Problems with a motor

[nicky123]

Im not sure if im posting in the right place.
Here is my problem:

3-Phase motor for air extraction

Motor 1:
220/440 Y V
HP:5
I: 15.8/7.9 amp
RPM: 1740
FP:0.78
n:79.4%
Tn/To: 20.4/47nm
FS 1.5

Motor 2:
220/440 Y V
HP:7.5
I: 21.8/10.9 amp
RPM: 3480
FP:0.87
n:77.4%
Tn/To: 20.4/47nm
FS 1.5

Problems:
1. The motor 1 was burned out after one month running because electricity failure ( the motor was running only with one phase when the light when out in the city )
2. The motor 1 was not extracting enough air.
Solution:
1. The electricity problem was fixed by installing some component to avoid working with 1 phase.
2. Motor 2 was installed.
3. The motor 2 now is extracting enough air.

Poblems:
1. After one day running, the motor 2 burned out.
2. Motor 2 was rewound with the same characteristics of motor 1. The motor is now running without problem since 2 month ago.

Problems:
1. Is not extracting enough air.

Overall, is seems like the extractor is working ok with motor 1 but is not extracting enough air, but with a motor 2 is extracting enough air but burned out after one day running.

Aditional data:
1. The motor is fed by a transformer of 30kva, this transformer also fed an air conditioner of 60.000 BTU/h and others minor electrical appliance.
2. The extractor was installed to extract air using 15m of duct.
3. I have no data about cfm or pressure required.
4. The extractor is 30'' size.

Questions:
1. Possibles causes why motor 2 burned out.
2. How can i know which motor fix ?
3.Possibles solution to make the extractor to extract enough air.

[keebler303]

Extreme Overload!

The fan and motor must be (reasonably) matched. You cannot assume that any motor will work. The power a fan needs is a function of the cube of the speed. If you double the speed (from 1740 to 3480) you have increased the power required by 8 times! So you actually need something like 40 HP to run that fan at 3480 RPM. So no surprise the motor burnt up. You need to get the right fan and the right motor to flow the necessary air.

Matt

[Al_The_Man]

The other thing to ensure it doesn't happen again, whatever motor, is to use the correct O/L' s on the contactor with coil interlock contact.
Al.

[nicky123]

Extreme Overload!

The fan and motor must be (reasonably) matched. You cannot assume that any motor will work. The power a fan needs is a function of the cube of the speed. If you double the speed (from 1740 to 3480) you have increased the power required by 8 times! So you actually need something like 40 HP to run that fan at 3480 RPM. So no surprise the motor burnt up. You need to get the right fan and the right motor to flow the necessary air.

Matt

The cause of all the problems was because of the people that installed the system, totally irresponsible and then they disappear. I have no idea about the requirement or about the extractor ( no information at all, no plate , in the equipment), I only have the data of the first and second motor.
I know that not any motor will work, i purchased a different motor to replace the first one because there is no other motor available, and i thought that with a higher speed it will extract more air.
The only information that I have for sure now is that with 5hp and 1740rpm it works, but is not extracting enough air, so, what can i do? with a 10hp motor and 3480 rpm will it work? or still not enough power to not burn out?
The other issue, is that I only have 30kva to run the motor, the air con and the minor electrical appliance, I cannot exceed 30kva, so, i cannot install a 40hp motor as you said because the electrical power is not enough.

[mike_Kilroy]

surely you have used clamp on ammeter to measure the current into each motor?


LIST THiS - how can we help if you dont give us current/load readings on what turns the stuff?

do u have 1 fan or 2?

why not throw a vfd on the motor(s)? it will protect it from overload, 1ph, and let you dial up the speed to see what the max speed is u can have with a given motor and not overload it??

u were told fan load goes up by the cube of the speed so you need to help your self with this type of analysis.

guessing will get us this:

1) load @ 5hp motor 1800rpm is less than 5hp
2) load @ 7.5hp motor 3600rpm is MORE than 7.5hp
not much to go on to help yourself!