Disassembling a Permanent Magnet Motor + 5

[ServoWizard]

Hello,

I have a 2HP AC permanent magnet motor and I need to remove the armature so that I can reduce the shaft diameter. I was prepared for a battle due to experiences with smaller motors but I under estimated this task. What is the proper method for removing the armature so as to prevent damaging it and the motor fields?

I chose this motor for my project based on Reliance Electric's statment that electronic braking is not require when using this motor, just short the fields. I comprehend their method because I use the same approach for a quick means of evaluating brushless servo motors and my Lincoln pipe line welder also uses that approach for melting welding rod, which is exactly what I would expect to happen happen when the fields are shorted but it would be melting copper? Surely there must be a resistor used between the field leads. If so how many Ohms?

Thanks

 

[Skogsgurra]

The standard procedure is to make a soft steel collar that fits the rotor and then press the rotor out of the stator into the collar. The collar shall be close to the stator so that the rotor slides into it without "seeing" air.

The collar will present an alternate path for the magnetic flux so that the rotor doesn't self-demagnetize. Make the collar thick enough not to saturate from the flux. A 10 mm (say 1/2") wall will usually be OK. Do not remove the collar from the rotor! Reverse operation when inserting the rotor again.

Best bet is to talk to the motor producer. They usually have contracted local repair shops that can do the work for you.

Gunnar Englund

http://www.gke.org

 

[stardelta]

The above method described by Skogs is the correct and safest way to do it, but its also the most expensive. This is how the trade do it unless you happen to have a length of steel pipe of suitable size to hand or dont mind paying to have something fabricated. Firstly remove both endplates, then cover the windings and comm` with a couple of nice thick coats of glass tape to minimise any impact damage. Place the field frame in a vice (attatched to a good heavy bench!)with the shaft pointing upwards. If your lucky it will have a tapped hole in the end and you can get an eye-bolt in it, if not you will have to fabricate a clamp of some sort. Place the hook of your overhead crane into the eye-bolt or clamp and press the up button.......easy and cheap! As the armature clears the frame the shaft will be attracted to the magnets and pull over to one side. It could damage the winding so be ready for it. Make sure you put somthing metalic into the field frame to stop the permanent magnets going flat as soon as the armature is out. Offcuts of steel bar or large bolts are ideal.

 

[Skogsgurra]

stardelta,

That may work wit a DC servo motor. But it will not be a good method if it is an AC PM motor. So it depends much on what motor type you have. I think that ServoWizard has an AC motor.

Gunnar Englund

http://www.gke.org

 

[stardelta]

Skogs,
The method works equally well for both types of motor I can assure you, furthermore I am sure we are not the only shop that does it in the way I described. We have in the past even gripped output shafts in vices and lifted the fields verticaly off, but it is risky and must be set up properly. Repair shops as I am sure you can imagine need to be competitive and keep costs to a minimum and in my view the cost in making a one-off tube is outweighed by the risk of dismantling in this manner. I am sure the makers and service agents will as you say use tubes but they will be dealing with large volumes of similar size machines and not one-offs.
ServoWizzard?
You are talking about reducing the diameter of the output shaft, are you aware of the fact that this will reduce the amount of power it can transmit and if you have a high torque application could even snap it? It would be a better option assuming its possible to increase the size of the pulley/coupling bore.

 

[ServoWizard]

Stardelta,

Your statement regarding the reduction of the shaft size is true however my application does not lend itself to an alternate solution. The motor shaft fits into a gear box and the shaft receiver is harder than my head.

Servo

 

[Marktal]

I am relativly new to this forum, and have read many 'old' posts up until i came across this particular one. I understand that most users of this forum are professional, both educational and practical. And i have noticed that some suggested solutions are calculated, expensive, drawn-out and unless you have a large $$ fund, unrealistic. However, all responses i have read are insightful, and problem-solving. Reading 'Star-Delta', response to this post, is quite refreshing. A simple, practical, arguably preffered and straight forward solution to a question. I suppose a mechanical background, where an actual 'hands-on' & cost effective nature is more common, seems to me, to be a realist insight. Good work Star-Delta.

 

[ServoWizard]

Stardelta,

Does the metal slug need to be the same diameter as the armature and how much time does one have to insert the slug? I'm working on a motor that cost $1,800.00 so I'm slowly proceding with caution.

Servo

 

[stardelta]

Servo,
I would initialy say take it to a local shop and let them do the whole job to your stated requirements then you have some come back in case of problems but before anyone can make any further comments we need some clarification regarding what it is you have exactly as there has been some confusion. Is it a DC motor with a permanent field or an AC motor with a magnetic rotor?

 

[Clyde38]

Hi Servowizard,
I have an alternate solution. If you have access to a machining lathe (or milling machine) you can mount the motor to the lathe and grab the shaft with the collet. Then remove the endplates. You can pull the rotor out of the stator (be sure that the collet is secure to the shaft). If indeed this is a modern PM motor from Reliance, the magnets will not demagnetize. You should cover the rotor magnets with tape for the machining operation. Reverse the procedure to put the motor back together.

 

[Barry1961]

Just a few thoughts....

Could you clamp the motor to a lathe bed then turn down the shaft while motor is running? I know some of the onsite machine repair companies do stuff like this.

Can you buy a new input quill for the reducer?

If you are turning down the shaft very much there is probably a sizing problem.

Barry1961

 

[stardelta]

Clyde?
Thats an interesting proposition, I could see it working assuming that the motor could be accomodated and the chuck and tailstock has enough travel to allow the rotor/armature to clear the field frame. I would be wary though of holding the shaft in this manner though as the weight may bend it slightly and just a couple of thou` run-out can lead to problems with vibration and shorten bearing life.

Barry?
I think you may thinking of in-situ machining specialists who can skim commutators and slip rings of certain machines without dismantling by driving them with another suitable motor. I could imagine trying to turn the shaft down of a powered up motor would be a next to impossible thing to do with any accuracy and far more difficult than dismantling it and setting up between centers in a lathe where alignment, cooling, tool-holding, speed and feed are not problems. If you wanted to loose just a few thou` over a short distance you could polish it off with emery cloth but I wouldnt consider trying to remove large amounts of metal in any other way than by using a properly set up lathe operation.