gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
141213-2009 EST
I constant see comments on this forum that there is high inrush because the load is inductive. This is a totally incorrect statement.
A true pure inductor can not have its current instantaneously changed. If the initial current in the inductor is zero, and you apply a large voltage to that inductor, then the instant after the application of the voltage the current is still 0. With the voltage maintained constant (applies to AC or DC) the peak current will never exceed its steady state value. True perfect inductor means an invariant value of inductance.
The above will also be true for a real air core inductor with any amount of eqivalent internal series resistance.
Motors and transformers do not fall into the category of true perfect inductors.
In a motor at stand still there is no counter-EMF generated. When a motor is first started if looks like an iron core (ferromagnetic) inductor with series resistance. If the rotor is kept locked, then the current variation with time would behave as defined by the the characteristics of the particular electro-magnetic circuit. When the rotor is released then it spins up to its full speed value. The rotation of the rotor produces a counter-emf (a voltage that opposes the source voltage) that is approximately proportional to speed. Thus, at full speed there is only a small voltage applied across the series resistance-inductance circuit and current is much less than at locked rotor.
In an iron core transformer with a non-zero flux state at the time of application of a voltage, and the applied voltage forces the flux to a greater value, then the core is driven more toward saturation, and the magnetizing current increases. Essentially the inductance is lowered. High inrush currents will result. The closer the initial flux is to saturation at turn on the greater is the peak in rush current.
A pure capacitor can not have its voltage changed instantaneously. Assuming no internal series resistance in the capacitor, then the application of a large voltage to a capacitor with zero initial charge (0 volts on capacitor) will have a large inrush current defined by the internal impedance of the voltage source. This current decreases to its steady state value with time. We can not define what happens with a theoretically perfect voltage source connected to a perfect capacitor.
The inrush current of motors and transformers is not because these are loosely classified as inductive, but as a result of other factors.
.
I constant see comments on this forum that there is high inrush because the load is inductive. This is a totally incorrect statement.
A true pure inductor can not have its current instantaneously changed. If the initial current in the inductor is zero, and you apply a large voltage to that inductor, then the instant after the application of the voltage the current is still 0. With the voltage maintained constant (applies to AC or DC) the peak current will never exceed its steady state value. True perfect inductor means an invariant value of inductance.
The above will also be true for a real air core inductor with any amount of eqivalent internal series resistance.
Motors and transformers do not fall into the category of true perfect inductors.
In a motor at stand still there is no counter-EMF generated. When a motor is first started if looks like an iron core (ferromagnetic) inductor with series resistance. If the rotor is kept locked, then the current variation with time would behave as defined by the the characteristics of the particular electro-magnetic circuit. When the rotor is released then it spins up to its full speed value. The rotation of the rotor produces a counter-emf (a voltage that opposes the source voltage) that is approximately proportional to speed. Thus, at full speed there is only a small voltage applied across the series resistance-inductance circuit and current is much less than at locked rotor.
In an iron core transformer with a non-zero flux state at the time of application of a voltage, and the applied voltage forces the flux to a greater value, then the core is driven more toward saturation, and the magnetizing current increases. Essentially the inductance is lowered. High inrush currents will result. The closer the initial flux is to saturation at turn on the greater is the peak in rush current.
A pure capacitor can not have its voltage changed instantaneously. Assuming no internal series resistance in the capacitor, then the application of a large voltage to a capacitor with zero initial charge (0 volts on capacitor) will have a large inrush current defined by the internal impedance of the voltage source. This current decreases to its steady state value with time. We can not define what happens with a theoretically perfect voltage source connected to a perfect capacitor.
The inrush current of motors and transformers is not because these are loosely classified as inductive, but as a result of other factors.
.