3 phase current rating - simple question!!

[Speedy]

Simple question ;

I have a 3 phase Solid State Contactor here on my desk.

It is rated 15A Load Current. Is this 15A per phase, i.e. 45A total OR 15A total.

I have asked a few people who should know but....

Cheers
Speedy

 

[Skogsgurra]

It is 15 A/phase - no doubt about that. And if you parallel the three contacts you can put 45 A through it. Assuming that the contacts share current perfectly.

You should be careful with AC/DC, though. A contactor like this can switch AC up to the rated current, but if you plan to use it for DC, you should find out what the rating is for your voltage and your load (inductive loads like magnets and such are very difficult and need special contactors).

 

[jdkasper]

Just an added note. If your connecting a three phase load, your total maximum current is 15A.

 

[busbar]

Two potential problems—paralleling mechanical or solid-state relay contacts by external jumpers may lead to unequal current sharing, depending on relay. Second, attempting to switch DC with an SCR-based relay may cause it to latch on until external power is removed [which can be seriously hazardous.]

 

[Skogsgurra]

busar,

Sorry, didn't notice the "solid state" part. Should not have mentioned DC at all.

 

[Speedy]

Basically we have these contactors switching a bank of cartridge heaters. The heaters are balanced across the 3 phases with a maximum of 3Kw per phase. With a 15A rating these should be fine.
Our problem is the contactors are failing, they leak across some of the phases. Some leak all 3 phases while others only 1. If it was a case of 15A total, 5 per phase then underating could explain what happened. If they were sized ok, then why are they blowing.

BTW I am not working directly with this problem, I'm leaving that to the company that built the m/c. I'm just trying to figure out myself what is going wrong.

Cheers,
Speedy

 

[sreid]

Resistance Heating elements have a much lower cold resistance (say 3x? lower)than when they are hot. Perhaps you are pulling 45 amps during warmup.

 

[dbaird]

Distributed line capacitance presents particular problems for contactors and their contacts. This occurs when a contactor is located a considerable distance from the load to be switched. Is your control long distance from your heater? Just a thought.....

David Baird
mrbaird@hotmail.com

Sr Controls Engineer

EET degree.

Journeyman Electrician.

 

[nbucska]

SCR based switches have considerable leakage -- so what
you consider failure may be normal. Check the specs.

The switch must be able to switch the cold starting current.
(see sreid) Measure the cold resistances and calculate it.



<nbucska@pcperipherals.com>

 

[jraef]

Speedy,
It should be rated 15A per phase, but of course anyone can be a bonehead so if you post the manufacturers data I can check it out. As nbucska said it is quite normal to see some leakage through SCRs, but what do you mean by leakage, and how are you measuring it? If for instance you are using a Fluke meter you may see line voltage on the output, but if you use an old jeweled analog meter it will read nothing. That is because the leakage is microamps and the meter burden on the analog meter will draw it down to nothing.

The 10 most common reasons for failure in SS contactors are:
1) Heat
2-7) More Heat
8) Voltage spikes
9) Component failure
10) Operating conditions (aka Heat)
How are they being controlled and is there adequate cooling in the enclosure?

&quot;Venditori de oleum-vipera non vigere excordis populi&quot;

 

[nbucska]

jraef:
I just looked up a 25A SCR -- it has around 1mA at 25 C
but 10mA at 125C --- so not microA-s.


<nbucska@pcperipherals.com>

 

[Speedy]

Folks,

Thanks for all your help. I looks like the SSRs were undersized. The manufacturers of the SSRs have verified this.
We had 2 sizes 15A and 25A. The 15A, while ok for 150A Inrush Current Resistance, can only handle 5.1 KW (25A- 220A but only 8.6 KW max).
Our max loading was 3Kw per phase, this may have edged slightly higher so hence the problem.

This 3KW was also blowing some of the 25A ones but to a lesser extinct.

Thanks again!!
In some cases, we only had one phase loaded but still blew the contactor across all 3 phases.

(Voltage was 230 with a pulsing load (1 sec on/off) as temp was approached

 

[RobWard]

Hi.
We had a problem with some SSRs on some brand new process ovens we had built.

Being a humble sparks, I couldn't figure out what was wrong and had to ask my uncle, the family electronics boffin, to help. The control voltage was 230v ac and was in phase with one of the phases on the load side and the SSR just didn't like it one bit,and after about 30 seconds or so the SSR got hot and called it a day.

So he stuck a capacitor in to move the control frequency out of phase and hey-presto everything was tickety-boo!

Then I had the pleasure of phoning the oven manufacturer and asking precisely how the ovens had passed the soak tests that we were assured had been carried out, when in fact the ovens couldn't have ever worked...

&quot;I love deadlines. I love the whooshing noise they make as they go past.&quot; Douglas Adams