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I hope this hasn't been asked already, I browsed around a little, and didn't see anything....I'm fairly new to motor control and read somewhere that you could soft start a motor by putting series resistors inline and shunting them with time delay relay contacts.....what would be the best way to go about selecting the value/wattage for these resistors? As a starting point, we could assume a 230vac 3phase 8 FLA motor.....




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Welcome bnbmark.

Please give more info regarding your load and type of application, not all loads could be started in the right way using series resistors.




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

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Hello bnbmark


Welcome to the forum.


The starting methoid that you refer to is usually called primary resistance starting and you can find some information on this at Motor Starting

The first uestion is to establish if the motor can be reduced voltage started. If it can, you then need to determine what the required start current is and then from that, you can calculate the value of the resistors required.

I would suggest that you also read Motor Control to get some back ground into motor characteristics and starting.


Essentially, the load dictates the minimum starting torque. The only way to reduce the start torque is by modification of the driven load. The motor acts as a converter of electrical energy into mechanical energy and determines the minimum start current to develop the minimum start torque. NB there are very larg differences between different motors!

Once you have determined the minimum start current, you can calculate the value of the resistors. - ohms law applies. The initial impedance of the motor can be determined from the rated Locked Rotor Current of the motor. The initial powerfactor of the motor will be in the order of 0.2

Once you have determined the start current and the start time of the motor and load, you can determine the power disipation of the resistors and now you have the complete specification.


You could try using my Electrical Calculations software to help you determine the start current requirements and start time requirements. You need to download it from http://www.LMPhotonics.com/busbar32.zip and then program in the speed torque curve of the load and the motor and also the speed curent curve of the motor.


Best regards,

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