Blower High Current Draw

[roydm]

I have another question for the motor gurus.
At a project in Korea we have 3 blowers that seem to be drawing very high current
The motors are 460 Volt 53 kW with a nameplate FLA of 77 Amps coupled to the blowers by belt drive
The speed has been checked with a tacho, both motor and blower are as expected.
The calculated current draw based on speed and discharge pressure is 68 amps but the motors are reported to be drawing 90 Amps.
They have already burnt out one motor but this may have been due to a bearing failure.

Is there any reason for a motor to pull higher than normal current other than just plain overloaded?

Thanks in advance

Roy

 

[electricpete]

Hi Pete. I was under the impression that the rotor impedance was not perfectly linear with slip frequency. This may be demonstrated by the action and theory of a double squirrel-cage motor

Double cage motor depends on deep bar effect. That would be important during start (slip = 1) and low speed, maybe up to slip = 0.5. Deep bar effect certainly is not going to alter the effective rotor resistance between s=0.01 and s=0.02.

The torque curve of most induction motors shows that torque is not linear with slip.

Correct, only at small slip is torque proportional to slip and that behavior changes as slip increases... noticeable deviation often by full load slip and nothing close to proportional by the time we reach breakdown torque at maybe s=0.1. You'll recall there were simplifications that I stated along the way such as neglecting leakage reactance. That means we don't expect the answer to be exact, but I feel it is certainly close enough to illustrate the point I was trying to make.

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(2B)+(2B)' ?

 

[waross]

Good call macmckim.
I stand corrected Pete.
But, when we compare the reported speed to the reported current I believe that it would be prudent to add measurement error as a possible complicating factor. If the speed and voltage are as listed, the current would be expected to be much higher.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[roydm]

Thank you all for the great suggestions.
Our Mechanical Engineer is going to site next week, I will report back if he finds anything.

Regards

Roy

 

[Strong]

I tried to submit this yesterday, but ocasionally the Submit Post button does not display!
You need to verify both suction and discharge pressure, since the delta-P is important. Air temperature for calculating air density is also important. Verify blower and motor nameplates and all specification documentation, since the blower's actual speed may be higher than design or motor is undersize. You could verify that blower rotates freely after hot shut down and measure casing temperature duringnormal operation to verify no excessive internal friction/rubbing is present.

Walt

 

[electricpete]

...Pete.
But, when we compare the reported speed to the reported current I believe that it would be prudent to add measurement error as a possible complicating factor..

So if we were trying to gage likelihood by this one consideration taken at face value, it would favor undervoltage or misconnection as the cause rather than overload. However, there can perhaps be small errors in the measurements and deviations in line frequency in some parts of the world and possible errors in data passed thru multiple people etc so I don't put huge faith in this aspect.

Sounds like we are in violent agreement.

If the speed and voltage are as listed, the current would be expected to be much higher.

I missed the fact that voltage was already measured, but I see it now.
I don’t think connections were checked. That can explain the symptoms seen (both slip and current taken at face value)
That remains something to look at (along with overload).


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(2B)+(2B)' ?

 

[Skogsgurra]

"Violent agreement"! I will use that next time. It can be a confusing factor in any discussion.

Jeff pointed out that there may be a problem with ducting and capacity. I would do this to find out if that is the problem:

1. Run each compressor separately and note power (or current) draw for each of them. If already too high - then quit the test. If good, then continue.

2. Run compressor #1 and start #2. If current then too high on both, the duct is the problem. If still OK then do next:

3. Start #3. If current too high now - the ducting is the problem.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

I violently agree Gunnar. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rhatcher]

Pete,

I don't think that this is an undervoltage problem due to misconnection for a higher voltage.

First, if the motor was connected wye instead of delta, the winding voltage would be 58% of rated and the resulting torque would be 33% of rated. For a motor connected single circuit instead of parallel, the winding voltage would be 50% of rated and the resulting torque would be 25% of rated. In either case, I don't think the motor would start or run for a constant torque load like this.

Second, and more importantly, there is no nominal voltage above 460 that would be achieved by reconnection for a higher voltage, whether delta-wye or parallel circuit-single circuit reconnection.

BTW, I hope that this does not result in a "violent disagreement." LOL.

 

[electricpete]

First, if the motor was connected wye instead of delta, the winding voltage would be 58% of rated and the resulting torque would be 33% of rated. For a motor connected single circuit instead of parallel, the winding voltage would be 50% of rated and the resulting torque would be 25% of rated. In either case, I don't think the motor would start or run for a constant torque load like this.

That's a good point. But please also note that the load during start depends on the system lineup. It is not uncommon at our plant for pumps to start in low-load lineup and operate at high load lineup (that would include recirc line wide open for pd pump). So it may be possible to rule out misconnection using your logic if we know more about how the machine is started, but we don't have that info at this point.

Second, and more importantly, there is no nominal voltage above 460 that would be achieved by reconnection for a higher voltage, whether delta-wye or parallel circuit-single circuit reconnection.

There does not need to be nominal voltage above 460. How about multipurpose motors designed for wye start / delta run but used with a simple DOL start MCC. If it is left permanently connected for wye during run, it results in the type of mis-connection I described (actually that has happened at our plant)

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(2B)+(2B)' ?

 

[electricpete]

clarification in bold:

It is not uncommon at our plant for pumps to start in low-load lineup and operate at high load lineup (that would include recirc line wide open for pd pump)

should be

It is not uncommon at our plant for pumps to start in low-load lineup and operate at high load lineup (that would include recirc line wide open during start for pd pump)

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(2B)+(2B)' ?

 

[electricpete]

Sorry.....should have said blower nott pump.
Still depends on system lineup. There may be unloader valves open during start also

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(2B)+(2B)' ?

 

[rhatcher]

Pete,
You make some good points. The breakdown torque at the "reduced" voltage would be about 80-100% of the rated torque at rated voltage so it is possible that the motor would run under load if it were started unloaded.

I guess we'll have to wait for roydm to give us the results of the investigation to see what is discovered.

 

[roydm]

Sorry Guys,
I'm still waiting to hear what they found if anything.

The site puts a low priority on giving us feedback.

Roy

 

[rmw]

Lot's of nice acedemic discussion here but my experience with information from site, especially when it comes in bits and drabs is usually wrong. I can't recount the times I've been given information that turned out to be incomplete, incorrect and downright wrong once I went to site to check it out for myself.

Make sure whomever you send to site is schooled in everything that he/she needs to check out and make sure that they have the tools. Also, warn him/her to be wary of who at site has some a$$ to cover because that often clouds the veracity of information as it is is doled out.

rmw

 

[roydm]

Ok, I have some solid data
There are 3 blowers, at any one time 2 are running, they ran each combination and all read very close, phase currents were within an Amp of each other

The motors are WEG model 172 787 000 out of Germany
Nameplate 460V 60 Hz 53kW 3560 RPM 77.1 Amps pf 0.92
not sure what all this means
3~225S/M2
IP55 Iso-KI.F delta180K S1 SF1.0 Amb 40°C

According to the blower curve with 35°C Ambient air 2580 RPM 0.7 bar it should draw 50kW

In reality all 3 blowers are spinning at 2496 RPM 0.68 bar
the motors are spinning at 3505 RPM (55 low) and pulling 81.5 Amps

I also have some historical data from when they only had 2 blowers and ran just one, then they drew ~79-80 Amps so the current has gone up a little.

Q/ If the blower is rated at 50kW is the 53 kW motor ok with a belt drive

Q/ Is the 35° difference in air temperature significant (more air mass)

I'm pretty much convinced that the motors were spec'd too small but you know how it is, blame electrical first.
Thanks for all the input so far
Roy

 

[burnt2x]

Any idea how big these blowers are? CFM?

 

[waross]

It looks as if the blowers are over-loaded. Another way of saying that the motors are too small.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[burnt2x]

The only way for a roots blower to draw more current from its drive motor is for the blower discs rubbing, constriction of either discharge or suction ports or a failed pressure relief valve. Any condition that will attempt to control the air flow will result into the equipment drive drawing more amps.

The static pressure at the discharge point (at the duct) does jibe with what the blower curve and the actual data you posted! A 0.7 pressure corresponding to a blower speed of 2580 rpm should be SP = (2496/2580) x 0.7 = 0.677 approx = 0.68 as you measured.

 

[roydm]

Since all 3 blowers exhibit the same symptoms I think we can rule out the disks rubbing.
The 53 kW motors are overloaded for sure, the question remains why.

Could it be the blower vendor's calculation is wrong or optimistic

Is it the motors are not really putting out 53kW and the actual HP is much less, Is there some
sort of standard for the motor manufacturers to follow

Is there a significant inefficiency in the belt drives requiring more HP.
I have suggested several times they could reduce the size of the motor sheave however knowing
the client if the blower is running slow they will complain about that.

Is the temperature difference (design 35°C actual 0°), more dense air meaning the blower is
moving more mass significant at all

Thanks for your patience.

Roy

 

[burnt2x]

I don't believe your blower will labor too much operating at a cooler temperature. IIRC, the rule of thumb is for every 4 deg C of temperature decrease there will be a 1% lower energy consumption due to a denser air being handled.
Also, I made a quick calc on your motor data: kW = 53, Voltage = 460, assumed motor efficiency = 0.9 and an assumed blower efficiency of 0.70 and I got a line amps = 105 A! Did the OEM assume a higher motor efficiency and a higher blower efficiency?