Motorcontrol using PWM and MOSFETs

[dannyjoer]

Hi everybody

I am doing a universityproject regarding speed control and regulation of an AC/DC universalmotor. The motor itself is from a washingmachine.


The PSU intended to supply the main current is malfunctioning for some reason. We have connected three 555 timers on a breadboard. The first 555 timer provides triggervoltage to the last two 555's. We wanna keep the current in the field windings constant, so that the flux is constant, therefore the second 555 is preset with a constant dutycycle. We then intend to control the speed of the rotor by increasing the dutycycle in the third 555 timer to control the current in the armature windings.

The 555's output signals is connected to the gate of two MOSFET's, connected to the field- and armature windings. View attachment 5552.pdf (The figure only shows the setting of the fieldwindings)

Now here is my problem. When i provide the breadboard and the "PSU-board" common ground, i get a voltagedropreading with my voltmeter of 100V between the two zeros in the boards. How is that possible? And how do I ensure common ground for the boards?

Do I have to galvanic isolate the whole thing?

I know its a lot of text, and please do ask supplementary questions if needed.

Thx.

Danny

 

[Marvo]

Is this circuit your own design? Why don't you just configure one 555 timer as an astable oscillator without external triggering? Your circuit is very complicated for a universal motor PWM chopper circuit.

Anyway as far as your common zero problem goes, what components are on which board? Are all the 555 timers on the same board and the two MOSFETS ON THE PSU board? If so you would need to common the 0v reference ground between the boards.

 

[dannyjoer]

Yes the circuit is my own design, ofcourse inspired by numerous google-hits. All of the 555 timers is placed on the bread board, and the MOSFET's and the diode rectifierbridge is placed isolated on a heatsink on the PSU board. The goal is to plug the PSU board directly to a powersocket (230 V RMS, 50 Hz). It seems like some of the components draw a large amount of amps.

The problem is that when I common the 0V reference between the two boards, simply by connecting a wire between them, the 100V drop messes up the 555 timers. Would it help to include a common reference throug an actual ground connection?