Having the ability to braze all of the rotor bars on one end of a squirrel cage rotor for an electric motor at one time without ring distortion is a remarkable achievement made possible by using induction heating. The savings in production time are made even sweeter by the consistency and precision you’ll see in brazes. This particular application is a good example to demonstrate the speed, efficiency and accuracy of induction brazing with eldec power supplies. Our article explains how a typical setup for squirrel cage rotors works, indicates the types of induction power supplies commonly used and includes a video demonstration.
Induction brazing of short-circuit rings in electric motor production
Our standard system set up for brazing short-circuit rings includes a coaxial transformer with a ring inductor mounted on a table and connected to an eldec medium frequency generator (MFG) by a cable assembly several meters in length. The short-circuit ring is positioned on the ring inductor, which has roughly the same dimensions as the short-circuit ring. The rotor with the rotor bars is fitted vertically into the trough-shaped release of the short-circuit ring. After adding the alloy in pellet form, the brazing process is started by simply pushing a button. The power supply output is controlled pyrometrically in direct relation to the pre-set specified temperature. The specified temperature is reached at maximum output after which the temperature is maintained until the previously added alloy has melted and the brazing process is complete. The running time up to this point can be pre-set; however, it can also be monitored and terminated manually, allowing full control of the system and process.
Placing the inductor below the rotor ensures heat is more evenly distributed between the ID and the OD. By contrast, a torch focuses the heat on the OD of the ring. Heating with induction can also get the ring and bars up to brazing temperature faster, which means less heat travels up the bars into the rotor stack. And, since alloy is typically drawn to the hotter areas of the joint, heating from the bottom with induction draws the alloy more evenly along the entire bar/ring connection. In addition, while utilizing an induction power supply with a pyrometer, it is possible to set the optimum brazing temperature and hold the temperature while the alloy melts and flows. Using a torch, on the other hand, is a manual process that must be performed by a very skilled operator.
Brazing short circuit rings
For small to medium electric motors we recommend using eldec’s ECO LINE medium-frequency generator (MFG) up to 150 kW, a pyrometer for temperature control and a custom manufactured, field-controlled ring inductor on a brazing bench that can be adjusted for various motor sizes. Virtually maintenance free, ECO LINE medium frequency generators operate quietly, take up little space and can easily be integrated into a cell controller thanks to a standard highspeed PLC and multiple control options. In addition, power supplies are available with single output or multiple output transformers. These connection options allow energy to be delivered sequentially (one side at a time), simultaneously (power provided in parallel and individually controlled) or symmetrically (power provided in parallel and working together, not individually controllable).
Brazing large squirrel cage rotors
When manufacturing large electric motors, such as those for wind turbines and large industrial motors, our standard system setup includes a coaxial transformer with a ring inductor mounted on a table and connected to an eldec medium frequency generator (MFG) by a cable assembly several meters in length. The size of the system needed depends on the size of the squirrel cage rotor and the speed necessary for production. Systems typically range in size from eldec’s MFG 50 to MFG 250; nevertheless, these powerful induction power supplies are available in the ECO LINE with power ratings ranging from 5 to 150 kW and frequencies from 8 to 40 kHz. eldec’s CUSTOM LINE medium frequency generators are built with power ratings up to 500 kW and larger for large industrial electric motors. The largest eldec squirrel cage rotor system in the US is 400 kW and brazes rings up to 40″ OD, 3″ thick.
All medium frequency ECO LINE generators may be built standard with a Power Boost feature that allows you to over-dive the 100% continuous duty rating by an additional 50% for up to three minutes before needing to drop back down to the 100% output power setting. Strengths include igbt transistor technology, precise energy dosing and an efficiency rating of over 90%.
A typical complete system includes:
- Induction power supply
- Coaxial output transformer (5-15m)
- Refrigerant chiller for water cooling
- Ring inductor for each size rotor to be brazed with the system
- Optical pyrometer for precise temperature regulation
- Brazing table for supporting rotor and inductor
eldec’s medium frequency generators (both ECO and CUSTOM LINES) have a wide variety of configuration options that can be matched to specific applications for electric motors and generators, including (but not limited to):
- Short circuit ring brazing on squirrel cage rotors and strand brazing on form wound stators for automotive and locomotive drives, and large pumps, industrial motors and wind generators
- Shrink fitting of stators into motor housings
- Shrink fitting of motor shafts and rotors
Watch the video
Our video below shows the system setup and brazing process for a large squirrel cage rotor for a wind turbine. Please note that the video in this example was sped up for demonstration purposes. Actual speeds depend on the size of your parts, the size and frequency of the power supply as well as the design of the induction heating coils.
Customized power supply design and manufacturing
At eldec, every new induction brazing system starts with a design review by our experienced engineers. Induction coils are then custom designed with optimal coupling distances and geometry to fit your specific part. After assembly, every power supply is fully inspected and tested to ensure quality workmanship and process capability prior to shipment. Once an induction coil is built, our process development staff can help you optimize your power settings to meet your specific project requirements. To learn more, give us a call or request a consultation and we’ll call you.