Motor pulling higher then rated amps.
Motor pulling higher then rated amps.
(OP)
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?
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?





RE: Motor pulling higher then rated amps.
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
RE: Motor pulling higher then rated amps.
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.
RE: Motor pulling higher then rated amps.
RE: Motor pulling higher then rated amps.
RE: Motor pulling higher then rated amps.
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RE: Motor pulling higher then rated amps.
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?
RE: Motor pulling higher then rated amps.
Besides the ambient temperature around the motor as dpc states, what is the ambient around the fuses?
RE: Motor pulling higher then rated amps.
RE: Motor pulling higher then rated amps.
Of course, maybe the motor is just overloaded by the process!
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If we learn from our mistakes I'm getting a great education!
RE: Motor pulling higher then rated amps.
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.
RE: Motor pulling higher then rated amps.
RE: Motor pulling higher then rated amps.
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.
RE: Motor pulling higher then rated amps.
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?