isolation transformer primary winding delay circuit

Thread Starter

whburling

Joined Dec 1, 2016
7
I purchased a "hospital" isolation transformer originally designed to isolate failures on the secondary winding of the isolation transformer from unknown equipment tied to the primary winding of the isolation transformer. A Hospital Isolation transformer ties earth ground to the secondary and to the grounded part of the outlet so that it appears to be identical (to an outlet checker) to the outlet to which the primary winding is connected. The transformer isolates any secondary winding load circuit failure from being seen by other devices attached to the power feeding the primary winding.
(that understanding was conveyed to me via Larry (search isolation W0QE YouTube) with his awesome simulation).

I want to convert the "hospital" isolation transformer to a "technician" isolation transformer in which all earth ground exposures are removed from any equipment connected to the secondary side of the isolation transformer. The conversion has been amply stated elsewhere in this and other forums. I will not repeat it here.

There is one feature that came with the hospital isolation transformer that I do not understand and hence can not consider if I should retain it or not in my effort to make the conversion of Hospital to Technician isolation type transformer.

That feature is a delay circuit that exists between the off/on power switch and the primary winding of the transformer. The timing is 0.5 second which
suggests that at least 30 full cycles of 120 VAC take place before current is allowed to feed the primary winding.

I can only think of one use for the delay. If the "hospital" isolation transformer lost power to its primary windings while the switch was "on", then it is
highly probable that a great number of devices are attached to the circuit that lost power. Hence when power is restored, the surge of current to start
all the low power devices in a hospital environment are likely to have completed their power up sequence well within the 30 full cycles. Why that is important is not clear to me as the transformer is a huge inductor which resists current surge (my understanding). Maybe the delay is to protect the
electronic filtering of input power of attached medical equipment from any repowering sequence that might occur after sudden power loss.

Any ideas? I am just making a wild guess.
bil
 

Thread Starter

whburling

Joined Dec 1, 2016
7
please see : https://forum.allaboutcircuits.com/threads/isolation-transformer-2.178488/
NSAspook provided the answer. Essence:

The actual circuit is composed of three parts. (1) a relay whose contacts are in series with the hot leg of the primary and one leg of the primary winding (2) a time delay in series with the hot leg of the primary and the relay coil whose other side is to primary neutral and (3) an inrush current limiter across the relay contacts.

The operation is as follows:
when the physical switch of the isolation transformer is turned on, the transformer is energized by current coming from the hot leg of the source , through the inrush current limiter, then into the hot side of the primary winding. The inrush current limiter
is a thermister which would normally remain in the circuit after it has done its job. It works by presenting a high 10 ohm resistance to the inrushing current and heats up upon dissipating energy. thermister resistance drops at higher temperatures., but the effective voltage drop remains constant. in my case, the isolation transformer would now only see a source voltage minus 12 volts. The relay and timer work to totally eliminate the voltage degradation of the inrush current limiter.

The design objective is to short out the inrush current limiter. this is done by putting relay contacts between the inrush current limiter wires and closing the short circuiting contacts after the inrush current limiter has done its job. rather than measure the
current, a timer is set to actuate the relay x seconds after the power switch has been closed, where x is a safe estimate of the time after which inrush protection is not likely to be needed.

The only part that needs clarification is to explain where the inrush is coming from. nsaspook offered us all two references
that lead to explanations. My understsanding (which could be very wrong) is that residual magnetism occurs in the core of the transformer. its magnitude and orientation varies depending on the state at which the transformer was turned off. Turning the transformer on at the wrong part of the voltage cycle forces the transformer to draw a large current to overcome the residual magnetism orientation. Ideally one could measure the residual magnitism orientation and magnitude and control the time
at which the source voltage cycle is allowed to be seen by the transformer. But at this moment such a device is not cost effective and hence we have to put up with inrush current in transformers.

Thank you all for helping me understand this issue. If i have missunderstood any part of the explanation, please put forth a more accurate story. I don['t care who creates the accurate story. I just am grateful people are trying to create stories that predict object behavior.
bil
 

nsaspook

Joined Aug 27, 2009
8,389
Residual magnetism (from random points in the cycle during power off) is one factor (one component of magnetizing current) that may increase or decrease the magnetizing inrush current.

From the link I posted: https://mospace.umsystem.edu/xmlui/bitstream/handle/10355/61852/Thesis_2017_Charlapally.pdf

1.4 Inrush Current
The main causes of inrush current are
i. Voltage angle during energization (Point of Wave)
ii. Remanence Flux
iii. Source Impedance
iv. Leakage impedance

Remanence Flux reversal can enhance (vs a demagnetized core) the level of inrush current causing very high peak currents that may trip the protection components of critical circuits.
When a transformer with positive remanence flux as shown in Fig 1-6 [2] is
energized at the zero crossing while going from negative polarity to positive polarity, it is
driven into deep saturation. At this point, the permeability is close to unity and the
transformer core acts almost like air and even a slight change in flux would draw huge
amount of current. Therefore, the transformer core must be demagnetized before reenergizing.
The soft-start circuit limits current from all causes during the cores electrical demagnetization cycle on power up.
 

Thread Starter

whburling

Joined Dec 1, 2016
7
Residual magnetism (from random points in the cycle during power off) is one factor (one component of magnetizing current) that may increase or decrease the magnetizing inrush current.

From the link I posted: https://mospace.umsystem.edu/xmlui/bitstream/handle/10355/61852/Thesis_2017_Charlapally.pdf

1.4 Inrush Current
The main causes of inrush current are
i. Voltage angle during energization (Point of Wave)
ii. Remanence Flux
iii. Source Impedance
iv. Leakage impedance

Remanence Flux reversal can enhance (vs a demagnetized core) the level of inrush current causing very high peak currents that may trip the protection components of critical circuits.


The soft-start circuit limits current from all causes during the cores electrical demagnetization cycle on power up.
I am so grateful for your participation. You encourage me to be more curious than I tend to be. In reality I read the entire
thesis, but need to play with the topic a bit more before I can really grasp what is happening.

So your addition (up above) rightfully included the four cases of inrush.

I am still not sure why remanence flux reversal (i am assuming that the statement means the state of the remanent flux is opposite of what should be expected if a voltage were applied with a magnitude of zero and increasing), would enhance the level of inrush.

when i finished the thesis my take away was that if I could match the applied voltage phase (which has an expected instantaneous flux angle and magnitude) with the actual flux angle and magnitude, then there would be no inrush other
than the other three sources of inrush.

So the question surfaces: can one measure the magnitude and angle of flux after one removes the applied voltage. that is what I would like to see.

I imagine the last flip of flux angle contains the only energy available to hold the flux angle in place to produce magnetism ( frozen flux angle). 1/120 of a second worth whose available magnitude can't be greater than the difference in previous magnitude and current magnitude. I can't believe that is enough energy to create a flux orientation that will survive
thermal and other disruptions.

So clearly I do not have a model to think with in my head that aligns with actual practice.

re: your question....is my original question solved. YES !!!!! it is thanks to you. thank you.

I am mystified why the inductance of the coil (forget the core) does not play a more major role. Does not inductance oppose
current inrush?

what might be valid is that there might be a lag in flux angle that probably does not . Hence when a voltage is applied, one can expect an instantaneous flux angle and if it does not correspond with the
 
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