I upgraded one of my projects with a stronger motor this week. Its a 220V AC motor(first AC I've done) and the idea was to turn on and off depending on the where its needs to be.

When I ran the program, I noticed that the display which shows its status displays random junk when the motor turns on and off.

The mcu controls the motor by controlling a mosfet which in turns opens and closes a relay. All works great except for the junk output. I read of all the possibilities and sat on the relay for a while.

If I remove the motor and just have the controller control the relay there is no interference. If I add the motor then interference is back. If I remove the motor and use an AC filament bulb then same issue.

I read a few articles about AC line interference but nobody ever gets to a "its working" stage. Is there a way this can be solved or should I place the controller in a steel box to shield it.

 

I upgraded one of my projects with a stronger motor this week. Its a 220V AC motor(first AC I've done) and the idea was to turn on and off depending on the where its needs to be.

When I ran the program, I noticed that the display which shows its status displays random junk when the motor turns on and off.

The mcu controls the motor by controlling a mosfet which in turns opens and closes a relay. All works great except for the junk output. I read of all the possibilities and sat on the relay for a while.

If I remove the motor and just have the controller control the relay there is no interference. If I add the motor then interference is back. If I remove the motor and use an AC filament bulb then same issue.

I read a few articles about AC line interference but nobody ever gets to a "its working" stage. Is there a way this can be solved or should I place the controller in a steel box to shield it.

What sort of sensors are you using? Using shielded cable is key to this kind of application.

 

Also insure the motor frame is earth grounded, and all metalic parts of the project, you may also have to bond the common of the controller supply, this is done by using a central star point ground plate to take all the grounds to incl the service ground.
http://www.automation.siemens.com/doconweb/pdf/840C_1101_E/emv_r.pdf?p=1
Max.

 

Since both a motor and a light bulb are inductive loads, relay contact bounce turns the inductance into a high voltage generator, and excellent source of both radiated and conducted noise. An appropriately rated capacitor across the relay contacts might squelch the noise at the source. Plus all of that grounding stuff.

ak

 

Since both a motor and a light bulb are inductive loads, relay contact bounce turns the inductance into a high voltage generator, and excellent source of both radiated and conducted noise. An appropriately rated capacitor across the relay contacts might squelch the noise at the source. Plus all of that grounding stuff.

ak

Would you recommend a capacitor in series with a resistor? or just the cap? Plus, wouldn't the cap need a resistor in parallel for it to be able to discharge between cycles?

 

A solid-state relay in place of the mechanical relay would likely eliminate the problem.

 

A solid-state relay in place of the mechanical relay would likely eliminate the problem.

Yeap... that's probably the simplest solution... how much current does the motor draw?
In fact... using an SSR would also allow you to eliminate the mosfet.

 

If the motor is not grounded even using a SSR may not cure it.
Max.

 

Is there now galvanic (optical, relay) isolation between the MCU and the motor?

 

Is there now galvanic (optical, relay) isolation between the MCU and the motor?

That's a very good point... complete isolation of the MCU from the rest of the system (including the relay) would further improve things

 

What sort of sensors are you using? Using shielded cable is key to this kind of application.

The sensors are nothing fancy, they are just pots on the adc pins. The wires I'm using on them aren't shielded. Would an unshielded sensor effect the lcd screen? I would expect iratic reading at worst.

 

If the motor is not grounded even using a SSR may not cure it.
Max.

He mentioned "random junk when the motor turns on and off" which would imply that it's due to relay contact bounce, and that would be eliminated by a solid-state relay.

 

Since both a motor and a light bulb are inductive loads, relay contact bounce turns the inductance into a high voltage generator, and excellent source of both radiated and conducted noise. An appropriately rated capacitor across the relay contacts might squelch the noise at the source. Plus all of that grounding stuff.

ak

I don't think this is the issue. If I remove the relay and just connect the motor to the power source just using a plain switch it gives the same result. The relay currently has a resistor in series and a capacitor across the pins together with a diode.

 

He mentioned "random junk when the motor turns on and off" which would imply that it's due to relay contact bounce, and that would be eliminated by a solid-state relay.

I'll give the solid state relay a shot. I have a solid state from an old machine.

 

Would you recommend a capacitor in series with a resistor? or just the cap? Plus, wouldn't the cap need a resistor in parallel for it to be able to discharge between cycles?

The cap is across the contacts where the AC load is , not across the coil where the DC drive is. There is no "discharg(ing) between cycles." Also, no series resistor; an X or Y rated cap is designed to sit across the power line continuously.

ak

 

I don't think this is the issue. If I remove the relay and just connect the motor to the power source just using a plain switch it gives the same result. The relay currently has a resistor in series and a capacitor across the pins together with a diode.

That resistor, capacitor, and diode are around the coil, not the contacts. Also, mechanical switch contacts bounce just like mechanical relay contacts. Another possibility is the start capacitor centrifugal switch, if you motor has one.

A solid state relay probably is the best solution. They create their own kind of interference, but most have circuits and components to reduce and/or snub this.

ak

 

The cap is across the contacts where the AC load is , not across the coil where the DC drive is. There is no "discharg(ing) between cycles." Also, no series resistor; an X or Y rated cap is designed to sit across the power line continuously.

ak

Sorry about my "cycles" quote.... I meant to say between motor "turn on - turn off" work cycle... not electric ac-cycles

 

That resistor, capacitor, and diode are around the coil, not the contacts. Also, mechanical switch contacts bounce just like mechanical relay contacts. Another possibility is the start capacitor centrifugal switch, if you motor has one.

A solid state relay probably is the best solution. They create their own kind of interference, but most have circuits and components to reduce and/or snub this.

ak

I see it now, sorry I'm still a bit green. With the inductive load you get something like a boost converter. When you rapidly open and close the switch you get a higher voltage than the input. I switched out the button to a more bouncible button and removed the relay. Its not difficult to replicate the result by tapping the button.

 

........ If I remove the motor and use an AC filament bulb then same issue.

An filament bulb is non-inductive, yet you still get interference with it when it turns on and off.

..... If I remove the relay and just connect the motor to the power source just using a plain switch it gives the same result. .........

I don't think inductive kick back is the source of your problem. In your troubleshooting you have removed both inductive sources, your relay and motor, yet you still get interference. I would look more in the direction of AC mains. Perhaps a voltage drop or a ground loop when the motor turns on. Connect your motor or instrumentation to a different AC outlet/circuit. Separate them AC wise.