Motor Control Center feeding 3-phase motors

[katwalatapan]

Hello,

I have a query concerning a Motor Control Center (MCC)feeding 3-phase 575V motor loads.

Do MCC usually have a feature to automatically trip the circuit feeding 3-phase motors, if a phase is lost? OR would the motor still run off two phases, overload and then eventually burn out?

Thank you.

 

[dpc]

If you want loss-of-phase protection, you need to specify and purchase it. It is not standard - at least for NEMA type MCCs.

 

[wroggent]

Took a quick glance at the Eaton overload relay user guide - it appears their C440 electronic overload relays feature phase loss protection. That type of relay is a pretty standard part for their new MCCs. Keep in mind that will offer protection only for individual motors that use the relay, it won't protect the whole MCC.

 

[dpc]

Again, I would not assume loss of phase protection is provided unless you specify it.

 

[wroggent]

I just bought two. I didn't ask for that specifically but they still quoted it that way.

 

[jraef]

Most solid state overload (SSOL) relays now come with phase loss protection based on current through at least one phase being less than some prescribed threshold value (typically <20%). That capability is essentially "free" once you go to the trouble to make a solid state overload relay. Some NEMA style MCC manufacturers are now quoting the simplest SSOL options as standard (we do). These simplistic SSOL relays are designed as direct cost replacements of bi-metal adjustable overloads so they often don't have a lot of extra bells and whistles, but phase loss is, like I said, a "freebie". But because not everyone does that, you are better off asking for it specifically rather than take chances.

Side note: Phase Loss Relays that are voltage based are, in my opinion, too easily fooled because if the phase loss takes place while a motor is running, the motor can act like a rotary phase converter and regenerate a voltage on that missing leg that the Phase Loss relay sees. That's why I prefer using a current based phase loss detection in the motor OL relay, because you can't fool it.



"Will work for (the memory of) salami"

 

[katwalatapan]

Thank you everyone for your inputs.

Also for older MCC units, that may lack Solid State Overload Relay and similar single phase protection, would it be possible to easily integrate phase loss protection onto the starters in the MCC?

 

[oldfieldguy]

In reference to older MCC's in many cases you can retrofit the newer solid state overload units in replacement of the existing thermal overloads, thus giving yourself the phase loss protection for individual motors.

This is an important consideration. One of my predecessors installed phase-loss protection on the MCC incoming main breaker. Loss of phase resulted in the tripping of the main breaker, a fine thing for protecting the motors, but it also killed power to lighting and auxiliaries AND the station was in the dark until somebody showed up to reset the breaker.

We ended up by tripping the incoming utility breaker on loss of phase. It was ahead of the automatic transfer switch, so loss of phase on the incoming utility, a pretty common situation, resulted in tripping the utility breaker and the transfer switch started up the station backup generation to maintain station operations.

old field guy

 

[jraef]

What brand of older MCC do you have?

"Will work for (the memory of) salami"

 

[DRWeig]

Can you fit these in the starter circuits:

http://www.kele.com/power-monitoring-and-protection/455-voltage-monitors.aspx

Disclosure: I'm a consultant for Kele, but these things are available lots of places...

Best to you,

Goober Dave

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[SteveWag]

One problem I have encountered is that the solid state motor overload device needs to be manually reset after detecting a phase loss. Some of our Waste Water plants have a “Standby” power unit that may be start delayed by as much as 3 min. That means that all DOL units will be tripped and require manual resets. Much better to use thermal OL’s and a three phase monitor. Insert a monitor contact in the holding circuit.
Steve

 

[jraef]

One problem I have encountered is that the solid state motor overload device needs to be manually reset after detecting a phase loss. Some of our Waste Water plants have a “Standby” power unit that may be start delayed by as much as 3 min. That means that all DOL units will be tripped and require manual resets. Much better to use thermal OL’s and a three phase monitor. Insert a monitor contact in the holding circuit.
Steve

Why would having a start delay make any difference? Is your backup supply tuning on one phase at a time? It's not supposed to work like that. Someone has something set up wrong, or you have a really crappy SSOL.

"Will work for (the memory of) salami"

 

[Arbab1]

Hello Guys,
I am trying to control the position for a 3 phase brushless DC Motor.I have to also control torque. So I have inner loop which is a current control and an outer loop which is a position feed back. Plz advise wether from what is the best place for feedback. Should I take it from the phase currents or the supply current. Also advise me the PWM control also. If any one has done it in simulink , that will be great.

 

[jraef]

Arbab1, your post is off topic, you need to start a new thread. It gets too confusing for people to track which question is being responded to.

"Will work for (the memory of) salami"