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Thread: motor switching question (mildly OT)

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  1. #1
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    Default motor switching question (mildly OT)

    I'm building a box that has electronically controlled relays that apply and remove 120V AC power to several different motors (all less than 1/3 HP, one motor per relay). Do I need something like a MOV or transient voltage supressor (back to back zener) or RC snubber or etc to deal with the inductive voltage spike when the motors are switched off?

  2. #2
    TR3driver Guest

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    Should be OK without, if you are using relays rated for motor operation. They have oversize contacts to deal with arcing from inductive kickback.

    But adding suppression certainly won't hurt anything.

  3. #3
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    Depending on how you drive the relays, You may require snubbers/diodes across your relay coils...

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    I need to look up more info about the relay board, but it is a bunch of relays controlled by low level electronic signals. I see some additional components on the board, so the coil or solid state part seems taken care of. What I am worried about is the load end or switching contacts. The relay says something like 250V 5A on it but I don't know what that says about its ability to switch an inductive load.

  5. #5

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    Yes, put MOV's at least across the input power line.
    The spikes WILL feed back into the panel board And may cause problems somewhere else in your house. The motor's will never be affected until a big spike comes along & punches a hole in the insulation.

    MOV's are in your power strips for TV & computer.
    Normally they are not too big. (Jules of energy)
    The more MOV's in the house the better!

  6. #6
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    Depends what you are driving the circuit board with

    With a PLC, Microprocessor, I/O board on your PC, then absolutely they are required. But as for the load side of the relay, they don't make a difference. The Back EMF is already there, and any reflected power will cancel out.

    But for use with motors your relays should have a HP rating. Otherwise they may lack sufficient RMS Amperage Braking capacity

  7. #7
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    Beanbag,

    You probably don't really need a suppressor for that HP and 1 cycle every 3 days, but it won't hurt. You can buy a RC snubber or you can make one. Do a search for RC contact snubber or such and you will get good info. The basic configuration, as I remember it, is a .47uF capacitor at 400+ volts in series with a 47 to 100 ohm 1/2 watt resistor. That combination is wired across the relay contacts (all contacts that switch the motor). Do a search to confirm the values and voltages and to see the wiring better.

    Robin

  8. #8
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    You talk about MOVs so I assume you are talking about protection for the load circuit.

    Your motors will probably work justfine with no surpression devices for the load circuit. However, there may be switching transenents that have an effect on other electronic devices in the shop or house/building. You may have to add some devices to prevent this. But you can install it and see what happens first. Then add whatever is needed.

    However, it is also common to use protection devices in the control or coil circuits of relays. You said you are using "electronic control" and most commecial "electronic" control circuits will have diodes or some other form of protection built into the coil circuit. This is necessary to protect the electronic devices (ICs or transistors) from damage from the back EMF (Voltage) when the relay is turned off and to keep spikes out of the power and ground circuits of the controller. These spikes can cause all manner of problems in logic circuits because they can look just like logic pulses to the ICs. If you are designing your own control circuit, I would strongly recommend that you take this into account. A DC control circuit usually uses reversed biased diodes across the relay coils. I am not as familiar with AC controls and am not sure what would be best.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  9. #9
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