What does Usable at 208 mean? Can a 230 motor be used at 206v? 4

[TimWPH]

Hey guys,

I have (5) new sidewall propeller exhaust fans with 3HP 115/230 volt, 60 Hz, single phase 1725 Rpm fan motors with a 1.0 service factor. At the bottom of the tag it also says usable at 208 volts 15.5 amps. The system they are operating on is 208 volt 60 Hz.

I am having trouble with several of theese motors never coming up to speed and kicking the breaker and the more we use the fans, more fans start to have the same problem. It started out with (2) doing this and now it is (3). They do not do this every time but after they have cycled on & off a few times they start to kick the breaker and continue to do this every time until cut off and left off for a while.

When they come on & run they will pull about 77 amps for 3 seconds and then come up to speed and pull about 16 amps. When it doesn't come up to speed it will run and pull about 77 amps until the 30 amp breaker supplying it trips. We have checked start & run capacitors and they all check good.

The panel they are being fed from is supplied power by a 460 three phase primary to 208v single phase secondary transformer. Each fan is supplied by a seperate circuit form a breaker panel. All the wiring has been checked and is of suffiecent size according to the NEC and was increased by one wire size every 100 feet as specified by the electrical engineer on this job. We have checked the fans and belts and everything seems to be turning freely.

With the fan not running its circuit is about 213 volts at the fan. With it running at speed it is about 213 volts at the fan. When it comes on and does not come up to speed it reads about 184 volts while it is pulling the 77 amps but the breaker panel the circuit is coming out of does not drop voltage.

I think the motor should be either a true 208 volt or 200 volt motor but the supplier assures me there must be a power problem because the motor says "Usable at 208 volts".

What exacty does usable at 208 mean? Does it mean it can be used at a voltage 10% below 208 and work fine or does it mean it it is O.K. down to 208 volts and below that it is not reccomended?

Any help would be appreciated.

 

[JHanson717]

How far, what gauge wire(s) from the breaker to the motor? You appear to be dropping 50 volts along the way...
That much sag could cause issues.

 

[rbulsara]

If the tag says so its OK to use on 208V systems.
Your issue seems to be breaker sizing and fan inertia.

77A is 5 times 15.5A and that is pretty normal. The acceleration time depends on the fan inertia.

NEC allows up to 250% MCCB , so a 40A breaker would be normal sizing and within the code. Check out NEC 430. You can even go higher if the load characteristics require it.



Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

VOLTAGE DROP.
Your voltage is dropping from 213V to 184V.
That's 213V-184V/213V=13.6%
Increase the size of your motor feeder conductors two more AWG sizes.
When the rule of thumb is obviously not working (was increased by one wire size every 100 feet) it's time to quit hitting your wife and do the calculations.
Or- use another rule of thumb.
After you do the calculations, you will increase the wire sizes.
Or, two AWG sizes reduces the voltage drop to about 9 V.
Your motor should now get about 204 Volts and should start easily.
("Usable at 208 volts") BUT NOT at 184 Volts.
The motor should start and run with nominal 208 volts. (208V +- 10%, minimum 187.2V)
You are using the motor at the extreme limit of its rating. I suggest restricting the air flow slightly to bring the current down to 15.5 Amps.
I have assumed that your current and voltage readings are accurate.

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

 

[TimWPH]

JHanson717 & rbulsara,

Thanks for the response.

The closest fan to the panel has about 50 feet worth of wiring and is fed with a 8 awg. It only drops to 200v at start up but it still never gets up to speed and trips the breaker after turning at half speed for 10 seconds or so.

The farthest one away is about 500 feet woth of wiring and is fed with a #6 awg.

I am used to seeing theese types of fans coming up to sepdd in a couple of seconds. Acoording to several electricains i have talked to a 30 amp circuit should be sufficent for a 3 Hp fan.

What does usable at 208 volt actually mean?

 

[rbulsara]

15% voltage drop on DOL is not a surprise, the key is to allow it to recover.

Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

If high inertia (starting) torque is determined to be the issue, using a boost autotrasformer (208 to 230V) would also help.

Try easy things first.

Rafiq Bulsara

http://www.srengineersct.com

 

[edison123]

The motor is actually designed for 230 V but it can also work at 208 V provided the voltage did not drop below 208 V. In your case, the voltage is dropping steeply to 184 V and since the motor torque decreases by the square of the voltage, the original 230 V motor is unable to run up to full speed thereby tripping your breaker due to prolonged inrush current.

Setting your supply voltage to 230 V should solve this problem.

 

[waross]

Good call Rafiq. The autotransformer should also reduce the 16 Amp running current to a safer level.
Too bad that a single 480 Volt line was not run to the fans.
With local controls, you could get a lot of fans on a circuit.

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

 

[TimWPH]

waross,

Thanks for the reply,

Would operating this 230v motor at 208 volt decrease its tourqe and make it difficult to get the motor & fan up to speed causing it pull greater amperage? The actual brake horsepower needed for the fan was calculated at 2.84 BHP.

Also if this motor is "usable at 208" would that mean it should work OK at 10% below 208 which is 187 volts.

My main question here is if I have a 208 volt system and told my supplier that should a true 208 volt or 200 volt motor have been supplied in lieu of a 230/115 volt motor saying usable at 208 volt?

As far as accuracy of the volt meter the meter we are using has not been calibrated so it may be a little off but we have used two diffrent ones and they are reading with in a 5 volts of each other. The other meter reads 189 volts when the motor is pulling 77 amps at start up.

 

[TimWPH]

Thanks edison123,

I think you answered my primary question if which what you are saying is this motor is only designed to operate within 10% of 230 which would be O.K. at actual 208 voltage but under that there may be problems.

My only problem with the transformer change is there are other items being powered for mthis panel that wont opeerate at 230 volts.

I am haggling with the fan supplier telling him they should have supplied actual 208 or 200 volt fan motors since they knew the system was designed as a 208 volt system (10% + ot -. This is a plan spec job designed by an engineering firm and they specified the wiring sizes, breaker sizes, voltages etc...

Would I have this same problem if a 208 or 200 volt motor had been supplied? The BHP calculted for this fan is 2.84 BHP. What would the BHP of this motor be at 208 or 10% below that?

 

[burnt2x]

what edison123 said! At 208 volts, your motor torque reduced to the square of the voltage reduction. The acceleration time got longer and the breaker tripped before your motor achieved full speed.

 

[edison123]

Yes, 208 V was probably the lowest minimum required.

The 'consultant' screwed up. They should have specced a 208 V when they knew the system voltage was 208 V.

You may think of an auto or booster transformer that would step up the motor bus voltage from 208 to 230 V

No, the BHP does not change because of motor voltage change. Only the motor load current would be higher at 208 V.

 

[waross]

You may use a small transformer for each motor, or a larger transformer feeding a dedicated fan panel. If his was my job, I would be pricing three alternatives.\;
1> Transformer for each motor.
2> One transformer and a voltage boosted fan panel.
3> A possible change to 480 Volts. One switch and circuit for all fans or a 480 volt panel and a circuit for each fan. (At this stage of the project this is probably not going to happen.)

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

 

[jraef]

There is another possibility you are all missing here.

These are single phase motors; 90% chance they are capacitor start. If his capacitor is bad or is not switching out of the circuit when it should, that may account for the increased current and breaker tripping. That would explain why the one with the shortest lead length is one of the problem fans! Bad start circuit, high starting current for too long, resulting in voltage drop, causes a loss of torque, result is a failure to start...

I would start with a thorough check of the starting circuit for these motors before investing in more hardware.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[edison123]

Jeff - But OP said

"We have checked start & run capacitors and they all check good".

 

[rbulsara]

TimWPH:

What does usable at 208 volt actually mean?

means what it says but within its characteristics. Available torque would be less proportional to square of voltage reduction or x (208/230)^2.

A boosting transformer is most likely to solve your issue. If not you could have motor torque mis-match with the fan. Not all 3HP motors are equal.

Rafiq Bulsara

http://www.srengineersct.com

 

[itsmoked]

The reality of "usable at 208" generally means "unusable at 207". What that also means is if the motor actually runs the load it will still likely be dead within a year if below 208.

You need to reduce the loads by blocking some of the air flow or boost the voltage. Perhaps run a boost transformer where all these fan circuits originate at.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

Another option is change pulley ratio on the fans. It sounds like the motors are very marginal for the application. Pulley ratio can change as belts wears or belt tension changes. Fan power will increase with the cube of the speed.

 

[jraef]

Jeff - But OP said

"We have checked start & run capacitors and they all check good".

D'oh! Missed that. I saw that he said they checked the fans and belts and they turned freely, but I looked right over that sentence. Damn these aging eyes...
Never mind.


230V +/-10% makes it 207 to 253.
208 works, but as Keith says, it's not 208 - 10% again, it's 208 - 1V.




"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376