3-phase motor, phase failure detection? - Page 1
Forum section: electronics / projects

author: Snake____1
author_rank: Contributor
timestamp: June 29, 2020, 07:58:23 pm
content: Hi, i have a customer requiring a cut off and detection after the contactor for a 3-phase motor. Making things difficult, this a motor with reversible direction.Any recommendations?I have checked out most common Phase monitor relays, these activate when in normal conditions (i.e i cannot control the contactor with the relay mounted after the contactor).

author: capt bullshot
author_rank: Super Contributor
timestamp: June 29, 2020, 08:11:34 pm
content: Two ideas plus one and another come to my mind:1. You've drawn fuses in the diagram. If the only purpose is to cut off the motor in case one fuse open -> Fuses and fuse holders with built-in fuse monitor exist. This is e.g. some kind of a pin that pops out of the fuse when it blows, this pin actuates a small switch built into the fuse holder. These switches in turn can be used to turn off the contactor.2. If a common phase monitor (I'm not common with these things) shall be used to detect a blown fuse and a possible upstream phase loss and shall be installed after the contactor, so the phase monitor trips under normal condition when the contactor opens -> look for an adjustable delay on the phase monitor or add some external delay module and build some (maybe contactor or relay) logic to "blind" the phase detector while the contactor is open. Maybe one can do this using a smallish PLC (I'm not common with these things, so no recommendation from me).3. Build your own phase failure detector from scratch, using current transformers monitoring the phase currents. If one drops to near zero while the other still have current -> this is your condition to cut the power.4. Motor protection relays exist. These are intended to cut the power at overload conditions, check if such a thing exists that includes phase loss detection (I'm not familiar with that stuff, so I can't recommend a particular manufacturer and model). Anyway, in case of phase loss, even a standard motor protection relay could trip after some time (check that too).

author: WattsThat
author_rank: Frequent Contributor
timestamp: June 29, 2020, 08:53:54 pm
content: Protection from what? Please explain the requirements.As drawn, there is no motor thermal protection. This is not optional.

author: duak
author_rank: Super Contributor
timestamp: June 30, 2020, 03:54:42 am
content: In theory, if the contactor has overloads (overcurrent detector) and when tripped disable the contactor they should detect when the motor is single phased (lost a phase) because the two other phases have to supply all the current. In practice, this doesn't always work, probably due to the overloads not selected or adjusted to a higher current. For me, one example was a compressor where one contactor pole became intermittant, and the motor either didn't start or just stalled and yet the overload didn't trip.I sort of remember that a 6 winding zig-zag transformer can be used to detect a phase out by measuring its neutral connection current to system neutral. Only when the phase voltages are balanced will there be no neutral current. This method should detect a missing phase before the contactor is enabled.Note that the if the motor is running and it loses a phase, as long as it doesn't stall it will generate close to the correct phase voltages. This could fool a voltage only detector.

author: filssavi
author_rank: Frequent Contributor
timestamp: June 30, 2020, 04:53:33 am
content: Unless you find some ready made phase monitor from the usual din rail automation stuff manufacturers (which should exist), or they need hundreds of these things just tell your customer to buy a VFD, it will have all the internal protection needed to avoid the situation you described. It is going to be cheaper and more reliable than you trying to come up with your own low cost version (just by sheer nre costs)

author: andy3055
author_rank: Super Contributor
timestamp: June 30, 2020, 05:46:22 am
content: You can easily get something like this:https://www.ee.co.za/article/motor-protection-relay-3-phase-motors.html

author: Snake____1
author_rank: Contributor
timestamp: June 30, 2020, 06:20:34 am
content: Thanks for all the replies!Thing is: This is a crane hoist motor, if a contactor switch burns out and the operator tries to hoist a load it will disengage the brake and the load will fall to the ground. The cranes are normally pre-current protector, so this does not have that. The motor has a cone shaped rotor and stator so it will move axially 2-3mm when energized to disengage the brake.I have attached a proper schematic.I have in-fact thought of rolling my own protection circuit, but this will take time as CE/UL are mandatory. This is a time sensitive matter, so if some off-shelf product exists that would be great.The system is 400v 50hz, the motor is 2/8pole (2speeds).

author: filssavi
author_rank: Frequent Contributor
timestamp: June 30, 2020, 07:13:32 am
content: I would search for something like the omron K8AK-PH, that should do what you wantI seriously discourage you from rolling your own, because these sort of protection tale a lot of fiddling and testing to get working right (I’m talking functional testing, not safety or compliance) as you need to be very immune to noise, yet you can’t do too much filtering or you will be slow.Also Different grids can do all sort of weird and wacky shit depending on conditions and loading

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2020, 09:49:48 am
content: Quote from: filssavi on June 30, 2020, 07:13:32 amI agree, a suiltable off the shelf product, which complies with the appropriate safety standards, is the only way forward here. The original poster should also consider getting it inspected by professional with the appropriate licence/certification, otherwise they might not be covered by insurance and could be in serious trouble, if it causes an injury/death.

author: duak
author_rank: Super Contributor
timestamp: June 30, 2020, 04:10:04 pm
content: This is for a Crane? Oh my! I wouldn't tackle this, especially if it's a new design, without understanding all the applicable codes and requirements. I'm in Canada and have no idea what the legal or other requirements in Norway might be.On the technical side, a VFD could be a very nice solution, however, I expect it would have to be rated for an overrunning load and would probably need some affordances for crane operation. eg., if the braking resistor fails, it should switch to DC braking rather than throwing an alarm and disconnecting.

author: filssavi
author_rank: Frequent Contributor
timestamp: June 30, 2020, 04:59:38 pm
content: Yea I haven’t commented on that but I would not touch that crane with a 10 Km pole, the fact that you are just a single phase fault away from a potentially fatal catastrophic failure is worrying at best, it probably violates a very long list of legally mandated standards and will result in criminal charges in most countries.There should be a mechanical brake somewhere that gets released through some sort of safety relay connected to a safety interlock loop, that makes shure, among other things that all three phases are energised.It this is a task from your employer, and the problem is not considered dangerous enough to either delay the product release, or maybe even recall the product I personally would seriously consider searching for a different job

author: WattsThat
author_rank: Frequent Contributor
timestamp: June 30, 2020, 06:01:24 pm
content: This is not a one-off, roll your own project. If you aren’t licensed, insured and work in this field, stay the hell away from it.Regenerative (4 quadrant) AC drives are the de-facto standard for crane operation. This is a job for a crane company, period. The big players in AC drives (ABB and Siemens) have crane specific software that implement all the required safety features and other high end requirements like anti-sway. These drives are only sold to licensed, trained companies that design, fabricate, install and maintain crane systems.Cranes are a very serious business, treat them with the respect they deserve and don’t believe for a second it is a simple matter of just detecting one failure method the customer has asked you to implement. The only drive system that scares me more than a crane is an elevator...

author: andy3055
author_rank: Super Contributor
timestamp: June 30, 2020, 06:04:47 pm
content: Quote from: Snake____1 on June 30, 2020, 06:20:34 amRolling out your own? Are you willing to take the risks? When things go wrong due to industrial accidents, the results are deadly. I hope you are thinking of this seriously! Think of what is happening to Boeing.

author: wizard69
author_rank: Super Contributor
timestamp: June 30, 2020, 06:34:22 pm
content: Phase detection relays of all sorts are available. The problem you have is detection after the contractor which means the phase detector will not be "Good" until after an attempt to energize the motor is made. Usually this requires a time delay from switch on until verification. The obvious problem here is the potential for uncontrolled movement over that short period of time. In this case forward and reverse would likely mean two phase detection relays.As others have indicated there are all sorts of standards, both local and international that one needs to comply with when dealing with lifting machines. As others have indicated I wouldn't touch this with a 10 foot pole, a fantastic errors and omissions insurance program and all the other corporate protections. It is something for a specialist to address. In fact I'm very concerned that a third party is even being involved here as the crane manufacture would be the best organization to address this. Sounds really fishy to me.Even buying phase detection hardware will not be easy as I would expect a need to have such hardware validated for use in a crane. You would have to check with the manufacture of the phase detection hardware to see if they even permit it to be used in such a case.Quote from: Snake____1 on June 30, 2020, 06:20:34 am

author: capt bullshot
author_rank: Super Contributor
timestamp: July 01, 2020, 02:57:18 pm
content: Quote from: Snake____1 on June 30, 2020, 06:20:34 amSo, this falls into a category I'm familiar with- "Functional Safety", laws, regulations, standards, certification, ... and worst case prosecutor, lawyers, judge, liabilities, ...It's grown to a real big thing in comparison to some decades ago.Just one advice left: If you aren't a professional safety engineer, stay away. If you can't, ask for assistance from a professional safety engineer or whatever in your country is the equivalent to our German "Berufsgenossenschaft" (workers' compensation board) - basically a thing that work with insurance of the workers, risk management etc.If you want to do that from scratch or buy something, you'd have to go through quite an assortment of failure analysis and risk management stuff and be able to prove that you've done that according to the applicable standards. It might result in a quite simple result, but with a lot of paperwork behind. If you can't provide the paperwork in case of an accident, you're toast.As this looks like it's safety relevant, CE/UL is mandatory anyway, but not sufficient.

author: filssavi
author_rank: Frequent Contributor
timestamp: July 02, 2020, 06:47:59 am
content: Quote from: capt bullshot on July 01, 2020, 02:57:18 pm100 times that, compared to safety certification like IEC61508 and all its offsprings (ISO26262 for automotive) UL and CE are a walk in the park, it’s just a whole different level of complexity, not only the design must be safe, also the whole development cycle must be certifiable. That means all decisions pertaining anything even remotely related to safety need to have a full paper trail behind them justifying why the decision has been taken, how, potential safety risks, etc.The cost of all the process is also much higher as a result, however there is a good reason for it, safety regulations are written in blood...