2025.05.02 – Understanding Soft Starters: Basic Concepts from the Eaton Wiring Manual 10/23

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What is a Soft Starter?

• A soft starter is an electronic device for starting three-phase motors slowly.
  It controls how voltage reaches the motor during startup.
• A soft starter uses a method called phase angle control to adjust the voltage.
  This avoids sudden jumps in power.
• A soft starter follows the rules of IEC/EN 60947-4-2 (International Electrotechnical Commission / European Norm).
  This makes sure it works safely and reliably.

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What Happens During Startup?

• A soft starter increases the voltage from zero up to the normal value (U_N).
  This takes place during the time called t-Start.
• This smooth increase lowers the starting current.
  Less current means less stress on the motor.
• The motor torque also rises smoothly.
  Torque is the force that makes the motor turn.
• A smoother start protects the whole system.
  It reduces wear and noise.

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Why Use a Soft Starter?

• Machines last longer when they start smoothly.
  There is less damage to gears, belts, and pumps.
• The system avoids shocks like water hammer in pipes.
  Water hammer is a pressure surge in plumbing systems.
• Vibrations in conveyor belts get reduced.
  This helps in precise material handling.

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What Is the Top of Ramp (TOR)?

• The top of ramp (TOR) is when the voltage reaches its final value (U_N).
  At this point, the motor runs at full speed.
• Bypass contacts close at TOR.
  These help save energy by skipping the power stage of the soft starter.
• Less heat reaches the semiconductors.
  Semiconductors are the parts that control current flow.

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Deceleration (t-Stop) with Soft Starters

• A soft starter can also reduce the voltage slowly when stopping.
  This helps avoid pressure spikes in pump systems.
• The deceleration time (t-Stop) must be longer than the motor’s natural stop time.
  This ensures safe stopping.

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How Soft Starters Control Voltage

• Voltage rises during t-Start and stays at U_N until the motor is fully running.
  After that, it stays stable until stopping begins.
• Then voltage falls smoothly during t-Stop.
  This protects the system from sudden stops.

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Understanding Ratings with Examples

• Example: 55A: AC-53a: 3-5: 75-10
  This shows how much current the device can handle.
• AC-53a is a standard from IEC/EN 60947-4-2 (International Electrotechnical Commission / European Norm).
  It tells us how the device manages motor loads.
• 3 means the current is 3 times normal at startup.
  5 means that lasts for 5 seconds.
• 75 is the percentage of time the soft starter runs during a cycle.
  10 means up to 10 starts are allowed per hour.

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Types of Soft Starters

• Two-phase soft starters are for simple tasks.
  They are smaller and cheaper.
• Three-phase soft starters are for more complex tasks.
  They handle higher power and more settings.
• Two-phase starters only work in in-line setups.
  Three-phase ones work in in-line or in-delta configurations.

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Choosing the Right Soft Starter

• You must know the motor voltage and motor current.
  Also the type of connection (star or delta).
• The starter’s current must match the motor’s rated current.
  Otherwise, the motor won’t start safely.
• The type of motor, load torque, and ambient temperature also matter.
  These affect how the soft starter behaves.

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Allowed Motor Connections

• Star and delta motor connections can work with soft starters.
  The choice depends on voltage and motor setup.
• Motors with a neutral point cannot use a two-phase soft starter.
  This causes overheating and damage.
• Always check that the motor type matches the soft starter.
  Incorrect wiring is dangerous.

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Safety Warnings

• A soft starter still has voltage at the output, even when stopped.
  This happens because of how thyristors work.
• High voltage can cause serious injury.
  Always use proper protection and isolation.

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Coordinating with Protection Devices

• Type 1 coordination protects people and equipment.
  But the soft starter might need replacement after a short circuit.
• Type 2 coordination protects both people and the device.
  The device keeps working after a short circuit.
• Always place fast semiconductor fuses before the power stage.
  This keeps damage from reaching the starter.

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Running More than One Motor

• Several motors can use one soft starter.
  But the total current must not go over the starter’s limit.
• Each motor needs its own protection.
  This includes fuses or thermal relays.
• The motors should be similar in size.
  Big differences in power cause trouble during startup.

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Slip Ring Motors with Soft Starters

• Slip ring motors can work with soft starters if resistors get removed.
  The rings on the rotor must be short-circuited.
• Some slip ring setups may still need a resistor group.
  This depends on the motor type.
• Slip ring motors start with high torque and low current.
  This must match the soft starter settings.

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Motors with Capacitors

• Some motors use capacitors to improve power factor (cos φ).
  These are called compensation capacitors.
• The output of a soft starter must not connect to capacitors.
  It would damage the starter.
• Capacitors must connect to the line through a separate contactor.
  This avoids voltage spikes.

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Features of the DS7 Soft Starter

• DS7 is a two-phase soft starter with a bypass relay.
  It’s smaller and costs less than a three-phase unit.
• It comes in 4 current sizes:
  4–12 A, 16–32 A, 41–100 A, 135–200 A.
• Power and control circuits are isolated.
  This improves safety and reduces noise.
• The front has 3 knobs to adjust:
  • t-Start (0–30 s): how long voltage rises
  • U-Start (30–100%): how low voltage starts
  • t-Stop (0–30 s): how long voltage drops
• Maximum: 10 starts/hour, 40 with fan.
  LEDs show if it’s running (green) or in error (red).

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