Re: AC amplitude modulation for inductive loads

Marcel Baum wrote:
"Joerg" <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> schrieb im Newsbeitrag news:qWoTi.36574$eY.32446@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Marcel Baum wrote:
I have got a linear pump which needs a sinusoidal excitation.
The pumpflow is controlled by the amplitude of the AC to the coils
The amplitude is controlled via a DC input signal which on itself can change very fast.
So far I have used a 24VAC pump, which is no longer available, the actual pumps have 230VAC (50hz). The rated power is roughly 150W.

My present design is powered from 24V DC and utilises a full (H-) Mosfet bridge.
The bridge is controlled by a MC 33035 which is a PWM motor controller with an analogue input and a direction signal. The analogue input gets a sine have wave with variable amplitude, the DIR input sees a 50hz TTL signal in phase with the sinus.
To go on with this design I would have to rectify the 230V, change to a 500V Mosfet bridge and to add some HV photocouplers to feed the hi side Fets of the bridge.

Does anyone have better suggestion how to directly modulate the mains AC instead of synthesising the AC from a DC source?
I probably should mention that the pump wants to see a sinus like (or trapezoid) excitation which limits the use of a triac .

Any ideas are welcome.


Just to avoid a misunderstanding: The PWM is usually not done with the bridge itself. It is done by a converter up front and that gets the sine-wave control signal. The H-bridge just switches.



Thank you for your response

The 33035 has got a pwm on chip, the high side Fet is responsible for the direction the corresponding low side one is pulse modulated according to the actual input voltage.
Its not that expensive here.
Its very much like in the LMD 18245 used with stepper motors (this one is really expensive).


You want to pulse the bridge to get the sine wave and rely on the motor windings to smooth things out? That can backfire the minute the motor manufacturer changes the design. Personally I wouldn't do that. EMC can also become quite a nightmare, as can motor noise.


What I was thinking about is to use an existing DC-AC converter (e.g. 24VDC to 230VAC) lay out and to add a control input for the output voltage. No idea if this exists already. Have you ever looked into something similar?


Well, they work pretty much like I described. A switcher around a 3525 generates a voltage between zero and the desired peak voltage, usually as a forward converter. Sinusoidal half-waves. Then the bridge toggles that.


Since this pump is part of a medical life supporting device it has to run on batteries for at least 20 minutes anyway. The most simple way is to run it from a UPS (having its internal batteries), or if not possible use a buffered 24V battery with a DC-AC step up converter.


I design medical devices for a living and running an AC motor at over 50V is usually frowned upon by the agency guys. Are you sure you can't get a 24V version anywhere? Also, nearly none of the parts vendors endorses life support applications but I am pretty sure you know that.

--
Regards, Joerg

http://www.analogconsultants.com/
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