HVAC Questions? Ask an HVAC Expert for Answers ASAP
Hello again... That is good work on your end. Phase protection for 3 phase systems is not a common feature, as a rough guess 99% of 3 phase systems run without any phase protection and rely on heat sensitive motor or compressor overload devices for protection.The 3 phase motors will generally not start if a 3 phase system loses one of its legs and 'single phases'.Some 3 phase motors will run if one leg of the 3 phase drops out once the motor has already started... generally low torque fan or blower motors, but not 3 phase compressor motors.
____________You can check the amperage on each phase... if there are no single phase motors wired to just 2 legs of the power, the amperage will be in the same range... for an example 31 amps. 29 amps and 33 amps on lines 1, 2 and 3... all on the same motor.That happens when the phases are slightly out of balance, that is common up to 10% or so... (I will elaborate and attach a few links)
'out of balance' means that the phases are not exactly 120 degrees apart as they should be. That happens when other users on the utility power grid are running single phase motors off of two of the legs of power, line 1 and line 2 for example.... _________My experience is that most electrical engineers I meet in industrial plants think 'back EMF' only applies to electronic circuits and not to motor circuits and definitely not back into the utility company power grid.... that is not true however.Back EMF = Electro Motive Force fed BACK into the incoming power line...That 'Back EMF' is generated by a running electric motors windings, 180 degrees out of phase with the incoming power on each leg Further explanation: A running motor to a slight extent also acts as a generator, and sends that current back into the power lines.. and that power, being 180 degrees out of phase with the incoming power, impedes that incoming power, and causes it to shift out of phase slightly.The electric motor is not designed to run efficiently with power that is not exactly 120 degrees (thats phase separation).So that the motor slows down a bit... when the motor slows down... it offers less impedance to incoming power, so draws *more amperage on that leg... and that is why you see different amperage readings on each leg of a 3 phase motor.These are called 'power factor' issues in the industry... if you read an electric bill you will see that the power company measures this 'phase shift' on average and adjusts your power usage charge accordingly since a motor running on less than perfect power draws more amperage... and runs hotter.
(that overheat is what trips the overloads on those motors, that is the most common motor protection.)Let me know if that makes any sense on your end or not.http://en.wikipedia.org/wiki/Three-phase_electric_powerhttp://en.wikipedia.org/wiki/Counter-electromotive_forceBoth of these articles are accurate.Summary: I don't know the details of how your particular system is arranged or what power issues there might be, to be safe I would replace the defective device.Let me know if this makes any sense on your end or not... we can go from there with no time limits if you choose to rate my answers to date positively.Thanks!Phil