big iron toasted?

[Walter Harley]

I need some real-world perspective, please.

There's a big power transformer in a power amp I'm working on. The amp is
supposed to produce 1000W RMS (though we all know how amp specs are...).
The fuse at the input is 10A for 120V mains, and the label says 1100VA. So
the transformer is somewhere around 1kVA.

Now, when I hook this transformer up to 120V, with no load connected, it
hums loudly and it draws 1.1A from the line.

Interestingly, the current (remember, no load connected) is only about 0.5A
at 105V, and there is almost no hum. The hum and the current go up very
steeply as I increase the voltage to 120V, using a variac.

The secondary voltages measure what I'd expect: 138VAC rms across the outer
taps, 31VAC rms across the inner taps. DC resistance, subtracting the
resistance of shorted probes, is 0R37 across the outer taps, 0R58 across the
inner taps, and 0R38 across the primaries (in 120V mode, which has the two
primary windings in parallel).

Does this seem right? Has this transformer gotten toasted somehow, or is
this normal? I've never seen that much no-load current draw or hum, but I
don't usually work with amps this powerful, either.

Thanks!

 

[Sam Goldwasser]

I need some real-world perspective, please.

There's a big power transformer in a power amp I'm working on. The amp is
supposed to produce 1000W RMS (though we all know how amp specs are...).
The fuse at the input is 10A for 120V mains, and the label says 1100VA. So
the transformer is somewhere around 1kVA.

Now, when I hook this transformer up to 120V, with no load connected, it
hums loudly and it draws 1.1A from the line.

Interestingly, the current (remember, no load connected) is only about 0.5A
at 105V, and there is almost no hum. The hum and the current go up very
steeply as I increase the voltage to 120V, using a variac.

The secondary voltages measure what I'd expect: 138VAC rms across the outer
taps, 31VAC rms across the inner taps. DC resistance, subtracting the
resistance of shorted probes, is 0R37 across the outer taps, 0R58 across the
inner taps, and 0R38 across the primaries (in 120V mode, which has the two
primary windings in parallel).

Does this seem right? Has this transformer gotten toasted somehow, or is
this normal? I've never seen that much no-load current draw or hum, but I
don't usually work with amps this powerful, either.

It sounds like it's saturating near full line voltage. Are you running
a 60 Hz on 50 Hz by chance?

However, may be normal if the transformer was designed to always be operated
with a load. If the amp is Class A or AB, there would always be a guaranteed
load on the transformer.

Microwave oven transformers are also like this.

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[[email protected]]

I need some real-world perspective, please.

There's a big power transformer in a power amp I'm working on. The amp is
supposed to produce 1000W RMS (though we all know how amp specs are...).
The fuse at the input is 10A for 120V mains, and the label says 1100VA. So
the transformer is somewhere around 1kVA.

Now, when I hook this transformer up to 120V, with no load connected, it
hums loudly and it draws 1.1A from the line.

Interestingly, the current (remember, no load connected) is only about 0.5A
at 105V, and there is almost no hum. The hum and the current go up very
steeply as I increase the voltage to 120V, using a variac.

The secondary voltages measure what I'd expect: 138VAC rms across the outer
taps, 31VAC rms across the inner taps. DC resistance, subtracting the
resistance of shorted probes, is 0R37 across the outer taps, 0R58 across the
inner taps, and 0R38 across the primaries (in 120V mode, which has the two
primary windings in parallel).

Does this seem right? Has this transformer gotten toasted somehow, or is
this normal? I've never seen that much no-load current draw or hum, but I
don't usually work with amps this powerful, either.

Thanks!

It could be that you have a few shorted/poorly insulated turns in one
of the secondary windings, I would recommend taking it to a
transformer manufacturer and having it insulation & flash tested to be
certain, or at the very least perform an insulation test yourself with
a megger, both between windings and to the core. The initial current
drawn by these types of transformer is usually fairly high, as is
generated mechanical noise so I wouldn't worry too much about that,
however, the rise in core temperature should not become appreciably
high when it is off-load. If this is happening I would look for a
replacement. It has generally been my experience that the last
component to fail in high power amps would be the transformer, but as
with all things, that would very much depend on what happened to the
amp to begin with. For instance, if the amp was given 230v
accidentally, whilst being set for 120v that may cook it, as could a
shorted bridge rectifier or dead short smoothing block etc.


regards
Alastair

 

[Walter Harley]

It sounds like it's saturating near full line voltage. Are you running
a 60 Hz on 50 Hz by chance?

Thanks, Sam. No, it's definitely on 60Hz.

However, may be normal if the transformer was designed to always be operated
with a load. If the amp is Class A or AB, there would always be a guaranteed
load on the transformer.

Class AB, so there would always be at least a bit of load. But it also does
this when the load is connected - that is, when I hook it up to the rest of
the supply circuitry and hook that up to the rest of the amp. I'm not sure
whether it quiets down when it's actually pushing a load.

Back-story: The amp is a Carvin FET1000. This is a class-AB MOSFET stereo
amp, with each channel consisting of a paralleled trio of MOSFETs. (The
MOSFETs are paralleled without the benefit of current-sharing resistors,
which is part of what caused the failure that landed it on my workbench.
But that's another story.) It's biased at 120mA bias current on each side,
plus a bit extra for the control circuitry, so when idle I'd expect it to be
drawing about 0.3A of current plus whatever gets wasted in heating up the
transformer.

Sounds like your underlying thought is that it's nothing to worry about?

 

[Walter Harley]

It could be that you have a few shorted/poorly insulated turns in one
of the secondary windings, I would recommend taking it to a
transformer manufacturer and having it insulation & flash tested to be
certain, or at the very least perform an insulation test yourself with
a megger, both between windings and to the core. The initial current
drawn by these types of transformer is usually fairly high, as is
generated mechanical noise so I wouldn't worry too much about that,
however, the rise in core temperature should not become appreciably
high when it is off-load. If this is happening I would look for a
replacement. It has generally been my experience that the last
component to fail in high power amps would be the transformer, but as
with all things, that would very much depend on what happened to the
amp to begin with. For instance, if the amp was given 230v
accidentally, whilst being set for 120v that may cook it, as could a
shorted bridge rectifier or dead short smoothing block etc.

Thanks. It did indeed see a dead short for a little while; that,
delightfully, is the failure mode of this particular amp design when the
output transistors blow.

If it's dissipating 130W (=1.1A * 120V) with no load, I would think it's got
to get pretty hot; but I haven't left it hooked up for more than 20 seconds
with full voltage, so I'm not sure.

But, I'm confused as to how the tests you suggested would find a shorted
turn? That does seem like exactly the thing I want to test for.

-walter

 

[Hal Murray]

If it's dissipating 130W (=1.1A * 120V) with no load, I would think it's got

to get pretty hot; but I haven't left it hooked up for more than 20 seconds
with full voltage, so I'm not sure.

What's the power factor? My guess is that it's far from 1.

But, I'm confused as to how the tests you suggested would find a shorted
turn? That does seem like exactly the thing I want to test for.

If there is a shorted turn, it will smoke pretty quickly. (That's
the same as shorting the output of a 1 turn secondary.)


I'd probably put a dummy load on it for a while and see how hot
it gets. Light bulbs might be a convenient technology.

Does it smell like it was smoked?

 

[Walter Harley]

What's the power factor? My guess is that it's far from 1.

Good point. I don't know what the phase relationship between current and
voltage is, nor do I know what the waveforms look like. The meter I've been
measuring current with is not a true-RMS meter (though I do have one). That
would certainly explain the steep rise in (apparent) current as I get close
to 120V.

If there is a shorted turn, it will smoke pretty quickly. (That's
the same as shorting the output of a 1 turn secondary.)

I'd probably put a dummy load on it for a while and see how hot
it gets. Light bulbs might be a convenient technology.

Does it smell like it was smoked?

It smells and looks okay. But it's a bit hard to tell; other parts of the
amp were smoked, plus the amp has been used to do PA in clubs, so there are
other scents interfering. There's definitely nothing obvious.

Putting it on a dummy load for a while is a good idea. Couple of 100W light
bulbs in parallel, maybe.

 

[Michel Savard]

I don't know about this particular transformer but I think that you are
wasting your time?
It is normal for a transformer to draw current even though there isn't any
load on the secondary side. It is just a big coil on the primary side. Take
a step up/down transformer or even an isolation transformer , they draw
quite a bit of current even without a load, it is just a big coil.

Look for your problem elsewhere.

 

[Sam Goldwasser]

Thanks, Sam. No, it's definitely on 60Hz.


Class AB, so there would always be at least a bit of load. But it also does
this when the load is connected - that is, when I hook it up to the rest of
the supply circuitry and hook that up to the rest of the amp. I'm not sure
whether it quiets down when it's actually pushing a load.

Back-story: The amp is a Carvin FET1000. This is a class-AB MOSFET stereo
amp, with each channel consisting of a paralleled trio of MOSFETs. (The
MOSFETs are paralleled without the benefit of current-sharing resistors,
which is part of what caused the failure that landed it on my workbench.
But that's another story.) It's biased at 120mA bias current on each side,
plus a bit extra for the control circuitry, so when idle I'd expect it to be
drawing about 0.3A of current plus whatever gets wasted in heating up the
transformer.

Sounds like your underlying thought is that it's nothing to worry about?

Not necessarily. I'd expect them not to skimp on the transformer. Unlike
a microwave oven where cost is everything, it wouldn't be likely they'd
let the transformer go into saturation just to save a couple bucks on
copper.

However, if the transformer doesn't get hot with the amp idling, then
it's probably OK. If it cooks just sitting there with no audio input,
then it could be as someone else suggested - an insulation breakdown
or intermittent shorted winding or a few. The buzz isn't the issue,
it's the behavior of current with respect to input voltage.

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[Sam Goldwasser]

I don't know about this particular transformer but I think that you are
wasting your time?
It is normal for a transformer to draw current even though there isn't any
load on the secondary side. It is just a big coil on the primary side. Take
a step up/down transformer or even an isolation transformer , they draw
quite a bit of current even without a load, it is just a big coil.

This depends on the transformer design. A "big coil" as you say is an
inductor. If the inductance is high, the current will be low. Various
losses and parasitics can contribute to primary current.

In the case of this transformer, I believe the behavior is non-linear which
suggests either core saturation or some sort of breakdown at higher input
voltage.

Look for your problem elsewhere.

As has been suggested, run it no load or with a dummy load for awhile
to see if it gets hot. If it doesn't get more than somewhat warm,
it's probably fine.

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[Walter Harley]

However, if the transformer doesn't get hot with the amp idling, then
it's probably OK. If it cooks just sitting there with no audio input,
then it could be as someone else suggested - an insulation breakdown
or intermittent shorted winding or a few. The buzz isn't the issue,
it's the behavior of current with respect to input voltage.

I'll fire it up with a light load for a while tonight at full voltage, and
see if it cooks.

 

[Walter Harley]

I'll fire it up with a light load for a while tonight at full voltage, and
see if it cooks.

I didn't have time to rig up a dummy load, so I just ran it for 30 minutes
or so with no load. It hums and vibrates, but it doesn't get hot at all.
Evidently the power factor is very low.

So, I'm going to presume that it's just saturating at the very peaks, and
not worry about it. Probably under load it'll quiet down.

Thanks, everyone, for your advice!

 

[Sam Goldwasser]

I didn't have time to rig up a dummy load, so I just ran it for 30 minutes
or so with no load. It hums and vibrates, but it doesn't get hot at all.
Evidently the power factor is very low.

So, I'm going to presume that it's just saturating at the very peaks, and
not worry about it. Probably under load it'll quiet down.

Sounds fine. Still may need to tighten it up or add glue or something
to quiet it down.

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[default]

I need some real-world perspective, please.

There's a big power transformer in a power amp I'm working on. The amp is
supposed to produce 1000W RMS (though we all know how amp specs are...).
The fuse at the input is 10A for 120V mains, and the label says 1100VA. So
the transformer is somewhere around 1kVA.

Now, when I hook this transformer up to 120V, with no load connected, it
hums loudly and it draws 1.1A from the line.

Interestingly, the current (remember, no load connected) is only about 0.5A
at 105V, and there is almost no hum. The hum and the current go up very
steeply as I increase the voltage to 120V, using a variac.

The secondary voltages measure what I'd expect: 138VAC rms across the outer
taps, 31VAC rms across the inner taps. DC resistance, subtracting the
resistance of shorted probes, is 0R37 across the outer taps, 0R58 across the
inner taps, and 0R38 across the primaries (in 120V mode, which has the two
primary windings in parallel).

Does this seem right? Has this transformer gotten toasted somehow, or is
this normal? I've never seen that much no-load current draw or hum, but I
don't usually work with amps this powerful, either.

Thanks!

The current rising sharply does suggest saturation. Is there a tap on
the xformer for say 105 volts? that it may be connected to? That
would explain it neatly.

One amp does seem like a lot but may not be - it is an inductor under
no load and part of that current is reactive and out of phase with the
voltage so doesn't use power . . . does it get hot just being plugged
in for several hours?

I've a couple of 1kw Crown amps, they eat about 3 amps at 120 at idle.
They operate class AB and are class A up to about 100 watts.

 

[Asimov]

"Sam Goldwasser" bravely wrote to "All" (24 Sep 03 17:39:10)
--- on the heady topic of "Re: big iron toasted?"

SG> From: Sam Goldwasser <[email protected]>

I don't know about this particular transformer but I think that you are
wasting your time?
It is normal for a transformer to draw current even though there isn't any
load on the secondary side. It is just a big coil on the primary side. Take
a step up/down transformer or even an isolation transformer , they draw
quite a bit of current even without a load, it is just a big coil.

SG> This depends on the transformer design. A "big coil" as you say is an
SG> inductor. If the inductance is high, the current will be low.
SG> Various losses and parasitics can contribute to primary current.

SG> In the case of this transformer, I believe the behavior is non-linear
SG> which suggests either core saturation or some sort of breakdown at
SG> higher input voltage.

Look for your problem elsewhere.

SG> As has been suggested, run it no load or with a dummy load for awhile
SG> to see if it gets hot. If it doesn't get more than somewhat warm,
SG> it's probably fine.

A big transformer can draw a lot of magnetizing current. Anyways, why
doesn't anyone suggest he do a "ringing test" to determine if there are
shorted turns? An insulation leakage test may be useful too.

Asimov
******

.... I know flyback, ultor, and 47 other dangerous words.

 

[Dave Martindale]

If it's dissipating 130W (=1.1A * 120V) with no load, I would think it's got
to get pretty hot; but I haven't left it hooked up for more than 20 seconds
with full voltage, so I'm not sure.

The power dissipation is probably nowhere near that. The 1.1 A is
probably all reactive current, with nearly 90 degrees phase shift with
respect to voltage. The only power dissipation is the reactive current
multiplied by the resistance of the primary winding, which is probably
only a few ohms.

Dave