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Need help with Relay chatter

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drjak

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I have this project consisting of a current sensing relay controlling a motor control relay. The motor is a small 120 v AC motor with approx. 2 amp draw. The two devices that I need to sense a current draw on are both 120 v AC. Either one activated needs to energize the motor control relay. I used a rather simple and inexpensive current sensing relay, that has a contacts rating of 50 ma. which is adequate to control the motor relay. The current sensing relay is designed to have at least 4 amps of AC current flowing in the wire passing through the sensing portion of the device to close its contacts. It states in the instructions, if you need to activate the relay on a lower current, you can loop the wire a few turns through the sensing portion to achieve this. Of the two devices that I need to sense current draw on, one of them has a 70 ma. draw and the other has 1.3 amps draw. In order to sense these lower currents I had to loop the wire a few times and as a result the increased sensitivity is such that external 120 v AC line spikes can cause the motor relay to chatter. (false trigger) In my searching for an alternate current sensing relay with higher contacts rating and a time delay circuit built in, which could be used to minimize the spike issue, the best I have found so far has a trigger current range of 100 ma. to 1000 ma. and a contacts rating of 15 amps. I could use this device to control the motor but my 70 ma. draw device would not trigger it and these guys are kind of expensive.
I would appreciate any suggestions with a way to stop the false triggering. Thank you.
 
Is the motor relay coil AC or DC?
 
In addition to the motor coil being 120 V AC. the two devices that I am sensing the current draw on are small 120 V AC. motors.
 
How can this work??

From my understanding the current sense relay latches the motor relay and turns the motor on.

Until the motor is on there is no current flowing through the sense relay, so how can this work?

Pete.
 
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If it's really noise, maybe a line filter in front of the current sensing relay would help?
 
The current sensing relay is not sensing the motor current. The small motor that I refere to in the original post that is being turned on and off is actually a squirrel cage fan. The two devices that the current sensing relay is sensing are a flex shaft grinder, which draws 1.3 amps and the other is a micro grinding machine that draws 70 ma. I appologize for the confushion. The project consist of 3 motors, one being the squirrel cage fan which has its own control relay, and that relay is being energized by the curent sensing relay. The current sensing relay is sensing one or the other of the two smaller motors (flex grinder, micro grinder) and when activated, applies 120V AC to the coil of the motor control relay.

Ken
 
"If it's really noise, maybe a line filter in front of the current sensing relay would help?" (ronv)

Are you refering to a snubber circuit? I have read a little about this but have no experience with it. What type of line filter are you refering to, and its placement in the circuit?
Thank you

Ken
 
I think I would use a line filter. Are you fairly certain the problem is caused by noise on the ac line? In other words things are just sitting there and the relay trips? If so put it between the line and your motors. If the problem is from the relay switching noise then a snubber across the relay contacts would be the ticket. I would try 1 Ufd in series with 120 ohms for that one. This should be a cap rated for "across the line" use. Here is a link to a line filter. 5120.1000.0 Schurter Power Entry Modules
 
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