AML Motor Home User Manual download pdf (Page 14)

Section 4

4. OPERATION OF VACUUM-COMPATIBLE STEPPER MOTORS (VCSM)

It is assumed that the reader is familiar with the production of UHV and the handling of UHV components.

The successful application of vacuum stepper motors requires an appreciation of their thermal as well as their

mechanical properties. Compared to motors operated in air the available cooling means for motors in vacuum are

much less effective, and until development of the B-series motors continuous operation was difficult to achieve.

Apart from extending the run time, operation at low temperature improves the outgassing performance of motors. For

this reason minimum running times and motor currents should always be pursued. Selection of the largest motor

possible for the application will result in longer running times, lower motor temperature and lowest outgassing.

Design mechanisms with balanced loads whenever possible, or arrange that either the static friction in the system or

the motor detent torque will hold position without the necessity of maintaining phase currents to produce a holding

torque. The 'h' command may be used to reduce the phase currents and produce a holding torque which is

intermediate between the pullout torque and the detent torque. Refer to section 4:6 for a full description of power-

reduction techniques.

Many applications that appear to require continuous running, for example substrate rotation for ensuring uniformity of

deposition or implantation, can be equally well performed by intermittent short periods of stepping at low duty cycle.

Stepper motors should not be disassembled as this partially demagnetises the permanent magnet in the rotor and

permanently reduces the torque.

4:1 OPERATING TEMPERATURE AND RUN TIMES.

The maximum recommended running temperature of AML motors is 175 Celsius, as measured by the embedded

type K thermocouple.

Current C-series motors have published temperature and time graphs for typical operating conditions with the motor

mounted by its flange. Continuous running can readily be achieved with care at medium phase currents. Run times

at higher currents can be increased by additional heatsinking at the other end of the motor.

AML motors are suitable for operation at 77K and they are believed to be suitable for use at lower temperatures.

Because the resistance of the windings at low temperatures is small the efficiency of the motor is much greater than

at normal temperatures. A resistance of a few ohms should be connected in series with each winding, in order to

present a normal load to the SMD2. The leads of the motor will be very brittle at low temperatures and should not be

allowed to flex. The normal mechanical and electrical properties of all materials are recovered on return to room

temperature.

4:2 OUTGASSING AND BAKEOUT.

Newly-installed motors will outgas, mainly due to water-vapour retention in polyimide. As this material is microporous

the water is released rapidly and the rate will subside after a few hours. The rate may be accelerated by running the

motor to self-heat it.

Baking at up to 200 Celsius is permissible, and a 24-hour bake at this temperature will normally reduce the outgassing

to its minimum.

Motors are typically operated at some distance from the chamber walls where the bakeout temperature is most often

controlled. If the temperature indicated by the motor thermocouple during bakeout is not high enough when the

bakeout period is well advanced it may be increased to 175 by applying drive power. The preferred method of doing

this is by using the "b" command described in Appendix B, since this energises both phases and keeps the motor

stationary in a half-step position. Keeping the motor hot by this means while the rest of the vacuum system cools is

recommended as this will prevent condensation on the motor. Where internal infra-red heaters are used for bakeout

it is advisable to shield the motor from direct radiation and to achieve the desired temperature during bakeout by

running the motor.

Irreversible deterioration of the winding insulation will begin to occur above 230 and the motor may subsequently

produce larger amounts of gas, even at lower temperatures.

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AML Motor Home User Manual Page 14

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