Electronics > Projects, Designs, and Technical Stuff
3-phase motor, phase failure detection?
Snake____1:
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).
capt bullshot:
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).
WattsThat:
Protection from what? Please explain the requirements.
As drawn, there is no motor thermal protection. This is not optional.
duak:
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.
filssavi:
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)
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