Correct Position of Soft Starter

It is a usual, two speed, 3 contactors, installation. The soft starter "softens" in this way the switch over to the second (the higher) speed.

It will depend on the soft starter design. Some look at back-emf to "know" when the motor is accelerating, in which case they would also know that there is no motor connected and may either fault, or wait. Many will wait for about 2-3 seconds before faulting, just for the reason you describe.

But not all soft starter designs are the same, so you cannot expect it in advance. You will need to ask the supplier specifically.

This soft starter sounds like a star-delta motor starter. A star delta-starter is a pair of contactors where selecting one will provide a star connection and the other a delta connection with interlocks to prevent dual operation and the release of smoke. The selection may be invoked by a simple timer or the back EMF method already mentioned. Now then, which contactor are you talking about? I'm guessing it is the primary contactor.....or not. I'm not really sure what your problem is. Does the soft starter not work or do you want to implement a new installation?

2 speed motor. 3 contactors. Usual schematics.

Low speed: the 3 low speed leads receive supply, 3 high speed leads are free.

High speed : low speed leads disconnect from supply and inter-close, high speed leads receive supply. Here the soft starter is put between the high speed contactor and the motor. The contactor begins to supply the 3 phases to the SS as well as its auxilliary NO contact closes the control input on the SS.

If you don't need two speeds, hardwire your motor for high speed, close a single contactor and then use the soft start to ramp up and down your motor. If you do need 2 speeds hardwire your motor for high speed, close a single contactor and use a variable speed drive to ramp up and down your motor and control speed.

Elroy

Elroy,

I don't recall that speed control was an intended function of soft starters, just reducing inrush current while the motor gently accelerates to its nameplate speed.

--John M.

The motor is taking 10 A at the high speed and makes the 16C breaker go off from time to time. I suspect also the GE SS was not intended for continious use - only for starting (need then to be by-passed during motor run time). Has lasted, however, 4 or 5 years by now.

Many newer soft starts have bypass contactors built into them to reduce heat and improve efficiencies. We have soft starts here that have run for 10 years or more with no issues. Check the soft start cooling fan. If it isn't working your problem may be caused by excessive heat as a result of the cooling fan failure.

Yuri B.

Others have suggested replacement of the contactors and soft starter with a VFD. This is often a good option, but you would want to carefully look at the motor to ensure that the higher voltages imposed on the windings don't cause premature failure. Also, there is growing evidence of bearing failures from currents induced into the motor shaft. With your soft starter, you may want to increase the ramp time a little--it is entirely possible that slow drift in electronics or breaker performance has caused a marginal (but working) application to shift so it is now marginal (but sometimes not working).

--John M.