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Help Needed! *soft Starter Failure Issues*


Clayton

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I maintain about 100 waste water pump stations. They all have soft starters, controlling submersible pumps. There are two starters in each station.

 

The soft starters are Cutler Hammer 701 or 751/751 series starters, motor currents are typically 3A-20A for most stations. The starter would start the motor maybe 6 times an hour.

 

The problem is that the starters are failing, sometimes within a year of installation. Its happening over a wide geographical area and Im scratching my head over the problem.

 

I am going to put a temperature data logger in a few stations, measuring switchboard air temp, and temperature of the heatsink on each starter to see if that will bring to light an over temperature problem.

 

The only other cause of faiure I can think of is corrosive gas attack from the ducting to the wet well, but that seems a long shot to me.

 

Is there anyone out there that can provide an idea of where I can start looking for problems? -or Anyone involved with waste water pump stations with similar problems? Are the CH brand of starters good and robust?

 

Any help would be greatly appreciated. Thanks and regards, Clayton.

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Hello Clayton

 

When the starters fail, is it the SCRs that fail, or is it the electronics that fails?

 

If the SCRs fail, it is typically a supply related issue.

If the electronics fail, it could be a supply problem, but it is more likely to be environmental, heat, corrosion etc.

 

If you can provide more detail, We may be able to help more.

 

Best regards,

Mark.

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Hi Mark,

 

I don't believe that it is the scr's that are failing.

 

The 701 series has an external bypass contactor, and a typical faulty unit will intermittantly ramp up 1-5 times before kicking in the bypass contactor and running normally.

 

This ramping up fault tends to happen more and more often, and eventually trips the motor overload. The motor overload trip will occour weekly for say two weeks, then quickly progress to daily

 

The 751/752 series has an internal bypass contactor, and typically gives a fault code indicating bypass dropout. (the same fault code would be given with a problem with the external 24v dc supply).

 

The more I think about it, temperature could be the issue.

The starters are housed inside a switchboard cabinet, say 0.4m deep, 0.9m wide and 1.2m heigh. There is no ventillation in this cabinet. The cabinet is located inside a poorly ventillated stainless steel outdoor plynth, which contains this switchboard cabinet, metering, scada cabinet etc.

 

I am intending to install temperature logging equipment soon to try and determine if this is the issue.

 

Thanks, Clayton.

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Hello Clayton

 

I do not know these starters, but it sounds like they are an analog design and the integration circuit is getting leaky.

This can be due to contamination, or to temperature.

 

In this country, Radio Spares have small sticky temperature labels that you can stick on to things to see what the maximum temperature that has been reached.

Have you opened up the starters and had a look at the circuit boards?

I would look for signs of corrosion.

 

Best regards,

Mark.

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  • 1 month later...

Hi Mark,

 

I have had a four channel temperature logger for a month, and have moved it around a few sites.

 

The switchboard air temperatures are getting up to 55 degrees and the days were not as hot as they can get. Im starting to think it could be a temperature issue.

 

The specifications give derating tables up to 60 degrees and I think we would be exceeding this in some situations.

 

Our switchboards have anti condensation heaters which could be adding to the problem too. What is the best method of forced ventillation for the switchboards?

 

Thanks Clayton.

 

 

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  • 3 weeks later...

Hello Clayton

 

I wonder how you are getting on with this problem. Derating is usually applied to the current rating to prevent the junction temperature of the SCRs from being too high causing premature SCR failure.

 

If the temperature of the electronics gets too high, then some components can begin to behave incorrectly. Electrolytic capacitors can dry out and fail early and go leaky affecting the ramp up time of analog timers.

High temperatures will affect the gain of transistors and the sensitivity of optoisolators and can cause voltage regulators to limit the current flow.

These results are all temperature related and not related to the current flow of the starter.

 

It sounds to me that you may have an issue with over temperature that is related to the temperature of the electronics rather than the SCRs.

 

To reduce the temperature, add a fan to draw the hot air out of the enclosure. Ventilation slots or louvres on their own have very little impact on the temperature rise.

 

Best regards,

Mark.

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