Motor Capacitor Configuration

[dknguyen]

For motor bypass capacitors, how do you arrange them? I have heard of two arrangements:

1. Capacitor across motor leads

2. Capacitor across motor leads, capacitor from motor case to +V, capacitor from motor case to -V

Can you just not attach a dead-end ground to the motor case to make it like a faraday cage if you wanted to stop EMI? There doesn't seem to be a lot of discussion on the differences between method 1 and 2, except that some say that method 2 might make high currents run through your motor casing.

Thanks all.

 

[Nigel Goodwin]

You've mentiond THREE methods there (although you didn't label number 3), I would suggest you use all three capacitors, and 'ground' the casing as well! - the cost is only three cheap capacitors.

 

[JimB]

One capacitor between the motor leads will stop series-mode signals

Two capacitors to "ground" from the motor leads will stop common-mode signals.

Grounding the case will create a Faraday Cage (maybe?), but dont forget the two leads from the motor, most EMI will be conducted and radiated from them.

JimB

 

[bananasiong]

One capacitor between the motor leads will stop series-mode signals

Two capacitors to "ground" from the motor leads will stop common-mode signals.

Grounding the case will create a Faraday Cage (maybe?), but dont forget the two leads from the motor, most EMI will be conducted and radiated from them.

JimB

Hi,
what is series-mode signals, common-mode signals and Faraday Cage??

 

[dknguyen]

A common-mode signal (or noise) is any signal that is present on BOTH wires.
For example:
Red Wire carries 5VAC + 1VDC
Black Wire carries 10VAC + 1VDC

Then 1VDC is the common-mode signal since it is present on both wires. If you reject this it means that you "compare" the signals on the black and red wire and filter out whatever was the same between the two.

A Faraday cage is a device using the fact that the electromagnetic field inside a conductive object is zero, regardless of the electromagnetic waves on the outside. it's basically a conductive box or sphere or wire cage or something used to shield whatever is inside from electromagnetic waves (or interference) outside the box.

I've never heard of the term series-mode noise,