High inertia and high starting current are the two most common challenges when starting IE3 motors direct-on-line (DOL). When you connect an IE3 motor directly to the grid, the inrush current at startup can reach 6-8 times the rated current, causing both voltage dips on the supply and sudden mechanical shock on belts, pulleys and couplings. This is exactly where a soft starter comes in. A soft starter ramps the voltage applied to the motor in a controlled way, limiting both the starting current and the starting torque, thereby protecting both the motor and the driven machine. In this guide we walk through soft starter compatibility with IE3 motors, correct sizing, connection types and the use of a bypass contactor, step by step.

Soft starter panel connected to an IE3 motor with contactor wiring

What Is a Soft Starter and Why Use One with an IE3 Motor?

A soft starter is a power-electronics device based on thyristors (SCRs). It gradually increases the voltage applied to the motor windings through phase control. Starting at low voltage, the motor draws low torque and low current; as the voltage rises to the rated value over a set ramp time, the motor accelerates smoothly. This matters especially for IE3 motors, because the rotor design of high-efficiency motors can show a slightly higher inrush tendency than standard motors. Low-resistance rotor bars optimised for efficiency can increase the instantaneous current spike at startup. By limiting this spike, a soft starter relieves the supply transformer, the generator and the fuse-contactor coordination.

For a broader framework on IE3 efficiency mandates, see our article on IE3 and IE4 efficiency requirements. For a general comparison of starting methods, star-delta versus soft starter is a good starting point.

The Difference Between Soft Starter and Star-Delta

Star-delta starting splits the start into two stages: the motor first runs in star at reduced voltage, then switches to delta at full voltage. This method is cheap but produces a current and torque transient at the transition; it also works cleanly only with unloaded or lightly loaded starts. A soft starter, by contrast, ramps voltage continuously and seamlessly; there is no transition shock, and the starting torque can be tuned to the need. For applications that require smooth starts such as pumps, fans and conveyors, the soft starter is clearly superior. Review starting current and LRA to clarify which method suits your load.

Setting the Starting Current and Torque Ramp

Two main parameters are set on a soft starter: the initial voltage (pedestal) and the ramp time. The initial voltage determines the first torque needed for the motor to break the load loose and begin turning. Set too low, the motor fails to start and heats up in a locked-rotor condition. Set too high, the current-limiting advantage of the soft starter is lost. Typically the initial voltage is set in the 30-50% range and fine-tuned per application.

The ramp time sets how many seconds the voltage takes to climb from the initial value to full voltage. For high-inertia loads (large fans, centrifuges, masses that are hard to accelerate) the ramp is kept long; however, an overly long ramp overheats the thyristors and the motor. That is why soft starter selection must account not only for motor power but also for starting frequency and inertia (J). You can also assess inertia and impact-load effects through our motor selection under impact load article. Most soft starters also offer a current-limit mode, in which the device holds the current at a set multiple (e.g. 3-3.5 times rated) while accelerating the motor.

Current Limit or Voltage Ramp?

The voltage ramp is simple and smooth; in generator-fed plants where voltage dip is critical, the current-limit mode is safer. Our articles on motor selection on generator-powered sites and generator kVA - motor kW matching explain how to solve the starting current problem on the generator side. To evaluate it alongside grid voltage fluctuation, the IE3 motor voltage tolerance article is useful.

Comparison of in-line and inside-delta soft starter connection diagrams for an IE3 motor

Sizing the Soft Starter by Current

A soft starter is selected not by motor power but primarily by the motor rated current, the starting duty type and the starting frequency. If one of two equal-kW applications starts twice an hour and the other twenty times an hour, the latter may need one size larger soft starter. Manufacturers usually label soft starters in "normal duty (AC-53a)" and "heavy duty" classes. A label such as "3-30:60-10" means: start at 3 times rated current for 30 seconds, 60% duty cycle, 10 starts per hour. When selecting, you must match these figures to your application.

To read the IE3 motor rated current from the nameplate and size cable, fuse and contactor correctly, our article on IE3 motor rated current: cable, fuse and contactor selection is a direct guide. If you are unsure about reading the nameplate, see reading the IE3 motor nameplate. For the relationship between rated torque and starting torque, see rated and starting torque of an IE3 motor.

Why Is a Bypass Contactor Needed?

If, after accelerating the motor, the soft starter keeps passing current through the thyristors continuously, it heats up and loses efficiency. That is why most modern soft starters include a bypass contactor that engages once the motor reaches full speed. The bypass contactor takes the thyristors out of circuit and passes the current directly through the contactor contacts. This keeps the soft starter cool, reduces the panel heat load and extends device life. In some soft starters the bypass is internal, in others external; where an external bypass is used, the contactor must be sized to the motor rated (continuous) current, not AC-1. To plan all protection equipment together, our electric motor protection devices article offers a checklist.

Connection Types: In-Line and Inside-Delta (3-Wire / 6-Wire)

A soft starter connects to the motor in two basic ways:

1) In-line (series / 3-wire) connection: The soft starter sits in series on the motor supply line. All three phases of the motor are fed through the soft starter. This is the most common and simplest connection. The soft starter is sized to carry the full rated current of the motor.

2) Inside-delta (6-wire) connection: The soft starter is placed inside the delta winding ends of the motor. In this connection the current through the soft starter is about 1/√3 (≈58%) of the line current. This allows one size smaller soft starter for the same motor, reducing cost. However, inside-delta requires 6 wires (access to all six motor leads is essential) and is sensitive to phase order; incorrect wiring causes the motor to start in reverse or improperly.

The inside-delta option requires a good grasp of terminal connections and star-delta bridging. Our articles on electric motor terminal connection: star and delta bridging and star-delta wiring diagram are very helpful here. To verify motor rotation direction and phase order, we recommend motor rotation direction and phase sequence.

Thermal Protection and Monitoring Motor Temperature

A soft starter protects the motor at startup, but protecting the motor against overload during continuous running is a separate matter. Many soft starters have built-in electronic overload protection; nevertheless, on critical motors the winding temperature should be monitored directly with a PTC thermistor or PT100. Motors are thermally stressed especially in long-ramp, frequent-start applications. Our article on motor temperature monitoring with PT100 and PTC thermistor details this protection layer. For classic thermal relay and fuse selection, refer to thermal relay, fuse and protection selection. You will find how duty type (S1-S6) drives selection in duty type selection.

When a Soft Starter, When a VFD (Variable Frequency Drive)?

A soft starter only smooths the start and stop; it does not change the running speed of the motor. If all you need is to limit the starting current/torque, a soft starter is economical and sufficient. But if you need speed control, energy savings (via the affinity law on pumps/fans) or precise speed regulation, a variable frequency drive (VFD) should be chosen. A VFD provides a soft start and also controls the running speed; therefore it delivers energy savings in variable-load applications. For comparison, review VFD with an asynchronous motor: when it is needed and energy savings on pumps and fans with a VFD. Some soft starters also reduce water-hammer on pumps with a soft-stop feature; that is an advantage you can gain without moving to a VFD.

Which Makes Sense for Which Application?

For constant-speed, continuously running loads that are only stressed at startup such as conveyors, crushers, large fans and pumps, the soft starter is usually the most cost-effective solution. For lines requiring frequent speed changes, proportional flow control or high precision, a VFD is essential. For the savings calculation of a high-efficiency motor + VFD combination, see high-efficiency motor + variable frequency drive. If you are considering a move to IE4, the mechanical compatibility in the IE4 transition article also guides you.

Frequently Asked Questions

Does a soft starter reduce the starting torque of an IE3 motor?

Yes. By lowering the voltage to limit the starting current, the soft starter also reduces the starting torque roughly in proportion to the square of the voltage. So for loads requiring high starting torque (a loaded conveyor, a jammed crusher) you must set the initial voltage and current limit carefully, and select one size larger soft starter if needed.

Can inside-delta connection be used on every motor?

No. Inside-delta (6-wire) connection requires access to all six winding leads at the terminal box and a motor suitable for delta operation. On single-voltage motors with only a 3-lead terminal box, only in-line (3-wire) connection is possible.

Does a soft starter work without a bypass contactor?

It works, but once the motor reaches full speed the thyristors stay continuously conducting, heat up and cause efficiency loss. In continuously running applications, using an internal or external bypass contactor both extends soft starter life and lowers the panel heat load; it is therefore strongly recommended.

Get a Quote

Contact us for soft starter sizing suited to your IE3 motor, in-line/inside-delta connection selection and bypass contactor planning. Let us determine the right solution together based on the actual motor current and duty type. To get a quote now, visit our contact page or call us at +90 (532) 345 49 86. You can review our entire high-efficiency electric motors product family and reach our product range from our homepage. For geared solutions, take a look at our worm gear reducers category as well.

Purchasing and Selection Checklist

Before ordering a soft starter, verify the following items:

  • Have you read the motor rated current (A) and power (kW) from the nameplate?
  • Is the starting duty type (start time, starts per hour, duty cycle) defined?
  • Is the load inertia high? Will current limiting or a long ramp be needed?
  • Will the connection be in-line (3-wire) or inside-delta (6-wire)? Are all six motor leads accessible?
  • Will the bypass contactor be internal or supplied externally?
  • Are thermal protection (PTC/PT100) and overload protection planned?
  • Is the supply generator-fed? Has current-limit mode been selected for voltage dip?
  • Is speed control required? If so, should a VFD be chosen instead of a soft starter?

After completing this checklist, follow the steps in our information to provide when requesting a quote article to get a fast and accurate price.