The Model 420/430 Power Supply Programmer provides the control logic and interface between the power supply system and magnet. Oscillations can occur for a number of assignable reasons which are normally easily corrected. The following is a list of reasons why such a power supply system may oscillate and suggestions on how to avoid this.
1. Stability Setting of Model 420/430 is too low
The stability setting of the Model 420/430 programmer effectively changes the control gain of the unit. The higher this setting the slower the response time but the more stable the control will be. If this is the cause of the instability then changing the stability setting within the Model 420/430 menu to 100% should stop the oscillations. To determine the optimum setting you can begin backing off on the stability setting 1% at a time from 100%. After reaching 95% you can continue to reduce it in 5% increments. Once you begin to observe oscillations increase the setting by 1-5%. You may desire to further tune the stability setting by making incremental increases in the stability setting if "ringing" at the end of a current ramp is observed and undesirable.
2. Running power supply on a short circuit
If you operate the power supply on a short with the stability setting less than 100% it will oscillate. Most power supply systems sold by AMI leave the factory configured for the specific magnet they will be operated with. It is a good idea to become familiar with your power supply by operating it on a short first (load cables connected together), however, you will see oscillations unless you changed the stability setting to 100%. After you are finished with short circuit operation be sure to restore the stability setting to it's factory recommended/configured value before energizing the magnet.
3. Diode Failure
Many superconducting magnets are protected by sets of diodes across different sections of the magnet. Occasionally a quench or other such stressful event may damage one or more of these diodes. Diodes normally fail closed. When this happens the effective load characteristic that the Model 420/430 sees is different. Under the certain conditions this may cause instabilities in the control loop and the unit will begin oscillating. If your system oscillates the first time you use it then diodes are probably not the problem. If your system has run well without oscillating for extended periods of time and then begins oscillating after some abnormal event (quench, etc.) then diode failure may be the problem.
4. Persistent Switch Heater Issues
Heater Current is too low - If the power being supplied to a persistent switch is not enough to make the switch fully resistive, i.e. it remains partially superconducting, then the Model 420/430 is not seeing the correct magnet load characteristics and the unit may oscillate. To correct this problem try increasing the switch heater current setting in the Model 420/430 by 3 mA. Caution: Overheating the switch can have the potential of permanently damaging the switch by burning out the heater wires. Do not increase your switch heater current more than 10 mA above the factory setting without first consulting the magnet manufacturer to make sure this switch should survive the increased power input.
Heater warm-up time is too short - To ramp a magnet up or down the persistent switch must be heated if one is installed. The superconductor in the switch is heated from 4.2K to an appropriate value above 10K or so, at which point the switch becomes fully resistive. The time it takes for this temperature rise to occur will vary somewhat depending on switch design and other parameters. If the Switch Heater Warm Up Time setting for the Model 420/430 is too low then it will try to take control of the magnet before the switch is resistive. This can cause oscillations. If this is the problem simply increase the switch heater time in 5 or 10 second increments until the oscillations stop.
The Heater is not connected or is mis-wired - If the heater circuit to the persistent switch is not complete the switch will not heat and thus the Model 420/430 will be effectively trying to control a short circuit load and oscillations will be observed. Check to ensure proper connections on the rear of the Model 420/430 and and the magnet top plate feedthru connector. Proper resistance values should also be measured across the switch heater at the feedthru connector. Any open circuit or abnormal resistance measurements may indicate mis-wiring or possibly a damaged switch.
- Stabilizing resistor not installed on magnet without a persistent switch - The Model 420/430 is designed for optimum performance on magnets with persistent switches. To achieve reasonable response times with a Model 420 and avoid oscillations on magnets without a switch, an external stabilizing resistor must be added to the circuit. The Model 430 will be stable with magnets without persistent switches. For Model 420's, 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, VL 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 (2Ï€L) as is practical, typically within 25%, with a maximum value not exceeding 20 ohms. The preferred location of the stabilizing resistor is directly across the current leads as shown in Figure 2-7 below. Locating the resistor as close as possible to the magnet maximizes the accuracy of the current which is actually running through the magnet.
On systems equipped with retractable/break-away current leads or where top plate space is at a premium, it may not be practical to locate the resistor close to the magnet. In this case simply follow the power leads back to the rear of the electronics and mount the resistor there.
The resistor can be located on the rear of the power supply system across the (+) terminal of the power supply to the (+) terminal of the Model 420 when a Model 601 Energy Absorber is not present.
On systems with a Model 601 Energy Absorber present, the resistor can be connected from the (-) terminal of the Model 601 to the (+) terminal of the Model 420 as shown in the figure below.
If you are unsure about how to best connect the stabilizing resistor then please contact an Authorized AMI Support Representative.
Model 420 ROM version 1.3 or higher is also recommended. The version is displayed when the Model 420 is powered-on. Please contact an Authorized AMI Support Representative to request ROM upgrades for the Model 420.
Model 430 ROM version 2.01 or higher is also recommended...