Magnetizers, Magnetizing Fixtures, and Test Equipment

This section is intended to provide practical guidelines for analyzing and selecting equipment for magnetizing, demagnetizing, and measurement systems. An overview of the information required to analyze a typical application and some of the reasoning for each item is provided. A checklist of required information and descriptions of various magnetizing and calibration systems follows. In many cases, several different magnetizing systems could be recommended for any given application. The most important aspect of the analysis is developing a complete picture of the requirements. This can happen only with an ongoing dialog between the product design and manufacturing engineers and the equipment supplier.

In the early product development stage, magnet suppliers can assist project engineers in the selection of the appropriate magnet material for any given project. With the extreme range of magnet materials available today (see Tbl. 20), a close working relationship with magnet producers and equipment suppliers is increasingly important.

Magnetizer equipment manufacturers can assist you in selecting the right equipment after the magnet material has been selected.

Checklist

TBL. 20 Magnetizing Force for Some Common Permanent-Magnet Materials

The following items should be discussed by the project engineers and the product suppliers:

_ Magnet material

_ Grade

_ Characteristics

_ Operating point

_ Orientation

_ Physical dimensions

_ Configuration or shape

_ Polar configuration

_ Magnetizing and calibration requirements

_ Percentage of demagnetization and required accuracy

_ Parts handling per cycle rate and system control

_ Operating environment

_ Measuring and testing

_ Options

Magnet Material

What does one need to know about the magnet material? The magnet grade and energy product, MGOe; the required magnetizing force for saturation Hc and Hci; the Br ; whether the material is non-oriented or oriented; and the operating point of the magnet or assembly. This information is used to determine the field strength required for magnetizing and demagnetizing, as well as the preferred direction of the field and calibration requirements.

ILL. 110 Magnet hysteresis loop.

ILL. 111 Magnet shape, dimensions and magnetizing direction.

Physical Dimensions and Shape

What physical information is required? In short, as much as is available: shape, diameter, axial length, and a drawing of the magnet with tolerance information. This information helps to narrow down the size of the magnetizing fixture and the amount of energy required.

Polar Configuration

The polar configuration information relates to how the magnetizing field will be presented to the magnet or magnet assembly. Some of the items to be discussed are the number of poles, spacing between poles (if crucial), physical location or alignment of the poles on the magnet, requirement for skewed-angle poles, and requirements for alignment with respect to other items in magnet assembly. This information helps to determine if the equipment required is a simple solenoid, solid iron and copper, or a laminated-steel multipole magnetizing fixture.

Magnetizing State

The magnetizing state is the step in the manufacturing cycle when the magnet will be magnetized. Typical information should include the individual magnet or magnet structure, the materials in the structure, how many magnets are in the structure, structure dimensions, and a drawing of the structure. This information also helps to determine the overall size of the magnetizing fixture, and whether there is any requirement for a long pulse or additional field above the material's recommended field.

Percentage of Demagnetization and Required Accuracy

This item relates to the operating point of the magnet or magnetic device and the tolerance or accuracy of the set-point requirement. Information to include is as follows:

_ The percentage of demagnetization (i.e., 5 or 20 percent from saturation).The percentage of demagnetization is directly related to the amount of energy required.

_ The accuracy requirement and acceptance band for the final set point.

The accuracy and tolerance window gives an idea of the resolution required by successive demagnetizing pulses and the amount of time it may take to reach this level. This will impact the processing cycle time.

ILL. 112 Polar configuration.

Parts Handling, Cycle Rate, and System Control

The decision here is how the magnet or assembly will be handled in the manufacturing (magnetizing) process.

_ One-at-a-time or bulk magnetization

_ Manual loading by an operator

_ Automated system

_ Cycle rate

_ PLC or system computer control

ILL. 113 Magnet magnetized in an assembly.

The number of units magnetized in one cycle has a direct impact on the size and complexity of the magnetizing fixture and the energy-level requirement of the magnetizer.

The cycle rate can have an impact on the decision to offer a dc electromagnet, capacitive discharge, or half-cycle magnetizer. It may also impact the system cooling requirements and fixture design. The cycle rate is one of the key factors in deciding between a half-cycle or capacitive-discharge system.

If PLC or computer control is desired, one needs to understand what external controls may be required to interface with automation equipment or system controllers and software.

Operating Environment

The operating environment considerations are as follows:

_ Whether the equipment will be used in a laboratory, clean room, or production environment

_ Any unusual temperature or humidity considerations

_ Available input power A laboratory magnetizer normally requires much slower cycle rates, potentially less energy, and less automation. In many cases, a laboratory magnetizer is not easily transferred into production.

Clean room requirements can be very restrictive and need to be identified early in the design cycle.

The selection of system enclosures to accommodate temperature and humidity extremes may be a consideration.

Measuring and Testing Options

There are many different ways to assure the quality of the magnetizing process, magnet material, or magnet assembly. The following options provide various levels of control over the manufacturing and magnetizing process.

_ A current monitor with a comparator circuit to measure and compare the magnetizing current at acceptable levels

_ Gauss-meters with comparator circuits to measure either the magnetizing pulse or the residual flux density in the magnet

_ Fluxmeters with comparator circuits in conjunction with search coils, embedded in magnetizing fixtures, to measure the total flux density of the magnet or assembly

_ Temperature monitors to measure temperature rise in magnetizing fixtures and to halt the magnetizing process if the temperature rise goes above a safe level

_ A PLC system controller or computer in conjunction with the preceding equipment to control the operation and store statistical information Individual manufacturing and quality assurance philosophies typically drive the decision to add these various testing options.

Magnetizer Types

Magnetizers may be generalized into four categories: the permanent-magnet magnetizer, the dc magnetizer or electromagnet, the half-cycle impulse magnetizer, and the stored-energy or capacitive-discharge magnetizer.

Permanent-Magnet Magnetizer. The older-style permanent-magnet magnetizers consisted of a large U-shaped permanent magnet (usually alnico V) having adjustable pole pieces. The maximum air gap length is usually about 1.25 in. With the gap adjusted for a length of 1 in, a field of about 3000 Oe is obtainable.

The advent of high-energy permanent-magnet material has enhanced the development of permanent-magnet magnetizers.

DC Magnetizer (Electromagnet). The dc magnetizer utilizes one or two coils of wire wound on an iron frame having a C-shaped configuration. Adjustable pole pieces allow air gaps of various lengths to be used. Direct current is applied to the coils, and the resultant electromagnetic force produces a magnetic field in the air gap.

DC magnetizers are limited to charging straight or slightly curved magnets. Many C-shaped and U-shaped configurations cannot be fully saturated on this type of magnetizer; since the main magnetizing path is straight, the leakage field around the pole pieces is curved.

The duty cycle of the dc magnetizer is usually rather short, due to the large amount of heat generated by the current passing through the coils.

The dc magnetizer is useful primarily where fields having a straight-line characteristic are required. The relatively long time base of the magnetizing field (2 s or longer) is important where magnet assemblies having high eddy current losses are to be charged. An example of this would be large magnets contained in a cast aluminum housing.

ILL. 114 Basic elements of a half-cycle magnetizer.

ILL. 115 Basic elements of capacitive-discharge magnetizers.

Half-Cycle Magnetizer. The advantage of a half-cycle magnetizer, is its ability to generate pulses at a very fast rate. The half-cycle magnetizer operates by picking off one-half cycle from the power line. It will pass a part of a half-cycle with 1, 2, or 3 cycles from a 110- to 600-Vac 50- or 60-Hz line through a magnetizing fixture or pulse transformer.

The half-cycle magnetizer is generally a fixed installation because of the necessity of connecting to a heavy power line.

The half-cycle magnetizer utilizes wound fixtures or current transformers with single-turn fixtures in conjunction with timing- and discharge-control circuitry.

Duration of the applied pulse is dependent on the line frequency and adjustment of the control circuits. Pulse duration up to 8 ms may be obtained from a 60-Hz line.

The magnetizing force which may be obtained is, of course, dependent on the amount of current that can be pulled off the power line. Rapid operation can be obtained with a half-cycle magnet magnetizer, since there is no capacitor bank requiring several seconds to recharge; however, water cooling of the ignitrons or silicon controlled rectifiers (SCRs) and magnetizing fixtures is generally required where rapid operation is desired.

ILL. 116 Magnetizing two U-shaped magnets simultaneously.

Capacitive-Discharge Impulse Magnetizer. This magnetizer operates on a stored energy principle. Voltage is stored in a capacitor bank, usually requiring several seconds, and the stored energy is then discharged through a unidirectional switch (ignitron or SCR) into a charging fixture or charging transformer. The duration and waveform of the pulse is dependent on the capacity of the bank, the inductance of the fixture or transformer, and, to some extent, the resistance of the fixture. Pulse lengths of 100 µs to several tens of milliseconds may be obtained.

The basic elements of a capacitive-discharge magnetizer are shown in Ill. 115.

Depending on the pulse duration, which can be varied with capacitive-discharge magnetizers, charging fields in excess of 100,000 Oe can be developed.

One of the outstanding features of the impulse magnetizer is its ability to saturate curved and multipole magnets, as well as the straight bar and disk types. The magnetizing field around the secondary charging conductor is circular in shape and there fore closely follows the configuration of C-shaped and U-shaped magnets.

Selection of Magnetizer. Analyzing the features of the four major types of magnetizing systems leads to the following conclusions. Where economy is a consideration, long magnetizing pulses are acceptable, and only two pole structures (generally) are to be magnetized, and where the required magnetizing forces don’t exceed 10,000 Oe, the electromagnet will prove satisfactory. Self-heating of the electromagnet can become a limiting factor when the time that power is applied to the coils is prolonged. Water cooling of the coils can help overcome this drawback. Certain types of multipole magnets can be magnetized using special pole pieces; however, the con figuration of the magnetizing field generally precludes placing more than two poles on a structure.

The half-cycle magnetizer is perhaps the most rapid means of magnetizing large production volumes of parts of similar magnetic structure. The half-cycle magnetizer can produce many pulses per minute, providing increased throughput over impulse magnetizers. However, self-heating of the fixture can necessitate water cooling to prevent fixture damage. The relatively long pulse length of the half-cycle magnetizer is useful where large volumes of highly permeable materials are to be magnetized.

The impulse magnetizer has most of the attributes of the electromagnet and the half-cycle system with some minor limitations. Pulse length and peak current can be varied by means of changing storage bank capacity and changing storage voltage levels. Speed of operation is not as rapid as the half-cycle method, since 1 to 20 s may be required to charge the capacitor bank. Cost can vary depending on the energy storage level.

Extremely high currents, and consequently high fields, may be obtained with capacitive-discharge magnetizers. The power-line requirement for this type of unit is relatively low, ranging from 1 A for the low-power models to 20 to 30 A for the largest magnetizers required. Line demand is minimal in most cases, with little or no line effects. And almost any type of magnet and pole configuration can be accommodated.

Ill. 117 shows a typical pulse from a half-cycle or capacitive-discharge impulse magnetizer. One of the most important parameters, as far as waveform is concerned, is the rise time to the peak magnetizing current. Current rise time deter mines the eddy current losses and resultant heat developed in the magnetizing fix ture or in the structure being magnetized.

ILL. 117 Magnetizing pulse characteristics: A = peak magnetizing current, B = rise time to peak, and C = pulse duration.

ILL. 118 Solenoid-type wire-wound fixture showing relationship of flux Hto current I.

Fixture Design for Impulse and Half-Cycle Magnetizers

The simplest type of magnetizing fixture used is the wire-wound solenoid. The number of turns, wire size, and length of the solenoid are all determined by the size of the magnet which is to be magnetized and the voltage and capacitance level of the capacitor bank. The waveform of the current pulse applied through the coil should closely approximate half of a sine wave for maximum efficiency Ill. 118 shows a solenoid-type magnetizing fixture.

Where high energy levels are applied to a coil, especially where short pulse lengths at high peak currents are involved, proper physical bonding of the coil turns is extremely important. The stress forces applied to the coil can be large in some cases, and insufficient bonding can cause the coil to be physically damaged.

A solenoid may be termed an air-wound solenoid or it may be surrounded with a ferrous return path to enable higher magnetizing forces to be achieved by the low reluctance return path. The ferrous return path may provide additional physical strength.

ILL. 119 Four-pole laminated-steel wire-wound fixture.

The impulse or half-cycle magnetizer's arc is widely used in the magnetization of straight bar magnets. The large amount of energy which can be produced by this type of magnetizer enables the saturation of unusually large volumes of magnetic materials. Magnetization of straight magnets is usually accomplished in a cavity-type fix ture employing a coil wound like a solenoid and a soft-iron return path. This structure has high efficiency, since virtually all of the magnetizing force produced passes through the cavity along the axis of the coil.

Magnets can be magnetized as individual units, in groups, or as completed magnetic assemblies. Saturation of a completed structure is generally preferred, since the danger of contamination by ferrous particles during handling is reduced, and demagnetization by open circuiting is prevented. Open-circuiting a magnet refers to the removal of the keeper or working-gap assembly from the pole ends. This, in effect, applies a demagnetizing force to the magnet and in some cases (determined by the magnet configuration) can cause a loss of as much as 6 percent of residual induction.

The impulse and half-cycle magnetizers also lend themselves to the utilization of multipole wound magnetizing fixtures, diagrammed in Ill. 119.The multipole fix ture is used for the magnetization of various sensors, generator rotors, alternator rotors, synchronous motor rotors, multipole stator assemblies, and a wide variety of multipole configurations of a cylindrical, flat, or curved nature.

Where a magnet surface is continuous, not segmented, or has salient poles, the pole location and the polar embrace are determined by the pole-tip configuration of the charging fixture.

The polar embrace of a structure which incorporates pole pieces is determined by the pole pieces themselves. It’s usually advantageous, although not always necessary, to make the multipole charging fixture from layers of ferrous material rather than carve it from a solid block of iron. Stock used may be transformer iron or cold-rolled steel sheets, ranging in thickness form 0.014 to 0.060 in. This minimizes eddy current losses and results in more efficient use of the magnetizing energy. The laminated sheets should be insulated from each other by suitable varnish or other material. Some shorting between laminations may result when the structure is slotted and the cavity is turned, but this will cause only minor losses.

ILL. 120 Copper and iron four-pole fixture.

ILL. 121 Two-pole wire-wound magnetizing fixture.

ILL. 122 Two-pole hairpin magnetizing fixture.

Multipole magnetizing fixtures can also be made as the so-called single turn, cop per and iron fixtures. Instead of many turns of wire on each salient pole ( for example, 20 turns of number 16 wire per pole), a heavy conductor (say, having a cross section of 0.5 in) is convoluted around each pole. This generally entails the necessity of milling and hard-soldering the copper conductor sections. A further accessory is also required, the low-impedance single-turn fixture. Magnetizing several poles simultaneously can also be achieved using this type of fixture in conjunction with pulse transformers. A typical four-pole fixture is shown in Ill. 120.

A multipole cylindrical magnet can be charged by either method; however, the method used will have an impact on the flux distribution characteristics of the mag net or assembly. It’s important to know how this may impact the end product performance. During the early product development stage, testing of different magnetizing techniques will allow the design engineer to determine whether better performance can be achieved with one technique over another. As an example, a two-pole motor can be magnetized from the outside in using a wire-wound laminated-steel fixture, as shown in Ill. 121. In this case, a completed motor could be inserted between the two poles and magnetized from the outside in-that is, all magnetizing windings are external to the motor housing.

Another alternative is to use a hairpin copper and iron single-turn fixture. In this case, the motor shell, consisting of the steel housing and magnet segments, is inserted over the copper conductors and the internal steel pole piece. The current-carrying conductors are actually placed between the magnet segments-hence, the term "magnetizing from the inside out." The hairpin fixture is designed to fit exactly inside the motor assembly, with the outside dimensions of the fixture matched to the inside dimensions of the motor.

When the energy stored in the magnetizer is released, flux is developed in the hair pin fixture that will magnetize the motor assembly.

Magnetizing from the inside can also be performed with wire-wound hairpin fixtures in which multiple turns of smaller wire are used in place of the single-turn cop per construction.

The hairpin fixture can be designed with a shim in its center to accept a gauss-meter probe. This permits flux density measurements to be made immediately after magnetizing, without removing the motor assembly from the fixture.

Conclusion

As one can see, there are many different ways to magnetize any given assembly.

With the number of choices available, it’s imperative that a clear picture of the overall system requirements be developed and communicated to the magnet and equipment suppliers. An early involvement can save time and money in moving the project into production and in ensuring that the system meets the production goals.