Soft starters vs Inverters

Hello Everybody

During these years I have been just in working with Inverters for AC Motors. Now I have some suggestions to use soft starters in some purposes which does not need any special control. I am still a novice in this field. I started to read more about the softstarters but it is mostly about what they can do. can any body support me with info. about what they can not ?
and can they be connected to multi motors just as we normally do with inverters ?
and is the speed changable using them ?
What about start up boost features or brake ?

Soft starters are just that, soft starters. They do not vary speed, basically only ramp speed on start up and limit voltage and current.

Usually there are adjustments available to alter current limit, adjust ramp rate, etc. Many also have a soft stop function. This is typically used for pumps for example where there is a large head of water moving through a pipe. DOL start and instant stop cause pipe hammer, at the very least, and also tends to break bends and joints in the line due to sudden changes in water flow.

Normally, when a soft starter runs a motor up to speed, one uses a bypass contactor to take the soft starter out of circuit and the motor continues to run at full speed. The starter I normally use for pumps are manufactured by Sprecher & Schuh. They are soft start/stop, have an automatic bypass contactor built in and also thermal protection built in, They are also sold re-badged as AB. AB own Sprechur & Schuh.

I presume you reference to inverters means VSDs (variable speed drives). These also have ramp, current limit etc functions but stay in circuit all the time to allow variation of motor speed. They also soft start and stop drives but are much more expensive due to the fact that they CAN vary speed. Soft starters cannot.

As for cost comparisons, below 10hp, there is very little difference between a softstarter and a VFD. Above 10hp, VFD's are more expensive to the extent that, at 100hp 460V, the VFD is about 4 times the cost of a softstarter.

There is an important performance difference between the two devices. A softstarter always sends the motor pulses at line frequency, typically 50 or 60hz. It's only function is to reduce the voltage thereby reducing inrush current and starting torque. Unfortunately, starting torque reduces by the square of the voltage reduction so, at best, you will be able to reduce starting current from the normal 6-8 times nameplate to around 2.5 times nameplate. At that reduction of inrush current, the motor will barely have enough torque to start itself and a couple of low resistance ball bearings. This is due to the fact that the motor is still being started dissynchronously with 50 or 60Hz pulses.

On the other hand, a VFD always starts a motor nearly synchronously so, at start, the inverter is at 1-2Hz or so. The result is that the motor develops much more torque per amp and, for the same load, will start well below nameplate amps instead of several times nameplate. Therefore, if your object is absolutely minimum starting current, the inverter always wins. If your object is to reduce starting torque or to simple cut inrush down a bit, then a softstarter is a cheaper option.

It is important to note that the amount of softstarting that is possible is entirely determined by the breakaway torque of the load. For example, if you have an outdoor conveyor that has to start after being covered with ice in a freezing rain, not much softstarting will be possible. In contrast, if you have a large fan running on two ball bearings, a very soft start is possible because of the low breakaway torque of the bearings.

Hope this clarifies this subject a little.