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SCR soft-start switch issue

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SCR soft-start switch issue

Posted by Mike Northrop on Jan 10, 2019 9:02 am

The issue I have is with an electronic (SCR) soft-start 240v switch rated at 6kw running 5kw IR heating lamps (restive load).  With the soft-starter in circuit, activation starts the IR heater tubes ok and they run for 20 mins, but on associated circuits connected to the consumer unit other lamps dim and a sound system goes off momentarily which is a real issue if I have a live music performance running.  With the soft-starter removed and the lamps activated from a normal switch there are no adverse effects on other installations , no lamp flicker. and no sound system dropouts.  
I have not come across a problem like this before.  I am wondering if there could be a phase related issue but this does not answer why the sound system is affected.
Any comments would be appreciated.

 

Re: SCR soft-start switch issue

Posted by Alasdair Anderson on Jan 10, 2019 10:24 am

I can't understand why you would want to use a soft-start switch with a purely resistive load. There do not seem to be any advantages to doing so as they are intended for loads with high starting currents such as motors to prevent the power source/distribution system from being overloaded by the starting current, and it just seems to be an added cost. Without more detail it is difficult to say just what the problem is (for instance you don't even say if it is single phase or three phase) but my guess at present would be harmonic distortion of the power supply as I assume the IR heating lamps at 5kW are a significant proportion of the total load.
I would suggest just use the normal switch.
Alasdair

Re: SCR soft-start switch issue

Posted by Mike Northrop on Jan 10, 2019 9:03 pm

Thanks or the reply.  I am using the soft start switch to extend the life of the Quartz IR heater tubes as they are mighty expensive and they get switched many times a day.  Without this the filaments get more stressed by the high cold start current.  The supply is single phase.  I have run some more tests today and it seems that only electronic equipment is effected, e.g. LED lamps, building emergency lighting and our audio equipment.  Normal filament lamps don't show any flicker effect at all.  Other parts of the same building on different supplies are also showing some flicker effects!

I suspect you are right that the problems I am seeing are a harmonics from the Triacs.  

My question now is could the circuit design be modified to prevent this effect with maybe an inductive choke or similar.?  

Re: SCR soft-start switch issue

Posted by Alasdair Anderson on Jan 11, 2019 9:41 am

Mike,
Reading through your first paragraph my immediate thought was that an autotransformer would be a better solution than an SCR Soft Start, so that would be a definite yes for your final question, in my opinion. The big problem is that SCR Soft Starters are so prevalent nowadays that everyone uses them even when an alternative arrangement would give a much better solution, probably because the older alternatives are now much less familiar to people.
Alasdair

Re: SCR soft-start switch issue

Posted by Mike Northrop on Jan 11, 2019 11:09 am

Alasdair
Thanks for your feedback.  I agree.  However I am stuck for the moment with the SCR solution.  I think I will go back to the supplier with a view that their product should not be generating mains-borne noise as it is, that effects other mains connected products, as it does and see what they have to say.  I am uncertain if there are any UK regs governing this, as there are for EMC but will have a search around to see.  

Re: SCR soft-start switch issue

Posted by Sparkingchip on Jan 11, 2019 11:29 pm

Where are the heaters, are they up the garden or somewhere similar sharing a long distribution circuit with the equipment you are having issues with?

http://www.consortepl.com/wp-content/uploads/36000409-Iss-01-QSC1-6kW-Feb-15.pdf

Andy

Re: SCR soft-start switch issue

Posted by Mike Northrop on Jan 13, 2019 1:10 pm

Thanks for the reply.   We are a live music venue regularly running performances in Chester for 200 people or so, with indoor and outdoor entertainment areas  The outdoor heaters are within 4 metres and connected to a single phase sub-main board.  This board also runs aircon and amin stage ring main and lighting.  This sub-main is fed from the main building 3 phase main which also feeds numerous other circuits and sub-mains around the place.  


The interference is noticeable on equipment / lighting connected to the same sub-main as the heaters affecting primarily our sound system and he LED stage lights.  The interference is also noticeable on equipment connected to other sub-mains e.g. bar, kitchen, but only really the LED lights and emergency exit lights, which flicker off / on when the soft-start switch is activated.

So far I cannot detect any damage to electronics and maybe that is to be expected.  However, we cannot have the sound system dropping out mid performance, even for a fraction of second.  Closer examination of our sound system is ongoing at the moment to track down exactly what is dropping out, e.g. digi-mixer, pre-amps, main amps, microphones or something else.
 

Re: SCR soft-start switch issue

Posted by Paul Skyrme on Jan 14, 2019 11:08 am

Three things jump out at me with this.
1. Is the fixed installation compliant in full with BS 7671, are the volt drops within design requirements for the loads etc.  Noting that neutral connections and wiring are as, if not more important than phase conductors and connections when you have the chance of triplen harmonics in the system.
2. Are the temporary parts of the installation in full compliance with  BS 7909?
3. If this control unit is bought in, is it used within its specifications, and is it CE marked in accordance with the LVD & the EMCD?

If these three things are in place, then you should not have the problems you are seeing.
Paul Skyrme MEng, BSc(Hons), IEng, MIET, MSOE, MBES, TechIOSH, CMSE®. #e5 Founder Member

Re: SCR soft-start switch issue

Posted by Mike Northrop on Jan 15, 2019 12:08 pm

  Thanks for the reply.  
The installation has just undergone it's 5 year compliance check and it's all signed off for commercial use.   Voltage drops were suspected at first but the test I ran without the soft-starter showed no effects on any other connected appliance.  The connections / cables to the dis board and then to the main board have all been checked by our electrician in the recent sign off.  

Any temporary supplies are put in place by our stage / lighting / sound tech sub-contractor for external events.  At this time of year we don't run external events so there are no temporary supplies in place.  It is all fixed wiring complying with BS7671.


I checked the soft-starter and it is CE marked on the circuit board.  One would then assume that the LV and EMC Directives are applied appropriately by the design & test authority.   The unit was supplied via a reputable UK company and is of Chinese origin.  I don't know any-more about it than that at the moment.

I ran some tests with our sound technician last night.  On this occasion the sound system did not drop out at all.  It may be that the times this happens is only when specific sound components are connected and these may not have been last night.  I will continue to monitor.  

Some of the LED lamps around the building still flicker.  At least one of the emergency lights visibly tripped for a second or so with the power on LED going off when the soft-starter was activated.  I am now thinking that all I can do is get the supplier involved and send the unit back to them but feel at the moment that I don't have enough to go on.
 

Re: SCR soft-start switch issue

Posted by Mapj Test on Jan 16, 2019 12:11 am

The clue is the other lamps dimming.
While the lamps are cold, their resistance is much lower than when hot (check with a meter, and compare with the result of a P=I.V and R=V/I calculation)
For a filament lamp the resistance is roughly 1/10th so the inrush is 10times the hot current. These are heaters, not lamps,  so we can expect slightly less dramatic effects, but I suggest you measure it, it could be 5 to 1.
- the effect of the prolonged start-up is that the load stays cool, and the current stays high, so the load presented to the supply takes the form of thin slices taken out of each cycle of far higher current than the design is rated for - hence it pulling down on the supply, and presumably causing the audio to go into low volt drop out, EM lights to register loss of mains etc.
There is  probably nothing wrong with the soft starter per-se, but the supply is presumably not up to supporting surges of several times the running load.

Can you verify the cold resistance of a heater please.
regards MAPJ2 that is MAPJ1 under his other alias

Re: SCR soft-start switch issue

Posted by Sparkingchip on Jan 16, 2019 8:06 am

Pre-warming of theatre lighting comes to mind.

Andy 

Re: SCR soft-start switch issue

Posted by Paul Skyrme on Jan 16, 2019 5:10 pm

Mike Northrop:
  Thanks for the reply.  
The installation has just undergone it's 5 year compliance check and it's all signed off for commercial use.   Voltage drops were suspected at first but the test I ran without the soft-starter showed no effects on any other connected appliance.  The connections / cables to the dis board and then to the main board have all been checked by our electrician in the recent sign off.  
Forgive me if I am skeptical about this, I suspect that it is due to deficiencies in the installation for the loads you have is what is causing this.  Whilst the install may be OK for "normal" stuff, you have something unusual, that is in regular use, and is a known load, that may need special consideration.
I am also very skeptical about the quality of I&T done "under" BS 7671 these days.


Any temporary supplies are put in place by our stage / lighting / sound tech sub-contractor for external events.  At this time of year we don't run external events so there are no temporary supplies in place.  It is all fixed wiring complying with BS7671.
OK, but, don't forget that the temps are still those that are moved around and plugged in for your small internal events too.

I checked the soft-starter and it is CE marked on the circuit board.  One would then assume that the LV and EMC Directives are applied appropriately by the design & test authority.   The unit was supplied via a reputable UK company and is of Chinese origin.  I don't know any-more about it than that at the moment.
The soft starter must be CE marked on the assembly to confirm that it is compliant you should also have adequate instructions etc. to ensure its safe and correct use, and under PUWER Reg 10, you have a legal duty to ensure that this is so.

I ran some tests with our sound technician last night.  On this occasion the sound system did not drop out at all.  It may be that the times this happens is only when specific sound components are connected and these may not have been last night.  I will continue to monitor.  
That is a possibility.
Some of the LED lamps around the building still flicker.  At least one of the emergency lights visibly tripped for a second or so with the power on LED going off when the soft-starter was activated.  I am now thinking that all I can do is get the supplier involved and send the unit back to them but feel at the moment that I don't have enough to go on.
 

Have you put a power analyser on the circuits looked for any harmonics, or any noise, a scope on the input circuit would also show some issues.
If the unit is correctly CE marked then it must not emit any adverse radiated or conducted interference above the levels for the environment it is designed to operate in, which must all be documented in the instructions for the unit.
The lighting and sound equipment must also be sufficiently immune to the allowable levels of conducted and radiated emissions specified in the standards for the relevant environment, which are different for domestic & commercial environments.
 
Paul Skyrme MEng, BSc(Hons), IEng, MIET, MSOE, MBES, TechIOSH, CMSE®. #e5 Founder Member

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