Toroidal inrush

[McMurdo]

I've got an interesting problem here trying to fix an audio amp, its a modern one.

It uses a toroidal transformer, the inrush is mitigated by a 10ohm NTC thermistor in series with the mains which is bypassed by a 30A relay when the amp has successfully powered up.

Initially the mains switch was welded. I fitted a new one, and powered the amp up on the variac. No problem. When I plugged it striaght into the mains (via an IEC lead) the fuse in the plug blew. Ok, it was 5A, so I replaced it with 13A, permitted on a standard IEC lead. This time, the 32A mcb in my board tripped.

Once again, trying it on the variac, it powered-up fine and it works great.

So, I replaced the NTC (sometimes these can carbonise and arc over) and checked the relay. Tried again, and, again the 32A breaker in the board went.

I noticed that the breaker only trips when the bypass relay comes in, not during the initial switch on surge. Looking at the current on an analogue meter, there are 2 surges: a small one thanks to the NTC, then a huge one (followed by the breaker tripping) when the bypass relay operates.

Any ideas? Strange toroidal magnetising problem?

[60 oldjohn]

Hi Kevin, are the insulated mountings in good order? Failing this ask Ed Dinning.

John.

[60 oldjohn]

Just another thought, are you using the original fixing bolt?

I hope TX has not got one or shorted turns.

John.

[Glowing Bits!]

An audio amp should not blow a 5A fuse, let alone 32A.
What amp is it? How many watts (output) is it rated for?
I'd expect a 500 watt amp to just about operate on a 5A fuse without any problems.
Toroids don't tend to whack the mains too badly on switch-on, it sounds like either the rectifier is short or the toroid has a shorted primary, do check to see if either live or neutral are going to earth.
If the rectifier is short, it will pull a hell of a load through the toroid primary.
These pointers may help, a Megger insulation tester will defo help, for the tranny side at least.

[McMurdo]

yes all ok, the transformer has a moulded nylon centre boss with a short bolt top and bottom, and the fault is present before the top bolt is in. I've had the toroidal out to check silly things like rubbing of the windings, all ok.
If I leave the amp connected to the variac it doesn't take the 32A breaker out but just blows the plug fuse, I suspect the variac is acting as a line reactor.

rick - If I wind the amp up on the variac it works fine, it's only when plugged directly into the wall that the breaker trips.

It's a crown xls3500 BTW, rated 2.7kW

[Boulevardier]

Is the mcb that's tripping an overcurrent type, or an ELCB?

Mike

[McMurdo]

its a 32A mcb in the main board. I've never had it trip before.

[ColinB]

Is the relay switching both the live and the neutral? Any chance of a short between the two? Some sort of nasty 'make-before-break' problem with the switching in the relay?

[ColinB]

Interesting thread, as I'm having the kitchen refitted at the moment, and the builders site transformer takes out the 16A breaker feeding the lounge (and the PC!)... An extension to upstairs solved that one

[Trigon.]

Is the bypass coming in earlier than it should? (It might need perhaps a 4 or 5 second delay?)

Cheers

[Radio Wrangler]

Has the thermistor gone a bit high resistance? some do wear out. It may be not charging the capacitor bank far enough by the time the relay comes in. This would fit the small surge followed by big surge observation. The aim is to split the total surge evenly.

DAvid

[Boulevardier]

Or, as Trigon suggests, the relay may be switching in too early. How is the delay in doing this effected.

Would be useful to know the power of this amp and rating of xformer. Is it huge? Is it likely that any normal inrush (even without inrush mitigation) would trip a 32A breaker - unless it was a massive amp?

Mike

[eddie_ce]

Usually in domestic situations it will be a Type B 32A MCB. The magnetic part of these generally trip with a load of 5x the rated currrent, in this case 160A. I would be extremely surprised if any "normal" amp came close to an inrush of this magnitude, especially if presented with an NTC in series with the incoming mains supply..

I am following this thread with interest.

[Glowing Bits!]

Given how powerful this amp is, a 5A plug fuse will blow.
Have the transistors been tested in any way?
Something ain't right with this, although a surge on power-up without the relay would certainly stress a standard wall socket, it seems odd how it would take out 32A.
The best course of action would be to check if anything is going to earth.
Are there any anti-surge filter components after (or before) the tranny?

[mhennessy]

Being a PA amp, this will be a chunky transformer, so it's not surprising it requires the soft-start. I'm in the habit of powering up the PA amps in the rack sequentially, as these beasts really are very different to domestic audio setups. The racks often have 32A feeds.

If you're worried about the transformer having shorted turns, see what the current draw is with all the secondary windings disconnected. It should be comfortably less than 100mA - IIRC, the 400VA transformer in one of my old designs takes about 10mA. Of course, this shoots up when you hit saturation, and it's worth checking where this happens - that could be part of the problems if your mains is a bit on the high side...

The soft start on that design is a chunky 22 ohm power resistor, and the fuse is a standard 20mm type rated at 4AT that has never failed. Without digging out the source code, I can't remember what the soft-start delay was, but it's pretty short - well less than half a second. The inrush caused by the transformer was big, but surprisingly short. I determined this by monitoring the current draw at startup using a current transformer connected to a DSO - that's something I definitely recommended for these sorts of challenges

It's interesting that the breaker on the rear panel isn't tripping first. MCBs are a combination of thermal and magnetic, and can be quite quick to open - the type letter indicates the speed. It's obviously faster than the one in the amp!

[kalee20]

Toroidal transformers can have huge inrush currents. Any transformer is most likely to saturate if you switch it on at the zero-voltage crossing. It's kindest if you switch on at a voltage peak. Of course, switchi8ng on at a voltage peak is the worst when you come to consider a rectifier/capacitor secondary load. Sometimes, you just can't win...)

One reason why toroidals are particularly extreme is that the core is very good at retaining magnetism from its previous energisation, whether seconds or days ago (think magnetic core memories). So, switching off at the end of a positive half-cycle, followed some time later by switching on at the start of another positive half-cycle, is almost certain to saturate the core. Then, the only thing limiting current, is the resistance of the primary winding.

Sod's law, of course, says that if there is a chance you will flick your switch at just the wrong instant, then you nearly always will.

However, adding a NTC thermistor, or even a simple resistor, of appropriate value, should tame this. A bypass relay needs to be just that: a bypass for the limiter, not a changeover relay (ColinB alluded to this), and moreover needs to be after a 'reasonable' delay, so that both secondary reservoir capacitors are charged, and that the transformer has had time to settle down - as Tigon and Radio Wrangler have already pointed out.

[Craig Sawyers]

Looking at the power draw and thermal for this beast from https://www.crownaudio.com/en/products/xli-3500

The maximum current draw from the mains when delivering 660+660W is 22.5A

[Welsh Anorak]

Nowhere near the same league, of course, but I've had several hi-fi amplifiers with toroidal transformers blowing their mains fuses randomly. In these disconnecting the secondary still revealed a big inrush, and in each case only transformer replacement provided a complete cure. The originals still seemed OK though!
Big amplifiers like yours do dim the lights when you turn them on though...

[McMurdo]

I suspect the transformer TBH, maybe some effect in the core that goes above my head. I did have it once in a halogen light that always worked on the variac but always blew the fuse when directly switched. As I've said before, once this is powered up, via the variac, or by leaving the variac set to 240 and using the front panel switch, it powers up ok and does not take any current.
The NTC is fed off the mains switch L and onto the transformer L. The relay common is also fed with this switched live and simply puts it the other side of the NTC when pulled in. There is a capacitive dropper-resistor-diode-optocoupler hung on the NC leg of the relay which operates the audio muting line of the amp, so no power can be drawn unless the soft start has completed.
The actual soft start time is set by the amplifier management system on the main control board and is around 1 second. The NTC has already been replaced as a precaution, it is approx 20ohms when cold. As can probably be seen from my pic, I pulled the relay and broke it open to look inside, but it appears blameless, and the contacts are pristine.

In my experience some of these NTCs are rated 2 ohm, 10 ohm, or up to 47ohm, the lower value ones do not always have a bypass resistor but the bigger ones do because their continuous current rating goes down as the cold value goes up.

As toyah willcox once said, 'its a quandary'

[mhennessy]

As i say, check the "magnetising current" - what the transformer draws from the mains with nothing connected to the output side...