Transformer Input Voltage Reduction
Transformer Input Voltage Reduction
(OP)
Hi folks,
Is the voltage input magnitude has an effect on the transformer inrush current?
If voltage is V(t) = Vm*sin(wt+theta)
at theta = 90 Deg, V(t) = Vm*sin(wt+90) = Vm*cos(wt)volts
assuming no residual flux on the core,
This yeilds to steady state flux which is
Flux (max) = Vm/(w*Np)
at Theta = 0 Deg, then V(t) = Vm*sin(wt+0)
= Vm*sin(wt)
Therefore maximum flux at 1/2 cycle is
Flux(max) = 2Vm/(w*Np)
If there is residual flux(Br)present it may go as high as
Flux (max) = 3Vm/(w*Np) depending on the amount of Br and its polarity at Theta=0 Deg.
My question, is reducing the input voltage going to significantly reduce the inrush current? Up to what percentage assuming negligible system impedance?
Putting a resistor in series sized to say develop an initial voltage drop of 50~60% is another way and will also increasing the inrush decay period (damping time), after which it is shorted out by a contactor. While closing in the contactor to shunt it out, a voltage spike may happen which in turn may saturate the transformer again. Is this correct?
If a transformer secondary windings are manufactured with taps, is it possible to provide on-load tap changing through contactors only assuming a dry type transformer with 208D-208/120Y of the 20kVA size.
Is the voltage input magnitude has an effect on the transformer inrush current?
If voltage is V(t) = Vm*sin(wt+theta)
at theta = 90 Deg, V(t) = Vm*sin(wt+90) = Vm*cos(wt)volts
assuming no residual flux on the core,
This yeilds to steady state flux which is
Flux (max) = Vm/(w*Np)
at Theta = 0 Deg, then V(t) = Vm*sin(wt+0)
= Vm*sin(wt)
Therefore maximum flux at 1/2 cycle is
Flux(max) = 2Vm/(w*Np)
If there is residual flux(Br)present it may go as high as
Flux (max) = 3Vm/(w*Np) depending on the amount of Br and its polarity at Theta=0 Deg.
My question, is reducing the input voltage going to significantly reduce the inrush current? Up to what percentage assuming negligible system impedance?
Putting a resistor in series sized to say develop an initial voltage drop of 50~60% is another way and will also increasing the inrush decay period (damping time), after which it is shorted out by a contactor. While closing in the contactor to shunt it out, a voltage spike may happen which in turn may saturate the transformer again. Is this correct?
If a transformer secondary windings are manufactured with taps, is it possible to provide on-load tap changing through contactors only assuming a dry type transformer with 208D-208/120Y of the 20kVA size.






RE: Transformer Input Voltage Reduction
RE: Transformer Input Voltage Reduction
I have one engineer insisting of a 208-208V, Delta-delta isolation transformer, a special transformer that will have a special primary design to limit the inrush to within 5x only. On its secondary, another transformer will be connected who is having a high inrush. Since the isolation transformer is 1:1, once the downstream transformer is energized, its inrush will be seen as a "load inrush" and will be reflected at this isolation transformer's primary..so there is no mitigation happening. The isolation transformer will juts only be another impedance into the system. Is my concept correct?
RE: Transformer Input Voltage Reduction
RE: Transformer Input Voltage Reduction
Actually, I am doing a document for me to present to the stakeholders the kind of worth of trying to mitigate it and what are the counter productive effects and uncertainties of doing so. Its actually associated with the UPS problem I have posted earlier and the best way to solve the problem is to connect these input transformers (of the rectifiers) to normal AC sources and provide manual transfer switches for incoming supply diversity.