Service – American Magnetics 4Q12125PS-430 High Stability Integrated Power Supply System User Manual

Page 159

Rev. 5

141

Service

Troubleshooting Hints

STATION CONNECTORS

and the connectors on the magnet

5.2.14 There is excessive LHe boil-off during operation.

Excessive LHe consumption is usually attributable to one or both of the

following: thermal energy being conducted into the cryostat or electrical

energy being converted into thermal energy within the cryostat. Analyzing

the circumstances under which the high boil-off occurs will help determine

what is causing the problem.

1. For magnets equipped with switches for persistent operation, verify

that the persistent switch heater power supply is operating at the

proper current for the installed switch. Excessive currents cause

excessive boiloffs. The typical switch requires approximately 45 mA

to function correctly. Refer to the documentation provided with the

magnet for proper operating current. See Figure 3-13 on page 73.

2. Verify that the protective diodes on the magnet are not turning on.

Damaged diodes may short causing current to flow through them

whenever magnet current flows and cause excessive heating. This

can be identified by observing a change in the apparent field-to-

current ratio since some of the current is bypassing the coil. If the

boil off rate returns to normal with the magnet de-energized, this

may indicate a defective diode.

3. Ensure that there are no inadvertent thermal paths between the

cryogenic environment and the 300K environment. Ensure all

transfer lines are removed from the cryostat; check the position of

break-away vapor-cooled current leads.

4. Ensure the LHe level sensor is not continuously energized if

continuous level indication is not necessary.

5. Ensure the vacuum in vacuum-jacketed dewars is of sufficiently

low pressure.

5.2.15 Cannot display the magnetic field strength, only current

Note

Setup menu limits are always required in terms of current.