Why will a motor run over nameplated amps. 1

[Jetlitho]

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.

 

[Skogsgurra]

I would not be too worried. Fan curves are not always very exact. Also, the slight overvoltage may push magnetizing current up a bit. Does the thermal trip?

Gunnar Englund

http://www.gke.org

 

[Mobius44]

A positive or negative pressure in the boiler room can affect the amp draw of a combustion air motor. You could measure the rpm and differential pressure of the fan, and double-check with your fan supplier that you are operating at the expected location on the fan curve. The high current draw makes it likely the motor is driving a load of about 3/4HP instead of 1/2HP, a motor swap may be in order, or you may have adjustable air dampers which could be used to reduce the load.

Sean

 

[Jetlitho]

In this case, I am the fan supplier. I sell them to a burner manufacturer, who in turn sold them to an oven manufacturer. There are four burners on this oven doing this. I have a very exact fan curve from when these were tested at the factory and there is not a place on the curve where it could put a 3/4 HP load on that motor, no matter what pressure it saw. The motor is still drawing way too many amps. Unfortuneatly when we tested at the factory, we only tested Static Pressure and Airflow. We used the fan laws to calculate the BHP curve, but I'm sure it's not off that much. We have sold this fan on these burners hundreds of times, and never had this problem. I have to think it's low voltage. I guess the voltage would have to be at about 370 to get an amp draw that high. Maybe lower since I'm fairly certain the motor isn't at full load. From your answers it sounds like motors don't normally stray from the nameplated amps unless there is an outside condition, like too much load, or low voltage. I thought maybe there was a variance that was normal, but it doesn't seem likely.

 

[waross]

I have three suggestions;
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

 

[Tmoose]

One dryer blower of 6 or 8 at a carwash kept burning up its motor. The electrician increased the heater/fuse size a lot to keep it running. It was running backwards. I was pretty surprised the load was higher, not lower. I forget the impeller design

 

[quark]

If the fan is backward curved, then there can't be any overloading due to the function of fan. If it is forward curved, then the fan may load the motor beyond what is shown on the performance curve. Try controlling the damper and see what happens.

 

[KJvR]

A respectable international motor supplier once (2002) supplied 2 large 415V motors for ventelation fans with 525V nameplates on the motors. Had a few people picking their brains for a while why the motors were overloaded.

 

[MikeHalloran]

Four burners, four fans, four motors, all of standard, proven design and long term manufacture, all exhibiting identical misbehavior on _one_ oven.

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

 

[Skogsgurra]

"Discard all assumptions, and you'll probably be done in an hour or two."

Some wordings should be in gold letters! PLS for that, Mike.


Gunnar Englund

http://www.gke.org

 

[nightfox1925]

I have similar opinion with Mobius44. Fans are considered variable torque loads and there are situations that defies good judgement. Take a very simple example of a vaccuum cleaner, you will find that the sound pitch of the motor runnig increases as you put some restriction on the suction. This increased sound pitch indicates that the motor speed up and speed is proprotional to the horse power (HP is related to speed times torque). To wrap it up, see the operating conditions of the load whether it tends to vary the motor torque in an increasing manner due to back pressure. An opinion that may be expanded further.

 

[Skogsgurra]

Re: Vacuum cleaner and restricted flow. It is actually the opposite. Increased speed is due to decreased load when flow is restricted. If you measure consumed power, you will find that power decreases when you put a piece of cardboard before the inlet.

And that is natural, you transport less air - you need less power.

Gunnar Englund

http://www.gke.org

 

[stardelta]

I concurr with Skogs on this one. We do a number of large fan motor repairs and overhauls most being in excess of 100hp and during testing sometimes find that the airflow going in has to be restricted otherwise the motor can overload. In some cases they wont even get up to speed without some restriction. It makes sense to me, no air can be drawn in by the fan so there is no air to move and the motor is doing no work, other than to rotate the fan.

 

[bigbillnky]

Jetlitho

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)

 

[UKpete]

bigbill, is it true that with a fan load the current goes up if the voltage goes up (assuming fixed frequency)? I accept that there will be a slight decrease in slip, and the fan power will go up by the cube of fan speed, but enough to make it draw more current at the higher voltage?

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.

 

[waross]

Good points UKpete

Is this installation on a grid or on an islanded supply system?
respectfully

 

[bigbillnky]

UKpete

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)

 

[Skogsgurra]

Mustn't forget about excitation current - which goes up as voltage is increased. It ain't simple.

Gunnar Englund

http://www.gke.org

 

[waross]

Those points are well taken, but for a reality check;
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

 

[rmw]

How about mechanical problems? Fan rubbing, although it would be rare to have four of four rubbing the same. How about binding up otherwise?

Fan HP draw is more than just fan laws.

rmw