AC motor inrush current damping

[KDC]

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.

[hgg]

Seems like you folks are over thinking this...

I 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 version
and its capacitor is 16uF.  I've found an 8uF and an 11uF which I will try both.  The specs say that the
motor 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 measure
more than 5k.  I've measured it in circuit and it runs smooth up to 5K.  After that and for the other half
of its turn it gives random values.  20K, 40K, 5K again, etc.   I've rotated it left and right many times
and it looks like its better now but I think it needs replacement.  I found a point now where you can
actually stop the gate but with quite some force and I am not sure that it will stay at the same value
for long.

[G7PSK]

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.

[hgg]

My bad.  Yes its a sliding gate.
It looks like that the power stop potentiometer needs replacement.

[hgg]

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 sized
thermistor will be the best solution.  I will order some and try again.

[digitalnomadjc]

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".

[hgg]

Hi,

From the magazine preview I saw, it looks like that he is using the solution with the resistor
which I assume that he will bypass after a delay.  There is also a ready made kit that sells
on jaycar electronics : http://www.jaycar.com.au/productView.asp?ID=KC5511

Do you happen to have the full article?
Thanks.

[hgg]

I am waiting for some thermistors I've order, to try them out on the garage motor but meanwhile I have
found 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 initial
inrush current.  At the same time capacitor C1 will be charging for a period that depends on
its capacity and R1 & R2.  When its fully charged, enough current will flow through the gate
of 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=687-0984

Thank you.

[KDC]

The SCR will switch on after the cap charges, it won't ramp up.  All or nothing.

[hgg]

I see.  Ok, thanks.