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DETERMINATION OF CAPACITANCE AND EQUIVALENT SERIES RESISTANCE BASED ON DC LINK VOLTAGE

IP.com Disclosure Number: IPCOM000248414D
Publication Date: 2016-Nov-25
Document File: 5 page(s) / 177K

Publishing Venue

The IP.com Prior Art Database

Related People

Asko Kavala: AUTHOR [+2]

Abstract

The disclosure relates to the determination of an equivalent series resistance, ESR, and capacitance, C, of a DC link capacitor related to a converter device, such as an inverter or a frequency converter, of an electric drive. The estimation of the equivalent series resistance, ESR, and the capacitance, C, is done by measuring the DC link voltage and motor stator current, preferably whenever the motor is stopped; e.g. during an internal DC magnetization sequence.

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DETERMINATION OF CAPACITANCE AND EQUIVALENT SERIES RESISTANCE BASED ON DC LINK VOLTAGE

The disclosure relates to the determination of an equivalent series resistance, ESR, and capacitance, C, of a DC link capacitor related to a converter device, such as an inverter or a frequency converter, of an electric drive.

Preventive maintenance of converter devices, such as frequency converters, may be based on pre-planned replacement of wearing or ageing components. Exchange intervals follow component lifetime expectations which are based on empirical knowledge or schedules defined by manufacturer. However, the lifetime of a component can vary significantly, because of different operating environments and applications. Hence, operating condition of the components, such as the DC link capacitor, may need to be measured reliably under field conditions.

Known measurement methods of the DC link capacitors often do not fit the requirements. For example, a lack of appropriate measuring devices for field conditions, and also their reliability and price have not contributed the measurements to become more common. The measurements have often been difficult to perform, and they could also include risks like managing the complex mechanical structure of the converter device. As a result, the execution of the measurements may have taken a lot of time causing too high a price for the measuring service. Because of this, the aged components are replaced based on empirical knowledge or schedules defined by the manufacturer. Accordingly, there is a need for developing a new measuring method.

In this measuring method, the principle is to use the electric drive's own hardware as much as possible for measuring purposes. The estimation of the equivalent series resistance, ESR, and the capacitance, C, is done by measuring the DC link voltage and motor stator current, preferably whenever the motor is stopped; e.g. during an internal DC magnetization sequence. There is no torque induced in the motor because the current flows only in one direction. Figure 1 shows a schematic of an exemplary converter circuit, where the DC link capacitor is modeled as a series connected ESR and C equivalent circuit. The DC link capacitor may be a single capacitor or a capacitor bank comprising

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several capacitors connected in series and/or parallel, for example. The capacitor(s) may be an aluminum electrolytic capacitor(s), for example. The thick blue line in Figure 1 is for helping to follow the current flow to the stator from the capacitor bank in discharging mode.

The capacitor voltage is applied to the motor stator winding by using semiconductor (e.g. IGBT) switches S1, S2 and S6. S1 is switched with the constant duty ratio and frequency. S2 and S6 are constantly closed and other switches are open. When S1 is opened current freewheels through the freewheeling diode D4 and thro...