How to calculate current draw when fan motor fails.

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Bugman1400

Senior Member
Location
Charlotte, NC
I have 6 fan motors that are controlled by one contactor. Each fan motor is a 3PH 208V 1/3HP motor and around 1.21A FLA. So, 6 healthy running motors draw about 6 x 1.21 = 7.26 amps. Besides a bearing failure, I would expect another failure mode would be a lost phase in the motor. So, what would be the current draw for 1 motor failure. I am only measuring the B-phase (w/ clip-on CT) so, I would need to know the current if the B-phase was on the same phase as the failure or if B-phase is not on the same as the failure. IOW, I would expect the B-phase current to rise if the failure is on A-phase.
 

Phil Corso

Senior Member
Bugman,

A couple of weeks ago I provided an EXCEL program covering a problem like yours, except it involved multiple Delta-connected Heater Elements.

Contact me at {Moderator's Note: Email address removed. Please contact via Private Message to obtain email address} for a similar study to help solve your situation. There is no cost!

Regards, Phil Corso
 
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Phil Corso

Senior Member
Bugman,

In general the active conductors of the affected fan will increase in current. Although the magnitude of the increase will depend on actual loading, i.e, 1/4, 1/2, 3/4, 1/1, of full load, there are several notable indications:

1) Two of the System line-currents will be equal!

2) The affected Motor's power-factor will improve!

3) The overall Systems power-factor will improve!

Phil Corso
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Despite the likely success of the Excel spreadsheet working to attain the desired result, I think that it's a spurious endeavor nonetheless. Attempting to find one bad winding in 6 3 phase motors running together by monitoring one phase with a CT is still not going to be useful information. Let's say you discover that it is NOT on B phase. So what? That means you have narrowed the problem down to A or C phases, one of 6 motors, so you went from a 1 in 18 chance of finding it to a 1 in 12 chance. Is that significant?

Also, this begs the question that if you have 6 motors running from one contactor, do you have 6 separate overload relays or are the motors self protected? You must have one or the other. If you have the proper protection, damage in any one winding of one motor must take that entire motor off line, so knowing which phase it is or isn't becomes moot.
 

Besoeker

Senior Member
Location
UK
Despite the likely success of the Excel spreadsheet working to attain the desired result, I think that it's a spurious endeavor nonetheless. Attempting to find one bad winding in 6 3 phase motors running together by monitoring one phase with a CT is still not going to be useful information. Let's say you discover that it is NOT on B phase. So what? That means you have narrowed the problem down to A or C phases, one of 6 motors, so you went from a 1 in 18 chance of finding it to a 1 in 12 chance. Is that significant?

Also, this begs the question that if you have 6 motors running from one contactor, do you have 6 separate overload relays or are the motors self protected? You must have one or the other. If you have the proper protection, damage in any one winding of one motor must take that entire motor off line, so knowing which phase it is or isn't becomes moot.

Perceptive as ever.
 

Sahib

Senior Member
Location
India
If the motors are partly loaded, the increase in current in other phases due to single phasing (loss of one phase) may not be sufficient to actuate the overload relay whether it is individual motor protection or not. In such cases more elaborate motor protection may be required other than simply keeping track of the individual phase currents manually.
 

Sahib

Senior Member
Location
India
Phil:
I take HP=KVA approximately. So in OP case KVA=0.3 and during single phasing
208*I=300. So I=300/208=1.44A. The individual
O/L relay wpuld trip.
"
 

Phil Corso

Senior Member
Sahib,

I cautioned the OP that the one-page Excel program is a "Static" analysis. It ignores consideration of the "dynamic" world! A study I participated in covering single-phasing illustrated that line-current magnitude varied greatly, depending on the motor's loading!

I provided some direction to the OP, that, if carried out, could be used to modify the program! But, I also advised that it would not be a single-page program!!

Regards, Phil Corso

Ps: An aside: Had you noticed that your kVA calc showed that American practice of presuming input kVA ~ output Hp, is valid!
 

Besoeker

Senior Member
Location
UK
If the motors are partly loaded, the increase in current in other phases due to single phasing (loss of one phase) may not be sufficient to actuate the overload relay whether it is individual motor protection or not. In such cases more elaborate motor protection may be required other than simply keeping track of the individual phase currents manually.

Even the most basic motor overload units I've used in the past 40 some years have had unbalance protection that would offer single phase protection.
That's not a criticism. It just might be your area of experience.
 

Besoeker

Senior Member
Location
UK
Ps: An aside: Had you noticed that your kVA calc showed that American practice of presuming input kVA ~ output Hp, is valid!
Depends how ~ you consider acceptable.
I've checked a couple of motor/drive systems for which I had to provide guaranteed performance data.
Your approach would have given me about a 20% error.
Contracts have been won/lost on a fraction of one percent.
All performance tested.
And failure to meet the guaranteed performance carried serious financial pain.

Getting it wrong wasn't an option.
 

Sahib

Senior Member
Location
India
Even the most basic motor overload units I've used in the past 40 some years have had unbalance protection that would offer single phase protection.
That's not a criticism. It just might be your area of experience.
Ordinary O/L relays with no voltage imbalance protection or single phase protection feature offers no protection against single phasing to motors loaded to less than 50-60% for extended period of operation.
 

Sahib

Senior Member
Location
India
Sahib,

A study I participated in covering single-phasing illustrated that line-current magnitude varied greatly, depending on the motor's loading!
Thanks Phil for your reply. So my calculation for motor fully loaded case
Phil:
In OP case KVA=0.3 and during single phasing
208*I=300. So I=300/208=1.44A.
"
may not be correct. What is the correct way to do it?
 

Besoeker

Senior Member
Location
UK
Ordinary O/L relays with no voltage imbalance protection or single phase protection feature offers no protection against single phasing to motors loaded to less than 50-60% for extended period of operation.
Let me try this again in a little more detail for your benefit.
All of the even the most basic motor overload units I've used in the past 40 some years have had current unbalance protection that would offer single phase protection.

Have you used any that don't?
 

Phil Corso

Senior Member
Sahib,

The EXCEL program was my response to the OP's only query, i.e, fault-detection. He did not ask about location or protection. The program shows two methods: one (obviously) was supply-line amp differential; the other was the change in input power-flow using the 2-Wattmeter method.

I must reiterate, the program is a "Static" analysis, meaning dynamic effects attributed to speed change, and the Affinity-Law as it applies to Fan drivers, are ignored!

Regarding your direct question related to a motor operating at 100%

The maximum line-current could be 200-250% of the motor's line-current rating. Of course, impact on current distribution and magnitude in the motor's windings depends on whether it's Wye or Delta-wired. For the former only two windings are affected; for the latter, three.

Do you want a copy of the program?

Regards, Phil
 
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kwired

Electron manager
Location
NE Nebraska
Ordinary O/L relays with no voltage imbalance protection or single phase protection feature offers no protection against single phasing to motors loaded to less than 50-60% for extended period of operation.
Is the motor in any danger unless current in at least one phase rises above full load rating? Protection designed to respond specifically to single phase conditions just happens to catch it sooner then a thermal type of protection.
 

Sahib

Senior Member
Location
India
Is the motor in any danger unless current in at least one phase rises above full load rating? Protection designed to respond specifically to single phase conditions just happens to catch it sooner then a thermal type of protection.
To your question:
The negative sequence current in the rotor
under such condition would cause damage in the long run.
 

kwired

Electron manager
Location
NE Nebraska
To your question:
The negative sequence current in the rotor
under such condition would cause damage in the long run.

About the only load that wouldn't stall is variable torque loads like fans, I have seen many failures of fan motors due to single phasing. Constant torque loads will usually stall and you will have significant enough current to operate overload protection.
 

Sahib

Senior Member
Location
India
Phil:
As you say in your last post line current is 200% to 250% during.
single phasing, it follows "KVA=HP" is inapplicable during single phasing.
Note: email address via PM for program. Thanks.
 
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