The starting method of a motor is chosen from four data: the torque the load asks for during the start, what the supply tolerates in current and voltage dip, how often the motor starts, and whether the process needs speed variation. Direct-on-line is the simplest and needs a strong supply; wye-delta reduces current and torque for unloaded starts; the soft starter smooths the start and stop of loads that do not need variable speed; the drive controls speed all the time. None is better in general: it depends on the application.
The starting method is a decision for the whole system
Starting a squirrel-cage induction motor by connecting it straight to the supply produces a current of several times rated. It also produces a torque that may be more than enough for the load. That current depresses the bus voltage and that torque stresses the drive train. Starting methods exist to control one, the other or both. Choosing well requires looking at the load, the supply and the process, not only the motor. For the overview of the motor see induction motor and its protections.
The four questions
- What torque does the load ask for at start? A centrifugal pump or fan asks for little torque at the beginning. A loaded conveyor, a crusher, a reciprocating compressor or a mill may ask for high torque from zero speed.
- What does the supply tolerate? The short-circuit power at the point, the transformer, the generators and the other sensitive loads on the bus determine the allowable voltage dip (see starting large motors and voltage drop).
- How often does it start? A motor that starts dozens of times per hour has different thermal and switchgear demands from one that starts once per shift.
- Does speed need to vary? If the process needs variable speed, the starting question is resolved with the speed control method.
Comparison of methods
| Method | Starting current | Starting torque | Speed control | Complexity | Typical use |
|---|---|---|---|---|---|
| Direct-on-line | Full (several times rated) | Full | No | Minimum | Small motors, strong supplies, loads that tolerate the jolt |
| Wye-delta | Reduced to about a third | Reduced to about a third | No | Low | Unloaded or lightly loaded start, medium motors |
| Autotransformer or resistors | Reduced per the tap | Reduced | No | Medium | Large motors on a weak supply, older installations |
| Soft starter | Limited by ramp or current limit | Reduced and adjustable | No (only at start and stop) | Medium | Pumps, fans, conveyors, compressors without variable speed |
| Variable frequency drive | Close to rated | High from low speed | Yes, continuous | High | Variable-speed processes or demanding starts |
The reduction to a third in wye-delta is the usual ratio of the method. It goes with a six-terminal motor connection whose delta voltage matches the supply. For the comparison between the last two, see soft starter or variable frequency drive and, for the soft starter itself, soft starter.
Guide by application
- Does the process need to vary the motor speed?
- Yes: Variable frequency drive
- No: Does the supply tolerate the direct-on-line starting current without unacceptable voltage dip?
- Yes: Direct-on-line starting, with the switchgear and protections verified
- No: Does the load start unloaded or with little load?
- Yes: Wye-delta or soft starter
- No: Soft starter with current limit, or drive if the torque requires moderate current
Some typical applications, as guidance:
- Centrifugal pumps. Soft starter because of water hammer, or drive if flow varies.
- Fans. High inertia leads to long starts; the soft starter limits current and the drive wins if there is flow regulation.
- Conveyor belts. They ask for considerable starting torque; the soft starter with current limit or the drive avoids jolts and spills.
- Compressors. Depending on type: screw usually starts unloaded, reciprocating may ask for higher torque.
- Crushers and mills. High torque, large inertia; they usually require a drive or a well-sized soft starter, with review of starting time and heating.
- Small motors in simple duty. Direct-on-line starting, if the supply allows.
This list does not replace calculation: with the real load and supply data, the starting study confirms or rules out the choice.
What else is reviewed
- Protections consistent with the method. Starting time changes with the method and requires reviewing the relay trip class (see thermal relay and trip classes).
- Starts per hour. Limits heating of the motor and the starter.
- Emergency generator. A motor that starts well from the grid may not do so from a generator of limited power.
- Harmonics and compatibility. Soft starters and drives have effects on the supply that are assessed in the study.
- Maintenance and spares. The complexity of the method influences later service.
Illustrative example
Invented values.
A 55 kW pump fed from a transformer shared with sensitive equipment (illustrative). The direct-on-line starting current, close to seven times rated, produces a voltage dip that resets controllers of other machines on the same bus. The pump runs at constant flow. With the comparison above: direct-on-line starting is ruled out by the supply. Wye-delta would reduce the current, but the transition and the dip at the changeover still have an effect. The soft starter limits current with a ramp and avoids water hammer with a stopping ramp. The soft starter is chosen. If in the future flow has to be regulated, the study will point to the drive and conditions will change.
Minimum data for a starting study
A reliable starting study begins with concrete data that is worth gathering before the design meeting.
From the motor: power, voltage, rated current, speed, locked-rotor current and torque, code letter, rotor moment of inertia and the maximum time it withstands with a locked rotor, all on the nameplate and in the manufacturer’s data sheet.
From the load: resisting torque curve as a function of speed (pump, fan, conveyor and mill are all different), moment of inertia reflected to the motor shaft and requirements for acceleration or controlled stopping.
From the supply: short-circuit power at the connection bus, transformer impedance, voltage dip allowed by the utility or by the other loads, and characteristics of any backup generator.
From the duty: starts per hour, running time, environment and continuity requirements.
With those data, the study calculates the motor torque curve against the load curve during the start with each candidate method, the resulting line current and voltage dip, the acceleration time and the heating. The result is a numerical comparison of the options instead of a choice by habit. It is the procedure described by the recommended practices for motor starting studies, which is not reproduced here in detail.
What tends to surprise. Often the motor is “too much” and the supply “not enough”, or the other way round. A low-torque-rotor motor and a heavy load may not accelerate with a soft starter at reduced voltage. A bus that looks strong can dip more than expected if the transformer is shared. And a backup generator of limited power can be the limit that forces a gentler method than the normal supply would allow.
After choosing. The chosen method is documented with its settings (ramp, current limit, starting time, relay class) and the result of the start test is recorded. That way, a later change of load or supply can be compared with the baseline.
Common mistakes
- Choosing the method by habit or by equipment price.
- Using wye-delta with loads that ask for high torque at start.
- Putting a soft starter on a load with very high starting torque without checking that it starts.
- Not considering the emergency generator in the selection.
- Ignoring starts per hour in the thermal sizing.
- Changing the method without reviewing the overload relay, wiring and protections.
- Not measuring the real voltage dip of the start.
Checklist
- Resisting torque curve and inertia of the load.
- Short-circuit power, transformer and allowable voltage dip.
- Starts per hour and duty rating.
- Need for variable speed.
- Candidate method verified by calculation or by a starting study.
- Protections and switchgear reviewed for the chosen method.
- Behavior with the emergency generator, if there is one.
- Start test with current and voltage measurement at commissioning.
When to call us
Choosing the starter is cheap to review and expensive to correct later. Our team can carry out your starting, voltage-drop and power-quality study remotely, worldwide, and, for installed equipment, test and commission it in Colombia, or elsewhere on request. Write to us through contact with the motor nameplate, the load type and the supply data.
Related guides
- Safety relays: how they work and are wired
- Medium-Voltage Motor Starter: Types, Protection, Testing
- Intelligent MCC: CENTERLINE 2100, SIVACON S8, MNS, Okken
- VFD Harmonics: Effects and Solutions
References
- IEC 60947-4-2, Contactors and motor-starters – AC semiconductor motor controllers and starters.
- IEEE 3002.7, Recommended Practice for Conducting Motor-Starting Studies and Analysis of Industrial and Commercial Power Systems.
- Motor and starter specification guides from manufacturers (WEG, ABB).
Frequently asked questions
Which starter suits a centrifugal pump?
A centrifugal pump asks for little torque at the start and increases it with speed, so it accepts a soft starter well, which also avoids water hammer with a stopping ramp. If flow must vary during operation, the drive is the option, and it also allows energy savings.
Why not use wye-delta with any load?
Because it reduces starting torque along with current, and a load that asks for high torque from zero speed does not accelerate in wye. In addition, the change from wye to delta produces a transition with high current. It suits unloaded or lightly loaded starts.
When is direct-on-line starting enough?
When the supply and transformer tolerate the starting current without trouble, when the load and drive train withstand full starting torque, and when the motor starts few times. It is the simplest and most robust solution; it only needs to be verified, not discarded out of habit.
Does a soft starter work for heavy loads that start under full load?
With limitations. The soft starter reduces voltage, and torque falls with its square; on loads that ask for a lot of torque from zero speed there may be no ramp that starts without current rising as much as in direct-on-line starting. In those cases a drive or a wound-rotor motor is considered.