Building Test Loop

[camerontwilliams]

Hi I am with a company based in Idaho called Melni LLC. We make innovative electrical connectors for the industry. Currently we are starting to build our lab to test our connectors in house. I need some assistance on what kind of transformer and other equipment we may need to test properly. We have 480V 3phase power coming from the building. We need to be able to supply a high current load through a loop with the connectors installed to read temperatures at steady states. Depending on the gauge of conductor being used the amperage can vary from around 100 amps up to 1200 amps. Please help me by explaining what kind of transformer is needed and if a variac is used to control the amperage or what kind of equipment I will need to accomplish this task. I know this may seem simple to most of you but I am not an electrical engineer so please take it easy on me.
Thank you,

 

[ScottyUK]

You may find a ready-made solution in the form of a "primary current injection set" which, although expensive, may still cost less than a custom-engineered solution. For example

http://www.isatest.com/products-container/instrument-transformer-testing/kam

or

http://smcint.com/product/raptor-primary-system-electrical-testing-equipment/

FWIW, we built a high power primary injection set using the core of a large bushing CT from a 275kV transformer, with a primary of heavy cable driven from a 60A variac. The secondary was multiple paralleled lengths of welding cable. The rough & ready calculations are fairly straightforward if you know the load you are trying to drive current through and have some vague idea of the core dimensions. Member scottf will soon tell me that they are pretty damned complicated to do accurately, and he is right. This thing was transportable rather than portable and wasn't something you could use anywhere other than a very tightly controlled environment and lacked even the most basic safety features which you would find on a commercially-available product.

 

[camerontwilliams]

Thank you for your response ScottyUK, as of right now we have been using a 100% duty cycle 600 amp DC welder as our power source. The problem we have been facing is when we measure the current at 300 amps we are getting very different temperature readings at steady state on our connectors than the test lab at UL gets. About 40C difference in temperature at the point of contact. Could this be a DC vs AC problem? Or could it be that the AC input voltage to the welder has been rectified to DC? From what I understand since it is a three phase full wave rectification we should not be getting inaccurate readings on our ammeter. But I do not know enough about this to say what is going on.

Any Thoughts?

 

[itsmoked]

Lots of thoughts..

Do you understand that various meters read Average or RMS and that a rectified current will read very differently depending on which of the two you use?

Are you trying to duplicate the UL test? If so where is the test procedure listing the exact setup and method?

If one of the parties is using AC current and the other is using DC current you will see considerably different results. Are you using different currents?

Any air flow differences will make very large temp reading differences.

Exactly how are you reading the temperature? In this situation the type of measurement tool will make a large difference in readings. If you're using a non-contact method do you realized that shiny verses non-shiny makes an enormous difference in readings?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[camerontwilliams]

Hi Keith,

Thank you for your reply,

Yes I understand that the meter may not be reading properly. I am not sure what our meter is reading whether it is average or RMS. It is a Flir CM174. I know that the power source we are using has three phase 480VAC input and produces a rectified DC current.

Yes we are trying to duplicate the test, I have the UL 486A-486B standard which describes the test method and has a figure showing the vertical arrangement of the test specimens. However the standard does not go into detail for whether to test AC vs DC or what to use as a power source etc. It only gives an assigned amp rating based on the conductor size used in the connectors, conductor lengths, TC placement etc.

Yes we are currently using a DC current while they test using AC, we have wondered and asked many times if this would make a difference. We have had a copious amount of people saying no but we still don't understand. Could you please explain how and why this would make a difference in heat rise.

We are testing under controlled conditions.

For temperature measurement we use type J thermocouples with a welded bead tip and a data logger to record the readings.

If you have any more input on this matter it would be greatly appreciated.

Thank you,

 

[waross]

At 300 Amps you may have significant skin effect with AC. This will produce higher temperatures than the same DC current.
Watts loss creating heat is described by I²R
Skin effect causes the effective AC resistance to be higher, thus higher I²R losses.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Thanks Bill.

Also your CM174 is indeed a "true RMS" meter so that's good. It means strange waveforms likely present with any rectification will be correctly measured.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[emarch]

Using current injectors DC or AC that do not have an output regulation that keep the output higly stable, both in value and in waveform, will give a bad result when making heating tests. Be aware that any source with just a variac or even a fixed transformer may give a Voltage output as stady as the main supply is, but as the load under test gets hot, its impedance raise up with the temperature, demanding an increase in the volatge output to compensate this increment, which if not corrected will get less current than expected flowing through your load, and of course you do not get then the expected temperature values..

As this tests generally takes a long time (hours..) either you have a person continuosly reading the ammeter and correcting manually the variac to keep an steady current output value, or you need to use a constant current electronic device which will keep the current value accurately exact.

This is the only way to get consistent results in thermal tests.

Hope it helps

Eduardo Marchesi

http://www.smcint.com

 

[mcgyvr]

I knew the name sounded familiar.. These connectors were on the US TV show Shark Tank.. Pretty neat

 

[LionelHutz]

Who's higher?

You could use a current controller and a transformer to provide a constant current. A soft-starter that also does current regulation can work for that. We setup a customer with a controller for each phase to drive a transformer for current testing up to something like 3000A and it seems to work OK for them.