Imagine you have a three-phase induction motor drive with an inverter that utilizes a braking resistor during the braking phase. While the motor acts as a generator, the electrical power generated is dissipated through the braking resistor controlled by a chopper. To protect the braking resistor from overheating, I am looking to implement software-based thermal protection. One challenge is the absence of a temperature sensor on the resistor, necessitating the need to estimate its temperature. My initial approach involves using the measured dc link voltage and current through the resistor to calculate its resistance using Ohm's law. By understanding the temperature dependence of resistance, I can then estimate the resistor's temperature. However, initial experiments using this method have resulted in unreliable resistance values, even after attempting to filter them with a low pass filter. Any suggestions on how to improve the reliability of resistance values using this method?
When considering the best approach for managing brake resistance in a drive, opting for a temperature switch may prove to be a more reliable and simpler solution compared to calculating it manually. An alternative option would be using a temperature sensor with an analog output to track the temperature fluctuations over time. For example, when working with an Allen-Bradley PowerFlex 525 drive, software is available to assist in selecting the appropriate resistor based on the expected duty cycle. By inputting the resistor information into the drive, it can then utilize internal algorithms to safeguard the resistor. While calculating the dissipated watts for the brake resistor may seem complex, it can be simplified by using the formula I^2 x R. It is important to refer to the brake resistor specifications for details on dissipated power, including duty cycle and wattage. Although obtaining synchronized samples of brake current and DC Bus voltage may be challenging and may not yield accurate results due to the constant nature of resistor resistance. However, the resistance of a resistor remains unchanged over time.
One potential solution could be a circuit breaker that can handle the thermal load averaging. Adding an auxiliary contact to disable the VFD in case of the breaker tripping might also be beneficial for optimal control of the system.
Your approach makes sense in theory, but remember that real-world resistors are not ideal and their resistance may vary with potential transient issues and temperature shifts. To improve reliability, it might be helpful to consider basing your temperature estimations on the amount of energy dissipated over a certain time period, which would effectively be the integral of power (voltage*current). As you’re interfacing this with software, you could take into account the fluctuating values and calculate an average over a set period. Additionally, if possible, try to account for ambient temperature, as it can also affect the braking resistor temperature. Lastly, confirming your inference model with some occasional manual temperature measurements would also enhance its robustness.
Your approach seems fundamentally sound, but measuring the resistor's temperature indirectly, as you're doing, is notoriously challenging due to a number of factors, such as changes in ambient temperature or variations in the material of the resistor itself. Instead of attempting to filter the measured resistance with a low pass filter, you may want to consider a more sophisticated signal processing technique, such as a Kalman Filter, which can provide optimal estimations even in noisy environments. Additionally, you might consider integrating a safer margin of tolerance for the resistor’s operating temperature in your system's design to ensure that the calculated temperature stays within safe parameters even under worst case conditions, to offer additional protection against overheating.
While I understand your approach, fluctuations in your measurements could be due to many factors, which is why you're seeing unreliable resistance values. Here's a suggestion: since you're using an induction motor drive with an inverter, it is capable of calculator motor losses. You could potentially take advantage of this by tracking motor losses over time (including parameter changes like load and speed), to more accurately estimate the power dissipated in the braking resistor. Also, remember heat dissipation of the braking resistor may not be linear, so considering factors like airflow or any heat-sink attached to the resistor could also help increase the reliability of your temperature estimates.
Effectively estimating temperature independent resistance in such circumstances is truly tricky. You could consider adopting a dual-slope method, a technique often used in ADCs. It’s built on the principle that the charging time of a known capacitor through a known resistor is measured, which could help provide more reliable resistance values. Alternatively, think about estimating the energy each cycle deposits into the resistor with the integral of V^2/R over time, assuming you have a good estimate for R at room temperature. You can then use the specific heat capacity and mass of the braking resistor materials to estimate its temperature rise each cycle. This method requires you to accurately measure voltage and time, and to also have a good estimate of R, but it might provide a more reliable outcome. Both methods of course have their own flaws and variables to consider, but they could be a good starting point for your dilemma.
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Answer: - One approach could involve using the measured dc link voltage and current through the resistor to calculate its resistance using Ohm's law, and then estimating the resistor's temperature based on the temperature dependence of resistance.
Answer: - One challenge is the reliability of resistance values calculated using methods like Ohm's law based on measured voltage and current, which can lead to inaccurate temperature estimations and ineffective thermal protection.
Answer: - Suggestions may include refining the calculations, adjusting the filtering process with techniques like a low pass filter, or exploring alternative methods for temperature estimation that provide more accurate results.
Answer: - Reliable thermal protection is crucial to prevent overheating and potential damage to braking resistors, ensuring the safe and efficient operation of the motor drive system.
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