Motor pulling higher then rated amps.

[yoyoe]

I have a diecast machine with two 30hp three phase motors running 460 volts. Nameplate rating on them is 37.5 amps.
Under load they each pull 40-43 amps. What I would like to know is this normal? I also have a problem with blowing the main fuses, LPJ 100 Sp. Amp meter says total amps through the fuses is 88. Why would this cause the fuses to get so hot they discolor and cannot be touched, but the leads are just warm?

 

[itsmoked]

Very sorry but a sound installation will NOT exceed the FLA on the plate. You are seriously overworking those motors and will continue to have problems.

Can you reduce their load? Change a gear ratio?

Fuses are resistors by nature. They have dissipation. If they are in stagnant air they will heat up.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

I have a diecast machine with two 30hp three phase motors running 460 volts. Nameplate rating on them is 37.5 amps.
Under load they each pull 40-43 amps. What I would like to know is this normal? I also have a problem with blowing the main fuses, LPJ 100 Sp. Amp meter says total amps through the fuses is 88. Why would this cause the fuses to get so hot they discolor and cannot be touched, but the leads are just warm?

1) Depending on the machine design, it may be "normal" for your motors to run with higher amps if the design engineer made the decision to use the Service Factor of the motor. We all can (and probably will) debate the wisdom of that decision, but the reality is, it happens all the time with OEMs.

2) Fuses blowing is not necessarily related, although it may be. If the end ferrules are hot and discolored, it may be a loose clip (or whatever the connection method is). That might explain why the motor leads are not as hot but in general, the motor leads have more overall mass to spread out the heat so may be that it wouldn't feel the same. It may also be that the motors are just too small and you are running into the top of the fuse curve every time they try to start the load. it may be that you are attempting to start too often and the fuse curves become your short-cycle protection. it may be that your source is undersized (wire, transformer etc.) and starting both motors is causing a voltage drop, which causes the current to spike, which blows the fuses. it may be a combination of any of the above.

Bottom line, you need more investigation.

 

[richerdick]

Are they wired correctly is it possible thqat the motors are in delta when they should be in star?

 

[jraef]

He used HP, not kW, LPJ is a NEMA fuse design, so the OP is likely in N. America somewhere. We don't have the same issues with Delta and Y would motors. Besides, the current would be double, not just 13% high.

 

[jraef]

"would" should read "wound" in the above post.



"Edit button, edit button! My Kingdom for an edit button!"

 

[dpc]

If the motors are drawing this current continuously, it will be a problem eventually. If the current is intermittent, it might not be as much of an issue.

Does the nameplate indicate a service factor for the motor and is the "nameplate" rating you quote the full load amps or the service factor amps? Is this a continuous duty motor?

How is the ambient temperature around the motor?

 

[gepman]

I agree with itsmoked that the motors are overloaded even if they are on the edge of their service factor. A loose fuseclip as mentioned by jraef is possible but another possibility is that there are harmonics and your ammeter cannot pick up the true RMS. You need to use a true RMS ammeter with a bandwith up to 51st harmonic so you can get an accurate measurement of your "heating" amps. There are several reasonable priced ones available with these specs.

Besides the ambient temperature around the motor as dpc states, what is the ambient around the fuses?

 

[jraef]

Harmonics? Where was there mention of a VFD?

 

[ScottyUK]

Two obvious things which haven't been mentioned are under-voltage conditions at the motor and severe over-voltage condition. The latter would give rise to harmonics if the core is sufficiently saturated. Is the 460V quoted by the OP the motor's nominal operating voltage? What is it really operating at, measured at the terminal box?

Of course, maybe the motor is just overloaded by the process!


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[gepman]

The load doesn't have to be a VFD to have harmonics on the system. I recently designed the power system for a facility with a 12kV feed and three 2500 kVA transformers. One of the 2500 kVA transformers had a 2800A load with about 2000A being VFD driven motors. The current waveform at the main (before all of the loads including the VFDs) had over 30% total harmonic distortion fundamental (harmonic rms/fundamental rms). The voltage waveform was much worse. I had anticipated this problem and left room for an Accusine active power harmonics correction unit.

VFDs (and other switching power supplies) cause distortion and harmonics upstream of the load even with line reactors (which reduce but not eliminate the problem). The architect didn't give me enough room in the MCC rooms to add the line reactors to all of the VFDs.

 

[gepman]

To expound on what ScottyUK has written but mainly concentrating on the over-voltage condition because that is counter-intuitive (at least to me), my graph of "effect of voltage variation on induction motor characteristics" (from NEMA MG-1) shows that at 10% overvoltage the FLA increases about 4%, at 15% overvoltage the FLA increases about 9%. At 10% undervoltage the FLA increases about 10%, at 15% undervoltage the FLA increases about 18%.

 

[yoyoe]

Thanks for all the ideas. I have a little more info.

The two motors are continuous duty motors with a service factor of 1.15. Each motor is direct connected to a hydraulic pump with a coupling. When the pumps are not pumping pressure they each draw about 24-26amps. When pumping high pressure they go to about 45amps each.

I checked the voltage while the motors were running and it runs between 455 and 458 vac.

One motor is a twelve lead the other nine lead. Originally the motors were wired wye start delta run. One was rewound to nine lead, the other is still 12 lead original.

My question is, is this normal amperage draw and I need bigger fuses or does it sound like the motors are bad.

 

[DickDV]

Does not sound like the motors are bad. They are simply overloaded a bit.

What is making matters worse is the voltage being on the low side. If there is a way to raise the voltage, aim for 485-490. Your overcurrent problem may just go a way.

Are the motors running over their temperature rating?