Forces on stator windings/ motor frame at start up

rejoe.koshy

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Join Date
Dec 2011
Location
kolkata
Posts
193
Hi guys,
I am facing an issue with a motor which is encountering a recurring failure of its mounting stud(its a flange mounted motor), which is found to be broken.

It is a dual speed motor ( dual pole windings 8p/2p) & there was issue with the controller which switches the motor from one speed to another & hence I by passed it.I have noticed that the issue of motor stud failure surfaced after I had bypassed the controller.

I am not able to understand the real cause behind this phenomena.Does the frame or the stator windings experience any force at start up which is causing this issue?

Pls help.
 
How big of a motor?

Yes, the motor housing has forces on it.
a dual speed motor is designed to start in a low torque fashion and then switch to high speed.

the reason is that you have a load holding onto the motor shaft.
when you start the motor in low speed, you have enough torque in the motor itself to overcome the drag or friction holding the motor shaft.

when the motor gets up to speed, you then change to the high speed of the motor and the shaft sees the high speed force holding the motor shaft, not what it takes to start the load.

the shaft torque is balanced out and the motor housing stress on the mounting bolts is pretty constant.

for example, let's say this is a 20 hp motor - dual speed.
bypassing the dual start system is telling this 20 hp motor to do the work of a 60hp single speed motor. the bolts are breaking because the shaft load is so great that the motor housing is trying to turn. eventually you will start this motor one to many times and all the mounting bolts will break and the motor housing will become the rotating pare for a few seconds.

this is what I remember from motors, generators class way back when.

james
 
Does the frame or the stator windings experience any force at start up which is causing this issue?
Does the thing attached to the motor experience any forces, and the answer is 'yes of course it does', how much force depends on how quickly you accelerate the load and the force required to maintain the speed. As any action has an equal and opposite reaction, then the frame and mounting will experience that force but in the opposite direction. The greatest force will be during acceleration, so if you want to reduce the load on the motor and frame you must control the acceleration. Other things that can increase the load is a misaligned power train and starting the motor with the load running backwards.
 
Does the thing attached to the motor experience any forces, and the answer is 'yes of course it does', how much force depends on how quickly you accelerate the load and the force required to maintain the speed. As any action has an equal and opposite reaction, then the frame and mounting will experience that force but in the opposite direction. The greatest force will be during acceleration, so if you want to reduce the load on the motor and frame you must control the acceleration. Other things that can increase the load is a misaligned power train and starting the motor with the load running backwards.

Thanks Bryan for the insights but need a little clarification on the same.

So, is the force experienced by the frame a function of starting torque or the load attached to the shaft?.I have seen motors turn over a little bit when started unmounted & without load.
Technically I guess the low speed winding would produce less starting torque & high speed winding would produce more torque.If thats so, then the force generated by the low speed winding would be less & the reaction force on the frame would also be less so no issues..Would that be a correct conclusion?
 
I have seen motors turn over a little bit when started unmounted & without load.
It did have a load, just a small one comprising the mass of the rotor and the friction losses. Anything that has mass will require force to change its speed (accelerate/decelerate). The bigger the change of speed the greater the force required. You say your motor is dual speed, are you sure it isn't Star/Delta, Star/Delta is a method for reducing the available starting torque of a motor, dual speed is normally used because you need two different speeds out of the same motor. However if it is two speed it will require less torque to start at the lower speed because you are accelerating the mass of the whole system to a lower speed.

I don't have huge experience with motors but I am surprised that you are breaking the frame. I would be looking at the mounting arrangement, perhaps you can spread the load by using larger washers under the bolt heads. Are the bolts of the right size? Check that one mounting point isn't being forced to take a higher load because of an uneven bed.

The more torque a motor can supply, the more quickly the load can be accelerated.
 
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"For every action there is an equal and opposite reaction."

The torque on the rotor is the equal to the output torque to the load. The torque on the stator is equal in magnitude to the torque on the rotor.

You may have cheap bolts on the motor flange, and the steel may not be to spec. You could also be using the wrong grade/class of bolt. http://www.fastenermart.com/understanding-fastener-grades-and-classes.html

Check with the manufacturer of the motor to see what they recommend. If you go with high strength bolts and that isn't the problem then you will start snapping keys or shafts instead of bolts.

If power is applied to a motor turning at high speed you can develop many times the normal starting torque. (I once saw this happen on a 750 hp 3600 rpm motor - not pretty.) This is even worse if, as mentioned above, the motor is spinning backwards. Make sure that a time delay exists between starts so the motor is spinning slowly, or not at all, before you start it again.
 
It did have a load, just a small one comprising the mass of the rotor and the friction losses. Anything that has mass will require force to change its speed (accelerate/decelerate). The bigger the change of speed the greater the force required. You say your motor is dual speed, are you sure it isn't Star/Delta, Star/Delta is a method for reducing the available starting torque of a motor, dual speed is normally used because you need two different speeds out of the same motor. However if it is two speed it will require less torque to start at the lower speed because you are accelerating the mass of the whole system to a lower speed.

I don't have huge experience with motors but I am surprised that you are breaking the frame. I would be looking at the mounting arrangement, perhaps you can spread the load by using larger washers under the bolt heads. Are the bolts of the right size? Check that one mounting point isn't being forced to take a higher load because of an uneven bed.

The more torque a motor can supply, the more quickly the load can be accelerated.

No its not a star-delta motor.Its a 2P/8P ( dual winding) 10 Kw motor installed in the hoist unit of an overhead crane.The starting torque generated by a motor is independant of the load mounted on its shaft.I understand the concept of action & reaction but what is still unclear to me is that the rotor experiences a rotating force (which is the cation force) & in turn the rotor generates a reaction force on stator but how does the rotor transmit the reaction force back to stator.?

Kindly bear with me for my lack of knowledge regarding this, just trying hard to get to the root cause of the issue.
 
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There are a lot of web pages that describe induction motors, you really need to search and learn.

In its simplest terms the 3 phase electricity in the stator creates a rotating magnetic field, that then induces currents in the rotor that creates its own magnetic field. The stator rotating magnetic field is essentially trying to pull the rotor round, if you were at one end of a rope and trying to pull something you would feel the force/effort required.
 
but what is still unclear to me is that the rotor experiences a rotating force (which is the cation force) & in turn the rotor generates a reaction force on stator but how does the rotor transmit the reaction force back to stator.?

Kindly bear with me for my lack of knowledge regarding this, just trying hard to get to the root cause of the issue.

Stator force is same as rotor force. Root cause to your issue is that you had bypassed low speed. Pole change motors are designed so that they have to be started via low speed.
 

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