Can I PWM AC 240V?


Senior Member
My girlfriend and I were looking at a largish saltwater aquarium today and thinking about setting one up. This tank (2 meters long) had a 240V AC power head (water pump) at either end, at around ten second intervals one power head would turn off and the other would go on full power. This cycling of the power heads creates a surge current similar to the effect of the swell washing over a coral reef. The fish and corals do better with this sort of set up. They had a $$$$$pricey german made controller to run the power heads.

To decrease the wear and tear on the power heads I was thinking it would be good to have a soft start using PWM. I've seen this on AC 240V power tools to reduce start up kick ( large angle grinders for instance) so I presume it must to be technically possible.

Im thinking of using a 40X1 picaxe to run the tank anyway so its no big deal to have PWM control of the pumps as well. The 40X1 could control the lighting, temperature, water level and provide alarms of pump, heater failures, water level drops through leaks etc.


Edited by - BrendanP on 19/08/2007 10:32:28
Are the motors 'universal' brush type or brushless inductive?

This thread discussed some points:-
<A href=';pagesize=15&amp;forum_title=No+new+posts+please%21+15&amp;topic_title=Using%2BPWM%2Bto%2Bcontrol%2BAC%2Bdevice&amp;forum_id=21&amp;topic_id=3897' Target=_Blank>External Web Link</a>

If it uses induction motors then this, I would guess, is NOT a simple job such as basic phase control switching as used in hand power-tools which use 'universal' brushed motors.
I merely give that link to reduce repetition of what was discussed before.
But no doubt it will be repeated :)

If someone has a relatively easy solution I'd love to know for my own edification.


Senior Member
I'm guessing that the pumps use shaded-pole induction motors. (25 years ago, I would have been sure of it, but a lot has happened in the motor field in recent years, and there's a chance you might have something in there that's more sophisticated.) You might open one of them up and take a peek at the motor to see just what you have.

If those are plain vanilla, single-phase shaded-pole motors, the only practical way of controlling the speed is by a variable-frequency/variable voltage device, that varies the frequency from, say, 20 to 50 Hz, with minimum voltage (150 vrms, maybe) at 20 Hz up to 240 vrms at 50 Hz.

The app note in the following link provides more information: <A href='' Target=_Blank>External Web Link</a>

Good luck!


Edited by - Tom2000 on 19/08/2007 21:10:23
A lot depends on how long the &quot;soft start&quot; is - this is assuming shaded pole motors in a centrifugal pump or other non-displacement type pump.

You can use phase control to modulate the effective voltage for shaded pole motors to slow them down. It is a favorite application for small mains powered &quot;muffin&quot; fans to lower noise and shaded pole stir plates in laboratories.

The problem with using voltage (effectively what you would be doing) to control speed using induction motors, is that they have very little start torque - but that isn't a problem with a centrifugal pump or fan or any application where starting torque isn't too important.

So current would go up (more &quot;slip&quot; in the motor with respect to synchronous running) and torque would be down). So heat/current goes up fast, as speed and torque go down.

But if your game is to just simulate a tidal wash it may work very well - providing a shaded pole motor and low starting torque pump and you aren't running in the low speed region for too long.

&quot;Too long&quot; is a variable that you have no idea of - but one pump is &quot;off&quot; and the other is &quot;on&quot; so there's a duty cycle for cooling, and your soft start may be over a period of just a few seconds.

It may just work - you'd have to try it unless something else contraindicates what you plan to do (like a pump motor with a starting winding or very long soft start period)