Electric Motor Rewinding
Electric Motor Rewinding is of critical importance to our customers and we are proud to have an individual that is skilled in the trade, and proud of his work. Electric motors get overworked, sometimes, and simply burn out. That's where the rewind department comes in. In today's world we all know about the "throw away" mentality that exists. And I guess I can understand a lot of it, but that may NOT necessarily be the case with electric motors. Sometimes the motor is "irreplaceable". That doesn't mean a different motor won't do the job, it means that at the "exact" point in time, and "under the exact circumstances", the motor that has just burned out is "irreplaceable". It might be available from the factory but that might take, days, weeks, or even months. It might be a "special" frame size or shape. It may be an "obsolete" motor that can't be purchased for ANY PRICE! And maybe, just maybe, it is absolutely "critical" that the machine the motor runs, be back in service in as short a time as possible. That's when the motor gets rewound. Look at the photo above that Carl's winding. It's small in size and horsepower, but we're rewinding it! Probably for one of the reasons mentioned previously. AND... it will be finished and ready to be back in service in a matter of HOURS, not days, weeks or months.
But don't be fooled either, the "throw away" mentality exists in the industry and it's not going to go away. There are a number of places within this website, in different topics, where I've used the term... "Time is Money", and you know that's a true statement! And rewinding takes time. So it isn't surprising that motors of higher horsepower are being "replaced" rather than "rewound". Just because of the shear economics involved, it may be possible, due to material and labor costs, to "replace" a 50 HP motor (an off the shelf standard motor), rather than rewinding it. But initial cost of purchase may NOT be the only thing that you should consider. Let's get into that a little further on in this topic.
Electric motors burn out in many cases due to "overload", and overloads cause the motor to "overheat". They overheat, past the upper limit of the insulation within the motor, and the motor fails. Insulation is rated by temperature "class". Long ago... the original insulation was paper and friction tape made of cotton. and that insulation class was Class A with a temperature rating of 105°C. We then improved upon that by using some man-made fibers and the next class was Class B, with a temperature rating of 130°C. The next step was to Class F with a rating of 155°C and today, the most common insulating systems used by the more "progressive" Electric Motor Repair Facilities (like A.R.&E.), are using Class H insulating systems with a thermal rating of 180°C. And even some of us are mixing in some insulating papers and components that are rated at 200°C.
Now, here's the rub, most manufacturer's are building their motors (at the factory) with Class F insulating systems, and there are even some out there still using Class B. What does that mean for you? A new motor will have an insulating system rated for only 155°C BUT, if you get A.R.&E. to "rewind" the motor, your heat withstand rating will be "at least" 180°C, and maybe 200°C! That's a full 25 to 45 degrees higher temperature rating in Centigrade. If you're still like me and understand Fahrenheit better... that means your "REWOUND" motor can withstand internal temperatures a full 77°F to 113°F higher than a "brand new motor" from the factory.
Want another reason to "rewind" rather than purchase NEW? The majority of the motor manufacturer's are warrantying their motor for 12 months from the date of installation OR 18 months from the date of manufacture. If it sits on a distributor's shelf for more than 6 months, you may have an issue. At A.R.&E. we guarantee our rewound and rebuilt motor for a full 2 years! So when it's rewound and assembled with new bearings, you'll have 24 months of service before you have to worry about the warranty running out.
What's In Our Windings
- A motor that needs rewound is first disassembled and tested. We perform a "core loss test" to establish if the laminations and iron in the motor frame is still good. Meaning, it will pass "magnetic flux" and create a solid magnetic field without creating any "hot spots" within the laminations.
- The frame is then placed in a "charring oven" (at approximately 650°F)which removes the varnish and insulating materials from the defective winding. This allows us to remove the old wire easily.
- The old wire is removed and the frame and laminations are then "sand blasted" (we actually use "glass bead") to clean any old varnish and debris from the frame and "winding slots" so the new wire will not be damaged when inserted.
- The frame and laminations are then cleaned with air and painted.
- Slot insulation (Class H) is measured and cut to proper length. It is then inserted into the lamination slots to keep the wire from touching the steel laminations.
- Coils of NEW copper magnet wire are made to duplicate the original winding. This copper magnet wire is insulated with a "specialized" varnish that is temperature rated at 200°C. This insulation is formulated to withstand high voltage transient spikes associated with VFD drives.
- The coils are inserted into the lamination slots and a "top stick" is forced in the slot (on top) to hold the coils in place and compress them into a tight bundle within the slot.
- The coils are connected together in the proper fashion and lead wire is brazed to those connections being brought out to the junction box for use by the customer.
- The ends of the coils are "laced" down with high strength lacing cord to compress the coils into a tight bundle. This will aid in holding the varnish in the winding during the dipping and baking process.
- The finished winding is then tested to make certain it is connected properly and there are no grounded or shorted coils.
- The stator is then taken to the "insulating varnish dip tank" where it is submerged in Class H Epoxy insulating varnish.
- When it is totally saturated, it is removed from the tank and allowed to drain.
- The varnished frame is then placed in the "curing oven" and baked at 350°F
- Once the stator is cured and cooled down (usually the next morning), the mechanics reassemble and test the NEWLY REBUILT motor and prepare it for shipment.
We only touched on a "core loss test" of the stator laminations in the previous section, so let's talk a little more about the tests that each electrical winding is subjected to when it comes into our facility.
- Any motor that we receive for repair is given a "megger" test as the first line of diagnostics. This test checks the windings to "ground" (the frame of the motor).
- If the winding "FAILS" the megger test, then it needs to be rewound, and that process is started.
- A second test for the stator, if it passes the "megger" test, meaning we didn't find a "grounded" winding, is a "hi-pot" test. Hi-pot stands for "high potential" test. A megger test is performed at maybe 500 volts, and never more than 1000 volts. The "hi-pot" test is to really see how good the insulation is. This test is performed at "twice the rated voltage plus 1000 volts". So if we're dealing with a motor rated at 480 VAC, we would hi-pot test it at 1,960 volts. This test will also give us a "leakage" value of the current that "leaks" through the insulation to ground, and that value is rated in "milliamps" of DC current.
- Assuming the winding passes the megger and hi-pot tests, the final test is a "surge test". This test subjects the winding to pulses of electrical voltage and the data is displayed on a screen and looks like an electronic oscilloscope. The pulses are fed to EACH of the phases of a three-phase motor and the resultant display shows those "traces" on the screen. If the winding is GOOD, the 3 traces can be superimposed on each other and you will see what appears as a single trace. If there are 2 or more wires shorted together in the coils, the traces will NOT superimpose and you will see the multiple traces. The motor then needs to be rewound.
Apparatus Repair & Engineering, Inc. uses the best materials that we can obtain from our wire and insulation suppliers. If you read the topics above, you know that the materials have changed over the years, and that's what "progress" is all about. So it is not uncommon for us to "modify" our standard insulation system package by adding (or substituting) a newly available and better component to our list of products. But in this short paragraph I'd like to outline our "standard" rewind material for you.
- Wire - NEW copper magnet wire, insulated with modified polyester resin and over-coated with polyamide imide resin, to resist pulse spikes, thermal rated for 200ºC
- Slot Insulation - Laminated multi-layer insulation paper designed specifically for slot liner. Composition can be N-M-N (Nomex-Mylar-Nomex) in varying layer thicknesses; i.e. 3-3-3, 3-5-3, 5-5-5, 5-10-5. A different material but similar characteristics is D-M-D (Dacron-Mylar-Dacron). This material is available in varying layer makeup like the N-M-N. These papers give us varnish absorption on the "outside" layers, and tear resistance of the center layer of Mylar. A third choice of slot liner is called "TufQuin™" and it is manufactured by 3M™. Their marketing description says: "TufQuin™ TFT is an inorganic hybrid insulating paper laminate is a 3-ply composite of 3M TufQuin™ Hybrid Insulating Paper bonded to both sides of polyester film for high-temperature electrical insulation applications up to Class 200(NEMA Class N)". All these papers are excellent choices based on their superior mechanical and electrical protection.
- Top Sticks (Wedges) - Formed and shaped to slide into the top of the stator slot to secure the coils in place. These wedges are made of the same (or very similar) material as the Slot Insulation. They are purchased in different sizes to fit different slots, and also different "shapes", for the same reason.
- Phase Paper - In a three-phase motor, the winding is made up of "groups" of coils. Each group is associated with a different "phase" of electricity (A, B, & C). Because there is a different phase in each "group", we need to make certain that Group A is electrically insulated from Group B, and likewise, Group B from Group C. To do this, we insert a "phase insulation paper" between the ending coil in Group A and the beginning coil of Group B. And the same for Group B and Group C. This paper is usually more flexible than the multi-layer composites mentioned above but has similar or identical thermal properties.
- Insulating Varnish - After rewinding a motor the new wires need to be "secured" in place. Electrical current passing through wires will cause the wires to "vibrate". If the wires are allowed to vibrate they will rub against each other, wear off the magnet wire insulation, and cause a short circuit. So we "dip" the wound stator in an insulating varnish, then bake to dry, making the stator a complete and solid mass of wire and insulating varnish. Our varnish is thermal Class H, Epoxy Resin. We have found it to possess extremely good mechanical and electrical properties and we have not experienced any failures due to a failure of the Epoxy Insulating Varnish.
The rewind process is a very important step in the process of repairing an electric motor and repairing it to the highest standards of quality and reliability.
A.R.&E. uses nothing but the best materials in their rewind department and follows a rigid, but efficient process during every rewind. From the smallest to the largest. Give us a call if you'd like to discuss our processes in more detail, or stop by the shop for a tour of our rewind department in action.