Motor burning

dear,

we had a similar kind of experience in a blower of high temperature grade in gas furnace.

we had ordered the blower for 1200 deg. C gas flow, but as soon as 600 deg. gas temp. reaches, blower started making noise, & found out that blades were touching the body!

similar to this, I can suspect for rotor / stator expansion with heat. If the manufacturer argues that it is the fault of fan, may be true, as two different metals have different expansions with heat.

The fault occurred is not basically an electrical fault. the electrical fault is caused by the mechanical system.

there may not be a winding fault at all. Pl. check, on cooling, motor may restart for some min. again. till it becomes hot.'

Remedies : to confirm wether the motor with vfd is burnt or with star delta one?

if it with vfd, 3rd harmonics plays the role.

if it is star delta, then most probably, due to pusating load of the fan.

you may have to derate the motor in this case.

As I understand it the problem motor is the one on the Star-Delta starter, not the one on the VFD. That actually makes sense to me.

Star-Delta, although a popular way to satisfy requirements for reduced voltage starting because it is cheap, is actually the most brutal way to start a motor. Mechanical damage caused by the transition torque spike is not at all uncommon. Were it my equipment, I would NEVER use Star-Delta, I only use solid state starting (soft starting). It can accomplish the same or better reduction in starting current and has no transition spike of current or torque.

When you try to accelerate with a Star-Delta scheme, you are first connecting the motor windings in Star, which gives you 33% current draw and 33% of normal starting torque from the motor. But this is never enough to fully accelerate a load so in order to finish the acceleration, you must at some point transition into the Delta winding configuration. To do so, the motor starter must open all of the contactors to avoid a short circuited connection. This is called an "Open Transition". When you do this, the motor slows down (because there is no power connected) and although that lasts only a second, it is enough to allow the flux (torque creating component of the magnetic energy) to collapse. Therefore when the motor is re-connected in Delta, the magnetic feld must me reestablished in the rotor. In doing so, there is a second Inrush Current spike and that is JUST AS HIGH AS IF THE MOTOR WERE STARTED DOL. Most people are unaware of that. Because the motor is already moving, the duration is much shorter, but that spike has a corresponding spike in shaft torque as well. To add insult to injury, there is a phase shift that takes place between the Star and Delta connections, and any residual magnetism n the stator causes an out of phase regeneration when connected in delta. This can cause a current spike that is HIGHER than it would be if you had just started DOL, in fact it has been mathematically proven to be as high as 2000% of FLA! Again, that current spike will have a corresponding torque spike. Together, the torque spikes are referred to as a "transition spike" and has been known to shear the motor shaft on occasion, especially with high inertia loads such as large ID fans on boilers. Of the 3 that I have personally witnessed, 2 were on boiler ID fans.

The solution used to be what is called a "Closed Transition Star-Delta" starting scheme in which a 4th contactor and a resistor bank is added to absorb energy off of the motor and avoid the spike. But this scheme ends up defeating the only advantage Star-Delta has over Solid State starting, low cost. A Closed Transition start delta starter will cost MORE than a Solid State soft starter. That is why you hardly ever see them any longer.

The other probelm you can have with Start-Delta, and one you should check out very carefully, is thatmany people are not aware of the roper way to connect the motor indings. Any error can result in damage to the motor.

DEAR JRAF,

Thanks for your comments......it's a new knowledge to me eventhough i have been working in maintenance field for the past 2 years.........thank u...

There may be problem in rotor of motor, If secondary winding is faulty then it is quite possible it will load on primary winding that is stator, there is possibility of open bars at NDE side. Pl. check the rotor on growler.

kelkar

Hmmm one would have thought that; a) The motor shaft and housing were out of alignment; and or b) If all the internals were to heat up, they they ought not to be expanding and or distorting enough to enable collision. Me thinks there is misalignment between the parts.

Rajant 2001,

Correct my understanding if I am wrong: Electrical testing shows no problems with an electrical origin. Motor runs fine no-load. Failure occurs under load after a short time delay. Both times the failure was a mechanical rubbing between the windings in the stator and the steel of the rotor, occurring on the non-drive end of the stator. Both failures were the star/delta motor with vane flow control; the VFD motor has not had any failures. Correct??

Although some failures can be directly blamed on the star/delta starting method as others have accurately pointed out, the failures you are experiencing are most likely related to mechanical issues. Somehow, it appears that a longitudinal force is being imposed on the shaft, to push it backwards in the motor housing. The bearings in this motor are probably not thrust bearings, so they do a poor job of resisting this type of motion. Possible sources of this force can be thermal expansion within the fan area, air flow giving a longitudinal load on the fan shaft, or some reaction with the regulating vane flow control. The mechanical problem would be caused by some component's installation or operation that is unique to this motor and not present in the other (because its load control is by the VFD instead of mechanical means).

I suggest you look at the bearing selections and determine if one or more thrust bearings should be utilized. Look at the longitudinal clearances in the coupling to make sure that any thermal expansion has room to be taken-up without butting the motor shaft to the fan shaft (consider a greater open space initially or a coupling that has this capability). Look at the configuration of the inlet and outlet flow (compared to the other fan) and see if this is causing an unexpected longitudinal load on the motor. Look at the regulating vanes in the open position and see if this is causing this unexpected load.

--Happy troubleshooting, JMM

In a good constructed motor you normally have a locating and a non-locating bearing.

A locating bearing is a bearing that provides radial support and at the same time locates the shaft axially in both directions. It must be fixed both, on the shaft an in the motor house.

A non-locating bearing can axially travel in the motor house.

When a motor is running at normal load, the rotor will heat up more then the stator.

This means that the rotor will expand more then the stator. This expansion can be up to 20 mm for motor in the range of 100 kW.

Probably one of the bearings has not enough clearance. This results in bending the motor shaft.