Why will a motor run over nameplated amps.
Why will a motor run over nameplated amps.
(OP)
I have a motor on a fan, specifically a Reliance P56H1426 1/2 HP that feeds combustion air to a burner. The fan curve shows that the fan should never load the motor more than about .40 HP and yet the motor is running at 1.2 amps when it is nameplated at .96. This rating is on 460V 3Phase service, and my customer actually has 480V 3 Phase service. I know of common problems that can cause this, like low voltage, overloading, incorrect rotaion, etc... We have ruled these out, so does anyone have any idea what would cause this? I would normally call it a faulty motor, but there are four of these on an over and they are all doing the same thing.





RE: Why will a motor run over nameplated amps.
Gunnar Englund
www.gke.org
RE: Why will a motor run over nameplated amps.
Sean
RE: Why will a motor run over nameplated amps.
RE: Why will a motor run over nameplated amps.
1> Check for low supply voltage, but I understand from your initial post that you have done this.
2> Look at the fan curves again and the fan laws. Consider the effect of a pressurized room and an unrestricted discharge on the flow volume and the subsequent loading.
3> Check the ammeter.
respectfully
RE: Why will a motor run over nameplated amps.
RE: Why will a motor run over nameplated amps.
RE: Why will a motor run over nameplated amps.
RE: Why will a motor run over nameplated amps.
Yes, you could get four mislabeled motors, but what are the odds that you'd get only four, not two, or eight, or twenty? What are the odds that all four would end up on the same oven?
Take your instruments, a jaundiced eye, and a child's open mind, and plan to spend a day measuring every facet of that particular oven's performance. Discard all assumptions, and you'll probably be done in an hour or two.
I'd bet on reversed rotation [ E.g., the phase sequence at the disconnect was wrong when the oven was installed, the oven wiring was adjusted to get correct rotation, and then the phase sequence was corrected upstream as part of someone else's service call on another piece of equipment. Actually happened, on a hydraulic pump. ]
Mike Halloran
Pembroke Pines, FL, USA
RE: Why will a motor run over nameplated amps.
Some wordings should be in gold letters! PLS for that, Mike.
Gunnar Englund
www.gke.org
RE: Why will a motor run over nameplated amps.
RE: Why will a motor run over nameplated amps.
And that is natural, you transport less air - you need less power.
Gunnar Englund
www.gke.org
RE: Why will a motor run over nameplated amps.
RE: Why will a motor run over nameplated amps.
Goodmorning. I have to believe that the problem is more related to a high supply voltage or reversed rotation. I am leaning more toward a high supply voltage.
A decrease in voltage supply to a motor driving a blower should actually lower the line current. The same holds true for any load in which the load increases in proportion to an increase in speed.
In the case of reversed rotation you would be placing a positive pressure into the oven. The fan would then need to overcome the design load of the blower and the increased load due to the backpressure of the air inside the oven.
My suggestion is to take a vsd with you in which you could adjust the output voltage to match the nameplate voltage to see what effect it has on the motor current. What a great point Mike made in discarding all assumptions.
Bigbillnky,C.E.F.....(Chief Electrical Flunky)
RE: Why will a motor run over nameplated amps.
Of course, on a constant torque load the opposite occurs i.e. increased voltage gives decreased current, almost in proportion.
Jetlitho, if we are discarding assumptions, is the supply frequency correct? Fan power is very sensitive to this, because of the cube law.
RE: Why will a motor run over nameplated amps.
Is this installation on a grid or on an islanded supply system?
respectfully
RE: Why will a motor run over nameplated amps.
Absolutely. As you have stated, there is a decrease in rotor slip when the voltage supplied to the motor increases. This causes a slight increase in the blower speed, and the blower is now moving more air and has increased in load. If more air is being moved, more work is being done. The motor is now outputting more HP(KW if you prefer).
If the current dropped in relation to the increase in voltage, the output HP would not have changed(HP=I*E*Motor Effiency*PF*1.73/746).
Bigbillnky,C.E.F.....(Chief Electrical Flunky)
RE: Why will a motor run over nameplated amps.
Gunnar Englund
www.gke.org
RE: Why will a motor run over nameplated amps.
The rated voltage is 460. The actual voltage is 480 volts.
That's an increase of 4.3%
The rated current is 0.96 Amps. The actual current is 1.2 Amps.
That's an increase of 25%
The slip of a 1750 RPM motor is 2.78%
If the slip was eliminated completely the speed increase would be 1800/1750 = 102.9%
With the cube law;
102.9%^3 = 108.8% increase in HP.
We will never achieve synchronous speed so the actual figure will be much less.
In conclusion it's hard to see a 4.3% rise in voltage causing a 25% rise in current, even with the cube law..
respectfully
RE: Why will a motor run over nameplated amps.
Fan HP draw is more than just fan laws.
rmw
RE: Why will a motor run over nameplated amps.