jucesanc
Member
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- California, Bay Area
I have a 3000 HP, 4 kV, 1800 RPM, and frame 8010 induction motor. If a magnetic center indicator is not an option, how would you determine the magnetic center?
How do you find the magnetic center of an induction motor?
- Thread starter jucesanc
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ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
Does that correspond to the top dead center position of a gasoline engine?
winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
The magnetic forces that spin a motor also tend to pull it in undesired directions, eg. radial forces that tend to pull the rotor into the stator and axial forces that work to align the rotor in the center of the stator.
The bearings always have to handle the radial forces, though these forces should be pretty well balanced.
In a small motor we don't care much about these axial forces, and just let the bearings handle them.
I don't have experience with large motors to say how the axial forces get dealt with.
From a quick read it seems that the basic test is to spin the motor uncoupled from the load and determine where the shaft lines up axially. Then adjust the components connected to the motor so that they aren't pushing the rotor away from this 'magnetic center'.
The 'center indicator' mentioned is some sort of set of marks (a pointer and a groove) that indicates when the rotor is in a previously tested magnetic center position.
-Jon
The bearings always have to handle the radial forces, though these forces should be pretty well balanced.
In a small motor we don't care much about these axial forces, and just let the bearings handle them.
I don't have experience with large motors to say how the axial forces get dealt with.
From a quick read it seems that the basic test is to spin the motor uncoupled from the load and determine where the shaft lines up axially. Then adjust the components connected to the motor so that they aren't pushing the rotor away from this 'magnetic center'.
The 'center indicator' mentioned is some sort of set of marks (a pointer and a groove) that indicates when the rotor is in a previously tested magnetic center position.
-Jon
jucesanc
Member
- Location
- California, Bay Area
No. The application is for an induced draft fan. I'm looking for a description of how to determine the magnetic center of the shaft.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
How you "find" it is to couple the motor to the load and rotate it, then let it come to a rest, it will always rest in the mechanical center because of slight tapers in the bearing designs to allow for lubrication etc. Mark the shaft position. Then uncouple it, removing the coupling completely from the motor (to give it some space to move) and re-start the motor, marking where the center moved to while it is running (taking proper safety precautions). That is your MAGNETIC center.
For those who don't regularly deal with large AC induction motors, the magnetic fields created by the stator windings don't always match up to the mechanical center of the motor, meaning front-to-back center (not the rotational center as implied in the TDC question). That mismatch is then exerting an axial (parallel to the shaft) thrust on the rotor, a deflection forward or backward depending on where that magnetic center is. As noted, this is ONLY a concern with horizontally mounted sleeve bearing motors. Ball bearings have a race that they run it, so the motor is mechanically held in it's mechanical center. But with sleeve bearings, that axial thrust means you need what's called a "thrust bearing" in the motor or the machine it is connected to. Knowing how much defection exists between the mechanical and magnetic centers will determine what the equipment designer has to deal with in axial thrust.
For those who don't regularly deal with large AC induction motors, the magnetic fields created by the stator windings don't always match up to the mechanical center of the motor, meaning front-to-back center (not the rotational center as implied in the TDC question). That mismatch is then exerting an axial (parallel to the shaft) thrust on the rotor, a deflection forward or backward depending on where that magnetic center is. As noted, this is ONLY a concern with horizontally mounted sleeve bearing motors. Ball bearings have a race that they run it, so the motor is mechanically held in it's mechanical center. But with sleeve bearings, that axial thrust means you need what's called a "thrust bearing" in the motor or the machine it is connected to. Knowing how much defection exists between the mechanical and magnetic centers will determine what the equipment designer has to deal with in axial thrust.
- Location
- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Perfect example of this concept in this video... At the very end of the clip you can see that it stopped oscillating, that is the magnetic center.
I'm sure I did know this as a student eons ago. Our lecturer was an electrical machine designer and taught us all sort of things like short cording and stuff.
I still have the notes but I'm too indolent to look through them.
I've dealt with induction motors most of my working life and never needed to check this. So I'm curious to know why you want to............
jucesanc
Member
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- California, Bay Area
To be honest, I'm just curious myself. I'm spec'ing a new motor and I requested a magnetic center indicator because a similar existing motor had one. The vendor told me that instead of a magnetic center indicator, they would provide a nameplate above the shaft with text describing how to determine magnetic center. But I haven't gotten any replies from the vendor with a description for this.
jucesanc
Member
- Location
- California, Bay Area
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
No, but the information wouldn't really be useful. The PURPOSE is to determine the deflection of the axial thrust. For that, you need to know both pieces of information. In your case, since you are replacing an existing motor, that information is actually somewhat critical (again, assuming it is a sleeve bearing motor, and if the old one had the indicator, it was). As someone explained to me once on a 3,000HP blower; do you want 3,000 horses pulling your motor shaft away from the blower coupling, while the blower is using 3,000 horses pulling the other direction against the same coupling? Even though the deflection may only be a few hundredths of an inch, force is force. you are going to want EITHER for the new motor to have the center in the same place / direction as the old one, or to come with its own thrust bearing built in. If it has the thrust bearing built into the motor, you probably don't need the indicator or to know where the magnetic center is.
Tony S
Senior Member
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- Resting under the Major Oak UK
I’ve only had to find a motors magnetic centre a couple of time in 40 years. The motors were coupled to gearboxes where any axial thrust would ruin the input shaft bearings. Once the centre was found spacers were fitted to the coupling.
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