phase failure relay

E

Thread Starter

engineer

i haven't seen phase failure relays being used for motor control applications in general (in industry). can the overload relay handle a failure situation?

and what exactly happens if a 3 phase motor is subjected to 2 phase (1 phase failed)?
 
i am not asking about applications, and the fact that i didn't get any response whatsoever suggests that similar to my case, others also have not seen these relays being used commonly. so that, sort of, answers my 1st query. now if somebody instead of giving hints like this, could be more direct (like writing something)...you know.
 
Hi engineer

Yes overloads can detect a loss of a phase, although not directly. If a phase is lost, the resultant currents in the other two phases will increase, and assuming the overload has been correctly specified and set, the overload should, after a time period, operate.

Larger motors will have more specific phase failure protection achieved using Motor Protection Relays, where VTs and CTs are used to measure the voltage and current respectively, rather than with smaller motors the overload is installed in line with the motor conductors, usually mounted directly below the main contactor in the motor starter.
 
B

BIll Schwarz

Don't count on the overload to protect the motor from a single phase condition. It will likely be damaged if it is running and you lose one phase.
 
yes, low HP motors are not considered worthy enough to have phase failure relay in their control.
 
Engineer... your comment seems a bit harsh. The use of three (3) Overload elements, instead of two (2), has greatly reduced the risk of single-phasing, open-winding, and phase-loss.

There are quite a few Topics in the Control.Com Archives. If your search proves inadequate for your needs, let us know.

Regards, Phil Corso
 
The criticality of the motor to the process should be the driving factor in determining what protection should be included in the motor control circuit. Not horsepower or size.
 
J
> i am not asking about applications, and the fact that i didn't get any response
> whatsoever suggests that similar to my case, others also have not seen these
> relays being used commonly. so that, sort of, answers my 1st query. now if
> somebody instead of giving hints like this, could be more direct (like writing something)...you know.

Patience, patience. This is not a race, it's a learning experience.

First, you have to understand that there are no universal truths, everything depends on something else. In this case, the answer to "Will an overload relay protect against loss of one phase" will depend on the answer to "what kind of overload relay are we talking about?" But first, let's get some background: the "What happens" part of this.

In a 3 phase motor, the loss of one phase is essentially an extreme phase imbalance. In any phase imbalance, Negative Sequence Currents are created that are going to themselves cause counter rotating torque in the rotor. So that means that with a given amount of total stator current, the rotor current will be higher to produce the same amount of work (as defined by slip). That is alone a good reason to monitor phase imbalance, not just phase loss. Although this seems not to be germane, we'll revisit this later.

In a total phase loss, the remaining two phase current values will increase by the square root of 3, because all of the motor power is going to have to come from just those two phases. So for the same amount of work load, the current on the remaining phases increases by 1.732. Is that an "overload"? It depends on what the current WAS at the given load prior to the phase loss. If the motor was not very loaded, maybe not! For example if the motor was only loaded to 50% of its capacity, and you lose a phase, and the load remains the same, then the current will increase by 1.732, but that is now still just 87% of the full capacity, so it is STILL less than FLC and there is no overload. Therefore a standard* overload relay will not trip. Why would it? If the motor had an FLA of 10A, but was only drawing 5A for running the machine, you lose a phase and it is drawing 8.7A, what's the problem, right?

This is where the extreme phase imbalance issues comes into play. Even though the current seen by the OLR is still below the threshold of tripping, the Negative Sequence Current is causing heating the rotor disproportionately higher to what the amount of stator current should cause. It's very common that if there is a phase loss on a motor that is only 50% loaded, it burns up without ever tripping the OLR.

So now enter the "what kind of OLR are we talking about" issue. NEMA style bi-metal and eutectic melting alloy OLRs are the type that will NOT trip on anything other than phase current. *So they are the standard by which the bad situation above is related. If you are in North America and you are using old NEMA style electro-mechanical OLRs, then you SHOULD be using a Phase Monitor Relay at least somewhere in your system. If you are using Fused Disconnects for your motor starters, EVERY motor starter should have a Phase Monitor Relay.

IEC style bi-metal OLRs however have a feature built-in called a "differential tripping bar". This has a spring balanced force against all three heater elements so as long as all three are exerting equal force, the differential bar remains stationary. But if one phase has less current, and is thus cooler and not bending as much (the principal of operation of a bi-metal OL), then the differential bar is allowed to shift and it is attached to the trip pawl of the mechanism. So that means the trip release mechanism moves a little closer to the bi-metal strips if one phase is not doing it's part. Therefore phase current imbalance, INCLUDING a complete phase loss, will have the effect of biasing the trip curve downward and protecting the rotor from the added heating effects of negative sequence currents. It is still by no means a perfect solution though, because it still depends on the loading of the motor. In general if the motor is under 50% loaded, it may never trip either, but the chances increase exponentially with load. So in essence, it's better than nothing.

The best practice now is to use Solid State OLRs, or SSOLs, but only those that have phase loss protection. Most do now, but it's worth checking. There are two reasons I like this solution:

1) It is based on phase CURRENT, not phase voltage. Voltage based Phase Monitors have the unfortunate flaw that they can be "fooled" by regenerative voltage of a spinning motor. Think of a Rotary Phase Converter, same thing. So if the motor is RUNNING when a fuse blows, as it going to be the case 99.99999% of the time in a fused motor starter, then a voltage monitor may never know. Useless. But a current based phase monitor cannot be fooled this way, and an SSOL is just that.

2) SSOLs are MUCH MUCH more repeatable and reliable than bi-metal OLRs. Over time, the mechanical components begin to wear out and in a lot of cases, they begin to nuisance trip. Most of the time, the response of the line electrician is, unfortunately, to turn it up, or put in a bigger heater. Then you lose the motor. Use an SSOL and seal the dial and you don't have that problem.

Good enough for you?
 
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