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Ac motor Soft start
I am trying to make a cheap ac motor soft start to control a water pump. I have seen soft start modules by Crydom and others but they cost a little more than I am willing to pay as well as they are a little overkill for my application. ...
By Trevor Thomas on 2 March, 2000 - 2:56 pm

I am trying to make a cheap ac motor soft start to control a water pump. I have seen soft start modules by Crydom and others but they cost a little more than I am willing to pay as well as they are a little overkill for my application. The soft start circuit will go between a dc-ac relay and the motor (120vac 1Amp max). Any suggestions.

Thanks in advance

By Fred Chwalek on 2 March, 2000 - 3:00 pm

Try, Aqua series drives. They have small "throw-away" drives for little money ( under $300, I think ).

By Yamashita Taiichiro on 2 March, 2000 - 3:01 pm

I guess, such function, you are hoping, would be implimented in your motor driver, doesn't they ?

For example, Mitsubishi's inverter driver has such function.

If you want to control a induction motor with some electrical parts, I wonder valiable resistors or some could be applied. But I'm sorry that I have no idea how to impliment such kind of analog circits.


By John Cassel on 2 March, 2000 - 3:02 pm

Build yourself a light dimmer that will ramp after it is started. This could be done with a simple RC network. Make sue that the Scr can handle the peek load and once the motor is upto
speed you may want to short the SCR with Relay contact.

It is a hack But it should work

By Trevor Thomas on 6 March, 2000 - 1:37 pm

Thanks for the reply..
I was looking into the ceiling fan speed controller to try to control the speed of the pump. I ran into problems with these. From my understanding there is 2 types.

1. The variable resistor (analog) the problem with this is that it is hard to control it digitally. The only way that I can see doing this is to attach a stepper motor to it... would work but not very elegant.

2. The other way is to use a TRIAC or a Phase controlled relay the problem with these devices are that they "chop" the AC signal. This is ok for soft start or stops, but for controlling the speed over a longer time the non-sinusoid wave form will wreak havoc on a AC induction motor.

I know that the X10 guys/gals (home automation) are working on speed controllers (modified light dimmers) to control ceiling fans in smart houses
but I don't know if how that is going along.. Hello X10 guys/gals any input??

I will try to explain the application in a little detail. In the big picture I am making a environmental control system that will attempt to simulate nature in the ocean. This system will control, light, circulation, temperature, dosing (pH and other trace elements) on a daily basis for 1 year.

Lights. Dimmable to simulate different times of day as well as cloudy days, and lunar cycles.

Circulation. By pulsing on and off 3 pumps I will attempt to simulate the motion of waves. I will also be controlling the speed of the pumps to
simulate different intensities(night->day, stormy days etc). I know that I could leave 2 of the pumps off the simulate a quiet time in the ocean but for other reasons I would like to be able to control the speed of the motors.

Temp. Self explanatory.

Dosing Perilistic pump controlled by the main system.

The controller will be controlled with a EPLD mainly but will use other devices where necessary.
If anybody has any more suggestions on speed control and soft start/stop for the AC motor suggestions would be greatly appreciated.


> -----Original Message-----
> From: []
> Sent: Sunday, March 05, 2000 9:11 AM
> Trevor,
> Based on data you provided, ie, 1 Amp, 120 Vac motor, try commercially
> available ceiling fan speed controller.
> BTW are you trying to control speed of a fish tank pump?
> Regards,
> Phil Corso, PE
> Trip-A-Larm Corp

By Jeffrey W. Eggenberger on 10 March, 2000 - 9:45 am

The problem is that squirrel cage induction motors are not controlled by the amount of current. The are controlled by the frequency of the ac signal. Triacs increase/decrease the amount of current to the device. Normally the
fan speed is switched by increasing/decreasing the number of poles of windings used in the fan. You can get a solid-state switch that will do this
from a remote location.

Jeffrey W. Eggenberger
Electrician: Industrial, Commercial, and Residential

1 out of 1 members thought this post was helpful...


The major problem with speed control of fractional-Hp single-phase AC motors is not the "chopping" action of the controller resulting in
premature degradation of insulation. Instead, it is the fact that the motor has no starting torque. Various designs produce starting torque,
but they don't all respond well to speed control. Varying success is dependent on the type of motor: universal-type; split-phase winding;
capacitor start; reluctance winding; etc.

In my experience an SCR or Thyristor based controller will work well if:
a) the motor is of the universal type (essentially a series-wound DC motor used in power tools); and b) it has no integrally mounted
self-cooling fan.

Regarding other motor types, try experimenting with a simple input-line L/C filter (consisting of inductors and caps) or a 100 Watt bulb in
series with the motor.

Regarding input control of the ceiling fan controller, substitute a digital-to-analog IC chip in place of the rheostat or potentiometer.

If you need additional information, please contact me.

Phil Corso, PE
Trip-A-Larm Corp has softstarts as little as $104. Very easy to install with programmable ramp up, ramp down and initial torque. Their us
address is "": .

By K.Srinivas Rao on 19 February, 2002 - 1:03 pm

Is there any other method to control the motor. i.e. without Triac. how about using FETs and trying the PWM method for AC motor????

By Garry Hallam on 22 February, 2002 - 9:46 am

I was looking into adding a soft start module to a DIY Aquarium wavemaker I built. I contacted Velleman Components who suggested using their part: K8028. Heres a link