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Blower High Current Draw

Blower High Current Draw

Blower High Current Draw

(OP)
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

RE: Blower High Current Draw

I would not expect the overloads to allow the motor to run too long at 96 amps if the FLA is 77.

I assume these are constant speed. Perhaps the wrong rated frequency and or voltage? Has anyone measured the actual voltage being applied to the motors?

RE: Blower High Current Draw

The speed must be measured accurately and MUST be at or above nameplate rated speed.
Your current is about 17% high.
If a 1760 RPM rated motor is dragged down to 1750 RPM you can expect the current to be at or above 90 Amps.
Also check the voltage PHASE TO PHASE.
The phase to neutral voltages may be perfectly equal yet the phase to phase voltages may be unequal. That also indicates a phase angle error. Both conditions cause excess current.

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

RE: Blower High Current Draw

Some possible causes of higher current:
overload
voltage above nameplate
voltage below nameplate
frequency below nameplate
unbalanced voltage (should show unbalanced current)
** miswiring of the motor. How many leads does this motor have?

=====================================
(2B)+(2B)' ?

RE: Blower High Current Draw

Quote (roydm)

the motors are reported to be drawing 90 Amps
All three phases or only one?
Please, attach a photo of nameplate.

RE: Blower High Current Draw

If it is a centrifugal blower, is the air ducting in place yet or is there a flow control device that is not functioning or left wide open? In a centrifugal machine, load is proportional to the cube of the flow. So if the blower was designed to deliver a specific flow rate at your calculated motor power, but the flow control element is misadjusted (or not there) and is moving just 109% of normal air flow, that will increase the load on that motor to 132% of your calculated amp draw as you are experiencing (although I'm not quite sure how you are calculating what the amp draw should be).


"Will work for salami"

RE: Blower High Current Draw

(OP)
Thankyou for the prompt response.
Unfortunately the motors are in Korea and I'm in Vancouver, it's extremely dificult to get good feedback from site.
I will discuss your response with the mechanical engineer.

I should have mentioned there are 3 blowers connected into a common duct 2 run with a backup.
The second blower has only recently been bought on line as the plant expanded.

Thanks

Roy


RE: Blower High Current Draw

(OP)
Further Information
Apparently they measured the motor terminal Voltage last night
472 Volts phase to phase,the nameplate is 460
I don't know they checked all ways
The motors ar less than 300 feet from the MCC

The mechanical engineer is traveling to site next week, I will arm him with a good Ammeter.

Roy

RE: Blower High Current Draw

Are you sure about the motor nameplate date? 77A seems kind of low for a 53kw motor running at 460V. I would expect a current of about 85A for this rating. When you consider running at a slightly higher than nameplate voltage (472V), it may be that 90A is to be expected.

I agree with DPC that the motor overloads, if set to match the nameplate current (whatever that is?), should have interrupted the operation before the motor winding was damaged.

If a drive end bearing failure is found to be the cause of the failure, then improper belt tension is the obvious culprit. Are the motors equipped with roller bearings on the drive end? If not, then this is something that should be considered.

Finally, are you using the blower curve to determine the power requirement (and hence the motor current) based on speed and discharge pressure? If so, don't forget to include motor efficiency and power factor in the calculation.
I = P / (1.732 * V * EFF * PF).
Including these factors and the higher voltage will easily take the calculated 68A into the range of 90A. If you did include these factors, then do not be offended. It is an easy mistake to make and a standard question to ask in cases like this.

RE: Blower High Current Draw

(OP)
I keep getting information in dribs and drabs

I was told that the blower is not a centrifugal, "it's a rotary piston (lobe type, the HP . speed curve is quite linear"

Waross,
Some new tacho readings were taken, 2 of the motors were 3500, the 3rd was 3504, the nameplate is 3560

R Hatcher,
The motor calculations were done by the blower vendor

From the nameplate
The motor is a WEG model 172 787 000 from Germany
At 460V 60 Hz 53 kW 3560 77.1 A pf 0.92
Both bearings are ball races 6314 C3

I mentioned to the engineer the drive end are often rollers for belt drive

Thanks again, the suggestions will be a great help for our Mechanical Engineer when he gets to site

Regards
Roy

RE: Blower High Current Draw

In a Positive Displacement "lobe" blower, the opposite of what I said is then true; if there is an additional RESTRICTION to flow, it will show up as an increased power load on the motor.

But assuming that the air flow is as it should be and that you already burned up a bearing, it points to a mechanical problem, one that is showing up as added load on the motor not directly related to air flow. A bad bearing does cause the motor to pull more current, until it seizes completely and the motor shuts down. But the belts being too tight can also show up as added motor load, which later becomes a bearing loss as well because of the axial force on the bearing that is was not designed for. Some mechanics have an attitude of "a belt can never bee too tight", but that's just not true.


"Will work for salami"

RE: Blower High Current Draw

High slip and current suggests
Most likely to last likely
Actual overload
Under voltage
Unbalance
Severe rotor bar priblem (un likely...would notice noise and current oscillation)
(sorry, typing on phone)

=====================================
(2B)+(2B)' ?

RE: Blower High Current Draw

The fact that the motor is running below the rated speed implies that is overloaded. This is assuming that the supply voltage is balanced.

As suggested by jraef, perhaps there is a flow restriction. Or, it may be that the ducts are not properly sized to allow for the additional flow of the two or three blowers (?) that are required to support the plant expansion.

RE: Blower High Current Draw

I would focus on the low speed. Why are the motors running slow?? Overload and low voltage are prime areas of investigation.

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

RE: Blower High Current Draw

I agree with Bill. While overload is most likely, it is not the only possibility. Undervoltage is another important thing to consider.

Use the equivalent circuit referred to stator side.
Simplifying assumption Xm=infinity and X1=X2=0.
I1 = I2 = V / [R2/s]
Pout = I2^2 *R2*(1-s)/s ~I2^2 *R2/s
Pout ~ {V / [R2/s]}^2 *R2/s
Pout ~ V^2 *s/R2
s ~ Pout * R2/V^2

From the last equation, we see that low V causes increased s for a given P (we also know it causes increased current).

Unbalance includes negative sequence which creates negative torque which must be overcome by positive torque. The positive sequence effectively sees a higher load since it is fighting the negative sequence torque. Slip and current increase in analogous manner as they would if the additional load came from the driven equipment. Additionally, these people may have recorded the highest current of the three. Still would have to be a very severe unbalance

Also increasing R2 (such as rotor problem) causes increased s for a given Pout from above equation. Increased I would be minor effects if retained (1-s) instead of simplifying it to 1.. also oscillation may lead to high reading at a moment in time, lower later. Hence this one least likely.

The last two are not as straightforward or prounounced in terms of the mechanism for increased curent, but deserve mention as lesser possibilities in the order above imo.

=====================================
(2B)+(2B)' ?

RE: Blower High Current Draw

Quote:

Increased I would be minor effects if retained (1-s) instead of simplifying it to 1..
That was explanation related to the previous equation standpoint. From a physical standpoint, this corresponds to increased rotor losses which must be supplied by input power.

=====================================
(2B)+(2B)' ?

RE: Blower High Current Draw

It is good to arm the mechanic guy with an ammeter. But you should make sure he also brings a voltmeter. Or, in other words, equip him with a reliable clamp meter with volts and all other functions. I would bring one of the most complete clamps I know - the F607. Very complete and still simple enough for a clanky to operate. But any clamp with voltage range will work. The good thing with the F607 is that you can also use it to measure actual grid frequency AND motor speed (small magnet on the shaft and pick up the field with the clamp. That is a very good way to see if the slip is too high. It checks harmonics, too.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Blower High Current Draw

Good points Gunnar

Quote (electricpete corrected)


High slip and current suggests
Most likely to last likely
Actual overload
Under voltage [or misconnection]
Unbalance
Severe rotor bar priblem (un likely...would notice noise and current oscillation)
(sorry, typing on phone)
I corrected to add misconnection, which can act similar to under voltage if you have reconnected windings to expect a higher voltage and you don't provide it. For example connecting in wye instead of delta or connecting groups in series instead of parallel.

Another thing to note it appears the slip was double nameplate while the current was something like 125%.
If it was an overload, we would expect these two quantities to increase by a closer ratio.
If it was undervoltage, the different ratio is expected. Let's say voltage is 75%.
Then current increases by roughly 1/[3/4]~4/3 but slip increases by 1/3/4]^2 ~ 16/9.
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. And predicting likelihood is not really that important... but just wanted to reinforce the voltage and connections should be checked along with other possibilities.



=====================================
(2B)+(2B)' ?

RE: Blower High Current Draw

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. The torque curve of most induction motors shows that torque is not linear with slip.
I understand that they have added a third motor to what was originally a two motor installation. I would be inclined to check discharge pressures and currents with one, two and then three blowers running. Also ask for clarification on the current checks; One phase high or all three phases high.

One phase high, look for LINE to LINE voltage unbalance.
This may also increase bearing currents and lead to premature bearing failure.

Three phases high, look for restrictions and high discharge pressures.

Uneven voltages and phase angles may be corrected by the use of a wye:delta isolation transformer or bank with the primary wye point left floating. If switching voltage transients may be an issue, they may be mitigated with special switching and the addition of a single pole contactor.

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

RE: Blower High Current Draw

I would like to bring up a possible mechanical issue with the blower system - 2P-75HP that is being V-Belt driven.
It appears the equipment may be European mfg (Blower & Motor). This mfg may use the same HP for 50HZ applications
and how is on 60HZ system - 2850 RPM to 3560 RPM and did not change the pulley to accomodate for higher 60HZ speed.
Higher speed, lower the torque from the 60HZ motor.

RE: Blower High Current Draw

Quote:

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.

Quote:

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.

=====================================
(2B)+(2B)' ?

RE: Blower High Current Draw

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

RE: Blower High Current Draw

(OP)
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

RE: Blower High Current Draw

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

RE: Blower High Current Draw

Quote (waross 11 Jan 13 7:45)


...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..

Quote (electricpete 10 Jan 13 6:23)


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.


Quote (waross 11 Jan 13 7:45)


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).

=====================================
(2B)+(2B)' ?

RE: Blower High Current Draw

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

RE: Blower High Current Draw

I violently agree Gunnar. grin

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

RE: Blower High Current Draw

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.

RE: Blower High Current Draw

Quote:

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.

Quote:

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)

=====================================
(2B)+(2B)' ?

RE: Blower High Current Draw

clarification in bold:

Quote (electricpete)


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

Quote (electricpete)


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)

=====================================
(2B)+(2B)' ?

RE: Blower High Current Draw

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

=====================================
(2B)+(2B)' ?

RE: Blower High Current Draw

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.

RE: Blower High Current Draw

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

The site puts a low priority on giving us feedback.

Roy

RE: Blower High Current Draw

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

RE: Blower High Current Draw

(OP)
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

RE: Blower High Current Draw

Any idea how big these blowers are? CFM?

RE: Blower High Current Draw

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

RE: Blower High Current Draw

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.

RE: Blower High Current Draw

(OP)
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

RE: Blower High Current Draw

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?

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