Tag: soft starters

Have you ever though why your motor has a shorter service life than that of others? Let us tell you, your motor may need a soft starter. Generally, the motor shall adopt a soft starter as soon as its power exceeds 20hp (15kW), otherwise it will affect the use and operation of other equipment in power grid and damage the motor. Why should the soft start controller be used? What are the benefits of using motor soft starter?

Three phase asynchronous motor has many advantages, but also has two fatal weaknesses: one is the poor starting performance, the other is poor speed control performance. With the development of power semiconductor technology and application of high power transistors in industrial control field, two typical products greatly improve the two weaknesses of asynchronous motors: Variable Frequency Drive and Soft Starter. The VFD improves the speed control performance of asynchronous motors, while the soft starter improves the starting performance of asynchronous motors.
With various functions such as soft start, soft stop, pump control and timed low speed operation, the 3 phase motor soft starter has incomparable advantages over traditional reduced voltage starter in power equipment control in the industrial and civil construction fields.

Frequent starting leads to excessively high temperature, overheating protection of the soft starter thereof operates (starting frequency of the soft starter shall be controlled within 6 times per hour, particularly in heavy load). Protective elements act at starting, which makes the contactor unable to by pass, the soft starter works for long time and thus causing the protection to act (check peripheral circuit). Heavy load and long starting time lead to overheating protection (reduce load as far as possible when starting). Parameters of soft starter are improperly set. Long starting time and low starting voltage (increase the starting voltage). Radiator fan of the soft starter can't work normally (replace a fan).

As everyone knows, direct starting of water pump would cause the water hammer effect. Namely when the water pump starts and stops, water impacts the pipe and cause a serious water hammer. When the power is off suddenly or the valve turns off quickly, a water impact wave generates due to inertia of pressure flow, which is like the knock of a hammer, so it’s called water hammer. Sometimes the force generated from water impact wave may be great and damage valve or pump, therefore, to avoid the negative effect of water hammer effect, the pump’s rotation speed should increase to the rated speed slowly when it’s started. Two methods are applicable, one is the variable frequency starting, another is the soft starting.

Soft starter is used for controlling AC induction motor in soft start & soft stop. It applies to various squirrel-cage asynchronous motor control of load, the motor can smooth starting under any working conditions, protect the drag system, protect motor from inrush current and voltage impact, ensure reliable motor starting. Smoothly soft stopping function can effectively solve the inertial system surge problem, and eliminate the drag system of inertial impact.

Current-limiting start mode
When the motor starts, the output voltage increases rapidly until the motor current reaches the set current-limiting value and then keeps below the set value. As the output voltage gradually increases, the motor speeds up. When the motor reaches the rated speed, the bypass contactor attracts, the output current drops rapidly to the rated current or below, the starting is thereof completed.
When the motor load is lighter or the setting value is rather larger, the maximum starting current may not reach the set current-limiting value. Current-limiting mode is generally applicable for situations that has strict restrictions on starting current.

As we all know, the direct starting current of motor is 4-7 times rated current, the motors with a low power (generally lower than 7.5kW) have few influences on power grid and are allowed to start directly, but the large motors with high power (rated current 100A) have the instantaneous current of hundred of amperes and are not allowed to start directly because such high current has a high impact on power grid and can influence the normal work of other loads on the same power grid.