Most Frequently Asked Questions about Vacuum Capacitors

1. What are the different types of vacuum capacitors your company offers?

Jennings Technology is the largest designer and manufacturer of vacuum capacitors in the world. Jennings capacitors utilize vacuum and gas as the dielectric. There are fixed and variable capacitor models. Fixed capacitors are preset during production and remain constant through out their life. Variable capacitors utilize a specially designed mechanical interface that allows the capacitance to be varied throughout a preset range.

2. What range of capacitance do your products cover?

Jennings has the broadest product line offering of vacuum and gas filled capacitors. The fixed and variable capacitor models cover a capacitance range from 1pF up to 5000pF. With this variety of products, voltage and current levels also vary greatly with maximum values of 60 kV and 1000 amps. Operational characteristics will vary greatly depending on the capacitor design. Be sure to verify all operational specs and product specifications.

3. Why is there a difference between test voltage and working voltage?

Testing voltage is the maximum 60 Hz voltage that can be applied to the vacuum capacitor without breakdown occurring. Capacitors are tested at this voltage to determine the general condition of the capacitor. Working voltage is the maximum peak voltage that can be applied continually.
A complete response to this question can be found in the Jennings TechNote C-2: "The Effects of Voltage on Vacuum Capacitors"

4. What's the reason for a vacuum dielectric?

A vacuum provides a dielectric constant that closely achieves a value of 1. With such a dielectric Jennings capacitor designs can achieve the maximum voltage gradient per given space, thus enabling the smallest compact envelope.

5. Can you specify different capacitance vs. turns ratio?

Yes, capacitance vs. turns ratio can be changed for all variable capacitors. These changes can be implemented in the factory during original production of the capacitor. Jennings offers a variety of turning ratios for most of the variable capacitor designs. Please consult with our Sales Department for details.

6. What is the maximum temperature your capacitor specifications are based on?

The maximum temperature is based on MIL-C-23183 specifications. Most ceramic capacitors are rated for a maximum temperature of 125C and glass capacitors at 85C. A complete response to this question can be found in the Jennings TechNote C-1: "Vacuum Capacitor Characteristics "

7. What is the average life of a Jennings capacitor?

The mechanical life of variable capacitors is related to length of stroke, speed of operation, bellows material, and application. Extensive mechanical life tests have been run and continue to run. Life test results are generally based off of a 50-75% stroke length and 200-300 rpm. A cycle is considered one complete back and forth rotation of the lead screw. The most recent capacitor designs are achieving approximately 3 Million cycles. Contact the Jennings Sales Department for details relating to the different capacitor models.

8. What standard options are offered on Jennings capacitors?

There are a variety of modifications and options available with Jennings capacitors. It is recommended that you consult with the Sales Department to determine the best options for your specific requirements. Some of the more popular options with our capacitors are: Non-magnetic versions, Mil-Spec qualifications, various turns vs. capacitance ratios, mounting thread type, and corona rings.

9. What is a Q factor?

The "Q" factor is the quality factor. It is a ratio of stored energy to dissipate energy. A complete response to this question can be found in the Jennings TechNote C-1: "Vacuum Capacitor Characteristics "

10. What's the standard warranty?

The standard warranty on all Jennings Capacitors and Relays is 3 years and 1 year for all contactors, interrupters and coaxials, from the date of manufacture. The Warranty Policy is as follows: Jennings warrants that, at the time of shipment, the products manufactured will be free from defects in material and workmanship and will conform to the specifications furnished by or approved, in writing, by Jennings.

11. How do I determine the maximum current of a capacitor?

The maximum allowable capacitor RMS current varies as a function of operating frequency, assuming there is a constant RMS peak working voltage. Standard derating curves are published in our catalog. A complete response to this question can be found in the Jennings TechNote C-3: "The Effects of Current on Vacuum Capacitors"

12. Does torque change with time?

Yes, the torque required to change capacitance values does change over time. The changes in torque will appear randomly and are not directly related to time or capacitor age. Additionally the changes in torque will be nominal and should never go higher than the rated torque specification. Changes in torque will be driven primarily by lubricant distribution, stroke length, and operational temperature.

13. How can I get the maximum cycle life from my variable capacitor?

Jennings offers a variety of different capacitor designs. Among these models are application specific designs. It is crucial that you chose a capacitor model that has been designed for your application. Contact the Jennings Sales Department for help in this area.
The other key factors that effect the cycle life of variable capacitors are operational temperature, redistribution of lubricants, capacitor stroke length and drive speed.
Operational Temperature: maximum cycle life will be achieved through operations at ambient levels of 20C. Elevated temperatures will lead to more stress on the individual capacitor components as well as to the lubricants used in the capacitor. Jennings capacitors are designed for temperature ranges up to 125C.
Distribution of lubricants: The least amount of stress on the lead screw and plug combination will result when the capacitor lubricant is evenly distributed over the entire stroke of the capacitor. It is recommended to stroke the entire length of the capacitor (regardless of working range) every 10,000 cycles in order to redistribute the lubricant. Capacitor stroke length: A shorter stroke length will provide the least stress on the bellows and the lead screw/plug combination.
Drive Speed: Tuning the variable capacitors at a slower rotational speed provides for less stress on the lead screw and bellows and thus will yield a longer life on the capacitor. Additionally hand tuned capacitors also exhibit excellent life characteristics due to the slow tuning speeds. High RPM tuning (>500rpm) tends to degrade cycle life.

14. Can I increase the current rating safely?

The rated current specification for each capacitor is determined by the maximum current the capacitor can continuously handle when the capacitor is set to maximum working voltage with normal convection cooling (at ambient of 25C). It is not recommended to go over the specified current level. Refer to TechNote #C-3 for further information on: "The Effects of Current on Vacuum Capacitors"

15. Is there a safety factor built into your voltage rating?

Jennings builds in a safety factor to all working voltage levels for its capacitors. The Peak Test Voltage is the maximum 60 Hz voltage, which can be applied to the capacitor without breakdown occurring. The RF Working Voltage of the capacitor is recommended as 60% of the Peak Test voltage.

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