC electrical machines used in power drive systems — Application guide — insulation, bearing currents and cooling of motors on drives
- IEC 61000-3-12, Electromagnetic compatibility — Limits for harmonic currents produced by equipment connected to public low-voltage systems with input current > 16 A and ≤ 75 A per phase
- IEC 60364-4-43, Low-voltage electrical installations — Part 4-43: Protection for safety — Protection against overcurrent
- Manufacturer catalogue data for IEC contactors, motor protection circuit breakers, soft starters and drives — AC-3 ratings, kW assignments, type 2 coordination tables and cable-length limits are published per product range