3 phase motor phase fault 1

[hallmg]

Hi,

We have a 415v motor which lost a phase but kept running.

the usual running current was 37A, the overloads were set at 53A, on two phases the motor was drawing 63A. Supply fuses were 60A.

Should not this fault have tripped the overloads?

 

[Skogsgurra]

Thermal overloads are not very exact. There is also a considerable delay before the protection trips at such low overcurrents. For how long did it run?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[dpc]

Agreed. In years past, I have seen numerous motor failures in a facility when the utility dropped a phase I'm told the newer OL relays are better, but I am not surprised that the OL didn't trip. It probably would have eventually, but for motors you care about, some type of phase loss detection is a good investment.

 

[itsmoked]

Thermal overloads kill motors all the time in single phasing cases. Some of the lousy overloads even cause the single phasing.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

The way a thermal OL relay works is that there are two defined points in time and current, and an I[sup]2[/sup]t curve between them. The first point is the "pick up" point, being where the overload begins to trip. That is NOT the actual setting, although the setting determines it. In IEC type OL relays, the pickup point is 117-118% of the motor FLA. for NEMA OL relays it is 125%. Below that point the relay will hold indefinitely, above it, the bar starts to move inexorably toward tripping within 2 hours. The higher the overload percentage, the faster it trips. The other end of the curve is determined by the "trip class", defined as a maximum time at 600% of FLA. Class 10 means at 600% it must trip no later than 10 seconds, Class 20 = 20 sec., etc.

63A on a setting of 53A is only 118%, I would not expect it to ever trip. Now, some IEC OL relays have a phase loss differential bar that biases the trip pickup point lower if you don't have current on all 3 poles. So it likely would eventually trip, but in minutes, not seconds.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

When I was doing a lot of work with motors, all overload haeters were not created equal.
As jraef stated, the tripping point in NEMA land is 125% of motor current.
Some overload heaters were rated at the tripping current. For a motor nameplate current of 53 Amps, you would select O/L heaters rated at the tripping current or 66.25 Amps.
Other O/L heaters may be rated for the motor nameplate current and for a motor nameplate current of 53 Amps, you would select O/L elements rated at 53 Amps.
One manufacturer used a look-up table and part numbers. A copy of the table was pasted inside the starter enclosure. The sizing took into account the heat rejecting ability of the enclosure. For a given current, a large enclosure may use a different size O/L heater than a smaller enclosure.
Yes is is quite possible that an overload relay with heaters sized for 53 Amps FLC will not trip at 63 Amps.
You really have to RTFI whan sizing O/L relays and heaters.
Fuses are not that exact.

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

 

[jraef]

RTFI: Read The "Factory" Instructions.

There are other opinions as to the true meaning of the F however.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[davidbeach]

Reminds me of when, (years and years ago) Dr. Ruth did a "Sex for Dummys" book and for once RTFM actually meant RTFM.