Soft starters can offer benefits for almost all motor starting applications. Typical applications are highlighted below.
Soft starter applications
The most common soft starter applications are:
Fans are approximately 30% of all applications. Fans usually have a big moment of inertia due to the big flywheel. Some fans can be started with reduced load torque, with a closed damper. This is called an unloaded start and will make the start easier (shorter) but cause of the high moment of inertia, the starting time might still be quite long.
Centrifugal fans are very often driven by one or more drive belts. During a direct start, these belts have a tendency to slip. The belts slip because the starting torque from the motor is too high during the start and the belts are not able to transfer these forces. This typical problem gives high maintenance costs but also production losses when you need to stop production to change belts and bearings.
The key to solving the problems with slipping belts is to reduce the starting torque of the motor during the start. By using a soft starter, the voltage is reduced to a low value at the beginning of the start. Torque control will improve the start even further. Then gradually the voltage is increased in order to start up the fan. The soft starter provides the ability to adjust the settings to fit any starting condition, both unloaded and fully loaded starts. Using a soft starter will also greatly reduce the high inrush current when starting the motor, and thereby avoid voltage drops in the network. Some soft starters have built-in underload protection which will detect the reduced current caused by a broken belt, give a warning and stop the motor to prevent damage. Besides, phase reversal protection will detect if the phases are connected in the wrong order which might cause the motor and fan to run in the wrong direction.
A fan usually has a big flywheel with a big moment of inertia making it a heavy-duty start. Select a soft starter one size larger than the motor kW size. Since the big flywheel of a fan will cause a long slow-down period before the fan stops, a stop ramp should never be used for this kind of application.
Pumps are approximately 40% of all applications. There are a lot of different types of pumps like piston pumps, centrifugal pumps, screw pumps, etc. The most common version is the centrifugal pump.
Starting up a pump is normally not a big problem electrically. The problem is the wear and tear caused by pressure waves in the pipe system created when the motor starts but especially when it stops too quickly. Due to the small flywheel mass and the high breaking torque of a pump, a direct stop will cause a very sudden stop of the pump creating water hammering and pressure surges. During a single stop, this is merely an inconvenience but when performing several starts and stops per hour day in and day out, the whole pump system will soon be worn out. This will create a big need for service and maintenance and even worse unplanned shutdowns.
By using a soft starter, the voltage is reduced during the start sequence with the result that the motor torque is reduced. During the start sequence, the soft starter increases the voltage so that the motor will be strong enough to accelerate the pump to the nominal speed without any torque or current peaks.
Also, during the stop sequence, the soft starter is the solution. A soft starter using a normal voltage ramp will for sure reduce the problems with water hammering but in many pump systems, this is still not good enough. The solution is to use a soft starter with torque control in order to reduce the torque and stop the motor in the most optimal way in order to totally avoid water hammering.
In addition, some soft starters are equipped with underload protection to detect pumps running dry, a kick start to start blocked pumps, and with locked rotor protection to prevent damage caused by pumps being jammed while running.
A pump usually has a very small pump wheel with a low moment of inertia. This makes the pump a normal start so the soft starter can be selected according to the kW rating. If more than 10 starts per hour are performed it is however recommended to upsize the soft starter one size.
Compressors are approximately 10% of all applications. Smaller compressors are often of the piston type and the load torque increases linearly with the speed. Screw compressors are often used when there is a bigger need for airflow and this type has a load torque increasing with the square of the speed. Drive belts are often used between motor and compressor but direct connections via some types of toothed couplings are also common. Most compressors are started unloaded.
Compressors started direct online are exposed to high mechanical stresses on the compressor itself, but also on drive belts and couplings. The result is shortened endurance. In cases where the drive belts have been used the belts very often slip during the start. The source of these problems is the high starting torque occurring with the direct starting method.
By using a soft starter, it is possible to limit the starting torque to a level suitable for all different applications. The result is less stress on couplings, bearings, and no slipping belts during the start. The maintenance cost will be reduced to a minimum. When using a soft starter, the starting current received is approx. 3 to 4 times the rated motor current.
A compressor is usually a normal start and then the soft starter can be selected according to the motor kW size. If the compressor is a heavy-duty start, the soft starter should be upsized one size. The same applies if more than 10 starts per hour are performed, upsize one size.
Conveyor belts can have a lot of different characteristics. The length can vary from only a few meters up to several kilometers and the belt can be horizontal or incline. Typically, the conveyor belt has a constant load torque with low to high braking torque depending on the load.
Conveyor belts often need a starting torque very near or just above the rated torque of the motor. A direct online start with a normal squirrel cage motor gives approx. 1.5 to 2.5 times the rated torque of the motor depending on motor size, type, etc. When making a direct online start there is a very high risk of slipping between the belt and the driving role depending on this high starting torque. Gearboxes and couplings are also exposed to high mechanical stresses. This result is considerable wear and tear and often high maintenance costs.
Sometimes fluid couplings are used to reduce the transferred torque. This method is expensive and requires a lot of maintenance.
By using a soft starter, the starting torque can be reduced to a minimum value and still be able to start up the conveyor belt. The setting feature of the soft starter makes it possible to adjust the torque to exactly the level that is necessary for the start. The result is the least possible stress on gearboxes and couplings and no slipping belts during the start. This will reduce the maintenance cost to a minimum.
When using a soft starter, you will receive approx. 3 to 4 times rated motor current during the start. In addition, some soft starters are equipped with phase reversal protection to detect conveyor belts running in the wrong direction. Soft starters may also have under-or overload protection to detect if the load is too low or too high, and then kick-start to be able to start jammed belts.
A conveyor belt can be both a normal start and a heavy-duty start depending on the characteristics. For a normal start, select the soft starter according to the motor kW rating. For a heavy-duty start, select one size bigger.
If more than 10 starts per hour are done, select one size bigger soft starter.
Crusher, Mill, Mixer, and Stirrer
Crushers and mills usually have constant load curves. These applications can have a very big flywheel and can be a very heavy-duty start. In most cases, both these applications are started unloaded, and when full speed is reached, the load is applied.
When starting direct online there will be high mechanical stresses leading to a shortened life length for all parts of the drive chain. In addition, the inrush current will be high and as a result of this in combination with the long starting times, there might be big disturbances in the network.
By using a soft starter, it is possible to limit the starting torque to a level suitable for all different load conditions. The result is less stress on the machinery and greatly reduced inrush current. For an unloaded start, the inrush current can be reduced to approximately 3 times the current while a loaded start may require 5 times the nominal current.
Crushers, mixers, mills, and stirrers usually have a very big moment of inertia so the soft starter is selected one size larger than the motor kW size. Since the big flywheel will cause a long slow-down period before the fan stops, a stop ramp should never be used for this kind of application.