AC/DC Switch Ratings for Motor Control: Which Rating to Use?

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When selecting switch ratings for a motor running off bridge-rectified mains voltage without filtering capacitors, it's important to consider whether the switch is on the AC or DC side of the bridge. AC ratings can be used if the switch is placed in the AC line to the bridge, while DC ratings should be applied if it's on the DC side. The discussion highlights that the lack of a zero voltage/current in DC switching diminishes arc extinction, making DC ratings lower than AC ratings for the same contacts. However, the motor's inductance can maintain current, allowing for some flexibility in rating selection. Ultimately, it's advisable to overspecify ratings and consult with company representatives for specific product guidance.
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I have a motor that is running off bridge-rectified mains voltage. There are no filtering capacitors across the rectifier output.

When figuring the ratings needed for any switches in series with the motor, would I use the AC or DC ratings of the switch?

My understanding is the biggest difference in the ratings between AC and DC switching for contacts is that the lack of a zero voltage/current diminishes the ability of the arc to break, hence why DC switching ratings are less than the AC ratings for a given set of contacts.

However, in this application, I still have the zero voltage/current so therefore I think I can use the AC ratings. Am I correct?
 
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If you intend to use it for AC purposes then use the AC switch ratings. However, it's always better to overspecify the ratings even if it means spending a little bit more money. I recommend contacting company representatives directly with questions regarding their products.
 
If you place the switch in the AC line to the bridge you can use its AC rating.

If you have it on the DC side use its DC rating. Even though your applied DC voltage voltage dips to zero twice per cycle , the motor's inductance maintains current and the bridge acts like a flywheel diode..

Draw yourself a full wave bridge with inductive load and observe how the bridge completes a current loop for inductive "kick". But that feature really eases things for ths switch if it's on AC side of bridge.

Better safe than sorry.
 
Jim, that was awesome as always.
 
Well golly, md, thanks for the kind words !

Hope your project works out smooth .

old jim
 
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