OT: Housing Voltage in an Autotransformer


n4buq
 

Please pardon the OT but I don't know where else to turn except to groups like this one for some things.

I have an older Powerstat 116 that I've refurbished (mostly cosmetic but have rewired it as well). This one of the models with just a two-wire plug and I decided to keep it original.

When I powered it up, I decided to check voltages in/around the housing just to make sure nothing was leaking through to the frame. To my surprise, when I checked between neutral and the frame, depending on where I test, I'm getting some significant voltages there. From neutral to the main frame, I'm getting 50VAC which is a bit alarming to me.

With power disconnected, I checked resistance between the frame and neutral and it shows infinity with a Fluke DMM so I don't know why there would be voltage on the frame.

The frame for this model is cast aluminum(?) with a flat, disk-like base where the windings sit on an insulated disk and a conical inside that rises up inside the windings. Is it possible that what I'm seeing is just an induced voltage from the windings? If not, I just don't see where/why there's voltage like that on the frame.

(If there's a more appropriate forum to ask this, please let me know.)

Thanks,
Barry - N4BUQ


Bob Albert
 

I am suspicious when a reading is about half the line voltage.  It's often caused by capacitors connected from both sides of the line to ground or neutral.
Bob

On Friday, May 20, 2022, 06:48:57 PM PDT, n4buq <n4buq@...> wrote:

Please pardon the OT but I don't know where else to turn except to groups like this one for some things.

I have an older Powerstat 116 that I've refurbished (mostly cosmetic but have rewired it as well).  This one of the models with just a two-wire plug and I decided to keep it original.

When I powered it up, I decided to check voltages in/around the housing just to make sure nothing was leaking through to the frame.  To my surprise, when I checked between neutral and the frame, depending on where I test, I'm getting some significant voltages there.  From neutral to the main frame, I'm getting 50VAC which is a bit alarming to me.

With power disconnected, I checked resistance between the frame and neutral and it shows infinity with a Fluke DMM so I don't know why there would be voltage on the frame.

The frame for this model is cast aluminum(?) with a flat, disk-like base where the windings sit on an insulated disk and a conical inside that rises up inside the windings.  Is it possible that what I'm seeing is just an induced voltage from the windings?  If not, I just don't see where/why there's voltage like that on the frame.

(If there's a more appropriate forum to ask this, please let me know.)

Thanks,
Barry - N4BUQ


Frank DuVal
 

Did you use the Fluke for this voltage measurement? Try shunting the red and black leads with a 10 to 100 K resistor to remove "ghost" voltage. I see this all the time using a Fluke in house/commercial wiring. Just capacitance coupling. If with resistor shunt, the voltage remains,  then hmm.....

Frank DuVal

WA4CWM

On 5/20/2022 9:48 PM, n4buq wrote:
Please pardon the OT but I don't know where else to turn except to groups like this one for some things.

I have an older Powerstat 116 that I've refurbished (mostly cosmetic but have rewired it as well). This one of the models with just a two-wire plug and I decided to keep it original.

When I powered it up, I decided to check voltages in/around the housing just to make sure nothing was leaking through to the frame. To my surprise, when I checked between neutral and the frame, depending on where I test, I'm getting some significant voltages there. From neutral to the main frame, I'm getting 50VAC which is a bit alarming to me.

With power disconnected, I checked resistance between the frame and neutral and it shows infinity with a Fluke DMM so I don't know why there would be voltage on the frame.

The frame for this model is cast aluminum(?) with a flat, disk-like base where the windings sit on an insulated disk and a conical inside that rises up inside the windings. Is it possible that what I'm seeing is just an induced voltage from the windings? If not, I just don't see where/why there's voltage like that on the frame.

(If there's a more appropriate forum to ask this, please let me know.)

Thanks,
Barry - N4BUQ


n4buq
 

Yes, this is what occurred to me as well but there are no actual capacitors involved.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Bob Albert via groups.io" <bob91343@...>
To: "tekscopes" <tekscopes@groups.io>, "tekscopes" <TekScopes@groups.io>
Sent: Friday, May 20, 2022 9:06:44 PM
Subject: Re: [TekScopes] OT: Housing Voltage in an Autotransformer
I am suspicious when a reading is about half the line voltage.  It's often
caused by capacitors connected from both sides of the line to ground or
neutral.
Bob
On Friday, May 20, 2022, 06:48:57 PM PDT, n4buq <n4buq@...> wrote:

Please pardon the OT but I don't know where else to turn except to groups like
this one for some things.

I have an older Powerstat 116 that I've refurbished (mostly cosmetic but have
rewired it as well).  This one of the models with just a two-wire plug and I
decided to keep it original.

When I powered it up, I decided to check voltages in/around the housing just to
make sure nothing was leaking through to the frame.  To my surprise, when I
checked between neutral and the frame, depending on where I test, I'm getting
some significant voltages there.  From neutral to the main frame, I'm getting
50VAC which is a bit alarming to me.

With power disconnected, I checked resistance between the frame and neutral and
it shows infinity with a Fluke DMM so I don't know why there would be voltage
on the frame.

The frame for this model is cast aluminum(?) with a flat, disk-like base where
the windings sit on an insulated disk and a conical inside that rises up inside
the windings.  Is it possible that what I'm seeing is just an induced voltage
from the windings?  If not, I just don't see where/why there's voltage like
that on the frame.

(If there's a more appropriate forum to ask this, please let me know.)

Thanks,
Barry - N4BUQ









n4buq
 

Yes, with the Fluke 25 (the older 25 model). I can try shunting. Perhaps just check that with an analog volt meter instead?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Frank DuVal via groups.io" <corvairduval@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, May 20, 2022 9:10:05 PM
Subject: Re: [TekScopes] OT: Housing Voltage in an Autotransformer
Did you use the Fluke for this voltage measurement? Try shunting the red
and black leads with a 10 to 100 K resistor to remove "ghost" voltage. I
see this all the time using a Fluke in house/commercial wiring. Just
capacitance coupling. If with resistor shunt, the voltage remains,  then
hmm.....

Frank DuVal

WA4CWM

On 5/20/2022 9:48 PM, n4buq wrote:
Please pardon the OT but I don't know where else to turn except to groups like
this one for some things.

I have an older Powerstat 116 that I've refurbished (mostly cosmetic but have
rewired it as well). This one of the models with just a two-wire plug and I
decided to keep it original.

When I powered it up, I decided to check voltages in/around the housing just to
make sure nothing was leaking through to the frame. To my surprise, when I
checked between neutral and the frame, depending on where I test, I'm getting
some significant voltages there. From neutral to the main frame, I'm getting
50VAC which is a bit alarming to me.

With power disconnected, I checked resistance between the frame and neutral and
it shows infinity with a Fluke DMM so I don't know why there would be voltage
on the frame.

The frame for this model is cast aluminum(?) with a flat, disk-like base where
the windings sit on an insulated disk and a conical inside that rises up inside
the windings. Is it possible that what I'm seeing is just an induced voltage
from the windings? If not, I just don't see where/why there's voltage like
that on the frame.

(If there's a more appropriate forum to ask this, please let me know.)

Thanks,
Barry - N4BUQ


n4buq
 

I put a 47k across the Fluke's leads and that drops the reading to about 200mV which I expect is normal.

I also used a Simpson 269 analog meter and, oddly, no matter what AC range I select, there's a small rise in the meter reading. That seemed odd but maybe "ghosts" do weird things?

At any rate, I think I'm good now. It just surprised me to see that.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Frank DuVal via groups.io" <corvairduval@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, May 20, 2022 9:10:05 PM
Subject: Re: [TekScopes] OT: Housing Voltage in an Autotransformer
Did you use the Fluke for this voltage measurement? Try shunting the red
and black leads with a 10 to 100 K resistor to remove "ghost" voltage. I
see this all the time using a Fluke in house/commercial wiring. Just
capacitance coupling. If with resistor shunt, the voltage remains,  then
hmm.....

Frank DuVal

WA4CWM

On 5/20/2022 9:48 PM, n4buq wrote:
Please pardon the OT but I don't know where else to turn except to groups like
this one for some things.

I have an older Powerstat 116 that I've refurbished (mostly cosmetic but have
rewired it as well). This one of the models with just a two-wire plug and I
decided to keep it original.

When I powered it up, I decided to check voltages in/around the housing just to
make sure nothing was leaking through to the frame. To my surprise, when I
checked between neutral and the frame, depending on where I test, I'm getting
some significant voltages there. From neutral to the main frame, I'm getting
50VAC which is a bit alarming to me.

With power disconnected, I checked resistance between the frame and neutral and
it shows infinity with a Fluke DMM so I don't know why there would be voltage
on the frame.

The frame for this model is cast aluminum(?) with a flat, disk-like base where
the windings sit on an insulated disk and a conical inside that rises up inside
the windings. Is it possible that what I'm seeing is just an induced voltage
from the windings? If not, I just don't see where/why there's voltage like
that on the frame.

(If there's a more appropriate forum to ask this, please let me know.)

Thanks,
Barry - N4BUQ


Ozan
 

On Fri, May 20, 2022 at 07:27 PM, n4buq wrote:
I also used a Simpson 269 analog meter and, oddly, no matter what AC range I
select, there's a small rise in the meter reading. That seemed odd but maybe
"ghosts" do weird things?
In most analog meters input impedance changes with the range. Picture of 269 says 5k-ohm/V in AC, you are changing the shunting impedance as well when you are changing the range.
Ozan


n4buq
 

Thanks, Ozan. I hadn't noticed that on my meter but, yes, 5K ohms/volt.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Ozan" <ozan_g@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, May 20, 2022 9:32:15 PM
Subject: Re: [TekScopes] OT: Housing Voltage in an Autotransformer
On Fri, May 20, 2022 at 07:27 PM, n4buq wrote:
I also used a Simpson 269 analog meter and, oddly, no matter what AC range I
select, there's a small rise in the meter reading. That seemed odd but maybe
"ghosts" do weird things?
In most analog meters input impedance changes with the range. Picture of 269
says 5k-ohm/V in AC, you are changing the shunting impedance as well when you
are changing the range.
Ozan



si_emi_01
 

If the analog meter movement is located close to the PowerStat, the induced magnetic field into the sensitive meter movement may cause that effect. The PowerStat and other Variac devices are Variable AutoTransformers - hence they have magnetic fields.

Magnetic field strength is attenuated by the cube of the distance. Move the Analog Meter a foot or two distance from the PowerStat. See if that makes a difference.

If the PowerStat is housed in Aluminum as you indicated, Aluminum is transparent (ie. provided no H-Field Attenuation), from about 100kHz down to DC.

Just a possibility.

Ross

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of n4buq
Sent: Friday, May 20, 2022 8:27 PM
To: tekscopes <TekScopes@groups.io>
Subject: Re: [TekScopes] OT: Housing Voltage in an Autotransformer

I put a 47k across the Fluke's leads and that drops the reading to about 200mV which I expect is normal.

I also used a Simpson 269 analog meter and, oddly, no matter what AC range I select, there's a small rise in the meter reading. That seemed odd but maybe "ghosts" do weird things?

At any rate, I think I'm good now. It just surprised me to see that.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Frank DuVal via groups.io"
<corvairduval@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, May 20, 2022 9:10:05 PM
Subject: Re: [TekScopes] OT: Housing Voltage in an Autotransformer
Did you use the Fluke for this voltage measurement? Try shunting the
red and black leads with a 10 to 100 K resistor to remove "ghost"
voltage. I see this all the time using a Fluke in house/commercial
wiring. Just capacitance coupling. If with resistor shunt, the voltage
remains, then hmm.....

Frank DuVal

WA4CWM

On 5/20/2022 9:48 PM, n4buq wrote:
Please pardon the OT but I don't know where else to turn except to
groups like this one for some things.

I have an older Powerstat 116 that I've refurbished (mostly cosmetic
but have rewired it as well). This one of the models with just a
two-wire plug and I decided to keep it original.

When I powered it up, I decided to check voltages in/around the
housing just to make sure nothing was leaking through to the frame.
To my surprise, when I checked between neutral and the frame,
depending on where I test, I'm getting some significant voltages
there. From neutral to the main frame, I'm getting 50VAC which is a bit alarming to me.

With power disconnected, I checked resistance between the frame and
neutral and it shows infinity with a Fluke DMM so I don't know why
there would be voltage on the frame.

The frame for this model is cast aluminum(?) with a flat, disk-like
base where the windings sit on an insulated disk and a conical inside
that rises up inside the windings. Is it possible that what I'm
seeing is just an induced voltage from the windings? If not, I just
don't see where/why there's voltage like that on the frame.

(If there's a more appropriate forum to ask this, please let me
know.)

Thanks,
Barry - N4BUQ


greenboxmaven
 

The stray capacitance between the winding and the frame and core is passing current and acting as a voltage divider. This is common with transformers and motors. I have a few military surplus power supplies that have .1 MFD. condensers in the power line filters. If you don't ground the case, there is about 60 volts on it and capable of giving a noticable sting, but not enough to trip a GFI. Using a three wire power cable eliminates the problem.

  Bruce Gentry, KA2IVY

On 5/20/22 22:27, n4buq wrote:
I put a 47k across the Fluke's leads and that drops the reading to about 200mV which I expect is normal.

I also used a Simpson 269 analog meter and, oddly, no matter what AC range I select, there's a small rise in the meter reading. That seemed odd but maybe "ghosts" do weird things?

At any rate, I think I'm good now. It just surprised me to see that.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Frank DuVal via groups.io" <corvairduval@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, May 20, 2022 9:10:05 PM
Subject: Re: [TekScopes] OT: Housing Voltage in an Autotransformer
Did you use the Fluke for this voltage measurement? Try shunting the red
and black leads with a 10 to 100 K resistor to remove "ghost" voltage. I
see this all the time using a Fluke in house/commercial wiring. Just
capacitance coupling. If with resistor shunt, the voltage remains,  then
hmm.....

Frank DuVal

WA4CWM

On 5/20/2022 9:48 PM, n4buq wrote:
Please pardon the OT but I don't know where else to turn except to groups like
this one for some things.

I have an older Powerstat 116 that I've refurbished (mostly cosmetic but have
rewired it as well). This one of the models with just a two-wire plug and I
decided to keep it original.

When I powered it up, I decided to check voltages in/around the housing just to
make sure nothing was leaking through to the frame. To my surprise, when I
checked between neutral and the frame, depending on where I test, I'm getting
some significant voltages there. From neutral to the main frame, I'm getting
50VAC which is a bit alarming to me.

With power disconnected, I checked resistance between the frame and neutral and
it shows infinity with a Fluke DMM so I don't know why there would be voltage
on the frame.

The frame for this model is cast aluminum(?) with a flat, disk-like base where
the windings sit on an insulated disk and a conical inside that rises up inside
the windings. Is it possible that what I'm seeing is just an induced voltage
from the windings? If not, I just don't see where/why there's voltage like
that on the frame.

(If there's a more appropriate forum to ask this, please let me know.)

Thanks,
Barry - N4BUQ


Jean-Paul
 

Barry bonjour As power electronics designer and manufacturing over the decades we have accumulated and used variacs in all sizes and types.

Any mains powered device with a metal housing must have a 3 wire earthed mains connection for safety.

The earth to châssis will also serve as a Faraday shield. Every modern Variac or powerstat is wired in this way. Old 2 wire devices are dangerous, and need a 3 wire mains connection and output recipticles.

Finally, all d'arsonval meters such as placed on metered variacs, are self Shielded as the armature, magnet and stator form a tightly closed magnetic circuit, and the toroïdal Variac has virtually no external magnetic field.

Bon week-end

Jon

PS The EEVBLOG forum has has a power electronics section.


Greg Muir
 

Barry,

The Simpson 260 measurement is an old trick used for many years. When you change to lower settings on the meter you are dropping the resistance of the input. If the voltage stays the same it indicates that it is merely intrinsic leakage of the device cause by capacitance or other coupling effects.

Greg