AC motor inrush current damping - Page 1
Forum section: electronics / beginners

author: hgg
author_rank: Frequent Contributor
timestamp: January 31, 2014, 08:31:47 am
content: Hi,I want to limit the initial 'kick' of the AC motor from a garage door in order for the cogs toengage smoothly and reduce the loud noise they make at the start. (It sounds like youare hitting the door with a hammer...)I was wondering if I can do it simply with an in series thermistor.Its an AC single phase motor, 270W, 1.1A, with 12.5uF condenser. (from the spec sheet).My question is, how do I choose the resistance of the NTC thermistor for an inductive load?Thanks.

author: DutchGert
author_rank: Frequent Contributor
timestamp: January 31, 2014, 08:34:07 am
content: I have only experience in DC motors but there we use a 'soft-start' with PWM or a series inductance (or a garantead length of cable) to reduce in startup current spikes.

author: hgg
author_rank: Frequent Contributor
timestamp: January 31, 2014, 08:39:05 am
content: I have also seen commercial soft starters for ac motors with thyristor circuits but I was wonderingif I could do it easily with just a properly sized thermistor. It works great with lamps.

author: SeanB
author_rank: Super Contributor
timestamp: February 01, 2014, 04:25:58 am
content: Series resistor and a relay to short it out. Simple circuit found in microwave ovens to reduce the turn on thump. If you have a dead cheap microwave it is the board found on the back side where the mains enters. Half is the input filter and the other half the limiter. Even the cheapest $50 microwave oven has one, and if you go hunting you can get a dead one for free.Looking at the microwave I was stripping yesterday ( dead magnetron so not worth repair) it uses a 30R 10W resistor and a 10A relay to short it out. delay is about 200ms between mains turn on and relay closing, but if you use a larger capacitor than the 100uF they typically put there it will be easy to increase it. You probably only want under 0.5 seconds of low power anyway.

author: hgg
author_rank: Frequent Contributor
timestamp: February 01, 2014, 09:27:35 pm
content: I've experimented a bit with a DC motor and the best solution here was to usean electromagnet (salvaged from a 12v relay) in series, together with diode in parallel.The inrush current completely disappeared. The inductance of the motor was 10mHand of the electromagnet 300mH.But here I have an AC single phase induction motor.I am not sure that this will work since the electromagnet will oppose the AC and willcause a permanent voltage drop. What do you think?

author: G7PSK
author_rank: Super Contributor
timestamp: February 01, 2014, 10:05:11 pm
content: If the door mechanism uses a cable put a spring into the cable. The only simple soft start for induction motors I have seen either use a resistance or an inductance which is switched out after the motor starts ( other than 3 phase where star delta is used) if the induction motor has a switched start coil which many do you could use the centrifugal switch to work your relay as well.

author: hgg
author_rank: Frequent Contributor
timestamp: February 02, 2014, 07:03:03 am
content: It does not use a cable. Its cog to iron rail. I think I will try an inductor but I will leave it onbecause I need to attenuate the power of the door which is extremely strong and dangerous.It does have a pot to regulate the power but it does not work well. Its either strong or dead...Can you suggest a suitable inductor to use in series with 230V?Do I have to measure the inductance of the motor in order to choose one?Its has three terminal. A common, an open and a close terminal, so I guess I would have touse two inductors. It also has a starting capacitor.Thanks.

author: hgg
author_rank: Frequent Contributor
timestamp: February 02, 2014, 09:04:41 am
content: Shall I use something like the following ?http://www.ebay.com/itm/AC-220V-Coil-10A-240VAC-28VDC-Electromagnetic-Relay-JQX-13F2ZA-8-Pin-DPDT-/350652527321?pt=AU_B_I_Electrical_Test_Equipment&hash=item51a484f2d9An electromagnetic relay, from which I will remove any moving parts & pins and leave only the coilpins which I will connect in series with the motor. It is rated for 220V/10A.

author: Strada916
author_rank: Frequent Contributor
timestamp: February 02, 2014, 09:12:43 am
content: That really coil is only rated at 28VDC. the contacts are rated at 240 10A

author: hgg
author_rank: Frequent Contributor
timestamp: February 02, 2014, 09:15:42 am
content: I am confused...28VDC its for VDC.Is the 10A rating only for the NO/NC contacts?Ok, its energized with around 28VDC and the contacts are rated 220V/10A...(what was I thinking? ...)So, is there any relay that will switch on with 220V?

author: Rerouter
author_rank: Super Contributor
timestamp: February 02, 2014, 09:20:22 am
content: correct, the coil would only be rated for a few 10's of milliamp at best,for 10A your talking a sizable inductor (remembering the wire its made of has to handle that 10A)

author: hgg
author_rank: Frequent Contributor
timestamp: February 02, 2014, 09:30:27 am
content: Maybe something like that and remove the moving parts?...http://www.ebay.com/itm/DC-220V-0-20mm-Stroke-Tubular-Solenoid-Electromagnet-/131092027986?pt=AU_B_I_Electrical_Test_Equipment&hash=item1e85b19a52This one is quite expensive though.

author: Rerouter
author_rank: Super Contributor
timestamp: February 02, 2014, 09:56:25 am
content: Get a transformer with line in, line out and a center tap at half line voltage on the output, rated at 10A, this is the size inductor you will need anyway, and the center tap wil allow you to switch to full mains once its started,its this or play with the motor start capacitor's value, it creates a phase shift to get the motor turning, the smaller the shift, the less grunt to turn over (lower value will mean less shift)

author: hgg
author_rank: Frequent Contributor
timestamp: February 02, 2014, 10:09:56 am
content: Ok, I will try your suggestion.Thanks for your help!

author: SeanB
author_rank: Super Contributor
timestamp: February 02, 2014, 01:11:06 pm
content: If you replace the start capacitor with a 10uf unit it will have much lower start torque. Though at around 5uF the motor will most likely not start.Look for a 3 phase soft starter, often many of them will only control 2 phases on the motor, and will operate with only a single phase if they do not have imbalance protection. You need a 208-400VAC unit in this case.

author: hgg
author_rank: Frequent Contributor
timestamp: February 02, 2014, 01:40:49 pm
content: QuoteSo lowering the capacitor value might be the best and easiest solution after all.I will try that first.(not sure I understood your suggestion on why to use a 3 phase soft starter on a single phaseinduction motor though).Thanks!

author: SeanB
author_rank: Super Contributor
timestamp: February 02, 2014, 01:51:40 pm
content: 3 phase soft starters ( at least the integrated ones that are wired up into a control cabinet with external contactor and control and overload) often only control 2 phases, and will happily work with only a common and one phase controlled to use with a single phase motor. Some are rated for that.

author: hgg
author_rank: Frequent Contributor
timestamp: February 02, 2014, 02:09:39 pm
content: I've just read on a different forum the following:Quoteand from another one:QuoteIs this true?If it is, then if I use a smaller capacitor I might overheat the motor and the capacitor.

author: SeanB
author_rank: Super Contributor
timestamp: February 02, 2014, 02:53:35 pm
content: First is for a motor with a start switch and a centrifugal switch to turn off the start winding after it is up to speed.Second is that your garage door is very likely a PSC motor, so will work with a soft starter. Try with the lower value capacitor first, as that will be cheaper. If the value is too low the motor will stall opening the door. With the duty cycle of a door motor it will not be a trouble with heating it up. It becomes a worry if it is on for hours on end like in a fan or aircon. There a failing capacitor will kill the compressor.

author: hgg
author_rank: Frequent Contributor
timestamp: February 02, 2014, 02:59:51 pm
content: I see.This is the motor actually:http://www.theelectricgateshop.co.uk/graphics/pdf/AG2000ING.pdfNot sure is its a PSC, but it does have a capacitor.Ok, I will try with a smaller capacitor and see what happens.

author: SeanB
author_rank: Super Contributor
timestamp: February 02, 2014, 03:09:25 pm
content: PSC motor. Makes reversing very easy. Try with a lower value capacitor, but be aware that with a heavy gate it might not open or close fully if there is dirt or debris on the track. Add a track brush to each end to help with this.

author: hgg
author_rank: Frequent Contributor
timestamp: February 02, 2014, 03:20:42 pm
content: QuoteYou cannot imaging how strong is this motor! The gate being solid iron and 3 meters long,you cannot stop it no matter how much force you put! It will not even complain.That is why I was thinking of an inductor that might also cause a voltage dropand reduce its power.But anyway, I thought that the capacitor helps only to start the motor. After that, it doesnot matter even if you remove it.

author: SeanB
author_rank: Super Contributor
timestamp: February 02, 2014, 04:09:16 pm
content: the capacitor is there to provide a phase shift to start the motor in a preferred direction. With these motors if you remove the capacitor it will not start, or may start running in either direction. You might want to install rubber bump stops as hard stops instead of steel to steel though. That will reduce the bang a lot.

author: matkar
author_rank: Regular Contributor
timestamp: February 02, 2014, 04:12:57 pm
content: Have you considered solving this problem mechanically? What is the distance between the cog and the rail? Your junction is probably to loose. The sound you're hearing is the result of air gap the cog travels before hitting the rail. Try to adjust the height of the motor to bring its cog closer to the rail. By minimizing this gap the loud sound should diminish.

author: hgg
author_rank: Frequent Contributor
timestamp: February 02, 2014, 04:23:04 pm
content: QuoteThe loud bang comes from the engagement of the motor cog on the rail that is mounted on the gate.QuoteEven at their closest distance there is some left to right play. Probably they did not match the cogand the rail properly. The motor though starts like a rocket. Unstoppable. That is why I want toreduce its power as well.

author: KDC
author_rank: Contributor
timestamp: February 03, 2014, 12:48:46 am
content: Seems like you folks are over thinking this...Resistor in line, timing relay to shunt it out. Sounds like you don't need it on for very long, just enough for the gears to meet cleanly before applying full torque.

author: hgg
author_rank: Frequent Contributor
timestamp: February 03, 2014, 08:23:05 am
content: QuoteI know, but I want to find the best and simplest way to solve the problem.I was looking at the manual but now that I opened the case, the motor actually is the 1000K versionand its capacitor is 16uF. I've found an 8uF and an 11uF which I will try both. The specs say that themotor draws 1.7 amps but I've actually measured 2.5 amps in one direction and 2 amps in the other...Same for both motor cables.It turns out that the pot of the power stop function is a bit dodgy. Its a 22K pot, but I cannot measuremore than 5k. I've measured it in circuit and it runs smooth up to 5K. After that and for the other halfof its turn it gives random values. 20K, 40K, 5K again, etc. I've rotated it left and right many timesand it looks like its better now but I think it needs replacement. I found a point now where you canactually stop the gate but with quite some force and I am not sure that it will stay at the same valuefor long.

author: G7PSK
author_rank: Super Contributor
timestamp: February 03, 2014, 08:42:38 am
content: I did not realise that you were talking about a sliding gate, I was thinking of an up and over when you referred to a garage door.On the board of the controller of a sliding gate there should be an anti crush adjustment which controls the motor torque this should be adjusted so the motor just moves the gate and there is not enough power to crush a person. The motor cog and rack should be adjusted so that there is 0.5mm between them any more and you will get a lot of backlash.

author: hgg
author_rank: Frequent Contributor
timestamp: February 03, 2014, 08:48:19 am
content: My bad. Yes its a sliding gate.It looks like that the power stop potentiometer needs replacement.

author: hgg
author_rank: Frequent Contributor
timestamp: February 03, 2014, 03:53:37 pm
content: I have just replaced the motor capacitor with lower 11uF & 8uF values but same thing.The motor starts full speed! It did not even blink... Its original capacitor is 16uF.Hmmm.Maybe I will have to build a ramp up AC circuit but I still think that a properly sizedthermistor will be the best solution. I will order some and try again.

author: digitalnomadjc
author_rank: Contributor
timestamp: February 04, 2014, 01:02:08 am
content: There is a project in July 2012 of Silicon Chip magazine that addresses this sort of issue - the project name is "power tool soft starter".

author: hgg
author_rank: Frequent Contributor
timestamp: February 04, 2014, 06:50:10 am
content: Hi,From the magazine preview I saw, it looks like that he is using the solution with the resistorwhich I assume that he will bypass after a delay. There is also a ready made kit that sellson jaycar electronics :http://www.jaycar.com.au/productView.asp?ID=KC5511Do you happen to have the full article?Thanks.

author: hgg
author_rank: Frequent Contributor
timestamp: February 12, 2014, 10:05:14 am
content: I am waiting for some thermistors I've order, to try them out on the garage motor but meanwhile I havefound the following article on Elektor for a simple soft start AC circuit.If I understand its function correctly, then at power on, resistor R3 will dissipate all the initialinrush current. At the same time capacitor C1 will be charging for a period that depends onits capacity and R1 & R2. When its fully charged, enough current will flow through the gateof TH1 and it will conduct bypassing the resistor R3.My first question is, do you know if the SCR will switch on immediately or gradually?Can you have a smooth transition from R3 to the SCR bypass?I am trying to replicate something similar on a breadboard but with a function generator,at 10Vpp / 50Hz sinewave. Will the following SCR be appropriate for that experiment?http://uk.rs-online.com/web/p/thyristors/6870984/?searchTerm=687-0984Thank you.

author: KDC
author_rank: Contributor
timestamp: February 12, 2014, 01:09:20 pm
content: The SCR will switch on after the cap charges, it won't ramp up. All or nothing.

author: hgg
author_rank: Frequent Contributor
timestamp: February 12, 2014, 01:18:21 pm
content: I see. Ok, thanks.