How to select correct motor soft starter?

Auto soft start selection methods

1. Pay attention to whether the load is a standard load or a heavy load. Different types of starters are selected according to the nature of the load.

If the load is a centrifugal pump (fire pump, sprinkler pump, water pump, etc.), the pump control function needs to be used to reduce the occurrence of system water hammer caused by the impact of liquid flow when starting and stopping, so the optional pump control function must be used. It is better to have a soft starter with underload protection or phase reversal protection.

If the load is a fan, use the soft start function to reduce belt wear and mechanical shock, and use the braking torque function at stop.

The loads are mixers, crushers, hoists, etc., utilizing dual ramp start and preset low speed operation to avoid mechanical damage.

The load is the conveying equipment, and the soft start and preset low speed functions can be used to realize smooth start and stop. The soft stop eliminates the anti-inertia impact of the drag system caused by the free stop, and avoids product displacement and liquid overflow.

2. When choosing a soft starter, it is not only necessary to consider the load condition and load category, but also the continuity of the load (working system) and the start-stop frequency. When the load is larger or the start-stop frequency increases, a product with a larger rating must be selected

3. Select the starter according to the nominal power, current and load properties of the motor.

Generally, the capacity of the soft starter is slightly larger than the working current of the motor. For some heavy loads in the cement plant, the first gear should be used, such as: crushers, ball mills, hoists, long belts, mixers, fans, etc. At the same time, due consideration should be given to cooling factor.

4. In other aspects, it is also necessary to consider whether the protection function is complete.

Such as overcurrent protection, overvoltage protection, single-phase grounding protection, upper and lower port open-phase protection, three-phase unbalance protection, phase reversal protection, etc. Without overload protection, it must be protected by another heating relay.

Some soft starters generally do not have short grid protection, and need to be matched with the capacity of the quick-release device and the product tube, which can protect the thyristor when the connected load occurs, and the low-voltage circuit breaker generally cannot protect the soft starter. , The incoming line reactor can limit the harmonic interference of the main power supply. It is recommended to use the incoming line reactor when several starters are connected to the same power supply.

Special attention should be paid to the wiring from the soft starter to the motor. Most of them have 3 outgoing wires, but some products also have 6 outgoing wires. The soft starter can be installed in the system that looks at the power speed factor compensator, but the capacitor must be located in the soft starter. to avoid damage to the thyristor of the soft starter due to capacitor discharge. In addition, an inductive coil is connected between the power supply and the capacitor. Most of the soft starters have auxiliary contacts for normal operation and fault signals for engineering design selection.

The soft starter is allowed to run under rated load conditions for a long time, and a bypass contactor can be used to short-circuit it after starting. After removal, pay attention to whether the motor running circuit has overload thermal protection function. In the actual application process, if the process conditions permit, when using a soft starter to start multiple motors, its capacity should be considered according to the largest starting load in the motor, which can greatly save investment costs.

5. Fully consider the impact of environmental factors on equipment capacity, such as humidity, temperature, altitude, dust and other objective factors.

The allowable installation altitude of the soft starter is up to 5000m, and derating needs to be considered above 1000m. For the specific derating factor, see "High Altitude Derating".

Ambient temperature:

kW: 40°C / 104°F

hp: 50°C / 122°F

Note: For applications that deviate from the above, a larger soft starter must be selected.

6. The quality of the power supply of the power grid and whether the inverter and the reactive power compensation device are running in parallel around them will cause the soft starter to operate abnormally or even be damaged. Attention should be paid to the design, selection and layout.

7. Pay attention to the number of starts

In some cases, the motor is required to start frequently, the load is large or the frequency of start and stop is increased, and a product with a larger rating must be selected.