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

 

[dpc]

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?

 

[waross]

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

 

[electricpete]

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?

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(2B)+(2B)' ?

 

[zlatkodo]

the motors are reported to be drawing 90 Amps

All three phases or only one?
Please, attach a photo of nameplate.

 

[jraef]

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"

 

[roydm]

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

 

[roydm]

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

 

[rhatcher]

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.

 

[roydm]

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

 

[jraef]

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"

 

[electricpete]

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)' ?

 

[rhatcher]

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.

 

[waross]

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

 

[electricpete]

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)' ?

 

[electricpete]

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)' ?

 

[Skogsgurra]

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

http://www.gke.org

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

 

[electricpete]

Good points Gunnar

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)' ?

 

[waross]

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

 

[macmckim]

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.