Motor Starting Methods Compared

Short answer

Choosing how to start a motor means balancing three things: what the supply tolerates, the torque the load asks for and cost. Direct-on-line is the simplest; wye-delta and autotransformer reduce current and torque in fixed steps; the soft starter adjusts them continuously; the drive also controls speed. This guide compares them and gives a decision criterion.

The problem of starting a motor

A squirrel-cage induction motor draws a current of several times rated at start, with a low power factor. If the supply is not strong, the resulting voltage dip reduces torque, affects other equipment and can prevent the start. If the load does not tolerate full torque, the mechanics suffer. Starting methods exist to balance current, torque, cost and complexity.

Each one is explained in its own guide in this series. This one collects the criteria for choosing among them.

Comparison table

Current values are relative to direct-on-line starting and are calculated for the ideal case; real values depend on the motor.

Method	Line current	Starting torque	Speed control	Cost and complexity	Remark
Direct-on-line	100 %	100 %	No	Minimum	Needs a strong supply
Wye-delta	33 %	33 %	No	Low	Six-terminal motor; transition peak
Autotransformer	k² (for example 42 % with k = 65 %)	k²	No	Medium-high	Adjustable taps; bulky
Stator resistors	k	k²	No	Medium	Dissipate heat; rarely used
Rotor resistors	Low	High	Only at start	Medium	Slip-ring motor only
Soft starter	Adjustable (current limiting)	Adjustable	No	Medium	Voltage or torque ramp; see soft starter
Variable frequency drive	Close to rated	High from zero	Yes	High	Full control; see soft starter vs drive

What decides the choice

1. The supply

Starting current is compared with the short-circuit capacity at the connection point and with the voltage dip the system tolerates. A simple decision rule: if direct-on-line starting fits within the allowable voltage dip (set by the utility or the installation code), no other method is needed. If it does not fit, use a reduced-voltage method or a drive. The calculation is covered in starting large motors and voltage drop. To calculate the voltage dip, current and starting time with each method, you can use our free motor starting tool.

2. The load

The torque the load asks for is compared with what the motor delivers throughout the start, not only at the beginning.

Load type	Starting torque required	Methods that usually work
Centrifugal pump, fan	Low, grows with the square of speed	Wye-delta, soft starter, drive
Screw compressor unloaded	Low if it starts unloaded	Wye-delta, soft starter
Loaded conveyor belt	High, almost constant	Direct, soft starter with a high limit, drive
Crusher or mill	High with inertia	Slip rings with resistors, drive, direct
Reciprocating compressor loaded	High	Direct, drive

3. The mechanics

If the torque shock damages couplings, gears or the load (water hammer in pumps, belt tension), choose a method with progressive acceleration: soft starter or drive. Wye-delta and autotransformer have a step at the transition.

4. Starting frequency

Frequent starts heat the motor and the starter. A soft starter is sized for a number of starts per hour; wye-delta, autotransformer and resistors have their own thermal limits.

5. Other functions required

Soft stop (stopping ramp): soft starter or drive.
Variable speed or energy saving: drive.
Braking: drive with resistor or regenerative braking; soft starter with braking in some models.
Integrated protection and communications: soft starter and drive.

Decision example (illustrative)

A 30 kW centrifugal pump at 400 V, fed from a transformer whose allowable dip at start is 10 %. The calculation shows direct-on-line starting produces a 14 % dip; the data are assumed.

Option	Estimated dip	Conclusion
Direct	14 %	Does not comply
Wye-delta	Close to a third of the current: about 5 %	Complies; centrifugal load with low torque
Soft starter	Adjustable	Complies; also avoids water hammer at stop
Drive	Minimal	Complies; justified if flow is also regulated

For a pump that does not regulate flow, the soft starter is usually the balanced option; with flow regulation, the drive pays back its cost in energy.

Decision tree

Which starting method suits?

Is motor speed control needed?
- Yes: Variable frequency drive
- No: Does direct-on-line starting fit within the allowable voltage dip and does the mechanics tolerate it?
  - Yes: Direct-on-line starting
  - No: Does the load start with low torque and are starts infrequent?
    - Yes: Wye-delta or soft starter
    - No: Soft starter with current limiting, autotransformer or drive

The tree gives a starting point and does not replace calculating the real start with the motor and load curves.

How the decision is documented

A well-made choice of starting method leaves a record with four groups of data. First, the motor data (power, rated current, starting current and torque, moment of inertia). Second, the load data (torque curve, inertia, starts per hour). Third, the supply data (short-circuit power, allowable dip). Fourth, the calculation result for each alternative. With that information, a later change of motor or load is assessed against the same basis rather than from scratch.

That record is also part of the documentation that customer and quality audits ask for: it shows that the selection was made with criteria and that the equipment settings (ramp time, tap, current limit) have a known origin. At commissioning, the measured starting current and time are recorded.

Starting in installations with local generation

When the plant is fed from a generator or a limited power source, the criterion changes: the source lacks the stiffness of the grid and a high starting current produces a larger voltage and frequency dip. In addition, the generator regulator needs time to respond, so staggered starting (large motors first, with gaps between them) weighs as much as the method chosen. It is common to combine soft starters or drives on the larger motors with direct-on-line starting on the small ones, and to sequence the starts from the control system so they do not coincide.

Medium-voltage starting: what IEC 62271-106 covers

In medium voltage, starting is operated by contactors and starters that IEC 62271-106 covers for indoor use, at frequencies up to 60 Hz and in systems above 1,000 V and up to 24,000 V. Three points of that standard change the selection compared with low voltage:

The contactor does not clear short circuits by itself. The standard states that the contactors and starters it deals with generally do not have adequate short-circuit breaking capacity. That is why it covers starters associated with a separate short-circuit protective device, and controllers in which the contactor is combined with that device, for example medium-voltage fuses. See also the medium-voltage contactor.
Starter types it includes: direct-on-line, reversing, two-speed and reduced-voltage, which can be autotransformer, reactor or rheostatic. The rotor rheostatic type is the one for wound-rotor motors and has resistors in the rotor circuit.
What is left out: circuit-breaker-based starters, equipment that also serves to adjust speed, and contactors or starters with semiconductors in the main circuit. In other words, the standard does not cover the soft starter or the medium-voltage drive; that equipment has its own standard and the manufacturer’s.

Reduced voltage. The standard characterizes the autotransformer or reactor by its rated voltage and by the number of taps that allow the starting torque and current to be adjusted. It also uses the starting voltage as a percentage of rated, the current it withstands for a specified time, the rated duty and the type of cooling. The standard tap voltage values are 50 %, 65 % and 80 %, and the locked-rotor torque values are 70 %, 100 %, 150 % and 200 % of rated torque. Because the autotransformer or reactor carries current only intermittently, the standard allows their windings to heat up to 15 K more than the limit in the component standard.

Starting duty. To assign the rating of a reduced-voltage starter, the standard assumes that the locked-rotor current at full voltage is six times the full-load current and defines two duty cycles:

Duty	Cycle	Rest afterwards
Medium	30 s on, 30 s off, repeated twice: three close-open operations	1 hour
Heavy	1 min on, 1 min off, repeated four times: five operations	2 hours

In practice, when a medium-voltage motor has long or repeated starts, the starter must be selected for that cycle and not only for the rated current. As with frequent starts in general, the limit is set by the heating of the autotransformer or reactor, and not only by that of the motor.

Common mistakes

Choosing by initial cost without calculating the voltage dip or the load torque.
Applying wye-delta to high-torque loads, which do not accelerate in wye.
Assuming the soft starter eliminates the current peak: it limits it, but with a heavy load the acceleration time grows and can trip the protection.
Installing a drive just to start when a soft starter solved it at lower cost.
Not considering the source: a generator responds differently from a transformer.
Changing the method without reviewing the protection and the motor wiring.

Checklist

Motor starting current and torque taken from the catalog.
Load resisting torque throughout the start.
Voltage dip calculated for each candidate method.
Starts per hour and duty defined.
Additional functions required (soft stop, speed, communications).
Protection and coordination defined for the chosen method.
Start test with current and time measurement.

Frequently asked technical questions

Can a method be combined with a drive? Yes; for example, a drive for the main motor and soft starters on the less demanding ones, or a drive with a bypass.
Does the method stay the same when the motor is replaced? Not automatically: the new motor may have a different starting current and torque.
Where is the limit of direct-on-line starting? In what the supply at the connection point tolerates; there is no single value in kW.

How we can help

Choosing the starting method is a study of the supply, the load and the motor. At Tiza Group we carry it out remotely, worldwide, coordinate the protection, and test the system before energizing it in Colombia, or elsewhere on request. See engineering design and studies, or write to us through contact.

References

WEG, Electric motor specification guide and soft starter catalog.
ABB, Softstarter Handbook.
IEC 60947-4-1 and IEC 60947-4-2.
IEC 62271-106:2011, high-voltage alternating current contactors, contactor-based controllers and motor-starters (clauses 1.1, 3.4.110, 4.109 and 4.111).

Frequently asked questions

Which method reduces starting current the most?

The variable frequency drive can start with a current close to rated, because it raises voltage and frequency together. The other methods reduce current at the cost of torque; the soft starter and the autotransformer do it adjustably and wye-delta at a single value.

Does a soft starter give more torque than wye-delta?

It depends on the setting. The soft starter can limit current to a chosen value and make better use of the available torque; in wye-delta torque is fixed at a third. At the same line current, the torque obtained is comparable.

When does no reduced-voltage method work?

When the load asks for a high starting torque, close to that of direct-on-line starting, such as a compressor or a loaded conveyor. There you need full voltage on a strong supply, a drive or a wound-rotor motor.

Which method should I choose if the plant is fed by a generator?

The one that limits current and starting peak to the minimum: a drive or a soft starter with current limiting, after checking the generator set size and its response.

What does IEC 62271-106 cover for starting medium-voltage motors?

It covers indoor contactors, controllers and starters for systems above 1,000 V and up to 24,000 V: direct-on-line, reversing and reduced-voltage starters using an autotransformer, reactor or rheostat. It does not cover starters with semiconductors or those that also adjust speed, and the contactor generally needs a separate short-circuit protective device.