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I am working on controlling a 220V 0.75hp AC motor with a Relay, and came across HFE9-3/12DST which by design is a dual coil, latching relay BUT with contacts in NC position.

I was wondering since latching holds on indefinitely until power is applied to opposite coil, would NO/NC settings compromise the safety/control of motor, ie I could setup the Relay in NO position, and keep it that way, until I need to move in the NC position, knowing that even if power is down, the Relay will not switch.

Additionally, I have the snubber circuit on the AC side for relay arc prevention when motor is turned off, and flyback protection circuit on the DC side when current is withdrawn from the coil. [DC Side Schematic1

datasheet Link : https://www.alldatasheet.com/datasheet-pdf/pdf/660894/HONGFA/HFE9.html?

enter image description here

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    \$\begingroup\$ If it really is mechanically latched, then I don't think the normal use of NO and NC apply. Better to think in terms of Set (close) and Reset (open) for the coil-to-contact relationship. \$\endgroup\$
    – AnalogKid
    Jan 13, 2021 at 0:54

2 Answers 2

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Your zener / rectifier diode flyback protection circuit is incorrect.

For this application, rectifier diode flyback protection would be adequate.

Here's the correct schematic, using two 1N4007 diodes.

enter image description here

Should my presumption of push button switch control be correct, the following circuit, employing a standard relay and a rotary on / off switch, should suffice.

enter image description here

MCB / fuse / thermal overload relay protection and earthing are not shown but are to be taken care of.

In my opinion, instead of a single pole relay, a contactor would be the right choice for motor control. The contactor, intended for motor duty, would have 3 sets of double-break contacts that would enable the neutral line also to be switched. It would also facilitate mounting of a thermal overload relay.

If the latch relay or a standard relay is a must then its contact could be used to drive the contactor.

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  • \$\begingroup\$ I am developing this project with final cost being a real concern! I was able to find another relay with similar characteristics but with NO contacts. As it stands, Relay is the most expensive part with costs being around $3 with MOQ as 1, and $2.4 with 10 units. In my initial research I wasnt able to find a single contactor for less than $15 CAD. Should I just add another relay on neutral line as well? I apologize for I didnt tell that I was using esp32 @ 3.3 volts to drive the unit and a SMPS will boost externally supplied 5v to 12V, in addition to a slow blow fuse on line side on AC line. \$\endgroup\$
    – Amogh Jain
    Jan 24, 2021 at 10:09
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    \$\begingroup\$ Hi Amogh, In a latch relay, 'NO' & 'NC' contacts wouldn't make a difference except for the coil required to be energised to close the contact. You may use a relay with 5V coils and avoid the SMPS. The cable should be rated at 125%, slow blow fuse at 175% and MCB (if used) at 250% of motor full load current. A second relay for the neutral is not a must. Instead you may look for a relay with 2 contacts. \$\endgroup\$
    – vu2nan
    Jan 24, 2021 at 11:34
  • \$\begingroup\$ Thank you so much for providing the fuse ratings !!! I wasnt quite sure of those! The relay that I have chosen is HFE10 has max load of 50A and can provide inrush current for motor upto 1-2hp(datasheet says 3hp, but our max load is expected to be 1.5hp), which seems sufficient for my use case. Lastly, I was wondering what you meant 'relay with 2 contacts'? Does it mean 'Double Throw' relay ? Can 'Dual Coil' relay be considered a relay with 2 contacts? \$\endgroup\$
    – Amogh Jain
    Jan 25, 2021 at 1:25
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    \$\begingroup\$ You are most welcome, Amogh! By 'relay with 2 contacts' I meant one with 2 sets of 'NO' or 'NC' contacts (one to switch the line and the other the neutral). A 'double throw' relay is one with 'change-over' contacts. A 'dual coil' relay only refers to one with a 'set' and a 'reset' coil. The motor rating is mentioned as 0.75 HP in the question but 1.5 HP in the comment. Please ensure that the relay ratings are not exceeded. \$\endgroup\$
    – vu2nan
    Jan 25, 2021 at 3:54
  • \$\begingroup\$ Thank you once again! I think I undestand relays a lot better now and will perform the field tests with 0.75hp motor. We chose a 3hp relay to ensure we are not bottlenecked in case some of our customers have a 1 or even 1.5 hp motor. Thanks! \$\endgroup\$
    – Amogh Jain
    Jan 25, 2021 at 10:03
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It is stable with power cycling.

On Off controls draw 1W so only 50 ms pulse is needed min and max affects heat rise.

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  • \$\begingroup\$ This correct . Critic -1 is wrong \$\endgroup\$
    – Tony Stewart EE75
    Jan 13, 2021 at 13:06

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