SPD in MCC
SPD in MCC
(OP)
We recently had a surge in a plant 'due to one the utility's fuses blowing.' The surge destroyed 3 contactors in one of our MCCs. We don't currently use any SPDs in the plant with a possible exception to small point of use units, e.g., integrated in a power strip. I've done a bit of reading on SPDs and I understand that there can be a great variability in there effectiveness, depending greatly on where or how they're installed. For example, wall units that potentially make use of long leads are less effective than (panel) integrated units because of the inductive reactance introduced by the leads. So far I've only looked at GE and Eaton's offerings for SPDs. Eaton offers a unit that supposedly is designed for installation within an MCC. Do any of you have experience with these units? The intention of installing an SPD in the MCC would be to avoid damage to the MCC components (and also connected apparatus) due to a surge from occurring again. Would the SPD function effectively in this capacity? Would the placement of the SPD in the MCC be a significant consideration, i.e., are the buses in an MCC of sufficiently low impedance to allow free placement of the SPD? What approach would you suggest for avoiding damages due to surges (internally or externally created) in a plant? MCCs are 1600A 480Y/277V from various manufacturers.
Thanks
Thanks






RE: SPD in MCC
We recently had a surge in a plant 'due to one the utility's fuses blowing.' Did the surge blow the fuses or did the fuses blowing cause the surge?
In what way were the contactors destroyed?
It may be wise to identify the origin of the "surge" before trying to mitigate future issues.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: SPD in MCC
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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RE: SPD in MCC
RE: SPD in MCC
An under voltage condition may destroy the coil. If the coil swells when if burns up, some electricians will replace the entire contactor.
A contactor should be able to interrupt reasonable overloads and over voltages without suffering damage.
I am leaning towards burned coils as a result of under voltage.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: SPD in MCC
RE: SPD in MCC
If you have an industrial facility with a lot of 3 phase motors that are already running when a phase is lost, those 3 phase motors will act like a "spinning reserve" to regenerate a 3rd phase, just like a Rotary Phase Converter does. But also just like an RPC, the control power must come from the "real" phases, otherwise it can drop precipitously and cause contactor coils to chatter. It's a very common problem and a key issue with regards to having phase loss protection that will shut down motor controllers.
Also as to the motor thermal overloads tripping on phase loss: common misconception. Basic thermal overloads will NOT trip on a phase loss. They only trip on INCREASED CURRENT. But if your motor is only lightly loaded, the current increase may not be enough to trip the overload relay quickly enough to avoid damage to components in the system. If you had Solid State Overloads that included Phase Loss protection (most now do), then that is fine. But if not, you cannot rely upon a bi-metal or melting alloy OLR to protect your system from damage due to phase loss. This is I know anathema to a lot of the sales literature for IEC style bi-metal OLRs, but it is true nonetheless. IEC overloads really only offer phase loss "sensitivity" in that the trip curve is biased to trip earlier on a phase loss, but it is not full protection. That might protect the contactors I'll admit, but the motor can still be damaged by the extra heating from the severe current imbalance that occurs. But if you are in North America and you had NEMA style overload relays in an MCC (as is usually the case), most of them don't have that feature anyway and you can definitely over heat the contactors on phase loss.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: SPD in MCC
RE: SPD in MCC
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: SPD in MCC
1.) Is the reduction of control voltage while the contactor is still closed due to the motor slowing down?
2.) For an RPC, if single phase 480V is supplied to two terminals, what should the expected voltage be on the third terminal when the RPC is lightly loaded, heavily loaded?
3.) If the phase that isn't used to derive the control power is lost (opened), would the chatter likely still occur? Example: The L-L voltage is normally 480V. Phase 1 and 2 power a 480/120V transformer, the 120V is used for control. If phase 3 is opened, wouldn't V_12 stay near 480V?
4.) Once the contactor opens, why would it reclose? Inductance of the control transformer? (v=Ldi/dt)
RE: SPD in MCC
But;
Phase loss;
The motors act as induction generators and attempt to support the missing phase. They are usually unable to fully support the non-motor loads on the lost phase and as a result the voltage drops on that phase.
If a contactor had a simple core it would tend to start to open every time that the sine wave passed zero. To avoid this there is a single turn winding across part of the core face called a shading coil. This causes a phase shift of the magnetizing force across part of the core face. This holds the armature firmly seated during the zero crossings of the supply voltage.
As the voltage drops, the shaded part of the core may not develop enough force to prevent the contactor from opening slightly only to be slammed closed by the next rising half cycle. Hence chatter with low voltage.
If the voltage drops so low that the contactor drops out completely, a large air gap is introduced into the magnetic circuit. As a result the inductive reactance drops considerably. This also drops the impedance and the current rises. If the voltage is not adequate to pull in the contactor, the coil may overheat and burn up with much less than rated voltage.
Remember that a phase loss is an abnormal event. Currents may rise to abnormal levels and voltage drops may increase to abnormal values.
Motors will seldom slow down more than a few RPM but they will draw more current. This will add heat but also the rotor heating may be disproportionally greater as the rotor transfers energy from the healthy phases to the faulted phase in it's role as an induction generator. Even though the motor current may remain below rated current for a motor with less than 100% loading, the motor may be overheated due to the extra rotor heating.
What's an RPC?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: SPD in MCC
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: SPD in MCC
We use a push button to supply power to the contactor relay; when the button is pressed one of the relay contacts closes, the contact is wired in parallel with the push button and allows current to continue to flow into the relay after the push button is released. It seems to me that if the contactor drops out, power will be no longer be supplied to the relay and thus the relay contacts can't change state until the start button is pressed again. We don't use DC, but the following image is basically what I'm talking about.
RE: SPD in MCC
That circuit can still chatter.
Many plants use remote control and switching of motor contactors and many pump and refrigeration applications use pressure, level or temperature controlled contactors all of which are at risk of low voltage failure without added protection schemes.
But we are all just guessing until we have more information concerning the details of your failures. I am sure that it seems obvious to you what has failed but not to us.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: SPD in MCC
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies