DC Varable Speed SCR circuit causing Earth leakage Trip - Page 1
Forum section: electronics / repair

author: hightowerMcD
author_rank: Newbie
timestamp: December 15, 2020, 08:33:32 am
content: Hi.I have a DC Motor Control Assembly, that seems to be tripping the earth leakage as soon as I turn the Potentiometer (i would imagine to the point where it activates the SCR's in the circuit) My First assumption is that there was earth leakage from induction through the system, (and I have not really ruled that out) -- since im pretty new to this.. I was hoping someone could tell me a little more about what is going on in this circuit?From the Attached, I can kind of see that the Bridge Rectifier is outputting 160vDC, however the positive terminal is not being used (are they getting this through the casing? is this a standard way to do this?Is there anything I can do to this circuit to improve the leakage? any help would be appreciated.Thanks

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 15, 2020, 10:00:02 am
content: Reason only one half of bridge used is it's all configured as a controlled rectifier so the thyristors are the other half of the bridge, just for added confusion the 4 thyristors are connected as two pairs so imagine them as just two (the missing bridge legs).I note this picture contains no EMC components whatsoever, the EMC from this kind of device will be very heavy indeed and your neighbours wont like you using it without lots of filtering and neither does your ELCB.Can you picture the EMC filters please ?

author: shakalnokturn
author_rank: Super Contributor
timestamp: December 15, 2020, 05:04:21 pm
content: The most likely leakage will be the motor itself, have you tried disconnecting it from the controller and measuring between its ground and one input terminal?Build up of brush powder in the wrong place can be enough.

author: picburner
author_rank: Frequent Contributor
timestamp: December 15, 2020, 06:11:14 pm
content: Following your pic, the circuit looks like the attached image.If you look closely you have inverted the cathodes with the gates of the SCRs.Unless you have special SCRs with K and G reversed

author: hightowerMcD
author_rank: Newbie
timestamp: December 15, 2020, 10:57:52 pm
content: Thanks all for your valuable Input! .1: I am connecting no load to this circuit at this point.2: The EMC Components, I attach an image of the board, where I think there is an Inductor.. otherwise there is none.. My intent would be to install some kind of EMC filter -- any idea what would work best for this type of circuit?3: @picburner - yes, it does look like the K and G are reversed.. I wonder if that is the problem with the circuit? Code on the SCR's say "S8025L - A7" which I would imagine should be A-G-K -- I get continuity between pin 1 and 3 (but not 1 and 2 and 2 and 3) -- does that mean that the middle pin is in fact the gate? -- would that be the reason that the circuit shorts as soon as I turn the pot?Once again. thanks for the help.

author: drvtech
author_rank: Regular Contributor
timestamp: December 16, 2020, 12:06:02 am
content: It's cathode, anode, gate. The gate is never in the middle. Here's the datasheet on Mouser's sitehttps://www.mouser.co.uk/datasheet/2/240/Littelfuse_Thyristor_Sxx20x_Sxx25x_Datasheet.pdf-775526.pdf

author: picburner
author_rank: Frequent Contributor
timestamp: December 16, 2020, 03:08:27 am
content: QuoteYes, there will definitely be problems if you reverse K and G.The pinout of S8025L is the one attached (top view).drvtech preceded me by indicating the correct pinout.

author: hightowerMcD
author_rank: Newbie
timestamp: December 16, 2020, 08:13:44 am
content: Ok. thanks all. So it seems like, beside the lack of proper filtration, there is nothing wrong with the circuit design .just a question, from this circuit, what type of filter would I want here (low pass, high pass, bands etc)

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 16, 2020, 09:25:53 am
content: Quote from: hightowerMcD on December 16, 2020, 08:13:44 amMaybe you could tell us how many Kw this motor is ?This type of controller is real bad on EMC and has been superseded by high frequency types using typically IGBT's requiring much smaller EMC components.I am very surprised you were able to buy this unit as it is certainly not compliant in it's present form, frankly you may find the cost of filtering components exceeds the cost of a modern unit that is properly engineered to include the required filtering.

author: hightowerMcD
author_rank: Newbie
timestamp: December 16, 2020, 10:10:09 pm
content: thanks @42 The Motor controller is a 1.5KW unit with a 1KW 6A (180v) DC Brushed motor.got it from China... so yeah, that explains why it is not filtered.. I will try to begin some search for IGBT.. but yeah, was hoping this was going to work.Thanks

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 17, 2020, 08:56:01 am
content: I hope you don't mind me saying this but don't buy cr*p from China.You might be able to get your money back from the importer if you report the product to the authorities because it's use and import would almost certainly be illegal in Australia and many other countries including the UK.