Difference between relay tables

Status
Not open for further replies.
wirenut1980

wirenut1980

Senior Member
Location
Plainfield, IN
Hi everyone, I am looking at the Square D relay overload selection catalog which can be seen here. I am troubleshooting for a customer that has a well pump motor that runs for about 10 minutes and then the overload relay shuts the motor down. The overload relay is a Square D bimetallic E80. Right before shutdown the current waveform peak current was 120 Amps, which gives an RMS value of about 84.9 amps. The motor has been refurbished and the nameplate full load current is 65 Amps.

The E80 thermal unit number is listed several times in the lookup tables next to different corresponding full load current ranges. What is the difference and how to tell which one is installed at the motor?

Btw, current and voltage waveforms did not show anything out of the ordinary that would cause the relay to trip.
 
What do mean not out of the ordinary? You say the motor is 65 A, but you have 84.9A prior to trip. I'd say that is out of the normal range.

Before changing the size of the OL, I would take a look at the operation of the pump. If it is drawing to much current, it is very likely you do not have enough resistance on the flow (output). This will most commonly be the cause of overcurrent on the motor. Assuming you have a way to cut back on output flow by use of a valve, put a clamp on ammeter on the motor conductors, turn on the pump. Watch the current and slowing close the valve and you will see the current drop back. Continue closing the valve until the motor current is in line with the rated FLA (possibly using SF if needed). If the flow is to low at the rated current for the motor, then a larger pump will be required.
 
Last edited:
What is also strange is that RMS data up until the trip time was showing 70 Amps on each phase. Then, approximately 12 cycles before the overload tripped the motor, waveforms were captured up until the overload trip, and that is where I saw the 120 Amps peak current. So maybe the flow starts out lower and in range of the motors full load current, but then flow picks up a bit after 5-10 minutes and causes the overload to act. Does this sound possible? I'll check to see if they can monitor the flow out of the pump.

Thanks!
 
wirenut1980 said:
The E80 thermal unit number is listed several times in the lookup tables next to different corresponding full load current ranges. What is the difference and how to tell which one is installed at the motor?
Click to expand...


The difference is the application. Look at table 16.383 and 16.384 (page 16-130 and 16-131 in the current digest) That table will tell you which table to look in for your thermal unit selection. The other tables are on p16-134 thru 16-152
 
. . .the operation of the pump. . .

. . .the operation of the pump. . .

Yes.

And, your current readings seem to confirm proper breaker operation.

What is happening that takes 10 minutes to change? Does the viscosity of the fluid being pumped change in the direction of higher viscosity every few minutes? In this case, it might not be the motor/breaker system at all.
One way to tell whether the problem is downstream (pump) or upstream is to read the motor shaft torque. Assuming the motor is good, the current draw is already telling you that the torgue goes up.

I guess the motor could have a heat-dependent partial short circuit in the windings. You'd need do a full load check on a known good mechanical load for at least 10 minutes to confirm this.

Another way may be to check the motor winding resistance when cold, when hot but before the breaker trips, and when hot just after the breaker trips. As the motor cools down I'd think you'd see an abrupt change in winding resistance to a higher reading if there is a heat-dependent partial short.

I wonder how the motor refurb people tested the motor, and for how long.
 
kingpb said:
If it is drawing to much current, it is very likely you do not have enough resistance on the flow (output). This will most commonly be the cause of overcurrent on the motor. Assuming you have a way to cut back on output flow by use of a valve, put a clamp on ammeter on the motor conductors, turn on the pump. Watch the current and slowing close the valve and you will see the current drop back. Continue closing the valve until the motor current is in line with the rated FLA (possibly using SF if needed). If the flow is to low at the rated current for the motor, then a larger pump will be required.
Click to expand...

Yep. Or, as this is a well, maybe it runs dry, also gonna overload the motor.
 
A well running dry will not overload the motor. In that case it would essentially be unloaded and therefore running at about as low of current as your going to see. The problem would be that the pump would become damaged because of high heat build up due to no water flow to cool it.
 
kingpb, I think with no water to move, there will be no load on the shaft to generate a back EMF, and the motor will overheat.
 
RayS said:
kingpb, I think with no water to move, there will be no load on the shaft to generate a back EMF, and the motor will overheat.
Click to expand...

Maybe I'm misintepreting what your saying, but it sounds like your saying that a motor is designed such that it cannot run continuously without a load attached to it? I think that may be in conflict NEMA MG-1.
 
RayS said:
yep, thats it. I dunno what the MG1 says, just going off what I recall.
Click to expand...

The inherent slip and impedance of the rotor of a squirrel cage induction motor provides suffcient back EMF to prevent an increase in current even as the load is reduced.
 
Lxnxjxhx what type of instrument would you use to test the or monitor the torque..I hope this is not a dumb question..:smile: as I head off to my manual for torque measurements..

One way to tell whether the problem is downstream (pump) or upstream is to read the motor shaft torque. Assuming the motor is good, the current draw is already telling you that the torque goes up.

edited to add

how would you vary the input of a well and would the restriction of flow be the only manipulation of the output..I am also feeling foolish at this point..
 
Last edited:
Varying the input is not a good idea, as the pump needs a certain amount of suction or you will have loss of head, and the pump will cavitate.

As far a output, you have three options: VFD, valve, or put in a smaller impeller. Certainly, changing the impeller will not allow any adjustment. The VFD may only be required to set the proper flow, and then it stays there forever, which means the cost of the VFD may not be economical. the valve, may be the most economical. If you need varying flow, then use an MOV, otherwise use a manual adjustment; set it and walk away.
 
measuring shaft torgue

measuring shaft torgue

Here's one group of people who say they can do it but I don't know if it is practical for this application.

http://www.mdi-sensor.com/?gclid=CNmVsbiV4pICFQtxHgodlQ4Vbw

The output shaft will be slightly twisted due to the torque, and the twist will depend on the strength and design of the shaft. If there are any mechanical engineers out there, they probably know how many minutes of arc or seconds of arc this twist would amount to.

I guess ideally someone would make a thing, maybe using strain gauges, that temporarily fastens to the output shaft and sends you the reading optically, with radio waves, or you might need a stroboscope to get the reading. Then you use a look-up table to correct the reading for the shaft design, diameter, material, etc.

Otherwise you could use current draw but that assumes the motor is good.
Here's a link connecting torque, speed, power
http://www.nsxprime.com/FAQ/Miscellaneous/TorqueHPSpeed.htm
 
Hi everyone, here's an update: The flow rate is pretty constant when the motor trips. I was told the most it changed was from 300 gpm to 309 gpm. Went back and did some more testing. The RMS current is actually right around 70 amps per phase, current imbalance is under 5%. Voltage is in range and nothing abnormal is happening when the overload trips. I am beginning to suspect that the wrong overload was either specified or installed, especially given that there are multiple E80 overloads with different current ranges for different applications. We are in the process or checking the actual catalog number to see which one is installed.

The motor was drawing close to it's namplate current and was not heating up...it was barely warm to the touch on the metal housing. Opening the cabinet door housing the overload allowed some cool air in there and the motor ran fine for around 20 minutes, at which point we closed the door and 3 minutes later, the overload tripped. A solid state overload might be the answer here.
 
Opening the cabinet door housing

Opening the cabinet door housing

Maybe the ambient cabinet temp is causing all the breakers to be very close to tripping?
 
Status
Not open for further replies.
Top