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Application Note

Using PRI In-Vacuum Stepper Motors

Revised 6 Jan 2003

Handling & Installing the Motors:

The motors are carefully cleaned to UHV standards and baked out before delivery to the customer, and should therefore not be touched with bare hands. The motors should never be immersed in cleaning solvents because the special vacuum-compatible lubrication system may be disrupted. Also, the motors have internal areas that may trap cleaning solvents. Should a motor or slide become fingerprinted, use a cleanroom wipe moistened with a mixture of freon and alchohol. If the contamination is extensive, return the item to PRI for disassembly and cleaning.

All three sizes of motors can be mounted with standard servo mounting clamps available from many sources. The motors may also be mounted with screws into the front face of the housing. However, use caution with the latter method to avoid screws which are long enough to protrude inside the case. These could contact the motor windings, causing shorts to ground or winding failure.

The Teflon insulated lead wires are almost impossible to strip with conventional mechanical wire strippers. A thermal stripper is strongly recommended. We use the Meisi model 4a Hot Tweezers, (Meisi Corp. 3350 Willow Lane, Westlake Village, CA 91361 Phone: 805-497-2626).

The color codes for the motor leads are as follows:

Motor Sizes A & B: Motor Size C:
Phase A Brown Phase A Brown
Phase B Red Phase B Red
Phase C Orange Phase C Orange
Phase D Yellow Phase D Yellow
Common Black Common A,C Black
    Common B,D White

Wiring Diagrams:

Size A & B Motors to SK-1 Motor Controller
Size C Motor to SK-1 Motor Controller


When designing your application, keep in mind that these motors are of the variable reluctance type, and thus contain no permanent magnets. Therefore, the motors have no detente torque when not energized.

If you plan to use the motors at cryogenic temperatures, please read our application note Using PRI In-Vacuum Motors at Cryogenic Temperatures.

Precautions to Avoid Overheating the Motors:

The problem is obvious: if power is applied to an object suspended in vacuum, and work and heat are not removed, the object will heat up until the losses due to radiation cause an equilibrium. The temperature rating of the motor windings is 125° C; at higher temperatures the insulating materials begin to degrade. This 125° C temperature can be reached in several minutes after the application of maximum input power to a thermally isolated motor. To make matters worse, the motor case temperature will only equal the winding temperature at steady state with no heat flow. In practical applications time constants of twenty minutes or more are common.

Fortunately, the situation is not quite as bad as it sounds. Motors are always connected to a load with some kind of mounting. This mounting can often be designed to conduct heat away. For example, the mounting can be a block of OFHC copper or similar material, bored out and split to clamp the length of the motor casing. This simple approach works best with motors that are operated intermittently at full voltage (operation of a minute or less with a duty cycle of 5% to 10%). For motors that must be operated continuously at full power, the mounting block can be mounted to the chamber wall. For the most demanding applications cooling water can be circulated through a jacket surrounding the motor case.

A thermocouple is useful for temperature testing during design of the motor application. Attach it to the motor case at the point farthest from the mounting. Testing should begin at about 20% of the full rated voltage for the motor, and sufficient time should be allowed for thermal equilibrium to be reached before increasing the voltage. Remember that time constants of twenty minutes or more may be expected.

Some general guidelines for driving PRI motors in vacuum:

Torque Ratings and Heat Buildup:

Torque ratings given in the motor specifications were obtained as follows:
On size A and B motors, torque was measured with 28 volts applied and dual phase excitation. Note that applying 28 volts continuously to an un-mounted motor in air will eventually burn out the motor. Therefore in vacuum this voltage must be reduced, and a very good heat sink supplied to permit continuous operation without damage. For a properly heat-sinked motor in vacuum, the full 28 volts should be applied for no more than one minute at a time (the stepper motor driver that we supply for these motors is adjusted to provide 24 volts to the motors).

The size C motor ratings were measured using 15W resistors in series with the common windings, and with up to 28 volts applied dual phase. The two resistors are required because voltages greater than 20 volts overload the motor, causing the maximum current per phase to exceed the 1.0 amp rating. For vacuum operation size C motors should be de-rated as described above for the A and B sizes.

Running Torque vs. Drive Parameters, Size C Motor:

This is slow speed running torque in in-oz, measured with a 15W resistor in series with each common lead.

Conditions Volts In Dual Phase Single Phase
Overloaded, 1.4A/phase 28 9.0 5.25
Overloaded, 1.4A/phase 24 7.75 3.75
1.0 A/phase 20 6.25 2.75

Steady State Case Temperature in Vacuum vs. Drive Parameters, Size C Motor:

Measured with 15W resistor in series with each common lead, and with the motor unmounted and lying on its side. This lack of mounting is the extreme worst case for heat buildup. The data show clearly that the motor must be mounted and heat sunk for continuous operation at high torque.

Volts In Dual Phase °C Single Phase °C
12 >150 146
11   130
10   108
9   96
8   81
7   68
6   58
5   50

Steady State Out Gassing vs. Motor Temperature, Size C Motor:

System baked out for 24 hours at 125°C, pumping with l/s ion pump only.

Motor Temp °C System Pressure Torr
25 4.0 x 10-10
58 2.8 x 10-9
68 3.4 x 10-9
81 5.4 x 10-8
96 1.6 x 10-8
108 5.2 x 10-7
124 3.6 x 10-7
127 5.4 x 10-7
130 9.0 x 10-7

Obviously, for continuous operation the motor temperature must be kept below 100°C for good vacuum performance. During these tests it was found that the motor could be operated intermittently at full power (20 volts, dual phase, 15W dropping resistors) for periods of roughly thirty seconds with the system pressure remaining in the 10-10 Torr range.

IMPORTANT: This application note is not intended as a substitute for user design and testing. With proper application design and testing these motors can provide long and reliable service in a variety of in-vacuum applications. Every application is different, however, and therefore PRI cannot guarantee these motors against burnout.

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