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Soft Start Soft Stop Circuit for Motors

The post explains how to make a triac based slow soft start and slow
end or slow stop circuit for heavy machine motors so that the motors are
able to gradual start stop actions instead of switching ON/OFF

The idea is basically intended to ensure less wear and tear on the
motor and additionally save electricity during the course of actions.

The idea was requested by Mr. Bernard Botte.

The Request
Dear mister Swagatam,
Sorry for my english , thanks anyway for any answer you will give
Before the question . I use different apparatus to handle wood using universal AC motor
originally made for a range between 230 to 240 volt 50hz (but I notice in certain part of my country 250
V too) because I need a lot off different kind of machine and that was only for hobby.
I buy the cheapest machines I can find ( I correct certain mechanical problems) for other machines
I use also a dimmer (home made based on the system used by vacuum cleaner and modified by
NINA67 ) and It work great.

But I also use a planer/thicknesser using a motor rotating at 18000 T/min. It seems made to not
pay any royalties to evict
copyrights. Before I had problem I tought it was a motor running at 3000
t/min (2700) multiply by 2 (like others) with a belt to reach a decent
speed of 6000 t/m (5400)
Sorry no. And I don’t use the dimmer.The motor run at +/- 18000 : 3 =6000 !!!
Knowing the cheap cost of that machine I use it like a “good father” not intensively etc
But one day there was a fume The
machine smoke and i dismount the machine to isolate the motor to evict
the fire . (the machine was under warranty but i need to make a lot off
kilometers to make an exchange. And there, they don’t tell me it was a well known and reccurent problem … but … they know it! )
In fact when everything was cold . I
look the axis who rotate he seems also shooting on the opposite side of
the gear belt at every start Like there wasn’t a grower . I show the
motor in a company saling different kind of motor.They make also
refurbishing but they explain to me that it was an “exotic” motor but
they set the same diagnostic .Start to fast
So come my question:
Could you please make a schematic to have a “soft start / soft ending ” for different universal
motors in fact if i use my dimmer
system based on BTA 16 800 cw (better than the other mentioned above) it
seems ok but i have only made 3 of them . I’ll want to integrate that
in every big machine . And use only the on/off switch.
I want use thus a button to “switch
on” and one to “switch off” or an on/off switch. But also a
potentiometer to select the minimum level (depending the power of each
motors) when the motor start running and a potentiometer to select the
timing (555) between the slow start and the full speed (maybe also
shortcut the triac with a relay to have full speed an a green led if it
is relevant (but it will be nice) for the switch off the timing maybe
reduce. Why at the end because the extra current and problems binded.
Note : I have seen this application
with “fpla” or dedicated processors but I am sure it can also be done
with discrete components.
Why i can not do that : because I
never study the motors correctly but I know for example that it is not
correct to start the motor with a zero crossing system because It give a
maximum current and that make the same trouble (FIRE!) with the couple
at start and max current …
I have seen this request in other
forum touching other job mechanic wood etc … with no answer and people
say also if it work with a potentiometer but when you change from a
machine to another you can make mistakes etc…
Regards Botte Bernard (Belgium)
please don’t put my adress on the net
Nb i like also in your presentation the datasheet because it’s no so easy to have it without paying

Bernard Botte

The Design
The requested idea of a soft start, soft stop motor switching circuit
can be implemented using a simple triac based dimmer switch concept, as
presented in the following diagrams:

Referring to the above diagrams, the first diagram shows a standard light dimmer or a fan dimmer switch circuit using a heavy duty triac BTA41A/600.
The section which indicates the “4 triac module” is normally occupied
with a potentiometer for enabling a manual speed control adjustment,
wherein a lower resistance adjustment generates higher speed on the fan
motor and vice versa.
In this soft start, soft stop design, this pot section is replaced
with the indicated 4 triac module which can be elaborately visualized in
the second diagram.
Here we see 4 triacs arranged in parallel having 4 individual 220K
resistors at their upper MT1 arm, and 4 individual capacitors at their
gates with different values, and with a sort of sequential order of high
to low.
When S1 is switched ON, the triac having the lowest value capacitor
switches ON first, enabling a relatively slow speed start on the motor
due to switching of the relevant 220K resistor at its MT1.
Withing a few millseconds the next subsequent triac conducts which
has the next smaller value, and adds its own 220K resistor in parallel
with the earlier 220K resistor, allowing the motor to gain some more
Identically, the third and the fourth triacs also sequentially switch
ON within the next few milliseconds, thereby adding two more 220K
parallel resistors in the range, which finally allows the motor to reach
its maximum speed.
The above sequential speed increase on the motor allows the motor to
achieve the intended slow start switch ON, as desired by the user.
Quite similarly when the switch S1 is turned OFF, the relevant
capacitors switch OFF in the same order but in a descending manner,
which inhibits the motor from a sudden stop, instead it causes a step
wise slow stop or slow end on its speed.

Feedback from Mr. Bernard:

Dear mister Swag,

First of all, thanks for your fast answer.
Because you tell me you have a timing problem I have changed my operating system
 to linux mint 18,1 ‘Serena’ so i haved to re-install all the program I need and
 test it (setup it!) So apparently everithings seems working OK !

About the first schematic

I notice you don’t give any value to the upper side schematics so I pick it up 
from « How to Make a Simplest Triac Dimmer Switch Circuit »  
Parts List for the above enhanced fan dimmer circuit
(C1) C7 = 0.1u/400V
(C2, C3) C8,C9 = 0.022/250V,
(R1) R9 = 15K,
(R2) R10 = 330K,
(R3) R11 = 33K,
(R4) R12 = 100 Ohms,
VR1 = 220K, or 470K linear => Replaced by genial 4 triacs module
Diac = DB3,
Triac = BT136 => BTA41 600
L1 = 40uH
About the second schematic so simple solution i never have dreamed !!!
to be tested asap Genial! we say in French.

I doesn’t know that you can use polarized condensators for such AC applications!
And also that 50 volt was suffisant ! I you have a moment to explain why -

Anyway maybe i’ll try it this weekend if i have all the component. I prefer use new
capacitors my stock never change since 1993! 
In fact i was trying  different ways using for exemple opto triac (MOC)  but i also need
 to pick the freq of the AC network, also another based on your schematic Kiln Temperature
 Controller Circuit but with up down counter  4516b and 555 etc,etc so complicated
Many thanks

Regards B.botte 
My Response:

Thanks dear Bernard,
The image which you had inserted in the conversation did not get
attached properly and therefore it was not showing, but I have corrected
it now and have posted it back in the article.
I have rated the caps at 50V because R9 is supposed to be a 33K or a
68K resistor which will drop the current significantly and not allow
the capacitors to burn, this is my understanding.
I have used polarized capacitors because the gate of a triac works
with a DC drive, but yes you are right, in order to make it DC for the
capacitors we need to add a 1N4007 in series with the gate 1K resistors.
Now with regards to this design, if suppose the idea does not
operate very smoothly or fails to produce the expected results, we could
modify the existing gate drive for the 4 triacs into optocoupler based
drivers, and perform the same sequential delayed switching but through
an external DC circuit.
So this circuit ultimately has the potentials to deliver the intended results, either this way or that way.

25 thoughts on “Soft Start Soft Stop Circuit for Motors

  1. Without reading every word of the post, I appears that this schematic is for 110AC volt motors. How can that same thing be done with low voltage DC motors?

    1. Thanks for your reply. Is there a schematic to do a "soft start" automatically, without using Arduino, etc. In other words, I want to flip a switch and the motor slowly goes from stop to full speed automatically. Top speed would be governed by how much voltage is applied to the motor. If I were using a 24VDC gear motor that has a top RPM of 50, I would like the unit to go from 0-50RPM in 15 seconds. It sounds like I should learn how to write code but I'm too old for that. It looks like the schematic you recommended uses a 100K pot to vary the speed.

    2. Thanks, you can try the concept which is explained in the above article, it does not require any coding, just some tweaking of the capacitor values and the system will provide you with required soft start.

      the R9 can be appropriately selected for fixing the maximum range

    3. sir ,can u please tell me what will happen if a single phase induction motor is running on given supply at rated rpm,during this motor shaft start running on above rated rpm due to load connected means to say if connected load start runs more than motor rated speed during supply is connected.??

    4. Hi Gurmel, sorry I did not quite understand your question, how can the load force the motor to run at a speed more than the its existing RPM?? please clarify this so that I can try t give my opinion.

    5. lets say load is our 2nd highly inductive around 5kw power induction motor.
      in my case i want build a system to reduce the starting current i dont care about torque or anything i just want to reduce starting cureent to mini say to running amp.

      so i will couple 1kw motor with load ie. 5kw motor to make it run at some speed and than connect it to main supply than i will turm off first motor.
      the whole purpose of this system is reduce satrting current to lowest with investing high cost vfd controller i think if we give supply to running motor than it will consume less satrting current instead of standstill motor .
      i hope you will understand

    6. OK I understood, and according to me there's no problem in doing that, and this will effectively solve the switch ON high current issue.

      just make sure the 1kW motor is instantly disconnected at the same moment when the the 5kW switch is turned ON

    7. yes here is real issue
      is there any circuit in which lets say i switch on of 5kw motor than instantly or if want some lag of say 20ms than first 1kw motor's switch should be off?

    8. that can be easily done using a two way contactor switch or a DPDT heavy duty relay.

      however the 1kW motor will need to be detached mechanically, so that the 5kW motor becomes entirely free from the 1kW motor load.

      the two could be coupled with gears, and this gear system could be mechanically separated in response to the contactor switching or the DPDT relay switching.

    9. sir i didn't find dpdt relay for rating 20 to 30 amp 230ac.
      i think high power relay are not available in india easily.
      do u know any source on net in India for this relay type?

    10. sir does coper is better or aluminium for wiring 5kw load conection from inverter to load?
      lets say if we take 6sq. mm wire of cu and al. which consume more current by itself?

    11. thankeww​ so much sir..
      sir i we interchange phase and neutral wire during runnig operation of single phase Induction motor than does something happen to motor or not?

    12. sir i have question please answer it.
      i have 5kva inverter with 96v batry bank .each batrry rated as 75ah c10 12v connected in series.lets assume inverter take loads upto its rated value 5kw continue than how many hour of backup i will get at full 5kw load ?

    13. sir i use dpdt relay as you advised and its works like a charm thankew sir.:)

      sir may be u know now a days home inverter are rated in kva .
      i have inverter of 5kva .But i can load it upto only approx 4100w of inductive load of motor.
      i think if i decrease or eliminate kvar demand of motors with capacitor than kw of motor will epual to kva (approximately).
      so than 4100watt will translate to 4100va (approximately).and than i can put more load of 900va on inverter.
      so sir do u know anything in regarding this ?

    14. thanks Gurmel, I am glad the DPDT switch concept is working for you,

      As for the mentioned query, Although I know the basic conversion:

      KW = Kilo volt * Current
      KVA = Kilovolt * Current * power factor
      When power factor =1, then KVA = KW

      however I have little knowledge regarding how to decrease the kva demand in motors with a capacitor, i have not yet studied this theory in detail so far, If possible I may investigate it and let you know soon….

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