AC Motor Control - Jolts When Switched Off - Page 1
Forum section: electronics / repair

author: TGService
author_rank: Newbie
timestamp: March 31, 2017, 09:17:12 am
content: Hello, first time poster though been watching the blogs on YouTube for years.I service industrial dryers and recently I've been seeing a new model which has a strange behaviour/fault. No matter what speed the motor is running at, when you switch it off the motor "jolts" as if it is going full speed for a second.I have attached some images of the control board and a little schematic of the circuit (I've not drawn schematics since high school physics so don't laugh).I am wondering if there is something that can be done to modify this circuit so the motor does not jolt when switched off.Other control boards I work with have choke capacitors which is the only major difference I see with them, but I don't think I know enough to go playing around with the circuit without some guidance.Cheers.

author: Armadillo
author_rank: Super Contributor
timestamp: March 31, 2017, 09:56:10 am
content: Has this circuit been modified before?Firstly the board label is a SCR, but your schematic shows a Triac.OK given is a Triac, shouldn't it be a Diac at the gate then a diode?

author: TGService
author_rank: Newbie
timestamp: March 31, 2017, 12:35:33 pm
content: No it's not been modified, that's the way they come from the manufacturer.It is a Triac on the board. The part numbers are JST41Z-800BW on older boards and BTA16-600BW on newer board, both of which have this jolt when switched off.You have lost me with the Triac vs Diac. I've never done any circuit design stuff past playing with basic stuff.

author: Armadillo
author_rank: Super Contributor
timestamp: March 31, 2017, 12:53:53 pm
content: Your schematic, the diode should be a Diac.How did you know is a diode? Have you verified it.?EDIT: Measure the "Diode" to verify it.

author: Seekonk
author_rank: Super Contributor
timestamp: March 31, 2017, 12:55:40 pm
content: If that IS the actual schematic, I see a couple problems. The resistor capacitor snubber should always be connected across the triac. This is probably a mistake in your schematic. Increasing the value of C by several orders would help. Any noise on the power line can cause the triac to self trigger. The gate just floats. A resistor from 1 to 2 might help, try a 4.7K.

author: bktemp
author_rank: Super Contributor
timestamp: March 31, 2017, 01:07:19 pm
content: The diode is marked DB3 on the PCB. DB3 is a diac and the ones I have look similar. So it is most likely a DB3 diac.

author: Armadillo
author_rank: Super Contributor
timestamp: March 31, 2017, 01:12:30 pm
content: Quote from: bktemp on March 31, 2017, 01:07:19 pmDB3 is a 32-40v diac

author: bktemp
author_rank: Super Contributor
timestamp: March 31, 2017, 01:25:44 pm
content: I doubt the diac is the problem if the speed control works correctly during normal operation.Seekonk has probably identified the problem: If the schematic shown here represents the actual circuit the disconnected snubber at turn off will probably make the triac trigger for another period giving the motor full power for a short time.The switch should disconnect the full mains input instead of the gate trigger + snubber. But that requires the switch can handle the full current.My guess is, somebody tried to make the circuit as cheap as possible by using the triac as the power switch and using a cheap switch not able to handle the full current. The snubber is connected to the gate circuit to act as a "pulldown" for the gate.

author: Armadillo
author_rank: Super Contributor
timestamp: March 31, 2017, 01:49:09 pm
content: Discussion;

author: bktemp
author_rank: Super Contributor
timestamp: March 31, 2017, 01:58:07 pm
content: That should fix the problem.

author: TGService
author_rank: Newbie
timestamp: March 31, 2017, 01:59:18 pm
content: My crap schematic seems to be causing problems so here is a stripped board with the tracks overlayed if that helps eveyone understant the circuit better.As for the Diac, thats just my lack of knowledge that I drew it as a diode.These board are from dryers that you can tell are made to a budget as someone was guessing above.EDIT: The Triac is fitted to the back of the board (see previous pics I posted) so the outer pins will be the opther way around from what the slik screen suggests.EDIT: Thanks Armadillo, should that revision be good for me to go ahead and test out?

author: Armadillo
author_rank: Super Contributor
timestamp: March 31, 2017, 02:30:16 pm
content: Revised;

author: TGService
author_rank: Newbie
timestamp: March 31, 2017, 02:43:21 pm
content: Thank you very very much. I tried out the first revision you posted and that has eliminated the "jolt".The switches used are cheap ones as someone mentioned, so I don't know if they would be good enough to be on the mains. This dryer is built to a budget.I have just remembered another issue we have with these boards. When you turn them on at low speed, every 10th or so time they will not come on. However if you turn the variable speed up a little then they come on.

author: Armadillo
author_rank: Super Contributor
timestamp: March 31, 2017, 05:39:52 pm
content: The thanks should also goes to bktemp and Seekonk.Have you tried to adjust the preset to eliminate the dead-band.?

author: TGService
author_rank: Newbie
timestamp: March 31, 2017, 08:27:20 pm
content: Of course, I really appreciate everyones help.I haven't myself but one of the other service guys has and still every 10th or so time the dryer will not start until the speed is increased. It does it as well with the new circuit layout you gave me.Is it possible the switch is not making proper connection and that bit of extra resistance in the switch is causing the problem?

author: Armadillo
author_rank: Super Contributor
timestamp: March 31, 2017, 09:08:52 pm
content: I think it will be inherent in this simple circuit design without feedback because we cannot assume that all mechanical and inertial are the same, all bearings are equal smooth etc.It is nominal then that you need to increase the torque to make that initial mechanical rotation, that initial push, that little bit.Maybe others can provides better answer for you.

author: james_s
author_rank: Super Contributor
timestamp: March 31, 2017, 10:11:10 pm
content: This is the reason most motor speed controls switch on at full and then dial down from there rather than the way light dimmers usually switch on at 0 and dial up.Might be able to modify the circuit further so that it briefly applies more power when first switched on, then drops back to the set power.