In-Circuit Capacitance and ESR Testing

Electronic Innovations were contracted to perform 5 yearly testing of Danfoss and Schneider variable speed drives (VSDs) at a water treatment plant. A total of 71 drives were tested, which consisted of 50 Danfoss 415V VSDs, 15 Danfoss 690V VSDs and 6 Schneider Altivar 415V VSDs. The DC bus capacitors within the VSDs were a focal point of the testing. These electrolytic capacitors have a limited life span. Failure of these capacitors causes failure of VSDs. Capacitors can be tested periodically and replaced when necessary to maximise the reliability of VSDs.

 

There are 5 important measurements when gauging capacitor state:

  1. Capacitance, which generally decreases as a capacitor ages (Venet et al., 1999).
  2. Internal equivalent series resistance (ESR). ESR increases as capacitors age and is a good indicator of their state (Venet et al., 1999).
  3. Physical inspection. Danfoss specify that a physical inspection of the capacitors should not show any deformation of the cases of the capacitors or electrolyte leaking from them.
  4. Leakage current. Danfoss warn that low voltage capacitor testers like digital multimeters (DMMs) are of no use in checking DC bus capacitors.The main concern is to ensure that the capacitors do not have excessive leakage current when the DC bus voltage is applied to them. This cannot be tested at a low voltage.
  5. Midpoint precision/equalisation. Danfoss specify the voltage across the positive half should be within 10% of the voltage across the negative half.

 

Additionally it is recommended to take capacitance and ESR measurements at the fundamental component of voltage ripples (Venet et al., 1999). In the case of three phase VSDs in Australia this is 300Hz. To this end Electronic Innovations designed and built an in-circuit capacitor tester that measures capacitance, ESR, leakage current and midpoint precision at full DC bus voltage (600V) by injecting current in the range of 5A to 10A at 360Hz through a capacitor bank. The capacitor tester is able to take these readings whilst a VSD is powered and running a motor. The tester is also able to perform low voltage motor impedance tests to calculate winding inductance and series resistance. See the below video a demonstration.

The following charts show some of the results we obtained. We found that measured capacitance was close to rated capacitance for all drives (Figure 1). There appears to be a linear relationship between drive power rating and bus capacitance for 415V VSDs (Figure 2). 690V VSDs also have this linear relationship but appear to be a step different from the 415V drives, i.e. lower rated capacitance for a given drive power rating. VSDs with higher DC bus capacitance had lower measured DC bus ESR (Figure 3). The Danfoss and Schneider 415V VSD motors had similar inductance between phases of each motor (minimum coefficient of variation 0.075% and maximum 5.631%). The Danfoss 690V drive motors had larger differences between phases of the same motor (minimum coefficient of variation 11.432% and maximum 14.725%). These motors are manufactured by ABB and we are still yet to determine the reason for this large variation in inductance (Figure 4).

 

Reference: Pascal Venet, Amine Lahyani, Guy Grellet, A. Ah-Jaco. Influence of aging on electrolytic capacitors function in static converters: Fault prediction method. European Physical Journal: Applied Physics, EDP Sciences, 1999, 5 (1), pp.71-83.