Hi folks,

I am a marketing engineer with Larsen & Toubro Ltd., India. We are currently dealing a lot with Soft Starters. I have noticed that in some makes , under/over voltage protection is provided along with earth fault protection. However, many other notable manufactures do not have these features.

In some of our tenders, in the spec sheet for soft starters, under/over voltage as well as earth fault protection is asked.

I would like to know whether such protection is essential for a soft starter. Also what purpose does this protection solve.
Can under/over current prtection serve the same purpose?

Hello MaVericK

Welcome to the forum.

I have several friends from L&T, seems to be a great training ground for engineers who move to NZ!

There are two issues with the under voltage, over voltage and earth fault protection. One is potential problems with the motor, and the other is potential problems with the starter.

Motor.

• Over Voltage will cause the motor to be overfluxed. This will cause a rapid increase in magnetising current, iron loss and a build up of heat in the motor.
• Over voltage will cause a reduction in the work current drawn by the motor.
• Undervoltage will reduce the flux in the motor. This will reduce the magnetising current and the maximum torque capacity of the motor.
• Under voltage will cause the work component of the current drawn by the motor to be increased.
• under voltage will increase the running slip of the motor.

Soft Starter

1. Under voltage will at some point affect the operation of the electronics, typically will cause the firing of the SCRs to be less reliable.
2. Overvoltage will stress power supply components i the soft starter
3. Severe Overvoltage may damage the SCRs in the soft starter.
4. Earth fault could damage the SCRs in the soft starter.

If you ignore the increase in iron losses due to overvoltage, then the major problem is due to excess current and the motor can be adequately protected useing the over current protection. This does leave a small potential for motor overheating due to increased iron loss if the voltage is high enough to increase the iron loss markedly with out exceeding the current rating of the motor. This would apply to motors that are running at light load and where the magnetising current rises to just under rated current. - a very rare situation. This can also happen if the supply frequency is a little low with the voltage at rated levels.
The only issue with low voltage, is that the motor may begin to stall due to lack of torque. This will once again be picked up by over current protection if it is a problem.
Earth fault protection is best achieved by the use of semiconductor fuses.

Conclusion, overvoltage and under voltage protection is not a major problem. It is probably more of an issue with contactor coils than with soft starters.

Best regards,

Thanks a lot Mark!! You seem to be an expert on the topic of Soft starters. Maybe its about the country, even AuCom Electronics is based in New Zealand!
Also, you have rightly pointed out that many L&Tites later leave for New Zealand as I personally know an ex L&T employee now working at AuCom Electronics!!

I will keep posting queries as the soft starter market is growing at a very rapid rate in India.
Just the other day I had an issue with the semiconductor fuses used for soft starter protection.

We had used an IMS20405 model in inside delta connection for a 389A pulper motor. And the customer had used Bussman 500 FMM fuses for the application. Needless to say, in a few days the fuses started blowing one after another. We later realized that the fuses were undersized and promptly replaced them with 630 FMM. Since then we have not faced any further problems.

LOL, Did you know that Marke is the founder of Aucom?

I also used to work for a soft starter manufacturer and we had this issue come up a lot because ours were among those without voltage sensing. I never came across a situation where over voltage protection would have helped and under voltage was indirectly covered in current sensing. That's not to say it isn't valuable, but as a recurring problem to deal with in my part of the world it didn't rate high. The biggest headache was in ground faults (earth leakage), because it tended to do very unusual things depending on the installation that made it difficult to troubleshoot.

The last product design I helped with before leaving added ground fault protection and phase reversal protection using the existing current sensing technology, but over/under voltage was still intentionally left out. The decision from management was supposedly based on the risk of sensing line voltage or the added cost of isolating it. I'm not sure I believed them, but that's what they said when the prototype came out without it. In trying to make my case for investing in redesign to add it I researched those that did include it. the surprising thing I found is that the majority of those that claimed they had it, actually only sensed one leg, usually the A phase. I'm not sure how useful that would be.

Dear MaVericK,


pay attention to the temperature of superfast fuses. If they are well positioned and properly cooled, they should be sized in a better way. It is not necessary to install a fan to cool fuses, but it is necessary to have a good thermal contact by conduction and two heatsinks with enough mass to receive the heat during start-up phase.


Regards
Mario

I think under-voltage and ground fault protection functions are extremely useful.

Marke covered it very well. The only thing I would like to add is that 'predictable' behaviour is important irrespective of what the product may be. As outline by marke, undervoltage may result in missfiring of SCR's or insuffient energy to fire the SCR's at all, or motors stalling and then automatically restarting when the voltage begins to rise towards nominal again etc etc etc. All of these items result in unpredictable behaviour which in my opinion is just not acceptable particularly if associated with high end soft starter products.

I too know of few people from L&T including the one at AuCom and another that actually worked with me in Australia for some time. L&T seems to be a pretty switched on organisation and industry in general appears to be bnefitting from ex L&T engineers.

Incidently, I suspect you will find the pre-arching and total clearing I2t levels of the 630FMM to be beyond that of the SCR's used within an IMS2-0405. The fuses should be connected inside the delta loop along with the soft starter, if however you have them positioned in the line circuit, that would explain the failures.

As an extra tip, if you place the fuses on the load side of the starter, they will still provide the same protection against faults on the output of the starter, however in this configuration your bypass contactor will shunt both the soft starter and the fuses thereby further reducing temperature build up within the enclosure. It will also greatly extend fuse life.

Feel free to U2U me if you require further clarification.

Regards,
GGOSS

Thanks for all the replies.

However, I still have some doubts about the earth fault protection. Can the motor also be successfully protected from an earth fault using a S/C fuse connected at the input side? Or are the S/C used to protect only the thyristors?

Hello MaVericK

The semiconductor fuses provide earth fault protection for the thryistors. If there is an earth fault in the motor, the fuses will prevent damage to the starter, but not the the motor. The earth fault is already in existance!!
One of the effects of the use of semiconductor fuses is, the fuses are energy limiting fuses and if an earth fault develops in the motor, the fuses fail before there is a major blow within the motor. This can make repair much easier as the damage is much reduced, but it can make fault finding much harder as the earth fault can be due to an insulation pin hole which stays as a pin hole rather than becoming a major flash over with melted copper everywhere.
I do not believe that you can use any external device to give you earth fault protection within the motor such that it prevents the damage from occuring.
Sensitive earth leakage dtectors can give early warning of a fault, but the fault is still there.

Best regards,

They should refer to it as ground fault detection.

Regards,
GGOSS