Under-Voltage Motor Operation 1

[Beggar]

I have a processing plant in China with a range of motors, most in the 3 to 20 hp range but one that's 200 hp. Most are driving centrifugal fans. They run about 18 hours per day.

Recently we've seen our supply voltage drop to as low as 340 volts (as opposed to the 380 VAC motor ratings).

My question is what is the result of this? I've seen that the motor currents have fallen along with a reduction of airflow through the fans.

The performance issues are apparent; my primary concern is the potential for damage to the motors themselves.

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[itsmoked]

Luckily, the fan loads drop quickly with speed. This is why the current declines. Can your facility stand shutting off a few motors during a sag? Would this help?

As long as the power dissipation in the motor doesn't increase without any major internal cooling loss the motors probably won't be damaged.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

But less work is being done...

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[ozmosis]

beggar
If your voltage goes down and your load is constant then current will increase. Is the fall to 340Vac over short periods (a sag) or is is for longer periods (minutes/hours)? If the operation at 340V is for long periods then I'd be also looking at other equipment too to make sure you are not running at max current.
Are all motors on a fixed speed control(DOL starter/Y-delta etc..) using dampers/vane control to restrict airflow? It's not that often a centrifugal fan is fitted so that even running at full speed with maximum allowed airflow, it runs at rated current but you never know and now running such low voltages then the quickest and easiest way is to measure current. If it is higher than the rated current on the nameplate @ 380V then the problems get are that the motors get too hot and therefore their life is limited. If the motors are starting often at lower voltages then this too puts extra heat into the motors assuming your breakers/fuses are not going on a regular basis.

 

[machmech]

Pay attention to entrance and exit conditions.The conditions at the entrance and exit to a fan greatly influence fan system efficiency.

• Use long, straight duct runs upstream and downstream of the fan.

• Use gradual slopes when ducts expand or contract. A slope of 1:7 usually works well.

• For single-inlet centrifugal fans, place the drive system opposite the inlet to keep the inlet clear of obstructions.

• Avoid spinning the air into the impeller of centrifugal fans. Bringing the air in axially produces the best efficiency unless the impeller is specifically designed for either pre-rotation or counter-rotation. For example, inlet guide vanes, sometimes also called pre-rotation vanes, are used to vary the air delivery of centrifugal fans.

• If duct elbows must be used near a fan inlet or outlet, install turning vanes. If an elbow is installed near the outlet of a centrifugal fan, have it turn in the same direction as the fan impeller. Doing the opposite—turning the air in the opposite direction from the impeller—is colloquially known as "breaking the back of the velocity profile" and leads to substantial pressure drop.

• If a centrifugal fan with inlet guide vanes is to be retrofitted with a VFD, remove the inlet vane assembly from the fan inlet and replace it with a smooth bell mouth in order to improve efficiency.

• For axial fans, use bell mouths, spinner cones, and tapered outlet sections for maximum efficiency.

http://www.wisconsinpublicservice.com/business/eba_32.asp#IDAXRY3

 

[Skogsgurra]

Going from nominal 380 V to 340 V is a ten+ percent reduction. That means a more than twenty percent reduction in tourque, which results in increased slip. The increased slip manifests itself as less air flow, which you have noted, but also as more losses (slip is like slip in a clutch - the more you have, the hotter it gets) in the rotor.

High rotor losses sometimes make the bearings run hot. Stop the fan and touch the shaft. If it is cool - no problems. If it is hot, but below 80 C - no problems. If it is above that, there might be a problem with the grease in the long run. As always; it depends...

Gunnar Englund

http://www.gke.org

 

[freefallingbody]

Beggar,

If I may summarise the effect of 10% less voltage on motor parameters,

Torque, starting &
maximum running Decrease 19%
Speed: Synchronous No change
Speed: Full Load Decrease 1.5%
% slip Increase 23%

Efficiency:Full load Decr. 2 points
¾ load Little change
½ load Incr. 1 to 2 points

Power Factor
Full load Increase 1 point
¾ load Increase 2 to 3 points
½ load Increase 4 to 5 points

Current
Starting Decrease 10 to 12%
Full load Increase 11%

Temperature rise Increase 6 to 7 degC
Max. Overload Capacity Decrease 19%
Magnetic noise Slight decrease

Hope this helps.

Dinesh

 

[aolalde]

If the current dropped, the temperature on the stator should be lower than nominal. The drop of speed reduced the load proportional to the cube of speed reduction and the torque should have experienced some reduction too. Probably your rotor will not have any problem, but check the shaft temperature as Skogsgurra suggested.
If you can live with the reduced air pressure and flow, the motors will not have any problem.

 

[Marke]

Hello Beggar

There are two types of motors that are commonly applied to fans.
These are 1) high slip motors with a high resistance rotor, and 2) standard cage induction motors.

These motors behave totally differently under low voltage conditions.
High slip motors are commonly used on fans because they offer an easy means of speed control for the fan. With a high slip motor, as you reduce the voltage applied to the motor, the torque capacity reduces and the motor will slow. As the motor slows, the fan torque reduces and the motor will operate at a speed where the torque developed by the motor equals the torque required by the fan. I would expect the current to reduce wwhen the voltage reduces and there will be no issues arising from operating at the reduced voltage other than the air flow will be reduced.

A standard cage motor has a very low slip. As the voltage is reduced, the available torque will be reduced and the slip will increase. If the voltage is reduced sufficiently, the motor will stall and draw a very high current. This type of motor is not designed to be operated at reduced voltage and the reduction of voltage with a fan load will have a totally different result than the high slip motor. As the voltage is reduced, the speed will drop by a very small amount. The drop in fan load will be very small. The magnetising current in the motor will reduce. The load current however, will increase because the load will not change significantly.
Reducing the voltage on a standard motor that is well loaded, will result in an increase in current and an increase in slip power dissipated in the rotor.
Provided that the motor is operating at reduced load, there will be no damage to the motor. If however, the motor is operating at rated load, there will be increased motor heating which will accelerate the aging of the insulation within the motor due to elevated temperatures.

Bottom line? Fan motors (high slip motors) will not be affected. Standard motors will have increased heating which could cause early failure.
It is possible that the small motors will be fan motors, but the 200hp motor will be a standard motor.
The voltage drop from 380 to 340 is in the order of a 10% drop in voltage. This should not cause major issues with short term voltage sags.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[aolalde]

Hi Mark, I got confused with your explanation since the original post stated: “ I've seen that the motor currents have fallen along with a reduction of airflow through the fans.”

 

[MacRae]

I know that Beggar is not asking for solutions yet, as there may not be a problem from the voltage sag, but I had a thought. If the motors do end up having problems, could running them on a VFD help? I know that certain VFDs have allowable voltage input ranges that are different from thier output valtages. Could they act as a voltage stabilizer so to speak? Or are they not able to respond dynamically to supply voltage changes?
Like I said before, just a thought.
Ian

Ian Rines
Harris Corporation
Palm Bay,FL

 

[jraef]

Could they act as a voltage stabilizer so to speak? Or are they not able to respond dynamically to supply voltage changes?

Essentially yes, although "stabilizer" would imply something dynamically bidirectional in its ability to respond, and a VFD is only capable of responding in one direction; down. It cannot create voltage that is not there (contrary to some popular beliefs).

You would need to specifically program the VFDs response to a low voltage input condition. You would tell if to lower speed (frequency) in order to maintain the correct V/Hz ratio to the available line and prevent the motor from losing torque. The drawback is that it can only work on applications where the speed drop and HP (kW) loss will not adversely affect the work process. Also, many VFDs are sophisticated enough to offer this, but not all are.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

My question is what is the result of this? I've seen that the motor currents have fallen along with a reduction of airflow through the fans.

As long as the current is dropping there is not a problem.
Also as skogsgurra has pointed out, the voltage drop is 10%. that is the acceptable limit for NEMA motors.

I think itsmoked summed it up well,

As long as the power dissipation in the motor doesn't increase without any major internal cooling loss the motors probably won't be damaged.

respectfully

 

[imok2]

Sorry fellows I'm not an Electrical Engineer but wouldn,t an increase in his power factor help his situation?

 

[itsmoked]

imok2 I would imagine more PF would come with more slip, which I believe causes more heat.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[imok2]

So are you saying that his I sq. R would increase even though the reactive current would decrease?

 

[Marke]

Hello imok2

If there was a significant voltage drop due to line losses, then yes, adding power factor correction would reduce the current and therefore reduce the voltage drop. This could serve to inrease the voltage at the motor.
Increasing cable size will also reduce the voltage drop.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[itsmoked]

imok2; I, for some reason, turned your question entirely around thinking you meant by more PF => "worse" PF. So now that I am thinking probably like you were I agree with Marke.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Reality Check;
Improving voltage by improving power factor is one of my pet peeves. I hear this quite often. Correct the power factor and the voltage will be fine.
Yes, improving power factor will improve voltage, slightly:
However;
Most cases of low voltage are due to excessive voltage drop on overloaded lines, or low supply voltage from the utility, or improper settings of the governor or Automatic Voltage Regulator on a generator.
The power factor of a loaded induction motor is typically over 85%.
If we improve the power factor to 100% we will reduce the current by 15%.
If the voltage drop on the circuit conductors is 10% it will now be about 8.5%
If, as is often the case, the voltage drop or low voltage originates in the utility system, improving the power factor for a single motor or even for an entire plant will usually not make much difference to the utility system.
Will improving the power factor increase the voltage?
Yes.
Will it improve the voltage enough to make a difference?
Probably not.
What if the voltage drop is on the motor feeders?
Size the feeders properly, with attention to the Fine Print Notes regarding voltage drop. On a long run, consider starting current when calculating voltage drop.
With adequately sized circuit conductors and feeders the voltage should be adequate no matter what the power factor is.
What if the voltage drop is on the utility lines, will improving the power factor at a motor increase the voltage?
Not enough to measure.
There are many causes of low voltage. Over loaded feeders or supply lines, improperly sized conductors, low supply voltage for a number of reasons, and least of all, poor power factor.
Poor power factor as a factor in low voltage is usually a small symptom of one of the other problems.
Once the primary problem is corrected, it is usually found that correcting the power factor doesn't make enough difference to matter.
I surveyed a sawmill a few years ago. The power factor penalty was considerable but that wasn't the most important problem. The 34.5 kv three phase line serving the mill extended about 20 miles from the last small city. Past the mill two phases and the neutral extended another 20 or 30 miles. The load was a small city about 20 miles out and a couple of small sawmills and at least one small dairy. All running on 2 phases and the neutral.
The heavy neutral current was causing phase shifts and voltage unbalances as much as 15%.
The soft starters would trip out on phase angle errors. We fixed that with bypass breakers.
Then came the local expert from the power utility who told management that the problem was our power factor and as soon as we fixed the power factor all the problems would go away.
The power factor was soon corrected (to avoid the penalties) and then management was on my case as to why the other problems didn't disappear.
"How much did the utility engineer charge for his advice?"
"He didn't charge us anything!"
"That's exactly what his advice was worth!"
"But he works for the power company!"
"You don't suppose that that may be part of the power problem here?"
If you ask me or most of the people here who work with motors if correcting the power factor will increase the voltage, we will truthfully answer "Yes".
Then if you ask us if improving the power factor will increase the voltage enough to make much difference to the problem at hand, the answer 99% of the time will be "No. Probably not".
I think that I said that before. No problem, I think it's worth saying again.
Find and fix the real problem first.
respectfully

 

[itsmoked]

Agreed!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com