American Magnetics 420 Power Supply Programmer - Manual (Page 50)

32 Rev. 7
Installation
Operation on a Short-Circuit
2.6.2 Short-Circuit or Resistive Load
If operating with a short-circuit as a load without the presence of a
superconducting magnet, the Model 420 must be manually configured for
stability. Normally, when the persistent switch heater is deactivated, the
Model 420 essentially sees a short-circuit as the load since the persistent
switch shunts all current flow away from any connected magnet.
Therefore, one method of operating a short-circuit is to indicate that a
persistent switch is present with the persistent switch heater deactivated.
The preferred method is to indicate that a persistent switch is not present
(see paragraph 3.2.2.4) and adjust the stability setting (see paragraph
3.2.2.1) to control the load. A stability setting of 100% will always allow
control of a short-circuit as the load, regardless of the state of the
persistent switch heater.
If the resistance of the load is increased, the stability setting must be
decreased to improve the transient response of the system. If the current
appears to lag, then decrease the stability setting until the system is
responsive. If the current appears to oscillate, increase the stability setting
until the oscillations are damped.
Note
If you have purchased a superconducting magnet with the Model
420, AMI will normally provide a recommended stability setting for
optimal operation of the magnet system. If you operate the Model
420 with a different load, be sure to restore the stability setting to
the recommended value when the superconducting magnet is
reconnected.
The stability setting is essentially manual control of the gain of a
integrator present in the control logic of the Model 420. Increasing the
stability setting decreases the gain of the integrator.
A special case is the energy absorber designs available from AMI. For
example, the Model 601 is an infinite-resistance device until 5 VDC is
achieved across its terminals. Once the 5 VDC “bias” is present, the Model
601 allows current flow with a nominal 2 m
Ω
series resistance. Therefore,
the Model 420 will require an “integration time” to overcome the 5 VDC
bias. Once the bias is achieved, the series resistance is minimal and the
Model 601 apears as a short-circuit. It is not possible to decrease the
stability setting to remove the integration time, since once the 5 VDC bias
is achieved, the load is a short-circuit and the system will become
unstable.
Note that when operating with a superconducting magnet in the circuit,
the integration gain of the Model 420 will be adequate to quickly “bias” the
Model 601 and achieve a proper current ramping profile.