The Siemens inverter has an external resistor used for heating measurement, and the DC output voltage is around 550 volts, while it should normally be about 10 volts. What could be causing this issue?
A: It sounds like you're referring to the braking resistor. This component doesn't operate all the time. Instead, it's controlled by either the built-in braking unit inside the inverter or an external one. The braking unit functions as a switch, typically using an IGBT (Insulated Gate Bipolar Transistor) as its switching element. It’s connected in parallel with the inverter’s intermediate DC bus. When the DC bus voltage becomes too high — often during regenerative operations, such as when lowering a heavy load — the braking unit detects that the voltage exceeds its set threshold. At that point, the IGBT turns on, allowing the braking resistor to be connected to the DC bus. This allows current to flow through the resistor, converting the excess energy into heat and thereby reducing the DC bus voltage back to normal levels.
In the normal state you mentioned, where the voltage is around 10V, the braking unit is likely not active, and the braking resistor isn’t engaged. However, if the braking resistor is activated, the DC voltage can rise significantly, up to around 550V. For example, the Siemens braking unit has a high threshold of 774V and a lower threshold of 673V. If the voltage you measured was much lower than these values, it suggests that the braking resistor was already working to bring the voltage down.
The braking resistor generates heat as it consumes the braking energy, which is not very energy-efficient or environmentally friendly. A more advanced approach would be to feed the braking energy back into the grid, but small-capacity inverters usually use a braking unit combined with a braking resistor due to cost-effectiveness, simplicity, and practicality.
There are also fault scenarios to consider. For instance, if the braking unit is faulty and remains on regardless of the situation, the resistor will stay connected, continuously generating heat. You should be able to notice this by checking the temperature of the resistor.
Determining the correct braking power can be complex. In many cases, engineers select the braking resistor and unit based on experience. It's possible that the resistor might be oversized, leading to excessive heat generation. In such cases, increasing the resistor's power rating (by adding parallel resistors, though total resistance must remain unchanged) can help balance the power. However, if the difference is too large, simply increasing the resistor’s power may exceed the braking unit’s capacity, potentially causing damage. In those situations, it may be necessary to upgrade both the braking unit and the resistor.
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