September AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 1 8 4 7 There is nouniversal guideline for so-called “safe stand- off distances” for employees with medical devices or people without them, because such distances are application-spe- cific and greatly depend on coil design details, applied pow- er and frequency, presence of other electromagnetic devices in coil surroundings, and other factors. The magnitude of an external magnetic field can be either computer mod- eled or measured. For example, at Inductoheat we use both techniques. In some cases, the induction coil is not the main source of EMF exposure and other electromagnetic sources (e.g., transformers, bus bar networking, electromagnetic lifting devices) might contribute to an even greater degree to the magnitude of EMF at the workplace. A number of devices are available to measure EMF exposure in the workplace, ranging in applied frequencies and designs. Based on our experience, three-axis measuring devices provide acceptable accuracy if properly used and calibrated. For example, for a number of years Inductoheat has been using the Holaday 3-axis VLF Magnetic Field Meter HI-3637 (Fig. 2). This device is capable of providing isotropic measurements of a magnetic field’s magnitude. The instru- ment is designed to conform to the requirements issued in the IEEE protocols and guidelines for suitable frequencies Following are some general guidelines for monitoring and supervision of EMF exposure: • Running conditions of the induction system: The ma- chine should be running with the maximum running parameters for output KW or the maximum allowable running current/voltage to the coil. • Measurement specifics: The induction machine enve- lope should be drawn and measurements should be taken at the operator station (or where other people may be present) and at a number of points around the perimeter of the machine at distances from the machine of 0.25, 0.5, 1, 1.5 meters, and more (what- ever is most appropriate) and different heights from the floor. Data should be reviewed and a report issued to indicate whether the measured values comply with safety requirements. All readings should be taken and properly documented including setup parameters for capacitor and output transformer settings. If neces- sary, measurements should be made to calculate the coil running current for the specified meter readings. Over the years, manufacturers of induction machin- ery have developed a number of ways to minimize external magnetic field exposure in cases where it exceeds maxi- mum permissible levels. This includes a number of patent- ed designs and proprietary techniques including passive and active magnetic shields, magnetic shunts, and Faraday rings, to name a few. For a few years, we scan hardened relatively straight shafts using a 30 kHz machine. Our new component has a 4.8-mm radial step (sharp shoulder) that overheated when we tried to scan harden. We heard that a single-shot inductor could reduce shoulder overheating and still provide sufficient hardening in the internal corners of the diameter transition, so we asked a local coil builder to make one for us. Corner over- heating is reduced, but now we have a 4-6 HRC surface hard- ness variation around the circumference of the as-quenched component. We didn’t have this with scan hardening. How can we fix it? Answer: An appropriate inductor/quench design and suitable process recipe should not produce such a wide hardness variation along the shaft circumference while us- ing a single-shot inductor. A number of possible causes ex- ist, with two of the most common discussed here: Heat-related factor. With scan coils, induced eddy cur- rent flow is circumferential, producing uniform temperature distribution at any instant of the heat time. Only minor heat variationoccurs in a flux fringing regionof coil terminals. Part rotation makes this impact negligible, normally providing veryuniformtemperaturedistribution (assuminganabsence of slots, keyways, holes, and similar geometrical discontinu- ities). Unlike scanning inductors, the majority of single-shot inductors produce predominately an axial eddy current flow rather than circumferential. Therefore, if the heating time is relatively short (e.g., 2-3 sec), the coil copper heating face is narrow, and rotation speed is insufficient, there might be a noticeable, nonuniformcircumferential temperature pattern at the time quenching is applied (Fig. 3). Increasing rotation speed during single-shot heating might solve this problem. The use of a serpentine-type inductor is another option. Quench-related factor. MIQ coils are often used with scan hardening, normally providing sufficiently uniform circumferential quenching. Quench rings/barrels/followers used in scan hardening also produce reasonably uniform quenching. With a single-shot inductor, there might be ob- Fig. 2 — Holaday 3-axis VLF Magnetic Field Meter HI-3637. 15