Wanted - small variable speed inverter

[Harry Bloomfield]

I'm looking to source a cheap 240v 30watt mains inverter, to enable me to vary the speed of a peristaltic pump, if any one comes across one or happens to have one surplus to requirements.

[Tabby]

You're not giving nearly enough info here.

[Harry Bloomfield]

on 19/02/2011, Tabby supposed :

Which information do you think it lacks?

[Dave Liquorice]

Does the pump motor vary it's spped in relation to the input frequency, voltage or some other factor?

At least I know I have a synchronous motor in my very similar request. B-)

Might be worth digging about the web and seeing if there are any suitable diagrams and building yourself. I found a very simple one but was square wave not even "modified sine wave".

[andrew]

How cheap?

We use altivar 31s for 1ph-3ph and thay are about GBP150. Very configurable for Hz, ramp and overload.

AJH

[Harry Bloomfield]

It uses a small synchronous motor to drive the pump, via reduction gearing. What I'm trying to do is use the setup to meter small amounts of the pumped fluid. There are two ways I can do it, either variable on and off periods to adjust the average speed of flow, or to alter the mains frequency fed to the motor.

I know the first method will certainly work, but the second method will offer a steadier flow, but I'm not sure how much the speed can be adjusted by. I'm looking for a range of 6L per hour down to maybe 0.2 litres per hour I think.

[Harry Bloomfield]

Oh much, much cheaper than that, it is just a little experiment at this stage.

Is that AJH as in AJH pumps?

[John Williamson]

Over that ratio of flows, you may have trouble with the motor running too far outside its frequency specs and overheating at the lower speeds. A 30:1 variation on the frequency is, I would say, too wide a ratio, though experimenting with a spare unit may prove me wrong.

Not a problem so much if it's a universal type, though even there, running too slow will reduce armature cooling.

[Dave Liquorice]

Ah right very similar to my request then. B-)

As has been pointed out that is a big range but I would expect medical pumps to be able to do a similar range but perhaps not

6l/hr...

How do they do it? Maybe they use a proper stepper motor rather than a true synchronous one?

[Mike Harrison]

A quick & dirty way to generate variable frequency mains is an audio amplifier fed into a backwards-connected mains transformer, fed from a signal generator ( or PC soundcard)

[Tabby]

Now you've told us its a synchoronous motor, and I'm going to guess a

240v motor. The Range of 6 to 0.2 means a min drive frequency of 1.7Hz, and I cant see any 50Hz motor coping with that without significant complication. I think the only answer that will avoid a good bit of work is forget it, and just use a timed on period at 50Hz, or if you're prepared to reduce voltage to 80-100v you could go down to a fixed 17Hz using triacs to invert every 3rd halfcycle.

Surely there are other options too, but you either need the skills to design & construct a non-trivial circuit, or to buy it ready made. Is changing the gearing instead an option?

NT

[John Williamson]

A stepper motor would be perfect for this application. Possibly even cheaper than faffing about with variable frequency inverters.

[Harry Bloomfield]

It happens that Tabby formulated :

No, I need the full speed range available on the 'dial'.

[tinnews]

I doubt *very* much if you'll get anything like that range of control by varying the frequency. I think you'll be lucky if a synchronous mains motor runs over a range of much more than 40 to 60 Hz (maybe

70Hz if it knows about american electricity).

In general inverters don't let you vary the frequency either.

[Tabby]

I cant see any sync motor working over such a frequency range, not unless you do a lot of careful control & mollycoddling. If you knew how to do that, you wouldnt be asking more basic questions.

Someone suggested a sig gen, audio amp and tranformer, that would fry the motor very quickly, and wuoldnt go much below 20Hz with most amps. The only realistic advice is forget it, your pump can't do what you want it to.

NT

[Harry Bloomfield]

No, not that way perhaps - so back to organising a method to turn it on and off for variable periods of time then.

[John Williamson]

Unless, as has been suggested elsewhere, the synchronous motor were to be replaced by a stepper motor. Then speed control from zero to full at

100% torque is available with no problems, and it can even be programmed to single step.

A D.C. or brush/ commutator type A.C. motor would be almost as good with a pulse width modulated controller, but you'd need to keep a close eye on the temperatures.

Unfortunately, sometimes you can't get there from here.

[Terry Casey]

In message on Sat, 19 Feb 2011

14:51:53 + 0000 Mike Harris>

fed into a

Blimey! How powerful is your signal generator?

About 1 mW into 600 ohms is what I'd expect!

[Terry Casey]

14:51:53 +

amplifier fed into a

Oops! Didn't spot the words "audio amplifier" ...

You'd have make sure that the amp's PSU would hold up to a continuous LF power output and that the heat sinking arrangements were good unless the amp is considerably overrated - it isn't the job they are designed for!

[John Williamson]

14:51:53 +

amplifier fed into a

Hence the audio amplifier. ;-)