Pure sinewave for medical equipment

I have an air mattress that uses a 7watt pump on 230v ac for filling it up. It needs pure sinewave and will be damaged if it is given modified or stepped waveform. On a power failure, the matteress deflates and is flat. A UPS gives out modified or PWM sinewave. Can a dynamo be used in conjunction with a motor driving it and the motor could be run on a UPS. The motor might die but would last longer than if the pump itself is put on something else other than pure sinewave. A voltage regulator could be used at the dynamo's terminal to give 220v. However I dont know what type of motor to use, RPM, power etc. They are said to have 90% efficiency. I need this mattress for my mom who is critically ill and is developing pressure ulcers and bed sores. A pure sinewave could only be produced using a rotary mechanism so I was thinking of using a motor for this.

There are low power (100-250W) sine wave inverters available locally. You will have to ask around on Hall Road or Brandreth Road and test the output before putting it to use to be 100% sure.


Sorry to hear about your mother. I hope that she gets well soon.

7 watts is fairly low power. While your proposal is sound, it can be expensive. My proposal is to use a 20 watt mono audio amplifier kit to power the motor. The amplifier can be driven from an oscillator based on a few transistors or an op-amp giving 50 (or 60) Hz output, sine wave. This setup can be powered from a 12 volt battery. A small transformer used in 12 Volt power supplies can be used to step up the output of the amplifier to 230 Volts to power the 7 watts motor. Shouldn't cost more than 7-800 rupees.


Thanks for your reply :) In this circuit, a charger would also be needed. I have an APC 750va UPS and it's THD is around 3%. The UPS is not working and the output is causing the load attached to it to flicker. I tried it with a bulb and it was flciekring. Called the person who sold it to me but theres no afrtersales service.

My APC UPS does charge the batteries so I could use that for charging and use a wien bridge oscillator and an amplifier. But the UPS as a 24v charger. Will this give a sinewave with a THD around the APC's output?


Your worry about THD is overrated by a margin of at least 1,000%. For heavens sake, we have to run a motor here not a high fidelity audio system. The UPS output can certainly be used for running the pump. Since it is very low power the regulation maybe OK, But just check to be sure.

Of course 24 volts is OK for running the amplifier, if you go that route. You will have to reduce it to 12 volts using a 7812 type regulator since most amplifier circuits of the type cited above run off 12 vdc (usually built around ICs designed for automotive use in mind).


Ok, thanks a lot for that info. I'll see what I can do to build this.