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starting voltage of a soft starter

Guest Anonymous

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Hi guys, i'd like to inquire about the initial or the starting voltage of a soft starter. i've set it in 50% of line voltage at 8 seconds ramp up time. when measuring the voltage during starting, my meter reads the full voltage and my current meter reads about 60% of rated FLA. Is there something wrong with my settings? or is it a normal observation?;q
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It depends upon the type of meter you have. Most DMMs are sensitive enough to pick up the leakage through the SCR (and it's protective system) as full voltage. The test would be to see if you still show full voltage when the soft starter is OFF (with line power still applied of course). Older analog meters will usually have enough meter burden that they will bury the leakage current and display no voltage.


The initial output voltage setting is in and of itself irrelevant. What you really want is for the output TORQUE to be just enough to cause the motor shaft to begin turning. Any less and you are just wasting thermal capacity in the motor, because current will be flowing but no usefull work will be done. Higher torque is usually OK, but reduces the maximum benefit of soft starting. So the typical procedure is to start off with the setting as low as possible, and keep increasing it until the motor just begins to rotate.


The 50% factory preset is nothing more than a starting point based upon the manufacturer's experience of knowing that most common loads will require at least that much. But I have had some start moving with as little as 10%, many at 25%, while many others don't budge until you get to 65 or 70%. I even had one that didn't start turning until 85%, but then I had a hard time justifying the fact that the user just spent a lot of money for very little reduction in starting torque!


60% FLA is a very good starting point if your motor is turning. Remember, normal starting current would have been 600% FLA, so you are really at 10% starting current.

"He's not dead, he's just pinin' for the fjords!"
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  • 2 weeks later...

Just to add to the excellent comments by Jraef, the driven load determines the minimum torque required to get the load spinning. Sometimes, there are changes that can be made to the driven load to influence this start torque.

The motor characteristics determine the start current required to develop that start torque and there are big variations between motors in terms of start torque for a given start current.


The initial start conditions (current, voltage or torque) should be set to ensure that there is sufficient shaft torque available to allow the lod to begin to spin as soon as the start command is given.


Due to the variations in motors and driven loads, there is no correct setting for all applications or even classes of application.


Best regards,

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  • 2 months later...

I understand this is a relatively old thread but wanted to add a few comments for those that might be reading the thread for the purpose of gaining some additional knowledge.


Looking back at the original post, anonymous states "I have set it (the soft starter) for 50% line voltage ....... my meter reads the full voltage and my current meter reads 60% FLA". This strongly suggests the meter/s used to take the measurements are not reading correctly, most probably becuase they are not true RMS reading meters.



If the initial voltage on the soft starter is set at 50% line voltage, then the voltage measured at the output terminals of the soft starter at the momment the start signal is given should be 50% line voltage not full voltage as noted. In this case the meter is displaying a much higher value than it should.



If the initial voltage is set at 50% line voltage, then the current measured at the momment the start signal is given should be equal to 50% of the locked rotor current of the motor. As most motors will have a Locked Rotor Current of 6 - 9 times rated current, the meter should in fact be reading in excess of 300% FLA here, not 60% as noted by anonymous. In this case the meter is displaying a much lower value here than it should.


It is clear from the original post that anonymous was unsure about what his meters were displaying and quite rightly he should have been. In my opinion he did the right thing and asked questions before accepting what his meters displayed. For others this may have resulted in a lot of lost time trying to identify why the soft starter was apparently not performing as expected.


If anything, I guess the above acts to highlight two things.


1. Make sure that you use quality instrumentation.

2. Ask questions if you are unsure about anything.


I hope you can put this to good use.




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