Motor current decreasing after a while

Some time ago I had to commission an asynchronous motor driving a fan (directly connected without a belt).
The motor seemed rather small for the application as the current was somewhat higher then the nominal current at 50 Hz
(4KW, Inom = 9.5A, actual current = 9.7 A). I started with a cold motor, and after a while the current decreased slowly
to about 9.5 A. I must say the motor had long motor cabled (120m) and no output choke.
Could an increase of the resistance of the motor windings due to temperature be the reason for the lower current?
At the time of the test the drive was in U/F control.
What if I should put the drive in sensorless vector control?
Would the drive then sense the higher motor temperature through current calculation and compensate for the higher estimated resitance, and would this result in a current independant of the motor temperature?

Could an increase of the resistance of the motor windings due to temperature be the reason for the lower current?

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Yes, copper wire resistance does increase with increasing temperature. 0.2 Amp increase is reasonble for that. Also as the motor bushings heat up, and the lubricant gets warm, they will provide less mechanical resistance (up to a point).

I agree with Lancie. 0.2 amps is probably within the margin of error of the drive variation with temperature. I think you are worrying over nothing...now if the current goes up by more than 10 percent...with no other changes...time to start worrying.

To add a third variable to the above posts.
If the air being moved by the fan changes temperature, the density of the air changes, and the load on the motor changes, by a small portion

I have seen this effect on blast freezing fans, 0 deg C to - 35deg C causing up to about a 5% change.

Gil it is much more about 20/30% depending on type of fan.
Yes the motorwindings will heat up and the cable too. so i would expect more current flowing to do same task.
so look in other part like if flow is running the amps will decrease.

So look in other parts, like if the flow is running then the amps will decrease.

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That sounds backwards to me. For every fan I have worked with, if the flow is blocked (not "running"), the current decreases. That makes sense to me, because if the damper is closed and the air flow is blocked, then the fan can not do any work, therefore the current should decrease.

shooter said:

Gil it is much more about 20/30% depending on type of fan.
Yes the motorwindings will heat up and the cable too. so i would expect more current flowing to do same task.
so look in other part like if flow is running the amps will decrease.

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Lancie's right. Assuming this is a centrifugal fan, then as you close the damper the flow drops. Fan power is a function of both pressure and flow. As you close the damper the system back pressure increases, but not as fast as the flow drops. The net result is reduced power draw.

That brings up another misconception. The motor doesn't push power into the fan, the fan pulls as much power as it needs from the motor. (At least until you reach the motor limit, which is an overload and results in a over current trip.) I explain it to my students this way: Imagine taking a 500 hp motor and mounting the vent fan from your bathroom on it. Do you think the motor will push 500 hp into that fan?

The point of this is that if the motor is heating up and the winding resistance increases, it will decrease the motor efficiency but not the fan load. That means that for the same fan load the output power from the motor won't change, but the electrical power into the motor will have to go up. That in turn means that if voltage is constant then current will go up as the motor heats up.

Sometimes on higher pressure blowers I see the blower efficiency improve after running a while - the air and fan temprature goes up so the clearances close up, so efficiency improves, so the horsepower draw drops for the same air flow and system pressure, so the current will go down. Usually on a fan the temperature change is so small that this doesn't happen but maybe ......

The effect of inlet air temperature on power draw is fairly straight forward - it is the ratio of the absolute temperatures. For you guys using Celsius that means C + 273 equals K (Kelvin). Higher temperature results in lower power. PLC_User, if you look at your fan catalog or manual it should give you the equations.

OK so we have about a 5 HP fan unit I assume squirrel cage and not van axial. VFD driven with VFD in volt/Hz.

QUESTION 1 where are you measuring current ie between VFD and motor OR on line side of VFD. Line side is preferable. Most instrumentation do not give a true RMS value on PWM.
2. You know for certain you are not changing RPM??

In this case I think it best to measure Kw and on line side of VFD. I do not think I would worry too much about a change in current from 9.7 to 9.5. I would be somewhat concerned running motor at full load continuously.

To be able to discuss any density change on air caused by temperature change we need the temp at start and temp when running ie after heaters or other heat source have brought air to "steady state" temperature.

Once again more info needed to accurately discuss.


Dan Bentler

If I may add a couple of things, Dan:

I always use the amp display on the VFD itself. I've found it to be more accurate than measuring on the line side with conventional instruments because of the harmonics. That's true even if you use true RMS instruments. And, as you point out, using a clamp on or such on the load side gives you a warm fuzzy feeling, but not accurate data.

I also wouldn't worry too much about running the motor at full load continuously. Assuming that the motor is either nominal 1.15 service factor or inverter duty that is what it is designed to do.

Yeah I forgot to mention the screen dispay on VFD as most accurate wihout expensive instrumentation.

Sigh
So much to remember
So little memory

Dan Bentler

Another reason for not measuring anything on the drive output leads is safety. While logic would indicate that this is a 480V circuit, in fact, voltages as high as 1400V are common. That means that normal CAT III 1000V rated test equipment is not adequate. Using them on a drive output puts the meter and the operator both a risk.