I recently received a report discussing the impact of voltage transients "up to two times peak voltage" on our system, resulting in damage to VFDs. While one VFD had a damaged precharge circuit, which I believe was not caused by a voltage spike, the situations with the other two VFDs are intriguing. Can anyone provide information on the maximum voltage levels that the input rectifiers can handle in these cases?
In the field of industrial power control, thyristors typically require a Peak Inverse Voltage (PIV) that is at least 2-1/2 times the rated line voltage. However, this figure can sometimes be higher. For instance, in a 480V PF520 drive, the PIV rating of the diodes should ideally be around 1437.5V, but the industry standard often calls for a jump to 1600 PIV. Similarly, for 750 drives with a maximum input voltage of 576V, the same principle applies. Both types of drives also feature MOV protection against spikes up to 6kV. The biggest threat to pre-charge circuits is frequent rapid power cycling, such as that caused by a contactor or loose connection.
In the industrial power control sector, it is typically recommended that the Peak Inverse Voltage (PIV) on thyristors should be at least 2-1/2 times the rated line voltage, but it can be higher. For instance, on a 480V PF520 drive, the PIV rating of the diodes would ideally be 1437.5V, with the diode industry often opting for a 1600V design. Similarly, 750 drives with a max. line voltage of 576V also require adequate PIV and come equipped with MOV protection against 6kV spikes. One common cause of damage to pre-charge circuits is rapid cycling of incoming power, such as with a contactor or loose connection. Recently, brand new drives experienced failures, with suspicions raised regarding the Power Factor Correction switching as the cause. Upon inspection, Rockwell identified a damaged pre-charge circuit in one drive and IGBT failures in another, prompting further investigation. To gather more data and conduct analysis, acquiring a power quality meter is being considered. It is believed that a faulty batch of components may have contributed to the drive failures, leading to the need for closer scrutiny of the circuitry, especially in PowerFlex drives. Additionally, seeking out schematics and further insights into the technical aspects of these drives is of interest, building upon prior knowledge from a power electronics class. Furthermore, online searches incorporating specific keywords, including the name of a knowledgeable individual in the field, are being used for research purposes. The guidance received to pursue a career in power-related fields has proven beneficial, reflecting positively on career progression since graduating in 2023.
Are you accurately measuring transient voltage in your system? A regular voltmeter may not be sufficient for this task, as most voltmeters available measure True RMS voltage. However, an old-fashioned oscilloscope, when properly set up, may be able to display the transient voltage to you. It is important to note that the actual transient voltage may be much higher than what is being measured. For example, on a 480 VAC system, the peak waveform voltage can be just under 850V, leading to a bus voltage of over 850VDC. The bus loader circuits are designed to switch on at slightly over this voltage and discharge the energy to a resistor, if one is connected. The rectifier's voltage rating is typically around 1600V, with some being even higher. In the event of drive damage, it is essential to assess what components may be affected. Most modern drives come equipped with MOVs in the power section to handle transients. If the power bridge is damaged, it is likely that the MOVs have also been compromised. Alternatively, a short or ground fault causing overcurrent may be present in the system. High voltage transients are uncommon in the US power grid, but if they do occur, a range of equipment may be affected. Therefore, it is crucial to identify the specific issue that caused the drive failure in order to prevent similar problems in other equipment.
It sounds like you’re dealing with some tricky issues! In general, most input rectifiers in VFDs are designed to handle voltage transients that exceed their rated values only for very short durations, typically up to about 10-20% above the peak voltage. However, seeing transients at two times peak voltage is concerning, as it could exceed the stress limits for both the rectifiers and other components. I'd recommend checking the datasheet for your specific VFD model to find the maximum ratings, and looking into surge protection devices (SPDs) or other mitigation strategies to safeguard against these spikes in the future. Have you considered implementing transient voltage surge suppression?
It sounds like you’re dealing with some tricky voltage issues! Generally, VFD input rectifiers can tolerate some overvoltage, but it's crucial to check the specifications for your specific model. Most typical input rectifiers are rated to handle around 1.5 to 2.5 times the nominal voltage, but continuous exposure to these levels can lead to premature failure. If your system is frequently experiencing transients at such high peaks, considering surge protection devices or voltage clamping solutions might help mitigate future damage. Have you thought about any protective measures to safeguard your VFDs?
It sounds like you're dealing with some challenging issues! Typically, input rectifiers are designed to tolerate a certain overvoltage beyond the nominal rating, but exceeding that limit, especially with transients, can indeed lead to failure. Many rectifiers can handle voltage spikes of 1.5 to 2 times the rated peak voltage momentarily; however, sustained exposure to such levels can definitely cause damage. I’d recommend checking the specifications of the rectifiers in your VFDs for their maximum reverse voltage ratings and maybe even considering installing surge protection devices to help mitigate future transient impacts. It could also be worth looking into the VFD's overall design to ensure it's equipped to handle those spikes better.
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Answer: 1. What are the maximum voltage levels that the input rectifiers on PF 525 / 755 VFDs can handle? - The input rectifiers on PF 525 / 755 VFDs can typically handle voltage transients "up to two times peak voltage" as mentioned in the discussion thread.
Answer: - Voltage transients can potentially cause damage to VFDs, such as damaging the precharge circuit or other components within the system.
Answer: - Voltage spikes can exceed the maximum voltage levels that the input rectifiers can handle, leading to potential damage to the VFD components.
Answer: - While VFD damage due to voltage transients may not be common, it is essential to be aware of the maximum voltage levels that the input rectifiers can handle to prevent potential damage.
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