motor softstarter troubleshoot 2

[QGeek]

Hi all. I have been working with a softstarter fault issue on my company's equipment for over a month now. The softstart is an Allen-Bradley SMC-3, and the fault is four-flash Phase-Loss/Open Load. I have initiated one fix so far, which initially reduced the faults entirely. What I suspected initially was that the signal from the reverser [5MREV] control block, which ran to both the contactor [5M] coil and the sfst START signal, caused the sfst to finish its prechecks in some machines/instances before the contactor had fully closed, thus faulting a Phase Loss/Open Load. I put the signals and power up on a scope on a non-faulting piece of equipment in my shop and proved this was possible. It took 5M 68ms to close, and A-B specs say the initial checks are done in ~31ms. IDK why this wouldn't happen every time.

The first solution I implemented was then to wire the sfst START signal through the AUX contact of 5M. This delayed the sfst START signal by ~69ms. I changed wiring on 13 machines in two facilities, and all the faults went away -- at least initially. One facility has since reported two of the same faults again in one month's time. This machine used to fault 3-5 times per day before the fix.

I have attached drawings. Sheet 7 is the Power circuit, Sheet 13 is the Control circuit, Rev A before my fix and Rev B after.

My next step is to change out the contactor and reverser and to set up the scope on the troubled machine. I think I need to go back with more than this. What do you all think?

 

[jraef]

It's a little difficult to decipher your schematic with portions of it missing. But where does the power for A1 of the soft starter originate?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

Do you initiate a start by putting power to the Soft Starter, or do you power up the Soft Starter and then initiate a start?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[QGeek]

On the sfst, A1-A2 is control power and is on all the time (+24VDC). START is initiated by sending IN1 high (24VDC).

 

[QGeek]

wire 21 is +24VDC control power
wire 22 is 24VDC common
wire 31 is essentially 21 after the master control relay
wire 32 is 31 after customer interlock relay.

 

[QGeek]

I appreciate your help. Please let me know what further info I can provide. I have a lot.

Here, for one, is the sfst sequence of operation, attached.

 

[jraef]

One somewhat remote but easy to check possibility:

You are using 24VDC control, but have a number of contactor coils and a lot of control circuit interlocking going on, including several to external limit switches, meaning distance. You may be suffering from voltage drop when that final contactor pulls in. Maybe not much or for vary long, but just enought to push it below the minimum for the SMC-3 power (A1-A2) at that critical moment. You can test it by temporarily bypassing (with the proper precautions) all other interlocks etc, and just powering 5M and the SFST. If the problem goes away, you at least know where to start looking.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[QGeek]

Let me ask you this: what about the possibility of the contactor coil dropping out? To me this would more closely describe the fault: phase loss/open load. Is a contactor coil likely to drop out momentarily?

 

[itsmoked]

They can if a voltage sag affecting their coil is too deep.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Yes, but generally on a "2 wire" control circuit like yours (meaning sustained control signal with no low voltage release), what happens is that a contactor coil drops out, then the resulting load loss lets the voltage recover, the coil tries to pull in again, drops the voltage, drops out the coil, drops the load, voltage recovers, repeat ad nauseum until something fries. We call that "chatter" of the contactor. But the "coil" of the SMC-3 is not a coil, it's a switch mode power supply. Low voltage just makes it turn off.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[squeeky]

It looks like you have a brake in your circuit. Look at break wear. The inertia, may have the motor still turning when you change direction. This will cause high current.

 

[QGeek]

Gents,

I have just returned from the job site. I waited for two days with test equipment set up. Finally a fault happened. My hand happened to be on the joystick at the time. As I Extended the machine, it began to drag for maybe 2 seconds, before stalling and faulting. Same fault: Phase Loss/Open Load. I came back and reread squeeky's comment. The equipment is less than a year old. I was not reversing at the time. My first guess is that the brake was sticking.

What do you all think?

QGeek