Meaning_of the IOT Fault "IOT does not indicate a failure; it indicates a condition of the system. The IOT circuit monitors each power semiconductor in the inverter and watches for improper operation. It is important to understand the actual function of the IOT circuit is to protect the VSD and make troubleshooting simpler. Basically, the assumption is that for normal operation, when the IGBT is supposed to be on, it will look like a switch, or have a very low voltage drop. If, on the other hand, the device is asked to handle more current than it is capable of, the voltage drop across it will rise dramatically. The IOT circuit, therefore monitors the voltage across each IGBT. When that voltage exceeds a predetermined threshold (approximately 8 V), the IOT circuit turns the device off within 1uS and sends an inhibit signal to all other IGBT gate signals as well as a stop command to the microprocessor. Because of this, within about 10-15uS of detecting a problem on one power device, the entire drive will be shut down. Note that the IOT circuit does not tell you that there has been an over current (although that will be the most common reason for an IOT) or that the device is bad. simply tells you that there has been too much voltage drop across the device during operation Causes of the IOT Fault There are four possible sources of an IOT condition. Incoming Transient/Power Faults Converter Hardware Faults Inverter Hardware Faults Load Faults Incoming Transient/Power Faults A transient disturbance on the power system can cause the DC bus voltage to increase rapidly. Since the VSD output frequency must follow the voltage, this means that if the DC bus voltage increases, there will be extra current on the output as the drive tries to accelerate the motor. The sudden increase of output current can result in an IOT condition. trospeed Advantage Variable Speed Drive Troubleshooting Manual Converter Hardware Faults Abrupt changes to the DC bus voltage will cause changes to VSD output frequency and output current which in turn can cause an IOT condition. Inverter Hardware Faults The VSD output current, voltage, and frequency are dependent on proper operation and switching of the output IGBTs. Any interruption to the switching of IGBTs will result in Incorrect operation which will most likely create an IOT condition. Load Faults Load faults are usually due to a sudden arc of electricity to earth-ground. Examples of load faults would be if one phase of the motor or cable experienced insulation failure which provides a low-resistance path to ground . Typically this will cause increased current through one VSD output phase and may result in an IOT condition. Measure With the possible sources of the I0T condition defined above, this section will provide a method of identifying which of the above sources IS most likely to be the cause of the I0T Please note that the process below identifies as many of the relevant steps in the source identification as possible. When Field Service assumptions can be made why a step was either skipped or combined with other steps, it is a best practice to include that reasoning in the field Service report. The flow chart in figure 1 and 2 provides the general steps in defining which direction the investigation should take in assessing each of the four sources of IOT condition as the root cause.

Meaning_of the IOT Fault "IOT does not indicate a failure; it indicates a condition of the system. The IOT circuit monitors each power semiconductor in the inverter and watches for improper operation. It is important to understand the actual function of the IOT circuit is to protect the VSD and make troubleshooting simpler. Basically, the assumption is that for normal operation, when the IGBT is supposed to be on, it will look like a switch, or have a very low voltage drop. If, on the other hand, the device is asked to handle more current than it is capable of, the voltage drop across it will rise dramatically. The IOT circuit, therefore monitors the voltage across each IGBT. When that voltage exceeds a predetermined threshold (approximately 8 V), the IOT circuit turns the device off within 1uS and sends an inhibit signal to all other IGBT gate signals as well as a stop command to the microprocessor. Because of this, within about 10-15uS of detecting a problem on one power device, the entire drive will be shut down. Note that the IOT circuit does not tell you that there has been an over current (although that will be the most common reason for an IOT) or that the device is bad. simply tells you that there has been too much voltage drop across the device during operation Causes of the IOT Fault There are four possible sources of an IOT condition. Incoming Transient/Power Faults Converter Hardware Faults Inverter Hardware Faults Load Faults Incoming Transient/Power Faults A transient disturbance on the power system can cause the DC bus voltage to increase rapidly. Since the VSD output frequency must follow the voltage, this means that if the DC bus voltage increases, there will be extra current on the output as the drive tries to accelerate the motor. The sudden increase of output current can result in an IOT condition. trospeed Advantage Variable Speed Drive Troubleshooting Manual Converter Hardware Faults Abrupt changes to the DC bus voltage will cause changes to VSD output frequency and output current which in turn can cause an IOT condition. Inverter Hardware Faults The VSD output current, voltage, and frequency are dependent on proper operation and switching of the output IGBTs. Any interruption to the switching of IGBTs will result in Incorrect operation which will most likely create an IOT condition. Load Faults Load faults are usually due to a sudden arc of electricity to earth-ground. Examples of load faults would be if one phase of the motor or cable experienced insulation failure which provides a low-resistance path to ground . Typically this will cause increased current through one VSD output phase and may result in an IOT condition. Measure With the possible sources of the I0T condition defined above, this section will provide a method of identifying which of the above sources IS most likely to be the cause of the I0T Please note that the process below identifies as many of the relevant steps in the source identification as possible. When Field Service assumptions can be made why a step was either skipped or combined with other steps, it is a best practice to include that reasoning in the field Service report. The flow chart in figure 1 and 2 provides the general steps in defining which direction the investigation should take in assessing each of the four sources of IOT condition as the root cause.

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