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Soft Starter With Motors In Parallel Connection


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Dear all,


I am now working on a project for a water treatment plant, which the customer would like to use soft starters to control waste water pumps in order to take care of the water hammer problems.


There are about 200 units of 1kW motors used to pump waster water, which are located about 50m away one from the other.


The thing is, due to budget limitation, the customer could not afford to use 1 soft starter for each motor and would like to use, says 1 unit of 15kW soft starter to soft start and stop 15 unit motors. The time between each start is about 2 to 3 hours.


Does any one of you have any experience in this kind of parallel connection application with soft starter?


If this is workable, what is the maximum number of motors to be connect in parallel?


I am afraid if anything happen on 1 motor it will cause the rest of the motors fail as well, would this happen? If this might happen, then the whole world would point the finger at the soft starter before even start to verify the actual cause of failure.


Besides this, is there any other hidden risk, which you guys can foresee?


All comments are welcome!




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Dear schow,

the application is possible, you know that is not a perfect solution.

You will find differences in acceleration and deceleration time on different motors, but this could be acceptable, I think.


I'm quasi-sure that if you will calculate the energy saving, a single inverter to drive 8 or 12 motors together should be a cheap solution, instead of using a soft starter. The acceleration and deceleration times will be more stable.






Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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Hello Schow...


Such applications are not uncommon.


Each motor must have its own protection; i.e., thermal overload relay and contactor or a Linestarter (consisting of 3-pole, AC, HP rated, contactor with integral overload relay. If the O/L senses a problem with its associated motor, it should open the contactor to disconnect the motor from the output of the Soft Starter.


How many motors can you connect in parallel ?


The limitation is how large of a Soft Starter can you obtain. Remember, the Soft Starter is a limited source of controlled voltage/current whereas the mains are theoretically an unlimited source,


The Soft Starter MUST be rated to continuously source the TOTAL CURRENT, which is determined by

adding up the nameplate rated current of all the motors that will be connected to the Soft Start unit.


For example: quantity 10 motors, each nameplated at 15 amps rated current, would require a Soft Start unit that can continuously source 10 x 15 = 150 amperes.


It would be advisable to check with the manufacturer of the Soft Start unit you are considering for additional guidance in specifing a unit or units for your multi-motor application. They are your best source for answers to your application questions.


Bitte schön !


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One thing to keep in mind it that in this scenario, the only way to do it is to start and stop ALL of the motors simultaneously, or at least in groups depending on the size of soft starter you are willing to buy. Many people think they can soft start motor 1, then soft start motor 2, then 3 then 4 etc. etc., re-using the same soft starter. Unfortunately they never think the circuit out completely, you cannot re-use the soft starter once it is already at full speed with one motor. The necessary isolation contactors to make it work properly ALWAYS exceeds the cost of just buying separate soft starters for each pump. Trust me, I have proven it to people countless times.


If it is OK to start them all (or groups of them) simultaneously, the next rule-or-thumb is to oversize the current rating of the soft start by 10% above the aggregate motor FLA's to account for some capacitive effects in starting multiple motors with different lengths of cable between them. You will find this mentioned in many soft starter manuals. This isn't an absolute requirement, just a precautionary one. Some soft starters have built-in extra capacity and may not mention this issue however.


Another trick of the trade is to use IEC style Integral Motor Protective Switches for each motor circuit downstream from the soft starter. These devices provide both the thermal overload and short circuit protection for the motor windings and leads, allowing you to use smaller wire from the MPS to the motor. Otherwise, you would need to also have separate fuse protection for each motor circuit in addition to a motor starter and overload relay.


Lastly, you need to determine in advance the consequences of one of the motors overloading. In a pumping scenario such as yours, you may need to shut down ALL of the pumps if any one of them overloads because the loss of one pump in series will mean a loss of head and failure to pump.

"He's not dead, he's just pinin' for the fjords!"
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