480VAC Vibrating Motor, Missing Phase?

[chrisreisinger79]

I have a 480VAC Vibrating Motor currently running in our industrial facility. I just discovered this, and we are currently in production, so my testing options are limited at the moment. Recently hooked a 4-20mA current transducer to one of the phases to monitor current of the equipment in our PLC (Allen Bradley SLC 5/05). Looking in our program, I was getting a 4mA signal into the PLC. I had production stop for a moment, thinking the transducer had been incorrectly wired or the dipswitch on the PLC card wasn't set to current. Both were correct so I had them start back up. Using a meter to measure current on that leg, I found that its actually showing 0 current. The other two legs are reading the expected current value.

My question is: Is it possible to start and run a 480VAC vibrating motor with only two legs?

 

[ptonsparky]

I have a 480VAC Vibrating Motor currently running in our industrial facility. I just discovered this, and we are currently in production, so my testing options are limited at the moment. Recently hooked a 4-20mA current transducer to one of the phases to monitor current of the equipment in our PLC (Allen Bradley SLC 5/05). Looking in our program, I was getting a 4mA signal into the PLC. I had production stop for a moment, thinking the transducer had been incorrectly wired or the dipswitch on the PLC card wasn't set to current. Both were correct so I had them start back up. Using a meter to measure current on that leg, I found that its actually showing 0 current. The other two legs are reading the expected current value.

My question is: Is it possible to start and run a 480VAC vibrating motor with only two legs?

Possibly. Typically not for very long.
How big is the motor? Are you sure it's not just a single phase vibrator?

 

[chrisreisinger79]

480VAC 3-Phase 3/4HP Vibrating Motor. Was able to verify that all three phases have correct voltage at local disconnect to the motor. I agree, I don't think it's a missing phase. So, the next question is:

What could cause the current draw on one of the phases to read at 0?

 

[ptonsparky]

480VAC 3-Phase 3/4HP Vibrating Motor. Was able to verify that all three phases have correct voltage at local disconnect to the motor. I agree, I don't think it's a missing phase. So, the next question is:

What could cause the current draw on one of the phases to read at 0?

An open in the pecker head, motor winding, disconnect or contact on the motor starter. Old style motor overload heater could be open but that would be rare. Operator error is another but we will discount that.

 

[jim dungar]

The laws of physics say, a three phase motor needs to draw current through all 3 legs in order for the shaft to begin turning, unless the shaft is turned by some mechanical means.
Once the shaft is turning, it is possible for the motor to continue to run even while pulling current from only 2 legs, however it will not be able to provide its full rating.

 

[winnie]

A vibrating motor is an interesting load, essentially a motor sealed in an enclosure with an offset weight attached to the shaft. This is an almost purely inertial load, with slight friction from bearings and air.

I'd expect a very low starting torque requirement. On top of this, the load that doesn't care about direction of rotation.

While a perfect induction motor develops no starting torque when single phased, a real induction motor likely has small residual rotor flux or slot reluctance flux. So it wouldn't surprise me if a real three phase motor would 'twitch' slightly if hit with single phase across the line.

So I wouldn't be surprised if a vibrating motor hit with single phase would start, and once it is running it will keep going.

-Jon

 

[synchro]

.. Using a meter to measure current on that leg, I found that its actually showing 0 current. The other two legs are reading the expected current value.

I suggest putting a clamp meter around the conductors for both of the two legs that were found to be drawing a significant current. If those two are the only conductors conveying current to the circuit then you should get a zero current reading. If you see a significant current, then one possibility is that the other phase is open somewhere but there's a ground fault on the motor side of this open circuit. This might be able to provide enough of a rotating magnetic field within the motor to get it going.

Are there any other vibration motors attached to the same structure as the one you mentioned? If there are other such motors then maybe they could be inducing enough rotation to get this one started.

Was there any performance issue that induced you to monitor these currents? For example, if two vibration motors were being used to get approximately linear motion, then if one motor was turning but out of sync and slipping relative to the other then that could upset the desired motion.

 

[chrisreisinger79]

I suggest putting a clamp meter around the conductors for both of the two legs that were found to be drawing a significant current. If those two are the only conductors conveying current to the circuit then you should get a zero current reading. If you see a significant current, then one possibility is that the other phase is open somewhere but there's a ground fault on the motor side of this open circuit. This might be able to provide enough of a rotating magnetic field within the motor to get it going.

Are there any other vibration motors attached to the same structure as the one you mentioned? If there are other such motors then maybe they could be inducing enough rotation to get this one started.

Was there any performance issue that induced you to monitor these currents? For example, if two vibration motors were being used to get approximately linear motion, then if one motor was turning but out of sync and slipping relative to the other then that could upset the desired motion.

This is the only motor for this piece of equipment and the only motor attached to the structure. This piece of equipment is part of a production line that requires all pieces of equipment to be running in order for the line to run correctly and not cause a bottleneck. Unfortunately, the system was designed with an interlock that only utilizes a NO auxiliary contact to determine if the equipment is "running." As expected, this doesn't account for equipment motion stopping for any other another reason besides the starter disengaging. Until I can get a motion sensor installed, the next best solution was to monitor current draw to determine if the motor was running. It's by pure coincidence that the leg we installed the current transducer on happens to not have no current draw. This does fall in line with comments from maintenance and production staff stating it seems as though the equipment is not moving/shaking as it normally does.

If I am understanding your suggestion correctly, I should take a single clamp meter around the other two legs together and check the reading. Under normal circumstance, I should see a zero current reading.

 

[winnie]

If I am understanding your suggestion correctly, I should take a single clamp meter around the other two legs together and check the reading. Under normal circumstance, I should see a zero current reading.

Exactly. If the motor is correctly single phased, then the current going down L1 should exactly match the current returning on L2, and if you clamp around both L1 and L2 you should read zero. But if what is really happening is that L3 is somehow being bypassed (say to ground) then you will measure net current on L1+L2.

Jon

 

[chrisreisinger79]

I was able to troubleshoot the motor on Friday...I also learned a little more information on Friday as well. When I opened the electrical housing on the motor, I immediately found that one of the wires had rubbed against the conduit entrance and had exposed the copper. I found out that earlier in the week, a fuse was blown during night shift production. At the time, maintenance staff attributed the blown fuse to the shaker becoming overloaded with parts. After hearing that and finding the broken wire, I believe the opposite occurred. I pulled new wire, and the current is reading correct on all three phases.

I'd like to understand better the electrical theory behind what happened here. Some background information on the plant. The switch gear for this equipment is a 480VAC Delta (ungrounded) system. At present, there is a ground fault in the system due to some malfunctioning metal halide lights fed from this switchgear. This is a fairly common occurrence in our facility...when the components in these lights fail they typically produce a ground fault.

The wire was still fused to the conduit entrance, but no longer causing the fuse to blow. When I found the wire (SOOW cable, THHN stranded) still fused to the conduit entrance, about half of the wire strands were still intact, the other half were completely severed. The initial contact earlier in the week caused the fuse to blow. Once the fuse was replaced, the resistance in the circuit was high enough to keep the fuse from blowing but allowed the current to flow to ground rather than back to the source? Is this due to the ground fault already present in the system or a design of the Delta system and the nature of the contact of the wire to the conduit entrance?

 

[ptonsparky]

Yes and yes. Not or, both.
The current was returning thru the raceway. It was acting as your phase conductor.

 

[synchro]

Perhaps the ground fault on the SOOW cable caused enough current to flow that it was able to modifiy the resistance of the metal halide ground fault that's on another phase before the fuse blew. Then there was enough resistance in the halide light ground fault that the fuse didn't blow when it was replaced (or maybe it even cleared that fault). Just speculation from this end.

 

[GoldDigger]

A vibrating motor is an interesting load, essentially a motor sealed in an enclosure with an offset weight attached to the shaft. This is an almost purely inertial load, with slight friction from bearings and air.

I'd expect a very low starting torque requirement. On top of this, the load that doesn't care about direction of rotation.

While a perfect induction motor develops no starting torque when single phased, a real induction motor likely has small residual rotor flux or slot reluctance flux. So it wouldn't surprise me if a real three phase motor would 'twitch' slightly if hit with single phase across the line.

So I wouldn't be surprised if a vibrating motor hit with single phase would start, and once it is running it will keep going.

-Jon

I also imagine that such a motor would do its job equally well regardless of the direction of rotation. And the presence of the weight can lead to the motor coasting down to a stop consistently in the same position, so any residual flux or other minor cause of torque would consistently instead of intermittently allow the motor to start.
One possible difference with opposite rotation would be reduced efficiency of any built-in cooling fan. As long as the motor is lightly loaded, that may not be noticeable either.