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Soft Start Circuit

A soft starter is an inexpensive, and quick, way to lower the current going into your motor. The type of soft start and contactor you'll need depends on what kind of soft start circuit your motor is connected in. TEMCo will help you figure out which soft start circuit is right for you.

Soft Start Circuit

What is a soft starter?

A soft starter lessens the initial surge of current that goes into the windings of a motor when it is started, also known as the inrush current. Soft starters reduce the strain on electrical circuits of the motor.


Soft Start Circuit

A soft start circuit can either be an in-line or an inside delta connection.

In-line Soft Start Circuit

An in-line connection is the most common type of soft start circuit. In this type of circuit, all three phases are connected in series with the devices such as the overload relay and the main contactor. The devices that are in series with the In-line connection have to be able to handle the rated motor current.

For example, a 100 amp motor has to have a 100 amp soft starter and a 100 amp main contactor.

Inline Soft Start Circuit

Inside Delta Soft Start Circuit

An inside delta soft start circuit means that the connected devices are inside the motor delta circuit but do not have to be in line with the starter. In an inside delta connection, the current is reduced to 58% of the line current.

For example a 100 amp motor would require a 58 amp soft starter, which is smaller and more cost-effective.

This type of connection makes it possible to purchase devices that have an existing wye/delta starter.

For an inside delta connection to work, the motor must be delta type and have all coil leads exposed. Wye type motors, and delta motors with internally bonded windings, cannot be used in an inside delta configuration

Inside Delta Soft Start Circuit


Main Contactor

While a soft start circuit does not require a main contactor, it does provide a means of disconnecting the power to the system in the event of a fault, overload, or external e-stop condition.

When connected in an inside delta soft start circuit, the main contactor can either be in the delta circuit or outside of the circuit. Either way, it will stop the motor, but putting it inside the circuit will put the main contactor under tension. wait what? tension?

If the main contactor is placed outside the soft start circuit, it must be chosen according to the rated full load current of the motor, but if it is inside the circuit it can be sized according to 58 % of the rated full load current.

Alternative A: Main contactor located in the delta circuit.

Alternative B: Main contactor located outside the delta circuit.

Soft Start Circuit Main Contactor


Bypass Contactor

Starters may also come with an external bypass contactor, a device which reduces heat and power losses in the softstarter. Once the starter reaches full current, the bypass contactor closes; the starter then becomes across the line.

Types of Bypass Contactors

There are different types of bypass contactors, including AC1 and AC3, which have International Electrotechnical Commission (IEC) standards that depend on the duty cycle of the device.

AC1 is used for loads that are non-inductive or slightly inductive, such as resistive furnaces. AC3 is used for starting squirrel-cage motors, switching off when the motor gets up to speed.

Bypass contactors are usually found inside the starter, but they can also be used externally.

Call us at 877.474.8209 to speak with an application engineer about your project.