3 phase motor single phasing troubleshooting - Page 1
Forum section: electronics / beginners

author: robsims
author_rank: Frequent Contributor
timestamp: September 24, 2022, 09:58:12 am
content: Hi,I have a question. This is a three phase system(see picture) with line voltages of 380 volts . If a line breaks during operation (when the contacts of the contactor are in closed position), what voltages will i measure at U-V, U-W and V-W ? Will i still measure 380 volts between points U-V, U-W and V-W ? Please, no guessing, i want only experienced members to answer. Many thanks in advance

author: themadhippy
author_rank: Super Contributor
timestamp: September 24, 2022, 11:35:34 am
content: 0v as the overload will have detected the increase in current and opened the contactor

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 24, 2022, 12:11:00 pm
content: The breaker should trip in single phase condition but before it trips the voltage is still full 380V. Some motor actually run in single phase condition for quite some time. The current draw on the 2 legs are very high and the motor gets very hot.

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 24, 2022, 12:56:43 pm
content: Full voltage will be measured across all three measurement points as shown in your diagram.If the motor U phase opens as you suggest, the thermal overload (if set and wired correctly) should trip and remove the control voltage from the coil of the contactor. A very lightly loaded motor may not draw sufficient amps to trip the thermal.If the motor U phase internally shorts to ground and the motor is lightly loaded, it may continue to run without tripping. That’s a maybe and it depends upon where in the coil the turn(s) become shorted.

author: Zero999
author_rank: Super Contributor
timestamp: September 24, 2022, 01:01:39 pm
content: The reason why you'll still measure a voltage, when a phase is disconnected is because the motor will act as an alternator. This can be done deliberately to create a three phase supply, from single phase, but it's better to just use an inverter.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 24, 2022, 02:00:55 pm
content: But then I have the feeling that you asked us to do your quiz. You don't have a motor to take care of.

author: robsims
author_rank: Frequent Contributor
timestamp: September 24, 2022, 02:57:42 pm
content: Many thanks guys for helping me out. I do a lot of self study. Not getting much help from colleagues, everyone is busy. But that doesn't bother me.Answers that were to the point were from WattsThat and BeBuLamar. But okay,.. to summarize things, if the U line is broken and the motor is still running and before the thermal overload trips or the contactor is opened i will measure the full 380 volts across points U, V end W, Am i correct?Part 2If a line is open (like the U line)A And the motor is running, it will still run but will lose power. The current draw in the remaining lines will be higher and the thermal element andcontactor will disconnect. In the worst case the motor will burn.B And the motor is not running, it will not start running when the contactor kicks in i assume.Question: If i'm not aware a line is damaged and situations A or B occur, how do i troubleshoot?

author: Jeroen3
author_rank: Super Contributor
timestamp: September 24, 2022, 03:35:21 pm
content: Source:Measure each phase to eachother or neutral/earth. (viability depends on earthing system)Use your phase sequence detector.Motor:Disconnect the cable on the motor and measure the motor windings.Also perform insulation test of the motor windings.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 24, 2022, 04:00:08 pm
content: My troubleshooting is relatively simple. Leave the contactor open, measure the 3 windings resistance. They should read the same. Measure resistance from any of the phase to ground and perhaps with a megaohm meter. If it's OK here. Energize the contactor and verify that you have all 3 phase voltage at the motor.

author: robsims
author_rank: Frequent Contributor
timestamp: September 24, 2022, 04:17:13 pm
content: Many many many thanks BeBuLamar and jeroen3. if there are others who have additional information they want to share please feel free to do so, thank you

author: themadhippy
author_rank: Super Contributor
timestamp: September 24, 2022, 05:32:29 pm
content: When measuring the voltage better to use a low impedance meter,you wont be the first to disappear down the wrong rabbit hole chasing a strange voltage thats due to alternative paths

author: rstofer
author_rank: Super Contributor
timestamp: September 24, 2022, 09:29:10 pm
content: If the motor is lightly loaded, you may not notice.The full load current will increase to 173% of FLA (if the motor is heavily loaded) and the overcurrent device should be set at 125% of FLA so, sooner or later, the thermal devices will open the control circuit and the starter will drop out.Google is always your friend:https://www.cbmconnect.com/three-phase-motors-single-phasing/https://blog.se.com/industrial-software/2019/03/07/motor-protection-three-common-mistakes-and-how-to-avoid-them/When it's really important, you add a phase-loss detector to the control circuit:https://www.mcmaster.com/phase-loss-detectors/Wye-Delta and resistance starters require a bit more thought. Again, a phase-loss detector is helpful.For the truly paranoid, there are fuses with microswitch trip indication that can interlock with the control circuit.https://www.crescentelectric.com/fuses/fuses/cartridge-fuses/600v-4-12a-class-j-td-fuse

author: elekorsi
author_rank: Regular Contributor
timestamp: September 28, 2022, 06:32:23 pm
content: I usually dont even measure voltages on the running motors, but only check all three phases with current clamp meterIt tells me almost everything that i want to know. If possible i do a visual check, listen for bad bearings and sometimes take my thermal camera to check the bearing housing if anything is happening there...On a stationary motor: low resistance check of the windings and insulation check (1000V) of windings to ground and if possible also insulation between the windings.

author: Seekonk
author_rank: Super Contributor
timestamp: September 29, 2022, 01:46:14 pm
content: I used to design three phase loss detectors. The very lowest cost el cheapo detector was just a resistor to A and capacitor to B and measure that junction voltage to C. All on the motor side. If I remember right, it is a little better if there is an additional resistor in series with the capacitor. then measure the resulting voltage with an under/over voltage detector.