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Bleeding resistor

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Niraj Joshi

Thu Oct 14 2010, 05:43PM

Registered Member #3291 Joined: Fri Oct 08 2010, 06:06PM
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Posts: 11

Hi guys,

I want to connect a bleeding resistor across the 5 kV High voltage transformer to serve its specified purpose.

Any1 tell me how to calculate the correct value of bleeding resistor.

The transformer details are:

500 VA, 230V/5000V. 50 Hz.

please tell me the value of bleeding resistor to be connected across the terminals of Transformer.

Sulaiman

Thu Oct 14 2010, 05:48PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

A transformer does not require a bleed resistor
because the winding is low resistance.

Niraj Joshi

Thu Oct 14 2010, 06:07PM

Registered Member #3291 Joined: Fri Oct 08 2010, 06:06PM
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Posts: 11

@sulaiman: At high voltages, the magnetic core stores a little amount of energy at high potential due to residual magnetism. The bleeding resistor serves the purpose of discharging this energy. Bleeding resistor is in mega ohms.

Proud Mary

Thu Oct 14 2010, 07:35PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Niraj Joshi wrote ...

@sulaiman: At high voltages, the magnetic core stores a little amount of energy at high potential due to residual magnetism. The bleeding resistor serves the purpose of discharging this energy. Bleeding resistor is in mega ohms.

It isn't easy to understand how this could be the case.

Bleeder resistors are used (a) to provide a stable minimum load on, for example, a choke input power filter circuit, to prevent the output voltage rising to a high value, and (b) to discharge harmlessly high voltage capacitors - very often those in high voltage filters - to render them safe.

You seem not to have understood the basic theory of transformers.

Dr. Slack

Thu Oct 14 2010, 07:38PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

+1 Sulaiman

Whatever residual magnetic energy a transformer core stores will not be "bled off" by a shunt resistor across the terminals.

The capacitance between windings and core will store a tiny amount of energy which would be bled by a large resistor from one terminal to ground/core, if you charged it, and were bothered about it, which nobody ever is.

radiotech

Fri Oct 15 2010, 07:00AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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"Naraj Joshi wrote "@sulaiman: At high voltages, the magnetic core stores a little amount of energy at high potential due to residual magnetism. The bleeding resistor serves the purpose of discharging this energy. Bleeding resistor is in mega ohms.

If you wish, measure the capacitance to earth of the high voltage winding and assume a voltage of 5000 RMS volts could charge it under certain conditions. Then calculate Megohms * micofarads *5 to find how many seconds it will take to discharge it to 140 volta DC.

Proud Mary

Fri Oct 15 2010, 07:56AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

radiotech wrote ...

"Naraj Joshi wrote "@sulaiman: At high voltages, the magnetic core stores a little amount of energy at high potential due to residual magnetism. The bleeding resistor serves the purpose of discharging this energy. Bleeding resistor is in mega ohms.

If you wish, measure the capacitance to earth of the high voltage winding and assume a voltage of 5000 RMS volts could charge it under certain conditions. Then calculate Megohms * micofarads *5 to find how many seconds it will take to discharge it to 140 volta DC.

I don't follow how some angular fragment of an alternating current is to enter suspended animation in order to charge up this notional capcitance between winding and core, or whatever.

radiotech

Fri Oct 15 2010, 11:29PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

Its a guestimate about the asymetrical peak voltage charging the dielectric between the winding and the core after 5 time constants.
Fuse blowing open at peak? (contactors opening at peak make a sound like a cannon sometimes.) When we rebuilt high voltage motors we used a semiconductor insulation to prevent just such a buildup. Our radios require a megohm leak to prevent high voltage the aerial from charging, then perforating the mains transformer core to winding insulation. We drain the aerial to the power cord.

Sulaiman

Sat Oct 16 2010, 07:48AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Niraj Joshi, I am concerned for your safety,
you have a LETHAL combination of voltage, current and inexperience.
I suggest that you post exactly what you intend to do here so that we may advise you, don't be shy - we all have to start somewhere.

Meanwhile -
Trust us - there is no need for a bleeder resistor across a mains transformer secondary,
unless you need a heater!
IF you are using this transformer to power some kind of energy-discharge system
(e.g. Tesla coil, Coilgun, Railgun, coin-shrinker, Marx-generator etc.)
then you may need to protect the transformer from it's load.

A 'universal' protection scheme for such a transformer would be expensive;
Voltage clamping using a suitable number of TVSs in series/parallel
Current limiting using a high-voltage (HV) high-rupture-current (HRC) fuse
A low-pass filter to protect against reflected high-frequency voltage/currents.
Google 'Terry Filter' for some ideas.

For hobby use I would put a spark gap across the secondary
that is adjusted to just about NOT fire under normal operating conditions,
and a suitable fuse in the primary.
Almost bullet-proof.

What kind of transformer is it?
Judging from the rating I suspect a normal type of mains transformer with no inbuilt current limiting
(as found in a Neon Sign Transformer or an Ignition Transformer)
So a fuse or circuit-breaker in the primary circuit is ESSENTIAL.

Niraj Joshi

Sat Oct 16 2010, 04:52PM

Registered Member #3291 Joined: Fri Oct 08 2010, 06:06PM
Location:
Posts: 11

please view the circuit diagram and the bleeding resistor connected across the transformer primary.

@ sulaiman: thank u sir for sharing ur experience with us. had i been afraid i would not have joined this community.

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