Strange Voltage Drop, over amperage on motor

Status
Not open for further replies.
G

gk351

Senior Member
Location
IL
Got a call because a grain bin aeration fan wouldn't work. ( 230V 1ph, 10-15hp, 42-61FLA, SF1.0) Overload on starter was tripped, reset, and amp check was 74 amps. No brainer, checked capacitors, bad. Replaced run caps, and same result. 72-74 amps. I noticed I get a 7-12 volt drop when running 1 fan, but the same voltage with 2 both fans running (2 fans total on site.) Ran wire from panel to fan directly, no change. Had motor rebuilt, they only replaced a bearing, and said it was fine. So When POCO comes out, voltage drop isn't as severe that day (naturally), but they agree to upgrade XFMR, but asked if I will replace motor first. No problem, get new motor, and exact same amperage draw 72-74A. POCO changes XFMR, situation gets a little better 68-70A running now.

The service pole is about 250' from grain bins. The bin is fed with 4/0 CU conductors(2 ungrounded, 1 Neutral, no ground wire in the Feeder.) I do believe its in conduit underground, but cannot confirm as I haven't done any digging yet, but the conductor is THWN insulation, so Im hoping they weren't that stupid. I don't believe the voltage drop I have is normal with this size conductor, and I cant explain the over amperage.

Do I have a conductor going bad? The symptoms aren't that bad for me to suggest that, but Im running out of options. With the length Im talking here, whats the best way for me to do some tests (i do have multi meter, no megger.) I've contemplated laying conductor on the ground and testing, but that's a lot of wire and labor. I dont want to tell the owner new wire is the answer, when that might not fix the problem.
 
gk351 said:
I don't believe the voltage drop I have is normal with this size conductor, and I cant explain the over amperage.

Do I have a conductor going bad? The symptoms aren't that bad for me to suggest that, but Im running out of options. With the length Im talking here, whats the best way for me to do some tests (i do have multi meter, no megger.) I've contemplated laying conductor on the ground and testing, but that's a lot of wire and labor. I dont want to tell the owner new wire is the answer, when that might not fix the problem.
Click to expand...

If the motor is driving a load like a fan, once it has come up to speed the power used to turn the fan at the motor synchronous speed minus the normal slip will be just about the same independent of the voltage that is presented at the fan.
That means that if the drive ratio (assuming belt and pulley) is such that the fan would normally draw the rated full load amps, then reducing the voltage by 10% can be expected to increase the "full load" current by about 10% rather than reducing it.

The motor and overload will not be happy, since it is handling more current in the windings just to deliver the same power.

1. You are right to look at the voltage drop, but the important number is not so much the drop but what the actual voltage is compared to the rated voltage of the motor.
2. If the two motor/fan units are otherwise identical, check so see if someone put the wrong ratio pulley on the one which is having problems. The power load delivered to the fan goes up roughly as cube of the speed, so a 10% increase in fan speed will take 30% more power from the motor.
3. If everything else checks out and the voltage is just too low, you may want to use a boost transformer setup to increase the voltage to the motor up to the nominal voltage.
 
What was the voltage with and without the motor running?

Some temp wires out on the ground is not a bad idea, could be a lot smaller than 4/0. Megger would be a lot easier.

What is up with the other fan though? Why is it not pulling high amps?
 
Is there current in the lines after the disconnect if the wires are disconnected at the motor? Or is all of the measured current going into the motor?


Sent from my XT1080 using Tapatalk
 
GoldDigger said:
If the motor is driving a load like a fan, once it has come up to speed the power used to turn the fan at the motor synchronous speed minus the normal slip will be just about the same independent of the voltage that is presented at the fan.
That means that if the drive ratio (assuming belt and pulley) is such that the fan would normally draw the rated full load amps, then reducing the voltage by 10% can be expected to increase the "full load" current by about 10% rather than reducing it.

The motor and overload will not be happy, since it is handling more current in the windings just to deliver the same power.

1. You are right to look at the voltage drop, but the important number is not so much the drop but what the actual voltage is compared to the rated voltage of the motor.
2. If the two motor/fan units are otherwise identical, check so see if someone put the wrong ratio pulley on the one which is having problems. The power load delivered to the fan goes up roughly as cube of the speed, so a 10% increase in fan speed will take 30% more power from the motor.
3. If everything else checks out and the voltage is just too low, you may want to use a boost transformer setup to increase the voltage to the motor up to the nominal voltage.
Click to expand...
These fans would be axial flow with blade attached directly to shaft. Hard starting.
 
How old is the install?
Open drying floor on the bin?
Back.flow dampers?

A megger would be best to check the underground but we had pretty good results for years with a good VOM and some of the hand testers. Most of the new ones are worthless because of their low ohm range.
Tpros and the like.
It is most likely not the service lateral if the one fan runs by itself with no problems but second does not. Check the FOP across the starter. Open it up. Check all the connections.
 
You are possibly moving too much air and this is overloading the motors. Fans and pumps in general are loaded by the amount of flow, not by speed or pressure, though those items can and do effect the amount of flow. The more air you are moving the harder the motor is working to get the job done, the more current it will pull. A simple check is to take a piece of cardboard place it over fan inlet, not a complete blockage but move it around effectively changing the inlet opening size and watch the amps go down as you make the opening smaller. I have played around to come up with a carefully selected restriction plate to mount on the inlet screen before on such applications.

Other thing may be to have them check the bin for unnecessary leakage and seal those leaks - it could make some difference if there is excess leakage through areas it doesn't need to be leaking from. You may be surprised how much air leaks around the base if it has some age, or around the unload auger penetration or other similar areas, or the fan plenum itself.

This can also be a situation of what kind of grain is in the bin, what it's moisture level is and how full the bin is possibly making it easier in your case to pull more air through the bin, and if they put in a different product, or fill it full it may perform differently.

I have seen voltage drop worse than you are experiencing and similar horsepower fans run fine. The biggest key is to not drop much below the 230 volt rating as well as not exceed the nameplate amps.
 
I happen to disagree with Kwire on this. Axial fan current goes up as the air flow is blocked. We have a small axial fan on a VFD in our shop. Blocking the flow causes an increase in current from 3.3 amps to 4.2. FLA is 4.2.

I would imagine a single phase reacts accordingly. The difference here may be the drive is adjusting to maintain rpm. Not quite apples to apples but I believe that is what I have experienced in the field before.
 
ptonsparky said:
I happen to disagree with Kwire on this. Axial fan current goes up as the air flow is blocked. We have a small axial fan on a VFD in our shop. Blocking the flow causes an increase in current from 3.3 amps to 4.2. FLA is 4.2.

I would imagine a single phase reacts accordingly. The difference here may be the drive is adjusting to maintain rpm. Not quite apples to apples but I believe that is what I have experienced in the field before.
Click to expand...
If it is indeed an axial fan, then he is correct, an axial fan does not work the same as a centrifugal in that regard. But if someone did change a pulley ratio or pitch angle on the blades, in an attempt to get more flow from it, that could explain it. People do that sort of thing all the time, not understanding that any change in flow results in a change in HP required. Down is never a problem, but up always is.

But before addressing the mechanical issues as an electrician, I would eliminate the last remaining electrical possibilities. Can you rent or borrow a megger from somewhere?
 
Last edited:
Jraef said:
If it is indeed an axial fan, then he is correct, an axial fan does not work the same as a centrifugal in that regard. But if someone did change a pulley ratio, that could explain it.

But before addressing the mechanical issues as an electrician, I would eliminate the last remaining electrical possibilities. Can you rent or borrow a megger from somewhere?
Click to expand...
If OP has what I think he has, I work on these all the time, it is a fan propeller mounted directly on the motor shaft, there is no pulley. They get 12-15 Hp rating out of what is essentially a 10 hp motor because the motor is located in the airstream and keeps it cooled and the motor probably has AO (air over) in its rating. Partially blocking the air inlet reduces current in the motor. The fan was either improperly selected or you don't have the conditions it was selected for causing the excess air flow and increased current. The fact it is taking out run capacitors and the fact that the voltage drop doesn't seem to change all that much with a second fan running makes me suspect overloading more than other issues and that is what I would look into first is just simply blocking some of the inlet and see what happens - is about the easiest thing there is to try for this installation also.
 
ptonsparky said:
Just tested a 1ph 230v. Amps increase as flow is blocked. 7.2 to 8 amps. Not totally closed off but restricted at outlet by cardboard.
Click to expand...

You need to block inlet, not outlet.

Blocking outlet means your trying to push the same amount of air through a smaller hole, which will increase pressure; causing motor to work harder.

Could be too that there is too much pressure drop in air path, and the motor is working hard to overcome the effect.
 
Last edited:
kwired said:
If OP has what I think he has, I work on these all the time, it is a fan propeller mounted directly on the motor shaft, there is no pulley. They get 12-15 Hp rating out of what is essentially a 10 hp motor because the motor is located in the airstream and keeps it cooled and the motor probably has AO (air over) in its rating. Partially blocking the air inlet reduces current in the motor. The fan was either improperly selected or you don't have the conditions it was selected for causing the excess air flow and increased current. The fact it is taking out run capacitors and the fact that the voltage drop doesn't seem to change all that much with a second fan running makes me suspect overloading more than other issues and that is what I would look into first is just simply blocking some of the inlet and see what happens - is about the easiest thing there is to try for this installation also.
Click to expand...

Probably not a factor in this installation at all, but in some cases of excessive voltage drop when running as well as hard starting, it may help to add PF correction capacitors at/near the motor, in any case after the switch or contactor.

The PF correction will reduce the current in the wires to the extent that the running PF is really bad, and therefore somewhat reduce the voltage drop.
Again, not likely to be significant in the OP's situation of a fully loaded motor. The PF of a motor will improve as it is more heavily loaded.

It is probably worth checking to confirm that the motor is in fact getting up to full running speed, perhaps even using a tachometer readout to see just what the slip factor is and comparing the two motors that way.
 
kingpb said:
You need to block inlet, not outlet.

Blocking outlet means your trying to push the same amount of air through a smaller hole, which will increase pressure; causing motor to work harder.

Could be too that there is too much pressure drop in air path, and the motor is working hard to overcome the effect.
Click to expand...

No, works the same way. I did not try it on the 1ph fan, but did on the 3.

Catch of all this is the type of fan he has. Centrifugal of axial.
We can talk all day but until that info is forwarded, we are just enjoying the discussion.
 
Ok I'm going to try to respond to all comments. I have tried the blocking the inlet trick. Yes amperage goes down to around 65-67 amps. This is a vain axial fan direct drive, stock fan blade. It has a sister fan on a another bin right beside it. Before POCO changed xfmr I had voltage at 244. When u start this fan, I've saw drops as low as 231-233. I haven't checked the volts since the change out due to weather. I was able to mimic the same voltage drop on the other fan, but not the same amperage conditions. But only once. I'm on the verge of buying a megger or fluke 1505/1520. I'm still not sure using a megger to test is gonna get me anywhere. I guess I could go buy 300' of 40/40/20 URD and quickly be able to determine if I do have a bad UG cable.

Fan specs, GSI group 28" vane axial fan, 1 ph
Baldor UCLE1015
 
Yes, you are going to see drops on starting. The motor has to provide enough torque to start, plus the torque to get fan moving. Without dampers on the inlet, it will have the highest torque requirement. If you had a speed versus torque curve to look at it would help explain.

Your voltage drop on starting is only about 5%, that's really not bad; or are you say you have a continuous voltage drop of 5% when its running? If that's on starting, how long does it take to recover to near nominal?

Keep in mind your motor is most likely rated for 230V not 240V, so it should be running just fine, unless it is undersized for the fan.
 
Status
Not open for further replies.
Top