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Smc50 Solid State Soft Starter

06/08/2021 6:57 AM

Good day, we are currently using the above soft starter on a agitator motor which is on a steel tank. On the same tank we have a pH probe, this probe only works when agi motor is off. The earths of the motor , soft starter, tank and tx feeding pH transmitter are in the same potential.

Hopefully someone has seen this issue previously and can offer some guidance.

ps, this is a brand new installation

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#1

Re: Smc50 Solid State Soft Starter

06/08/2021 2:04 PM

Sounds like your probe is picking up electrical noise from the agitator...have you tried any alternative grounding or noise suppression techniques?

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#2
In reply to #1

Re: Smc50 Solid State Soft Starter

06/08/2021 7:35 PM
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#3

Re: Smc50 Solid State Soft Starter

06/08/2021 11:50 PM

It would be helpful to know just how the pH probe is not working, what do you see, and the manufacturer and model of the instrument, including the mount. Also information on the soft-start, such as, what starting mode are you using, what is your bypass mode?

Mounting a pH probe in a steel pipe with a fast moving liquid in the pipe is done on a regular basis, so it probably has something to do with your probe signal routing and raceway. The lead between the probe and the transmitter is very susceptible to RFI and EMI, so a steel conduit is often used.

The soft-start, which may not really be needed, could generate significant EMI in certain run modes. This noise is tolerable for most instruments, given suitable distance and/or shielding. You could try starting the motor across the line, and don’t use the soft-start. Or configure the soft start for Full Voltage start. If for some reason your liquid movement is creating a static charge or the mixer impeller is cutting a magnetic field somehow, you could try uncoupling the mixer and just run the motor.

Even though the earths of the equipment are theoretically at the same potential, it is possible that the actual routing of the bonding conductors could introduce noise, especially grounding having to do with the motor. These should be kept separate from the instrument ground until they come together at the actual earthing point.

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#4
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Re: Smc50 Solid State Soft Starter

06/09/2021 12:21 AM

The signal from sensor to transmitter is RS485. These soft start does not have a by-pass, it stays on line even at 50Hz as per manufacturer.

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#5
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Re: Smc50 Solid State Soft Starter

06/09/2021 1:53 AM
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#6
In reply to #4

Re: Smc50 Solid State Soft Starter

06/09/2021 7:23 AM

That doesn’t sound correct. It is probably self contained sensor transmitter with RS 485 output. The millivolt output of the probe is safe. The configuration of the soft start & model are also important things to help diagnosis. All things mentioned before still apply.

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#7

Re: Smc50 Solid State Soft Starter

06/09/2021 5:54 PM

Once the soft starter is finished ramping, it is across-the-line (DOL), so would not be contributing any significant electrical noise.

Most likely you have not properly routed your communication conductors away from the power cables and/or you have not properly shielded (screened) it or connected the shield to earth at only ONE end.

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#9
In reply to #7

Re: Smc50 Solid State Soft Starter

06/10/2021 7:00 PM

This type of soft starter remains in use once 50Hz is reached. Unlike other installation were a by-pass contactor that takes over once full speed is reached.

This has been installed as per manufacturers instruction's.

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#14
In reply to #9

Re: Smc50 Solid State Soft Starter

07/08/2021 12:08 PM

Nonetheless, once the ramping is done, the thyristors are hatted fully on and they are basically just conductors

Unless…

The SMC-50, without the integral bypass contactor, still offers the “Energy Saver” feature (Nola Circuit). If you have that enabled, that does retard the firing angle of the thyristors for as long as the motor is perceived to be less than fully loaded. I won’t go into the merits or theory on the Nola Circuit, but in my opinion, it’s detractors outweigh any possible benefits (if any at all). If that is enabled in programming, disable it and see if the issue persists.

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#8

Re: Smc50 Solid State Soft Starter

06/09/2021 6:24 PM

Did you include terminating resistors?

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#10

Re: Smc50 Solid State Soft Starter

06/11/2021 2:17 AM

In what manner does the probe malfunction?Over range, noisy,erratic output?More information will more helpful to diagnose this problem.

Ph probes have a very high input impedance,on the order of Gigaohms,so any stray voltage differential can affect the signal.

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#11

Re: Smc50 Solid State Soft Starter

06/14/2021 8:44 AM

It would be well worthwhile running a separate ground/earth connection from the <...pH...> equipment and isolating it from the <...agitator motor...earth...>; a separate "instument earth/ground" is the best way to prevent rough ol' motors and their controllers disturbing instrument signals, particularly high impedance ones like the glass electrodes found in <...pH...> equipment.

Next is cable segregation.

Next is crossing signal cables with power cables at right angles.

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#12

Re: Smc50 Solid State Soft Starter

06/16/2021 10:19 AM

RS 485 is pretty immune to EMI unless you have done something really odd like earthing probe supply line both at controller side and probe. It should be grounded star-like only at controller side or be tottaly floating with some kind of isolation device according to original design to avoid ground loops. Check installation. S.M.

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#13

Re: Smc50 Solid State Soft Starter

07/03/2021 9:57 PM

Friend,

Some other ideas from my past experience:

--Use a shielded cable between the soft starter and the motor, and ground/bond the shield only at the soft starter.

--Use a ferro-resonant type control transformer for the power supply for the equipment connected to the pH transducer. This will remove almost all power supply noise to it and keep the voltages constant regardless of temporary or "long-term" peaks and sags. These transformers are somewhat expensive but very good.

--Use a shielded cable for your connection between the pH probe and the controller, and again, terminate the shield only at one end (as others have mentioned).

Thanks--JMM

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