Devices, such as overload relays, will open the circuit in order to prevent the motor from overheating if there is overload.
A normal soft start will typically use a class 10 relay, while a heavy duty soft start will use a class 30 relay, since a longer starting time is required.
The relays may also allow temporary overloads, without causing harm to the motor.
Protection is provided for all currents up to the locked rotor current.
If the rotor becomes locked, stalled or jammed, it can cause the current inside the motor to spike.
The soft start is designed to recognize if the current stays at a level of current that is too high for a period of time that is longer than the initial startup. The soft start will shut the motor down if this becomes the case, in order to prevent the windings from burning out.
Phase loss is also referred to as single phasing. This occurs when a 3 phase motor loses current in one of its lines, causing the other two lines to carry extra current.
This can be caused by a broken wire, a blown line fuse, a damaged relay contact or a broken terminal contact.
Phase loss can result in damage to the windings because of the uneven current distribution. It can also result in fluctuation in torque and RPM, or even overloading of the generator.
When the direction of the phase in the circuit becomes reversed, potentially causing damage to the motor.
A soft start may also come with an external bypass contactor, a device which reduces heat and power losses in the soft start. Once the starter reaches full current, the bypass contactor closes; the soft start 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.
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