Blower motor overload.

kiran

Member
Join Date
Jul 2003
Posts
43
Hi,

I need some help to find out the reason for the blower motor tripping .

On one of our machines we have three 10hp 575v blower motors. These motors are started one after the other with a delay. All motors are from same manufacturers, same frame size,specs etc. When the blowers start only the second blower motor overload relay trips at the starting and this happens only some times. All the motors have same overload relay. I have checked the overload setting, Looked for lose connections everything seems to be okay. :confused: Could anyone suggest a reason/solution for this.

Thanks.
 
I'm assuming that you've got three phase power? I would check the amperage on each leg of power as it is starting and see if one leg is pulling more amps than the others - and recheck the amps as it's running. More amperage on one leg could indicate a loose connection. Also you may have some contacts going bad in your contactor. Also if you overoads have heater elements in them remove and inspect those - (Sometimes they can burn up) If you have a fuse on each leg...one of the fuses may be blown which would cause the motor to single phase. If you're really stuck you could try swapping one overload or starter, or both with one of the other ones that works right to see if it may be going bad...you know overloads CAN and DO go bad.
 
Last edited:
debounce

Another possibility may be in the control program. Debounce the contactors: if it goes off keep it off for a few seconds, then allow restart.
 
Fans, can be a problem.

All motors are from same manufacturers, same frame size,specs etc.
But, are the blowers themselves the same on all three motors? The fan load on a motor is proportional to the cube of the speed. In other words, as the fan speeds up the load on the motor is not linear and if these three blowers are not the same, then the load on the motor is not the same. Has anyone changed a blower out and replaced one with a different one? Are the pitch angle on the blades, number of blades, SCFM rating all the same on each blower? Makes a big difference in the operation if one is different.
 
Basic troubleshooting

Because the problem is intermittent, I would not blame the control program immediately.

If all three are indentical, first swap two of the starters (both contactor and overload relay). If the problem stays with the motor, replace, or swap, the motor. Now if the problem still stays with the motor confirm the starting control logic. If the problem follows the starter, continue this process of swapping devices in pairs until the problem follows a specific device.
 
If the motor OVERLOAD is tripping there is a 95% chance there is a mechanical problem with the blower or the parts that connect it to the motor. Make sure all three power fuses are good, and then let the the millwrights/mechanics look at it. Let us know how it turns out.......
 
kiran, I know this sounds too basic but please don't take offense. Overload blocks generally measure current and evaluate it over time to approximate motor temperature. In view of that, the primary question is "Is the starting current higher on the second motor?" More than likely it is. It may be only slightly higher than the other two motors but it may be just enough to cause it to trip.

Assuming you find that the starting current is slightly higher, you should first check to see if the mechanical load on the motor is what it should be. You might have a tight bearing or the rotor may weigh slightly more for some reason. I doubt that the fan load itself is the problem because you say this is a starting rather than a running problem.

Finally, you may well have a motor that simply draws a few more amps under high slip conditions than the others. It is not that uncommon that identical motors behave differently especially under extreme conditions like near-locked-rotor starting. It may also indicate partial failure of motor windings.

Assuming that the motor is just different or that unimportant mechanical differences are the cause, you have little recourse but to go up one size in o/l heater. Maybe someone else has some kind of trick that can be played but, short of softstarting, I don't see anything else reasonable to do. I have heard of two-stage o/l's. These have a current level for starting and a second, lower level for running. I've never used one however.
 
I agree with Dick. Be sure that the amps measured on each leg of output to motor do not exceed the nameplate value of motor once up and running. You may have a small difference between legs as this is proportional to the voltage at each leg. It is not uncommon for one leg to have a higher voltage to ground than another and this will show up on amp readings. Ol settings are set at 125% for motors with 1.15 SF and or 40C temp rise. If your motor is 1.15 SF and 40C temp you can set OL to a max of 140% of FLA to allow for startup in the event that 125% would not allow stArt up. I assume you are using IEC stlye starters. If you are, typical ol is class 10 (designates trip curve values) You could go up to a class 20 / 30 to allow more delay for start up. You must be sure that the motor nameplate amps are not exceeded in the actual (RMS) amp readings prior to any OL adjustments. If the amps exceed the nameplate value the motor is trying to work harder than it was designed to (although it can produce HP X SF no problem) and will eventually fail.
 
as i work a lot in refrigeration with lots of blowers.
i always start them all at same time as when i start one blower the other fans will start turning backwards thus giving problems as the current is too high when they have to turn correct way.
change the delays to very short interval as 2 seconds or something like it.
as soon as motorcurrent drops and this will be as soon as blower is on 50% speed start next one.

and check the currents first maybe one motor or ocr is faulty.
check all phases and all wires.
so each motor six wires.
 
Do you have any type of dampers on your blowers? We use them to help during start-up and to control amperage (load) while running. We use ampmeters with a high and low setpoint which adjusts the damper to keep the load where we want it. I realize this is from left field, but I'm just curious. Randy
 
motor starters

Is the motor starter (contactor +overload) is the same for the three motors. are you sure that the seconed one is a motor type.do you have a soft starter or not?
 
startup load

I agree with RWhite. If your second blower is 'free-wheeling', you'll be pulling more amperage than normal. Using an ampmeter, you should gradually shut down the air flow from the blower until you get within the RLA specified by the motor mfg. However, this is more likely a run-time problem than a startup problem.

I'm betting on a weak overload!
 
I have experienced similar problems in a blast freezer.
The second blower impeller was being turned backwads as the air from the first motor "short cycled" via the second blower.Therefore the second blower pulled more current on start than usual.Try shortening the delay time between startup if possible.

Hope this helps

Gunner
 
another idea

Hi

When you say blowers, does the air travel through some distance through ducting before it reaches its destination?

I had a blower motor (we'll call it a fan) that tripped out just after coming back from a rewind...Eventually found out that some of the air ducting was split & allowed more air to be drawn than it was designed for. The ducting was sealed back up & NFT with the motor



(NFT.. No further trouble) :)
 

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