Can this film capacitor be used in output LC filter of SPWM inverter? - Page 1
Forum section: electronics / projects

author: onemilimeter
author_rank: Frequent Contributor
timestamp: September 19, 2015, 10:44:20 pm
content: The circuit of SPWM inverter is shown in Figure-1.Can the film capacitor (shown in Figure 2, which is designed for DC filtering and DC-link circuit) be used in the output LC filter of the SPWM inverter?Kindly recommend a suitable (film) capacitor which can be used in the output LC filter of the SPWM inverter.Figure-1: Single-phase SPWM inverter (output voltage 230VAC; fundamental frequency 50Hz; PWM frequency 20kHz)Figure 2a: Film capacitor (Recommended Applications: DC filtering and DC-link circuit)Figure 2b: Film capacitor (Recommended Applications: DC filtering and DC-link circuit)

author: sarepairman2
author_rank: Frequent Contributor
timestamp: September 20, 2015, 12:47:01 am
content: real X and Y capacitors have been designed to fail in specific modes. (i.e. short and open 100% of the time)

author: SeanB
author_rank: Super Contributor
timestamp: September 20, 2015, 11:38:01 am
content: You probably want to be looking for AC motor run capacitors, which are meant to be used in this sort of application. I would guess you are looking for a value of around 50uF.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 20, 2015, 11:59:57 am
content: You might be able to use it. For an LC filter on the output of an inverter you generally need a capacitor with a high ripple (or AC) current rating but really high dV/dt rating. IGBT decoupling (aka "snubber") capacitors have extremely high ripple current ratings relative to their capacitance value, but their dV/dt rating is usually inadequate for this job, depending on how fast the switches in the inverter switch, how high the DC link voltage is, and how much series inductance is used in the filter (which depends on whether the filter is just for reducing the dV/dt of the inverter waveform, or for actually integrating the voltage into something more sinusoidal rather than square).

author: onemilimeter
author_rank: Frequent Contributor
timestamp: September 20, 2015, 01:00:06 pm
content: Quote from: SeanB on September 20, 2015, 11:38:01 amThanks. I'm looking for a value of 10uF~30uF.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: September 20, 2015, 01:01:20 pm
content: Quote from: AcHmed99 on September 20, 2015, 12:22:38 pmYes... I just did that this morning... Hope to get reply from them tomorrow (Monday)...

author: onemilimeter
author_rank: Frequent Contributor
timestamp: September 20, 2015, 01:04:38 pm
content: Quote from: MagicSmoker on September 20, 2015, 11:59:57 amThe output of the H-Bridge is a high frequency (20kHz) pulse train... then the LC is used to filter out the high frequency harmonics to produce a near pure sinusoidal voltage waveform. Kindly advise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 20, 2015, 03:14:21 pm
content: This is an AC inverter?You'll be filtering both sides, right? Having the load common flying on the inverter output is a messy idea.Use MKP type film capacitors, rated for AC voltage (AC line ratings optional).Motor run caps won't have low enough ESL to be effective, and are rated for much more AC line voltage than needed.Tim

author: onemilimeter
author_rank: Frequent Contributor
timestamp: September 21, 2015, 09:34:18 am
content: Quote from: T3sl4co1l on September 20, 2015, 03:14:21 pmThanks. Yes... it's an inverter (DC-to-AC). By the way, how can I solve the "flying load common"? Kindly advise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 21, 2015, 11:07:29 am
content: Well, you can have a neutral (one side grounded) AC output by using a half bridge and twice the supply voltage (+360 and -360V DC). Or an isolation transformer.If nothing else, you need to filter both sides of the output. Treat it as two half-bridge converters. Don't worry about common mode. The fact that the two sides are being used in a complementary way matters very little. Series L, C to ground. A standard off-the-shelf EMI filter after that will probably help even more. (The proof is if you can listen to AM radio without disturbances while the thing is running.)Tim