AMI 420, Installation, Operation And Maintenance Instructions, Page: 49 / 134

Rev. 7
31
Installation
Magnets w/o Persistent Switch
2.5.7 Third-Party Power Supplies
The Model 420 has been designed to function with a wide variety of third-
party power supplies. Please contact an AMI Technical Support
Representative for compatibility with specific models. Custom
modifications can be made to accommodate supplies that are not
compatible with the standard Model 420 configurations.
2.6 Special Configurations
The Model 420 has been designed for optimal operation with a
superconducting magnet (i.e. a very low resistance, high inductive load)
with a persistent switch. The Model 420 is capable of operating other
loads, however, some modification to the instrument settings and/or
connections must be considered. Two commonly encountered
configurations are 1) superconducting magnets without a persistent
switch, and 2) operation on a short-circuit or resistive load.
2.6.1 Superconducting Magnets without a Persistent Switch
For superconducting magnets without a
persistent switch, the Model 420 requires
the addition of an external stabilizing
resistor in parallel with the magnet per
Figure 2-7. If the stabilizing resistor is
omitted, the system current will oscillate
when attempting to charge the magnet.
The general guidelines recommended by
AMI for selecting the resistor value and
estimating the power handling
requirement are as follows:
where R is the required resistance in
ohms, L is the magnet inductance in
Henries, V
L
is the voltage limit setting of
the Model 420 in volts, and P is the
required power rating of the resistor in
Watts. For best results, R should be
chosen as close to the calculated value as
is practical, typically within 25%, with a
maximum value not exceeding 20 ohms.
Stabilizing Resistor
Figure 2-7. Illustration of
stabilizing resistor in parallel
with the magnet.
R 2
π
L
≤
P
V
L
( )
2
R
------------- =