killivolt

http://www.electro-tech-online.com/a...g?d=1206370704

The above schematic was in (Electronics chat forum.)

I wanted to know if the back emf from the AC motor in parallel to mains. Would somehow effect the rectifying circuit thru the transformer causing problems to the control circuit ?

Thank you, kv

dknguyen

You are worried about "stuff" generated from the motor passing through the transformer? I wouldn't worry about it.

BTW, BEMF stays inside the motor for the most part, especially if you are applying a voltage to the motor terminals. If you disconnected the mains and then rotated the motor, the motor would act as a generator and travel through the transformer.

killivolt

Ok, I got it. This low voltage control stuff has my head spinnin. Thanks

second286

Killivolt:
I wanted to know if the back emf from the AC motor in parallel to mains. Would somehow effect the rectifying circuit thru the transformer causing problems to the control circuit ?
Thank you, kv

Indeed it could killivolt, along with other obvious problems, good question! According to the schematic, the single phase AC motor should have a RC snubber in parallel with the motor to suppress any transients during the relay switching of the inductive motor load. Also there should be a zero crossing detector within the control circuit to allow relay switching to occur only when the AC voltage across the motor is passing through 0 volts.

killivolt

Are you kidding me.............. better talk to (dknguyen) I'm so confused ?

I pretty much would trust anybody right now cause I'm just chasing(snipes)

killivolt

[quote=dknguyen]You are worried about "stuff" generated from the motor passing through the transformer? I wouldn't worry about it.

BTW, BEMF stays inside the motor for the most part, especially if you are applying a voltage to the motor terminals.

(=dknguyen, you have some esplainin to do)

RetiredHAL

Hi, Killivolt, that is quite a control circuit for switching a simple motor.
Probably there are situations where this is type of control is necessary, but generally speaking there is not a problem with Back EMF. In the Household motors switch in and out all the time (refridgerator, freezers, washing machines, dishwashers, Garage doors,etc.) all these produce inductive kicks that reflect into the household circuits. Clocks , TV's etc all use Bridge rectifiers and do not get damaged.
When our washing machine switches on the lights flicker. This is caused by the voltage going low due to the sudden current drain. This type of "disturbance" or "transient" produces quite a voltage spike but it never seems to do any harm to my clocks, computer ,radio or TV on the same circuit.
Zero Crossover detection and Prediction/Compensation cctry may well be desirable in applications where there is a continuous switching eg in telephony/communication type ccts , but I think it is "overkill" in the cct you present.

Just my Opinion though

Cheers
RH

killivolt

Here snipie snipie. Here snipie snipie. I know that an improper ground plan in this example can produce undesired frequency's and random electron's floating around on the board are not harmless ? Which lends more credibly to better isolation as in the pre-prescribed suggestion above.

If anyone can lend a hand to move this further for my awkward brain please feel free to further confuse me.

................... whooooooa ! kv

RetiredHAL

Hi KV, I don't understand your reply.
My reply in this post reflected my experience with transients due to switched circuits. I used the household cct experiences to illustrate that Back EMF is not normally a problem when switching a motor on or off . I hope that someone else (more experienced) can enlighten you with your problem.

Cheers RH

killivolt

Both are credible and seem to allow my mind to dive into nowhere. I feel you have valid points as (dknguyen) I've seen her math and would have been happy until someone took the ball and went back the other way ?

....................... Thank you for your help. I just need starting points and if I can't get past these points then I will forever be confused with applications. I have worked on products with these sort of problems too.

Engineers and software developers fighting for control over such circuits resonating to infinity and beyond.

Designed obsolescence is what we are really talking about here if not designed correctly who cares I.E. (Microsoft) then it just becomes a pain for the user and field service people.

dknguyen

The Back EMF isn't something that "comes out of the motor". Think more of it like something that fights the AC voltage source so massive amounts of current don't flow through the motor.

But switching inductive loads will produce voltage spikes and those could travel through the transformer. WHat happens is that if you have current flowing through an inductor and you suddenly cut it off, the inductor will still try to keep the current flowing (or at least changing very gradually). It does this by increase the voltage across the inductor (similar to increasing the voltage across a resistor to get more current flowing through it). This is where the voltage spike comes from.

For a unidirectional DC motor you can put a diode across the motor to let the current have a path to flow between the motor terminals (letting the current reduce gradually and getting rid of the voltage spike). For a bidirectional motor circuit where current flows both ways, obviously a simple diode won't work since it will short-circuit in one direction. So you put diodes across the MOSFETs in an H-bridge to try and do the same thing. In an AC motor, what you can do is put a resistor and capacitor in series across the motor terminals to help absorb the inductive spike. (YOu should actually put it across the transformer since that is the thing you are trying to protect. Putting it right at the motor doesn't shield the transformer from the inductive kick caused by the inductance in the wires of the line.).

So yeah, I agree with the others. I didn't notice the switches by your motor. BUt those can cause spikes that will go through your transformer. Not the BEMF though.

bigjoe4

No that will be fine.

__________________
Blatent advertising deleted!.

killivolt

No,that will be fine ? whaa


I got it I just happened to be reading and trying to understand what an H bridge does your expansion is right on target.

Thanks from one old man. ................. kv

This will open a lot more doors for me....... whoooooa !

b00vy

hello im new to your site and forty years old , and just started my hnd
in elec and elect can you do superposition of a circuit with j numbers
ive only done them with ohms (sorry if this is a daft question its an assignment) cheers

rjvh

i am not 40 years young and have my hnd working already a long time
i know i am alergic for j numbers but it works better than ohms
you're excused for this inteligent question as i give you the only proper answer for it,you can report to your superior (that give you this assignment)
that you're close to finish it