Brake Resistors in Frequency Converter Applications

Krah adds dynamism to the PTC market with the new RXLG PTC series

Roger Renfordt, Krah
Bert Weiss, Rutronik

Self-protecting PTC brake resistors are often used for frequency inverters in the low power range as well as for servo controllers. Currently, there are very few manufacturers of these special brake resistors for these applications. The RXLG-PTC series from Krah means new options with the required dielectric strength are now available in the power range from 35 to 140W. The RXLG-PTC series is a group of PTC brake resistors for smaller frequency converters and servo controllers. Krah has developed them together with well-known suppliers from the PTC industry and thus offers design engineers a wider range of components with the required dielectric strength.

The RXLG-PTC resistors are available for rated loads of 35, 70, 105 and 140W. The resistance value when cold is 350W for the 35W type. At higher rated power, the resistance value of this type decreases to 175W (70W), 120W (105W) or 90W (140W). As is typical for PTC, the resistance tolerance is ± 35% and the breakdown voltage is only reached at 1100VDC. The new resistors are assembly compatible with existing systems.

To ensure the PTC resistors can consume as much energy as possible without being damaged, they are made of polycrystalline titanate ceramics. Ceramic as such is not conductive, but if it is specifically doped with foreign atoms, it has the properties of a PTC thermistor with a positive temperature coefficient. The resistance value of these ceramic bodies increases as the temperature rises. Above a certain temperature the resistance value increases almost exponentially. The resistors then no longer consume any more power, thus protecting themselves from overloading. The transition temperature of the RXLG-PTC resistors is roughly 140°C.

Optimized cooling

The focus of development was not solely on the optimum doping of the ceramics. Krah also developed the one-piece aluminum profile body in-house to ensure the real needs of industry are met. Both contribute to the improved performance of the RXLG PTC series compared to existing systems. The compact design enables a reinforced mounting base without exceeding an overall height of 11mm. The 98 x 34mm mounting base (for the 35W model) and the cooling profile allow the resistor ceramic to dissipate heat particularly well. Depending on the heatsink used, a further increase in performance is possible. This results in a wide power range and an array of application options for the RXLG-PTC resistors.

The resistors meet the requirements of Underwriters Laboratories and bear the UL Recognized Component Mark. They can therefore be used in products and systems that demand the UL quality mark and are available at www.Rutronik24.com.