Phase loss on a lightly loaded motor 1

[JPGQC]

Hi guys

I'm trying to learn more on motor protection. Topic of the moment: single-phasing protection.

I know about standard loss-of-phase and unbalance relays.

My question is: for a lightly loaded motor, say 50% of nominal HP, would a phase loss be dangerous for the motor?

Because of the light load, the current in the remaining two phases would not be high enough to be seen by the overload relays.

But what would be the impact of the negative sequence current in the rotor?

Thanks!

 

[itsmoked]

Another good reason to always use electronic phase loss protection instead of cr@ppy thermals. I have never seen a single phased motor that hasn't burned out but I can't say for sure that any of them were 'lightly loaded'.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Somewhere around 1/4 loading may be OK but that's a guess based on one winding doing the work of three and possible other factors.
Remember that this was SOP for the old phase converters built from a three phase induction motor and a box of capacitors.
Most in service motors will have more load than that and will burn out. The ones that survive, you and I won't be called to look at them. That may explain why we don't see them Keith.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wayne440]

Speaking of phase converters - would not a lightly loaded 3 phase motor also "generate" the missing phase in such a case?

 

[jraef]

Negative sequence currents set up counter rotating torque in the motor, so the motor essentially "fights itself" to continue turning. That means you get less torque per ampere of input current. It also means that for a given current draw by the motor, the heating effect is increased. But quantifying that is difficult at best. Most data relates to the effects of VOLTAGE unbalance because it is more quantifiable, even though the direct effect is from the resultant current imbalance. However the generally accepted status is that a motor can still end up being subjected to damaging thermal stresses from single phasing even if the connected mechanical load is reduced below 50% of rated. The figure I have always used is 33% load but is not based on anything other than something I read once when being asked about de-rating a 3 phase motor to work on a 1 phase supply (assuming they got it started somehow). I have been told by others it's 25% load (which jives with Bill above). The point is, it can't take much and if the motor is that lightly loaded, why is it on?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[jraef]

Speaking of phase converters - would not a lightly loaded 3 phase motor also "generate" the missing phase in such a case?

Sure, from the standpoint of voltage. That's why voltage based phase loss detectors can be so easily fooled. But the unused phase getting a voltage induced onto it does not mean that winding is creating work output on the rotor, the business end of a motor.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[rockman7892]

How can a voltage be generated on the missing phase? Would an example be a delta wound motor still reading 240V between the missing phase and the other two phases?

 

[wayne440]

Looks to me as if the "lightly loaded" motor could quickly sustain damage since it will be "generating" current into the dead phase(s). I'm aware that the motor would produce far less than rated power under said conditions.

 

[waross]

An induction motor generates a high back EMF. That is why the real current of a free spinning motor is relatively small.
The energized phase induces a magnetic field in the rotor. As well as inducing the back EMF in the active phase, this field also induces an EMF in the other windings. With no load connected to the open phase lead, the EMF will be close to line voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rockman7892]

Waross

When you mention "no load" being connected to the open phase lead are you referring to a load such as something other than a digital meter that would impose a small load?

 

[waross]

Consider a three phase lighting panel with one motor connected. If one phase of the lighting panel is lost while the motor is running the motor will attempt to replace or feed the devices on the missing phase. The motor may or may not be successful depending on the load being back fed, the size of the motor and the load on the motor. A meter, either analog or digital, will be a negligible load in most instances.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[amptramp]

I have seen a small knitting plant (6 - 8 machines) being supplied by a phase converter (can't remember if it was static or rotary) keep running when the entire plant was single phased (due to a failure in the wiring to the phase converter). All the machines kept running until the machine with the largest(HP/KW) motor was stopped. Then all the machines stopped and would not restart.

 

[jraef]

Such is the case to be made for phase loss protection, especially one that looks at imbalance and/or current but not just voltage detection.

Once a 3 phase motor is already turning and then the phase is lost, it might keep going as long as the load is light enough and/or there is another larger motor still spinning in the system. But that largest motor now is acting as a rotary phase converter for all the rest of the loads and will likely overload quickly. Once it does, the remaining units will eventually trip off line too.

By the way, this is a similar scenario to what accounts for "load contributions" to a fault.




"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[roydm]

Wouldn't the Out of Balance trip provided in most motor overloads trip the motor before any damage is done.
I know I have often used a 3 phase starter to supply a single phase motor and had to connect two overloads in series to prevent the Out of Balance trip.
Roy

 

[waross]

It depends on the type of overload used. Older NEMA overloads did not have an unbalance feature. We are discussing the case of a lightly loaded motor. At very light loads where the single phasing motor is not overheating, the unbalance feature may not trip the motor.
It depends!

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[roydm]

So if the motor is lightly loaded it should be OK running without one phase, if the load goes up motor will stall and trip the overload.
Slightly off Topic
I used to see the odd two phase motor installed in shearing sheds (NZ), I think the idea was to save on copper feeding remote farms.
Roy

 

[dpc]

So if the motor is lightly loaded it should be OK running without one phase, if the load goes up motor will stall and trip the overload.

I would never make that assumption without having a lot more data.

First, I would not assume a lightly-load motor will not overheat when single-phased. This will depend on the type of motor and the rotor construction.

Second, old-style thermal overload relays will not reliably protect a motor from destruction via single-phasing. They just won't.

I once had the pleasure of visiting a sawmill where virtually every three-phase motor was destroyed in about 30 minutes when one phase of the utility supply was lost. AFAIK, not one tripped their overloads prior to failure.



David Castor

http://www.cvoes.com

 

[waross]

roydm, Do you mean three phase derived from two primary phases and a primary neutral with an open delta transformer connection? True two phase has been gone in most of the world for generations and didn't save much copper.

http://en.wikipedia.org/wiki/Two-phase_electric_power

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Besides, true 2-phase power would not save any copper. You need at least 3 wires for 2-phase, maybe 4.

That said, there is probably a semantics issue at play here. In the US we at one time had true 2-phase power; 2 separate sets of phase windings that were 180 degrees out from each other. Elsewhere in the world, they refer to "2-out-of-3 phases" as "2 phase" power where we refer to that as "single phase" power. It is 3 phase power, with the phases 120 degrees out from each other, but with only 2 of them used. The MOTOR however would be called a single phase motor here in the US.

Semantics semantics semantics...


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[davidbeach]

Ok, semantics then...

In a 2 phase system the two are not 180 out of phase but are in quadrature, or 90 degrees apart. 180 wouldn't get you a rotating field.