Re: FAQ - Ballasting a power transformer.
Posted: Mon May 26, 2014 5:19 am
Just to add
The reason a ballast is used is to add a real impedance to the circuit to limit dI/dT and dV/dT both of these terms are very destructive to all systems so limiting voltage and current transients allows control circuits to act.
As Richard and Chris have said a value somewhere between 5% and 50% of the power available is about right unless you know all the parameters of the power supply and load where you may confine these limits to a tighter range.
Taking into account response time of the supply an unballasted system will appear to have negative resistance which is the most damaging of all conditions for the supply ie this can be asking the supply to to cope with more than twice of what it is capable of.
ie the time lag of the supply tends to reinforce not negate what is happening.
The Ballast slows system response so that the supply can correct /and /or limit what is going on.
Bode plots and their theory give the insight into this--it all comes down to phase margin (a measure of lag in the power supply)
Happy Ballasting
The reason a ballast is used is to add a real impedance to the circuit to limit dI/dT and dV/dT both of these terms are very destructive to all systems so limiting voltage and current transients allows control circuits to act.
As Richard and Chris have said a value somewhere between 5% and 50% of the power available is about right unless you know all the parameters of the power supply and load where you may confine these limits to a tighter range.
Taking into account response time of the supply an unballasted system will appear to have negative resistance which is the most damaging of all conditions for the supply ie this can be asking the supply to to cope with more than twice of what it is capable of.
ie the time lag of the supply tends to reinforce not negate what is happening.
The Ballast slows system response so that the supply can correct /and /or limit what is going on.
Bode plots and their theory give the insight into this--it all comes down to phase margin (a measure of lag in the power supply)
Happy Ballasting