APPLICATION NOTE CRA2512 Industrial Controls Use of the Low Ohmic Resistor CRA2512 in Inverter Drives T he 3 phase inverters are used in variable speed control applications. They can be found in a wide spectrum of automated controls including paper machines, presses, fans in cooling towers, pumps for water supplies, or axial, transversal and hoisting movements in cranes and many more. + For Use in: Paper machines Presses Cooling tower fans Water pumps Crane hoists DC 3 Phase AC Motor Inverters provide accurate and efficient control of standard motors creating cost-effective solutions. AC motors are less expensive than DC motors which would otherwise be used. Low ohmic resistors are an economical way to monitor and regulate the current used in the 3 phases by connecting them between a transistor on Figure 1. Typical schematic of a 3 phase each leg and ground. An A/D inverter driving a motor with low converter and microcontroller ohmic resistors used for control. calculate the current and establish what actions are needed by the PWM controller. The accuracy of the control relies upon the current measurement of resistors remaining within the budgeted tolerance. Current Measurement Resistor R1 is the Resistance measured at Temperature T1. Data sheets specify the TCR under conditions where the part is not placed under power. The T’s resistance of the device is measured at ambient temperature (+25 °C) and then again at the temperature extreme (-55 °C or +125 °C). However the surface temperature of the chip will increase when placed under power and this in turn will affect the resistance. Furthermore, The Temperature Coefficient (TCR) of a resistor is used the actual ambient with the following formula: temperature can be quite high under normal operating TR1(T2-T1) conditions. This is because the TC = TR2-TR1 x 106 circuit board will be confined in an enclosure with other TC = Temperature coefficient in PPM/ °C TR2 = TR at ambient temperature T2 heat-generating components TR1 = TR at ambient temperature T1 106 = Conversion factor to PPM with little or no air movement. APPLICATION NOTE CRA2512 Industrial Controls Use of the Low Ohmic Resistor CRA2512 in Inverter Drives T he CRA2512 series is rated to 3 watts of continuous applied power. It consists of a copper nickel alloy plate with solderable terminals and has the dimensions of a standard 2512 size resistor. Its high power density allows for some PCB space reduction. The TCR is specified as ±75 PPM/°C with the resistance being measured typically between an ambient temperature of 25 °C and either +125 °C or -55 °C. As a rule, power components such as shunt resistors are run at 30 - 50 % of their rating in real applications. This allows for operating at higher ambient temperatures and should result in a longer mean time to failure (MTTF). Commercial inverter drives are very compact and the temperature within their enclosure can often exceed 80 °C when running at full load. It is therefore more useful to understand how the resistor will perform under power at higher ambient temperatures. Shown below are resistance readings from a unit powered at 1 watt at three different ambient temperatures. The matrix also shows the temperature on the surface of the units. Power = 1 W T = 25 °C T = 85 °C T = 125 °C Resistance 10 mW 10.045 mW 10.075 mW Surface Temperature 48 °C 101 °C 138 °C The results show a remarkably low drift even under load conditions and at high operating temperatures typically found in real life applications. For more information on the CRA2512 and other resistive products from Bourns, please visit www.bourns.com Americas: Tel +1-951 781-5500 Fax +1-951 781-5700 EMEA: COPYRIGHT© 2008 • BOURNS, INC. 02/08 e/N0804 “Bourns” is a registered trademark of Bourns, Inc. in the U.S. and other countries. Tel +41-(0)41 768 55 55 Fax +41-(0)41 768 55 10 Asia-Pacific: Tel +886-2 256 241 17 Fax +886-2 256 241 16