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Manfred2
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Join Date: Oct 2010
Posts: 76

Soft Start Relay

01/01/2012 1:31 PM

Hi,

I have just destroyed a soft start relay (MCST2450AS, soft start at 50 amps). It was used to limit the inrush current into a 240 volt 45 amp variac. I was thinking of replacing it with a more robust relay (MCSS2490CM, soft start/soft stop at 90 amps). I found the part number in the Crydom catalogue, however, when I type this number into the parts search it does not recognise it. Also, I am in the UK, so, does anyone know the answers to the following questions:

1. Is the Crydom soft start relay the best way to limit inrush current in a variable transformer?

2. Where can I purchase the MCSS2490CM relay (or its equivalent if it is now obsolete) in the UK?

3. Is a phase angle control relay similar to a soft start relay? If so what are the similarities, benefits if any and would this be any better at controlling inrush current?

Thanks.
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SolarEagle
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#1

Re: Soft Start Relay

01/01/2012 2:40 PM

Find new number...
http://www.crydom.com/en/Products/Catalog/AdvancedWebPage.aspx?CategoryText1=Control%20Relays&CategoryText2=MCS%20Series%20Microprocessor%20based%20soft-start%20/%20soft-stop%20controller%20-%20Ratings%20from%2025A%20to%2090A%20@%2048-530%20VAC&CategoryText3=NA&CategoryText4=NA&CategoryText5=NA&GridPage=2&filter2=

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Manfred2
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Join Date: Oct 2010
Posts: 76
#12
In reply to #1

Re: Soft Start Relay

01/02/2012 9:41 AM

Thanks for the update Solareagle.
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tcmtech
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Location: South of Minot North Dakota
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#2

Re: Soft Start Relay

01/01/2012 3:47 PM

Why do you need a soft start system on a variac?

Typically they are intended for motor loads.
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PWSlack
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United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

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#3
In reply to #2

Re: Soft Start Relay

01/01/2012 5:12 PM

Exactly.

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Manfred2
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Join Date: Oct 2010
Posts: 76
#9
In reply to #2

Re: Soft Start Relay

01/02/2012 9:15 AM

When the variac is first switched on it draws in excess of 40 amps.
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Manfred2
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#11
In reply to #2

Re: Soft Start Relay

01/02/2012 9:27 AM

They are used for motors, however, they are also used for transformers.
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PWSlack
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#4

Re: Soft Start Relay

01/01/2012 5:13 PM

Did it close onto a short circuit with inadequate protection? What is the overload trip set at? What are the fuses set at? What is the VA of the load?

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Manfred2
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#10
In reply to #4

Re: Soft Start Relay

01/02/2012 9:19 AM

If there is no load attached the variac still draws in excess of 40 amps for a fraction of a second (inrush current) and the current input meter displays this. The trips are 30 amp D type MCBs.
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tcmtech
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#13
In reply to #10

Re: Soft Start Relay

01/02/2012 11:24 AM

So why are you running a 45 amp rated variac on a 30 amp breaker system?

To me thats just asking for inrush current issues.
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Tornado
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#5

Re: Soft Start Relay

01/01/2012 11:31 PM

If you are trying to reduce magnetizing (inrush) current to the variac primary, a zero-crossing relay might also work. I'm not sure whether you need either of zero-crossing or soft-start, though. Maybe just a better selection of circuit breaker or fuses on the variac primary?

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Rockyscience
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#14
In reply to #5

Re: Soft Start Relay

01/03/2012 11:57 AM

You are correct...Zero crossing SSR is what is needed.

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Manfred2
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#15
In reply to #14

Re: Soft Start Relay

01/03/2012 1:20 PM

I thought that it should be switched in at the peak of the voltage and not zero crossing. If the mains is applied when at its peak value, inrush current is at its lowest. Conversely, if the mains is applied at the zero crossing point, inrush current will be maximum. So how does a Zero crossing SSR achieve this? If i am obviously missing something, then please explain as I am not an electronics guru. Thanks,
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garth
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#6

Re: Soft Start Relay

01/01/2012 11:33 PM

Yes why do you need a soft start relay? Are you tripping a sensitive circuit breaker due to magnetising current?

Why not fit a series surge resistor in the line then by pass it for normal operation.

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brich
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#7

Re: Soft Start Relay

01/01/2012 11:44 PM

Variacs...

http://sound.westhost.com/articles/variac.htm#s1

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tom
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#8

Re: Soft Start Relay

01/02/2012 4:01 AM

You could use one of my solid state relays. They seem to be imune from surge failure and will operate up to 80 amps at 240v. On test I had one switching power to a large pulse transformer for a Sonar transmitter continuously for over 24 hours. Last year I supplied 3phase versions to BAe and they found that the 440V, 15A, SSR was happily taking 12x it's rated current for 500mS while starting up a big fan motor. They also found that the SSR produced 30db less noise than the equivalent Crydom SSR so you don't need line EMC filters. (The SSRs are designed to operate well within the EU and FCI EMC limits).

After 9 years of development, on my own, I now have a patent and am about to go into full production.
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Users who posted comments:

brich (1); garth (1); Manfred2 (5); PWSlack (2); Rockyscience (1); SolarEagle (1); tcmtech (2); tom (1); Tornado (1)

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