Tuesday, July 29, 2008

What's the Voltage Rating of an RG8-U Coaxial Cable?


Output cable and connectors are not trivial items for power supplies where output voltages can be 100,000 volts or higher. The cables and connectors used must function together as a system to safely and reliably access and provide the power supplies output for customer usage.

In many high voltage power supply applications, a shielded polyethylene coaxial cable is used. Polyethylene cables provide excellent high voltage dielectric isolation characteristics in a small but robust form factor. The shield conductor provided in a coaxial cable functions as a "Faraday Shield" for the center conductor of the cable that is referenced to the high voltage potential. If any breakdown in the main insulator occurs, the high voltage current will be bypassed to the grounded shield conductor that surrounds the main insulator. This inherent safety feature is one benefit of using a coaxial high voltage output cable.

RG8-U has long been used as a high voltage output cable in the high voltage industry. There is a variation of RG8-U that utilizes a solid polyethylene core. Specifications for this cable do not specify actual "high voltage" ratings, since this cable was not designed and fabricated with high voltage usage in mind. So the reality is, there are no high voltage ratings for RG8-U. Over the years others in the HV industry have used this cable at 20kV, 30kV and even higher voltages. Spellman does use RG8-U cable, but limits it usage to applications where the maximum voltage that will be applied to the cable is 8kV or less.

For voltages above 8kV where a coaxial polyethylene cable is desired, Spellman uses cables specifically designed and manufactured for high voltage usage.

These cables are of the same general design; as described above but the insulating core material diameter has been increased appropriately to obtain the desired dielectric insulating capability required. Frequently higher voltage versions of these cables utilize a thin semiconductor "corona shield". This corona shield is located between the metallic center conductor and the main polyethylene insulating core. This corona shield helps equalize the geometric voltage gradients of the conductor and by doing so reduces the generation of corona.

A high voltage cable and connector system can only be as good as the materials used to make it. Using cables that are designed, specified and tested specifically for high voltage usage assures that these materials are used within their design guidelines.

Tuesday, July 15, 2008

What Is the Difference Between Voltage Mode and Current Mode?

Power control, (a.k.a. power mode or power loop) is a third control mode that can be added to a variety of Spellman power supplies to provide another means to control and regulate the output of the supply. Voltage mode and current mode are the primary controlling modes of most units. Taking the voltage and current monitor signal and inputting them into an analog multiplier circuit, creates a power feedback signal (voltage x current = power). Using this feedback signal with an additional programmable reference signal in conjunction with error amplifier circuitry, a programmable power mode can be created.

Power control is typically used in two types of applications. The less common application is where the power into a load is the needed regulating parameter. A critical heating requirement may have very specific regulated thermal need. Using power mode, voltage and current limit levels can be established, and power mode will provide constant power to the load, immune from any impedance variations from the load itself.

The more popular usage of a power mode is in the area where a power source or load might be rated or capable of more current at reduced voltage levels, but limited to a particular power level. X-ray tubes frequently have this type of capability. If the maximum voltage were multiplied by this "increased current" capability, a power level above the rated power level would result. Power mode can address this problem by limiting the power to the maximum rated (or present) level.

Thursday, July 10, 2008

Where Can I obtain Information On High Voltage Safety Practices?

One of the most comprehensive publications regarding high voltage safety practices is an excerpt from IEEE Standard 510-1983 known as "The IEEE Recommended Practices for Safety in High Voltage and High Power Testing." This information is available from Spellman in the form of a printed document included in our "Standard Test Procedures and Safety Practices for High Voltage Power Supplies" handout. Please contact our Sales Department for a copy. Safety Procedures

Monday, July 7, 2008

What is Solid Encapsulation?

Solid encapsulation, also referred to as "potting," is an insulation media used in a variety of Spellman's supplies. The "output section" of a high voltage power supply can operate at extremely high voltages. The design and packaging of the high voltage output section is critical to the functionality and reliability of the product.

Solid encapsulation allows Spellman designers to miniaturize the packaging of supplies in ways that are unobtainable when utilizing air as the primary insulating media alone. Improved power densities result, providing the customer with a smaller, more compact supply.

Additionally, solid encapsulation provides the feature of sealing off a potted output section from environmental factors. Dust, contamination, humidity and vibration typically will not degrade or affect the performance of an encapsulated high voltage output section. This is especially important where a supply will operate in a harsh environment, or where a unit must operate maintenance free.

Wednesday, July 2, 2008

What is Corona?

Corona is a luminous, audible discharge that occurs when there is an excessive localized electric field gradient upon an object that causes the ionization and possible electrical breakdown of the air adjacent to this point. Corona is characterized by a colored glow frequently visible in a darkened environment. The audible discharge, usually a subtle hissing sound, increases in intensity with increasing output voltage. Ozone, an odorous, unstable form of oxygen is frequently generated during this process. Rubber is destroyed by ozone, and nitric acid can be created if sufficient moisture is present. These items have detrimental affects on materials, inclusive of electrical insulators.

A good high voltage design takes corona generation into account and provides design countermeasures to limit the possibility of problems developing. Spellman engineers use sophisticated e-field modeling software and a Biddle Partial Discharge Detector to ensure that each high voltage design does not have excessive field gradients, preventing partial discharge and corona generation.