snubber circuit design 2

[skrab]

I'm using a low cost VFD in a hoist application. I am controling the drive with some small ice-cube (120vac) relays. I need to eliminate any potential noise from the relay's coil. How do you size the R and C?
thanks in advance.

 

[jbartos]

Presently, manufacturers offer transient voltage suppressors (TVS). General Electric has used MOVs (Metal Oxide Varistors) that are essentially nonlinear resistors. Will this suffice or will you still intend to design the R and C? [sig][/sig]

 

[skrab]

Thank you for your timely reply. Unfortunately, this particular manufactor does not offer any suppression device accessories. I thought the MOV would only limit transient voltage peaks, correct? I've always assumed the main application of using a varistor across the coil would be to protect the device it is connected to. In this case, I am more concerned about the VFD. Without having a simple method to test this protective circuit, I prefer to use the RC if it will offer greater protection. My goal is to prevent an intermittent noise related problem down the road.

 

[xnuke]

For some good application notes on transient suppression, try:

http://www.littelfuse.com/ASP/Search/detail.asp?ID=460

If the link does not work, go to

http://www.littelfuse.com

and search "MOV."
Maybe something in there will help. I know at least one of the papers compares various devices, including MOVs and snubbers. Best of luck.
[sig][/sig]

 

[jbartos]

The MOV absorbs inductive energy stored in the relay coil. The function of R-C snubber in parallel with the contactor or relay coil is to absorb the coil inductive energy and suppress its voltage transients during turn-off. TVS, back-to-back Zener diodes and MOVs are considered solutions. (The inductive energy causes voltage transients if the relay coil is deenergized, since Vcoil = Lcoil x di/dt. Coil energy Wcoil = (Lcoil x squareI)/2.). The MOV, R-C and TVS shall not be rated energetically less than the amout of energy stored in the coil since they may become damaged. Ideally, the parallel combination of Lcoil with R-C forming impedance Z = R + jX shall have X=0 to achieve the resistive current through Z. Then, R and C values can be determined, and the resistor and capacitor sized depending on parameters of "small relays."
The Variable Frequency Drive (VFD) may need input filter(s), perhaps R-C or L-C. Also, it may need output filters, R-C or L-C. The filters are passive (can be active too, but more complicated) and mitigate harmonics. There are also EMI input and output filters which suppress the electromagnetic interference. Next, threre are different principles of VFDs available on the market. Please, would you have more information available? [sig][/sig]

 

[jbartos]

Reference: Billings C. Eng. MIEE, K.H. "Switchmode Power Supply Handbook," 2nd Ed., McGraw-Hill Book Co., 1999
There is Chapter 18 "Snubber Network" including
1. Snubber Resistor Value in Ohms
R=Timeoffmin/(2 x Csnubbercapacitorvalue)
Timeoffmin time in microSec
Csnubbercapacitorvalue in microFarads
2. Snubber Capacitance in microFarads
C=Ip x Tf/(2 x 70% Vceo)
Ip is maximum current in Amps
Tf is fall time in microSec
Vceo is rated voltage

 

[jzakr]

Iv'e been working on an application that controls a small
ASCO solenoid. I used a 47 ohm in series with a .1uf cap
for noise suppresion. The solenoid was completely messing
up my PIC16F628 microcontroller. I tried a MOV at first but
that didn't stop the problem. I also remember using a snubber
for a pump motor(1/3hp) circuit it ended up being 100 ohms 1/2 w
with a .1 uf cap rated at 200 for a 120 VAC application.
But.. like other posts say it all has to do with the speed of
the wackie pulse. Incidentally the solenoid above was 24vac
and I believe about 7 watts or so,, not very large but it was
twinking out my circuit at both turn on and turn off modes.
The above suppresor completely eliminated the problem.
You can determine the values by trial and error but at least
overrate your components.