Latching relay options for motor?  SSR?  mechanical? - Page 1
Forum section: electronics / projects

author: viper
author_rank: Regular Contributor
timestamp: August 24, 2025, 08:37:52 pm
content: I need to switch some motor loads. I need a latching type relay that would not need power in either the on or off state. voltages of 100-300V, and steady current of 5-15A, but inrush on starts can be as high as 80A. There are many mechanical relay options, but I'm wondering if I can/should abandon any SSR solutions? This is not something that needs switched very often. I have found one mechanical solution but they are insanely priced!I've never heard of a latching SSR so maybe that is out?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 24, 2025, 08:54:36 pm
content: Quote from: viper on August 24, 2025, 08:37:52 pmYou can get NO and NC SSRs but the NC simply use depletion MOSFETS, if you wanted to preserve the state, you would need a battery for the gate voltage.Possible in theory, with 2 MOSFETS and 2 optos for SET and RESET and a CMOS latch and a sufficient voltage small battery.Since your motors need power, you could use a smaller latching relay to drive a larger standard relay. Both can then be high volume, low cost choices.

author: Zero999
author_rank: Super Contributor
timestamp: August 24, 2025, 09:04:11 pm
content: There isn't enough information to properly answer this question. Is this: DC, AC, three phase?

author: viper
author_rank: Regular Contributor
timestamp: August 24, 2025, 09:14:40 pm
content: Sorry, this is an AC single phase application. Actually would also apply to 3P but in those cases, there is usually an MCC where the coil power can be interrupted.Wiz, your thought is good and something I might want to eval.

author: wraper
author_rank: Unknown
timestamp: August 24, 2025, 09:29:46 pm
content: Does not sound like a good idea from a safety standpoint.

author: Zero999
author_rank: Super Contributor
timestamp: August 24, 2025, 09:39:23 pm
content: The norm is a big mechanical contactor.I was also going to mention safety: what is the motor driving? Does it have an emergency stop? If so you quite likely need a safety relay.

author: themadhippy
author_rank: Super Contributor
timestamp: August 24, 2025, 11:08:08 pm
content: Quotewhy the no power requirement? theirs gonna be power at the switching contacts for the motor so why not also use that to power a contactor

author: viper
author_rank: Regular Contributor
timestamp: August 25, 2025, 12:05:17 am
content: Sounds like I need to stick with a mechanical relay. To answer a few question, this is a remote 'safety interrupt' only. I can't recall the specific code in NEC but there is an out as only a safety, not a means of disconnect. The only purpose of the relay is to save the motor. Consider it like "do nothing unless temp sensor goes high" sort of thing. Otherwise contacts stay latched and life is good.

author: Psi
author_rank: Super Contributor
timestamp: August 25, 2025, 12:10:29 am
content: After being activated you can reduce the voltage to the 'holding voltage' to save energy, if that helps at all.It would definitely increase lifespan if it's always energized.You can also try to find one with the lowest holding voltage.Assuming the latching option doesnt work out.

author: Simmed
author_rank: Frequent Contributor
timestamp: August 25, 2025, 02:06:19 am
content: Quote from: Zero999 on August 24, 2025, 09:39:23 pmupvote this

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 25, 2025, 01:59:01 pm
content: With a few hundred Volts and 10A you're in the big contactor range, and I'm not sure whether there are latching versions of those. Safety is a concern, but there are plenty of installations that start and stop automatically (such as pumps operated by a float switch).When the motor is running, it needs plenty of power, and the energy for a relay or contactor is not a problem. So I assume this installation is not always powered. I would probably go for a two way solution. Use a small signal latching relay to remember a bit, and then only switch the big contactor and the motor if there is power available. If you add a uC, you can remember plenty of bits in an EEprom or similar.

author: LaserSteve
author_rank: Super Contributor
timestamp: August 25, 2025, 07:32:53 pm
content: My company has made SSRs for over 30 years, and out of 156 possible stock products only one family of output module is "Normally On" and none store their last state. The normally "On" module is for situations where it controls a larger SSR. If you need hundreds / thousands of these, I'm sure we could design one with a memory for you, however there would be an NRE charge.if it is safety of life, or a LARGE motor, the standard methodology these days is to use a PLC, and the SSR drives a larger contactor. One thing to consider is how the motor powers up. Does it have a starter or is it small enough that it just starts spinning when power is applied? Considering the PLC is probably going to burn 10 seconds doing a self-test if it does not have backup power, how fast do you need to re-start? What happens during a power failure?What sensing is already on the motor?If this is very simple, and the motor only rotates one direction, a Normally on SSR switched by an open collector sensor, controlling a contactor might be inexpensive. Our SSRs will require a logic voltage as well, 5, 12-15, and 24VDC are the options.Need more details to advise you.Steve

author: wraper
author_rank: Unknown
timestamp: August 25, 2025, 09:04:27 pm
content: SSR makes no sense for constant ON operation, you'll basically have no advantages + all downsides. They're expensive, less robust, likely to fail short, and have significant energy loss/heating under load. For example, with 10A continuous load, expect about 12-15W of heat you also need to dissipate somehow. On other hand, normal relays are very reliable with infrequent switching.Quote from: viper on August 25, 2025, 12:05:17 amNormally ON + safety interrupt is a contradiction essentially.

author: viper
author_rank: Regular Contributor
timestamp: August 25, 2025, 09:24:43 pm
content: Quote from: wraper on August 25, 2025, 09:04:27 pmI agree with this. I think I need to stick with a mechanical relay. And and guess due to my poor wording, when I say "safety", I mean SAVE the motor, not life. The normal operation of the motor leaves the relay in the N.C. state, where it should live most of it's life. But if sensors are triggered, indicating if the motor is not stopped, it will self destruct, the relay is instructed to simply open, removing power to the motor, and creating an 'alarm condition', which would need to be manually resolved AFTER removing all power from the circuit.I should have thought about the zero crossing and holding of an SSR. I think their advantages of fast switching and high cycles is not needed here. I was just chasing 'cheaper'.