3 phase motor phase fault
3 phase motor phase fault
(OP)
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?
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?






RE: 3 phase motor phase fault
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: 3 phase motor phase fault
RE: 3 phase motor phase fault
Keith Cress
kcress - http://www.flaminsystems.com
RE: 3 phase motor phase fault
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
RE: 3 phase motor phase fault
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
RE: 3 phase motor phase fault
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
RE: 3 phase motor phase fault