The Many Types and Uses of Soft Starters
A Soft Starter is a device that starts motors with reduced power supplied at start-up. Reducing the power reduces potentially damaging electrical and mechanical shocks on the system.

As the name implies, starters "start" motors. They can also stop, reverse, accelerate and protect them. Whether it's a small fan, or piece of mining equipment, electric motor are often the driving force behind them. Electric motors consume 60% to 70% of all energy used in the United States.

Soft Starters are a combination of a controller and overload protection.
  • CONTROLLERS - turns electric current to the motor on and off. A contactor is a controller that is controlled by an electromagnet.
  • OVERLOAD PROTECTION - protects a motor from drawing too much current and "burning out" from overheating. The overload relay is the motor overload protection used in soft starters. It limits the time the overload current is drawn and protects the motor from overheating.
Soft Starters place a device called a reduced voltage starter, or soft starter, between the motor and the incoming utility line to regulate the amount of current fed to the motor. Soft Starters enable the AC induction motor to speed up in smaller, resulting in less current drawn than with a traditional motor starter. Due to decreased voltage, torque is also reduced resulting in a soft, or easy start. Soft Starters are used on all types of AC and DC motors. They are most commonly used with the AC squirrel cage induction motor because of its simplicity, ruggedness and reliability.

Why Soft Starters are Needed
  1. To avoid overloading the power distribution system
  2. To avoid unnecessary wear and tear on equipment by reducing starting torque
A typical NEMA design B motor can draw six to eight times its full load operating current when it's first started. If the utility's power distribution network is loaded to capacity, the current inrush from starting up large motors can result in anything from flickering lights to brownouts. It can also result in nuisance tripping of circuit breakers and protective devices on the system. Many utilities impose limits on the amount of power customers can draw at any one time, enabling a balance in their distribution system. Reducing voltage to motor terminals at startup reduces the current surge.
Soft Starter from ABB
ABB Soft Starter
Soft Starter
ABB Soft Starter
Types of Soft Starters
There are five main varieties of Soft Starters:
  1. Primary Resistor
  2. Auto Transformer
  3. Part Winding
  4. Wye Delta
  5. Solid State
1.) Primary Resistor -
Developed in the early 1900's, this simple unit is one of the first soft starters placed into operation. Fig. 4 shows that there is a resistor for each of the three phases of current. Resistors resist the flow of current. When the motor is started, the resistors resist the current flow resulting in a voltage drop. Approximately 70% of the line voltage is sent to the motor terminals at startup. A timer closes a set of contacts after the motor has accelerated to a pre-determined point. This removes the resistors from the circuit and lets full power through to the motor. Primary resistor reduced voltage starter diagram Primary resistors starters are known for their smooth starts. They offer two-point acceleration, or one step of resistance. For extra-smooth starting, add additional stages of resistors and contactors.

2.) Auto Transformer -
Auto transformer starting is one of the most effective methods of soft starting. It is preferred over primary resistor starting when the starting current is drawn from the line must be held to a minimum, yet the maximum starting torque per line amp is required. Instead of using resistors, this starter uses taps on transformer windings to control the power input to the motor. Taps are usually set up to provide 80%, 65% and 50% of the line voltage, respectively.

These taps provide built-in flexibility. Activating any one of three taps on the windings allows different amounts of current to the motor. In Fig. 6, the motor is receiving voltage through the second of the three taps. This type of starter can supply more current to the motor than other soft starters, while keeping voltage low. The transformer steps up the current making it greater than the line current input during startup. Autotransformer reduced voltage starter 3.) Part Winding -
The part winding method requires dividing the motor windings into two, or more, separate sets. These identical winding sets are intended for parallel operation. At startup, power is applied to only one set of windings. As the motor comes up to speed, power is applied to the other winding set for normal running. When windings are energized in this manner, they produce reduced starting current and reduced starting torque. Most dual voltage (230V/460V) motors are compatible with the part winding starter at 230 volts.

4.) Wye Delta -
Wye Delta starting requires the motor have connection points to each of the three coil windings. These are specially wound with six leads for Delta and Wye connections. Fig. 8 illustrates the winding configurations as they are connected at startup.

It is called the Wye Configuration because it is shaped like the letter "Y". This connection results in line voltage applied to an electrically larger winding, reducing the line current. It provides 33% of the normal starting torque and 58% of the normal starting voltage.

After a pre-determined time, the starter electrically switches the windings over to a Delta Configuration. This configuration resembles the Greek letter "delta". The windings are connected in their normal run configuration with every winding receiving full voltage.

An important consideration with this starter is at the transition point, where the starter switches from Wye to Delta, the motor MUST disconnect and reconnect. This type of Wye Delta starter is known as Open Transition and can have a momentary hitch in operation, allowing a momentary current inrush.

Closed Transition is another type of Wye Delta starter. It uses an extra contactor and set of resistors to keep the motor on-line during the transition. It eliminates the inrush concern and the cost is slightly higher than the open transition version. Wye and Delta Configurations 5.) Solid State -
The newest soft start method is the solid state type. It replaces mechanical components with electrical components. The key is the Silicon Control Rectifier or SCR. During motor acceleration, this device controls motor voltage, current and torque. Fig. 11 shows how the solid state soft starter controls the current draw and the starting torque. The SCR has the ability to rapidly switch heavy currents. This allows the soft starter to provide smooth stepless acceleration - the smoothest of any of the soft start methods. Current and Torque vs. motor speed diagram Order of Events when a Motor is Started (SOLID STATE):
  1. Start Contacts (C1) close
  2. SCRs gradually turn on and control motor acceleration until it approaches full speed
  3. Run Contacts (C2) close when SCRs are fully on
  4. Motor is connected directly across the line and runs with full power applied to motor terminals
Solid state reduced voltage starter diagram
Choosing a Soft Starter
Soft starters have properties that are more suited to specific applications. To make the proper selection, the following is needed:
  • Motor nameplate full load amps, locked rotor amps, HP rating, motor torque/speed curve if available.
  • Starting and stopping requirements - Longer start and stop times allow for smoother operation. Pulse start and/or jog option is useful for dough mixers, coal handlers or plastic extruders. Pump applications require smooth stops to prevent water hammer damage.
  • Torque requirements of machinery driven and load inertia.
  • Number of starts required per hour - Heat dissipation could pose a problem if the number of starts is excessively high.
  • Overload protection requirement - Overload protection is based on class. Class 10 starters trip of the current draw is 6 times the motor's fill load amps for more than 10 continuous seconds. A Class 20 starter trips in 20 seconds.
  • Electrical service range (line voltage)
  • Enclosure type
There are wide assortments of soft starters available including 200-690 VAC input, up to 700 HP at 230V and up to 1600 normal duty amps. Before making your final selection, consult a soft start application specialist.
Soft Starter Video Overview