Capacitive-Discharge Magnetizing

Getting the Most Out of Your Magnets

Magnets are useless until they are magnetized. No clever or innovative electric motor design can overcome the fact that if the magnets are not magnetized, the motor won’t work. While enormous resources are often committed to electric motor design, optimization, and production efficiencies, it’s easy to overlook the magnetizing process. Improperly magnetized magnets can offset gains made in design and production engineering. Advancements in higher-energy-product mag nets also create new challenges to motor producers who chose to take advantage of these materials. This section will assist the electric motor producer in finding the optimal magnetizing process for motor production, improving the performance and reliability of the final product.

A magnet is magnetized when it’s exposed to a large magnetic field. Large magnetic fields require large electric currents, which require large amounts of energy. A magnetizing system converts energy from a power supply into magnetic fields used to magnetize the magnet. Magnetizing systems consist of two basic components: the magnetizer, which stores and releases energy in the form of an electric current, and the magnetizing coil, which converts the large electrical current into magnetic fields to magnetize the magnets. The magnetizer can be thought of as an energy storage and release system, while the magnetizing coil directs the energy from the magnetizer into useful work that magnetizes the magnets.

Let us first consider the magnetizer, which stores energy to produce currents.

There are several ways to store electrical energy:

_ Batteries

_ Utility power lines

_ Inductors

_ Rotating devices (generators)

_ Flywheels

_ Electrical capacitors

Given the large fields usually required for the magnetizing process, pulse cur rents rather than continuous currents are used to create magnetizing fields. Because of the brief but intense currents used in the magnetizing process, electrical capacitors have been the usual component used to store electrical energy in magnetizing systems. Ill. 123 shows a block diagram of a basic magnetizing system. The instantaneous energy in a pulsed magnetic field can be extremely high. Five million watts is not uncommon.

ILL. 123 Block diagram of magnetizing system.

Storing Electrical Energy in Capacitors

Since magnetizing systems usually use electrical capacitors to store energy, they are often referred to as capacitive-discharge magnetizing (CDM) systems. The energy stored in a capacitor or bank of capacitors is mathematically expressed by the formula where

C = capacitance, F

V = voltage across capacitor, V

E = energy stored in capacitor, J or W.

Commercially available capacitors usually don’t have sufficient energy storage capacity. Therefore, individual capacitors are assembled into capacitor banks using series and parallel combinations of capacitors. Ill. 124 shows a CDM system with Y capacitors connected in series and X capacitors connected in parallel. In a typical CDM system, a capacitor bank will produce from a few hundred to tens of thousands of joules. Tbl. 21 relates the energy of some typical CDM systems, their storage energy, their materials, and the approximate sizes of magnets they can magnetize.

The specifications for capacitor banks used as energy storage for CDM systems should include the following:

_ Energy storage capacity

_ Electrical voltage

_ Maximum instantaneous current

_ Estimated life (number of charges and discharges)

_ Internal resistance

ILL. 124 Connection of capacitors in magnetizing system.

TBL. 21 Typical Capacitor Specifications

Special precautions must be taken to ensure that the capacitors can withstand the repeated charging and discharging required in CDM systems. This is especially important in high-speed production applications. Also, the instantaneous current may exceed 10,000 A, which can cause internal heating in capacitors. Internal heat buildup can cause capacitor failure.

Two types of capacitors are used for energy storage in CDM systems, electrolytic and oil-filled capacitors.

Electrolytic Capacitors. Due to the materials used in the manufacture of electrolytic capacitors, the maximum voltage rating is about 500 V dc. Higher voltages can be reached by using them in series.

A major limitation of electrolytic capacitors is their inability to be charged in the reverse direction. This limits them to an exponential waveform that is discussed later in more detail.

The size of an electrolytic capacitor with a 3500 µF capacity at 300 V is approximately 6.7 cm diameter by 14 cm long. It weighs 1 kg and stores 157.5 J of energy.

Electrolytic capacitors are usually much cheaper than oil-filled capacitors.

Oil-Filled Capacitors. Because of the materials and manufacturing methods used, oil-filled capacitors are usually much larger in size than electrolytic capacitors and can be charged in either direction, allowing them to produce either an exponential waveform or a half-sine oscillating waveform. Oil-filled capacitors contain less capacitance for the same size of package. Typical values might be 2500 V at a capacitance value of 1600 µF, weighing 22 kg and storing 500 J of energy. The size of this capacitor is 13 × 34 × 33 cm. Tbl. 21 lists a number of factors that pertain to electrolytic and oil-filled capacitors used in CDM systems.

Most voltages used by CDM systems require extreme care, both in operator safe guards and fixture design. The higher operating voltages of oil-filled capacitors increase the precautions that must be taken. Also, the cost of components increases as the voltage increases.

Electrolytic capacitor systems weigh less and occupy less space than oil-filled capacitor systems. Energy storage per unit of weight or volume is usually not a critical factor in choosing an energy storage system. However, the lower energy density of oil-filled capacitors can contribute to a longer capacitor life and increased charge/discharge cycles for oil-filled capacitor energy storage systems. This advantage of lower energy density becomes more crucial when the application requires large peak currents and rapid cycle rates, which tend to shorten capacitor life.

In addition to longer capacitor life, oil-filled capacitors have the advantage of producing either an exponential, half-sine, or oscillating waveform. Since electrolytic capacitors can produce only the exponential waveform, most of the energy from the capacitor bank is dissipated into the magnetizing coil each cycle, while the half-sine waveform dissipates only a small portion of the capacitor-bank energy into the magnetizing coil. The half-sine energy that is not dissipated by the magnetizing coil recharges the capacitor bank in the opposite direction. With proper electronic circuitry, it can then be discharged through the magnetizing coil in the next cycle. The advantages are savings in input energy and reduced heating in the magnetizing coil.

Connecting the Magnetizer to the Magnetizing Coil

The switch to transfer energy from the energy storage system to the magnetizing coil can range from very elementary to very sophisticated. In one application, the switch was a short piece of magnet wire about 2 mm in diameter. The wire was clamped between two blocks. The energy storage system was a bank of lead-acid batteries, with a 2-mm length of copper coil in series with a magnetizing coil. In fractions of a second, as the current built up in the circuit, the magnet wire became hot to the point of violent disintegration. When the magnet wire disintegrated, it worked as a switch to shut off the electric current so that neither the magnetizing coil nor the energy source would be damaged. By choosing the size of magnet wire, the peak current could be controlled. With this technique, safety glasses are an absolute must because the disintegrating wire causes copper to be splattered about during each magnetizing cycle.

A far less spectacular but preferable switching method is to use a solid-state electronic switch, such as an SCR. SCRs are easy to control electronically and are very reliable. In some CDM systems, an ignitron vacuum tube is used. The ignitron tube has some disadvantages:

_ It has a limited life.

_ It uses mercury, which is a hazardous material.

_ It may require water cooling.

However, the ignitron will take more punishment in terms of exceeding its electrical ratings than an SCR. If its specified electrical ratings are exceeded, an SCR may be destroyed. If the ratings of an ignitron are exceeded, it may sustain only minor damage and continue to operate.

CDM systems can be designed to produce magnetic field pulses specific to the application. The factors to be considered are the following:

_ The magnitude of the magnetic field required

_ The duration of the magnetic field required

_ The size of the magnets or magnetic assembly

_ The cycle rate of the magnetic field pulse

_ Whether the magnet will be magnetized by itself or as part of an assembly

Magnitude of the Magnetic Field

The magnitude of the field required is dependent on the coercive field of the material. Virtually all magnet producers and suppliers provide this data. The coercive field is a measure of how difficult it’s to both magnetize and demagnetize the magnet.

The higher the coercive field of the magnet, the more difficult it’s to magnetize.

Tbl. 22 indicates the range of coercive field values for different types of magnets.

A rule of thumb is that the applied field should be 1 1/2 to 4 times the coercive force of the magnetic material. The coercive force of high-energy materials may reach 15,000 Oe or more; therefore, magnetic fields of more than 50,000 Oe may be required to reach magnetic saturation. The magnetic fields in a fixture can be measured using either a gaussmeter or a fluxmeter.

TBL. 22 Magnetizing Force Required for Saturation of Common Commercial Magnets

Duration of the Pulsed Magnetic Field

The time that the magnetic field must be applied to the magnet material is more complicated, and unlike the coercive field of a magnet, cannot simply be looked up in a data table or specification sheet.

Many magnetic materials are metallic, and metals conduct electric current. As the magnetic field changes, eddy currents are produced in the material, which produces an opposing magnetic field. The magnetic field in the metallic material (i.e., the mag net) is the sum of the magnetic field generated by the magnetizing coil minus the field generated by the electric current flowing in the magnet to oppose this field.

This effect is commonly known as Lenz's law.

We can understand this phenomenon mathematically. Lenz's law can be expressed as follows:

... where e is the voltage generated by Lenz's law to produce the eddy currents, df is the change of magnetic flux caused by the CDM current pulse, dt is the time interval

... of the magnetic flux change, and k is a simple proportionality constant. Note that the negative sign in front of k is a result of the eddy currents opposing the change in magnetic flux.

It can be shown that eddy currents are proportional to the rate of change of magnetic field with respect to time by

I_eddy = K

…where I_eddy is the magnitude of the eddy current and dHCDM is the change in the magnetic field produced by the magnetizer over a time interval dt. From Eq. (3.17), we see that if the rate of change of the magnetic field produced by the magnetizer is high, the size of the eddy currents will also be high. Since the eddy currents act to oppose the applied field, large eddy currents can cancel out a substantial fraction of the field produced by the magnetizing fixture. This problem can be overcome by simply reducing the rate at which the magnetic field increases. This requires a longer magnetizing pulse width. However, lengthening the duration of the magnetic field pulse causes heating in the magnetizing coil.

While eddy currents complicate the magnetization process, they can be dealt with effectively. Since eddy currents depend on the resistivity of the magnet, ferrite mag nets, which are electrical insulators, are highly immune to the effects of eddy currents during the magnetization process. Unfortunately, alnico, samarium cobalt (SmCo) and neodymium iron boron (NdFeB) magnets are all decent conductors, as indicated in Tbl. 22, and so eddy currents are produced in these materials during pulsed magnetization. Since alnico magnets have relatively low coercive fields, the effect of eddy currents can be overcome by simply using large enough applied fields to overcome the opposing fields that result from the eddy currents. Eddy currents are more difficult to overcome with SmCo and NdFeB magnets.

The difficulty with magnetizing SmCo and NdFeB magnets is that since these magnets have such large coercivities, it’s highly impractical to simply "overpower" the effect of the eddy currents. This creates more problems than it solves. To begin with, extremely large amounts of energy must be stored and released to overcome eddy currents in this manner. As we shall soon see, this requires a very large magnetizer, which is both expensive and takes up potentially valuable space on the factory floor. Huge currents flowing through the magnetizing fixture dissipate tremendous heat-heat that can damage the fixture, even possibly causing catastrophic failure. If the magnets are in a steel assembly, large forces due to the rapidly charging magnetic fields can cause flexing of the steel assembly, which can break the magnets inside. At the very least, additional time between magnetizing firings is required for the fixture to cool. This reduces production throughput. An effective method of magnetizing rare earth magnets balances the large currents required to produce a large field against longer pulse times with reduced eddy current loss effects.

A seldom-mentioned advantage of bonded rare earth magnets is that they are nonconductive; therefore, eddy currents are usually small in these magnets. The rubber or plastic binders are insulating and electrically isolate the conducting grains from one another. Bonded magnets are easier to magnetize than similar-sized bulk magnets for this reason.

When balancing the high peak currents required to magnetize rare earth magnets against long pulse durations to minimize eddy currents, the following formulas are helpful....

These equations describe the peak current ip in the magnetizing coil and the angular frequency ? of the current pulse. The duration tp of the current pulse is then...

The symbols R, L, and C refer to the electrical resistance of the magnetizing circuit, the inductance of the magnetizing coil, and the capacitance of the magnetizer.

The resistance is usually small compared to the inductance, and can therefore be ignored for most analysis. In this case, the peak current ip is simply expressed as....

The inductance is proportional to the number of turns squared on the magnetizing coil. The capacitance is the capacitance of the capacitor bank in the CDM system. In typical CDM systems, the time to the peak magnetizing field will vary from less than 1 ms to more than 20 ms. The capacitance, inductance, and resistance of the CDM system determine the magnetic field pulse duration.

TBL. 23 Energy Requirements of Typical Magnetizing Applications

Energy storage, J Size and material of magnets or assemblies 2,500 Large alnico; medium ferrites; small SmCo and NdFeB 15,000 Virtually all alnico assemblies; large ferrite; medium SmCo and NdFeB 30,000 Almost all ferrites; large SmCo and NdFeB

Size of the Magnets or Magnetic Assembly

Because it takes energy to create magnetic fields, a magnetic field over a large volume will require more energy than the same field over a smaller volume. Thus, the size of the magnet or magnetic assembly to be magnetized is important. One can describe the magnetizing process as a transfer of energy from the magnetizer to the magnets. The energy difference between an unmagnetized magnet and a magnetized magnet is proportional to the square of the magnetic induction times the volume of the magnet. The energy requirements for a given magnet material scale linearly with the volume of the magnet. Tbl. 23 provides the energy requirements of typical magnetizing applications.

Cycle Rate of the Magnetic Field Pulse

The cycle rate of the magnetic field pulse is an important consideration for the magnetizing system. For high-volume electric motor production, the magnetizing system must be designed to handle a high cycle rate. For prototype and laboratory use where the cycle rate is of little importance, a more versatile magnetizing sys tem than that used for production purposes is usually preferred. The rate of magnetizer charging is an important consideration in the choice of the capacitors used.

Any energy that is not dissipated in the magnetizing coil as heat or energy to magnetize the magnets must be restored in the capacitor bank. This fact has advantages and disadvantages. Since only a small part (as little as 10 percent) of the energy is dissipated in the magnetizing coil or used to magnetize the magnets, about 90 per cent of the energy will be returned to the capacitor bank. This greatly reduces the source energy needed to recharge the capacitor bank. However, the capacitor bank is now charged in the opposite direction, and the next magnetic pulse will be in the opposite direction unless the proper electronic circuitry is employed to reverse the current through the magnetizing coil.

Since electrolytic capacitors cannot be charged in the reverse direction, a CDM system that produces a half-sine waveform must use oil-filled capacitors. The additional cost of the electronics and oil-filled capacitors can double the capital cost of a CDM system. The operating energy savings are usually not sufficient to warrant the additional capital cost of using the half-sine waveform, unless technical reasons warrant its use. If magnetizing-coil heating is a serious problem, the use of oil-filled capacitors and the half-sine waveform may well be the best choice.

Where magnetizing-coil heating is not a problem, even when refrigerator cooling is required, electrolytic capacitors and the exponential waveform are usually the most cost-effective solution.

The cycle rate mostly affects the heat generated in the magnetizing coil. The cycle rate for production CDM systems varies from a few parts per hour to 1 or 2 s per part. For most applications, with cycle rates above 200 parts per hour, the magnetizing fixture will require cooling. However, the space available for the magnetizing coil and the magnitude of the magnetic field required can greatly alter this general rule. The most convenient cooling method is by pumping a refrigerated liquid through the magnetizing fixture.

Magnetizing Magnets Individually or as an Assembly

A magnet which is built into the final assembly, such as a dc motor, is usually harder to magnetize than the magnet by itself. This can result in a larger CDM system, lower cycle rates, or more complex magnetizing coils and fixtures.

Metallic magnet assembly housing will dissipate energy during the magnetizing process in the form of eddy currents. This is especially true in the case of magnetic metals, such as steel alloys (including magnetic stainless steels) and laminations.

Nonmagnetic metals such as aluminum are also a source of eddy current loss, but to a lesser extent than magnetic steels.

However, the advantages gained by assembling unmagnetized magnets are usually more than worth the payment required in magnetizing problems. It’s difficult to assemble magnets once they are magnetized. In fact, this is a potentially dangerous operation to do by hand. This is especially true with large magnets and magnets with high energy products, which are very difficult to handle when magnetized.

Magnetizing Coils

The magnetizing coil is usually the weak link in a CDM system. It’s on the receiving end of an electrical energy jolt that can reach an instantaneous value of 5 million W.

The magnetic forces caused by the high currents in the magnetizing coil windings can cause physical movement of the windings, resulting in shorted turns or causing the windings to open.

The sheer number of electrons flowing in the windings can cause the coil winding to blow apart if the windings have sharp bends. Heat buildup can soften encapsulating epoxy, allowing physical movement of the conductors. Heat buildup can also weaken the winding insulation, creating the possibility of shorted turns. The high current pulses (a few hundred to more than 10,000 A) will cause instantaneous heating of weak spots in the magnetizing coil winding. These hot spots can be at connection points or at flaws in the copper conductor.

The magnetizing coil must be designed to do the following:

_ Withstand a few hundred to several thousand volts.

_ Accept a few hundred to several thousand amperes.

_ Withstand the mechanical forces caused by the electrical current flow.

_ Produce the magnitude of magnetic field required for the material being magnetized.

_ Generate a magnetic field pulse of sufficient duration to allow the magnetic field to fully saturate all of the magnetic material.

_ Have a sufficient cooling system so that the required production cycle rates can be realized.

_ Be safe when operators come in contact with it. The safety issues must deal with both the electrical voltages and physical forces.

The high voltages involved require that special attention be given to insulation.

The proper wire should be used, with insulation specifications that meet or exceed the maximum voltage of the CDM system. Special care must be taken when making connections. All metal parts of the fixture should be securely grounded with wire equal or superior to the magnetizing coil conductors in size and insulation specifications. The operator should be precluded from coming in contact with the magnetizing conductors. This can be done by encapsulating the magnetizing coil in epoxy, placing the magnetizing coil out of reach of the operator during the magnetizing field pulse, or both. It’s usually necessary to encapsulate the magnetizing coil in high-strength epoxy. This encapsulation serves two purposes:

_ It keeps the conductors from moving during the magnetic field pulses.

_ It provides safety for the operator.

The magnitude of the magnetic field is determined by the amount of energy applied to the magnetizing coil. The duration of the pulse is greatly affected by the number of turns on the magnetizing coil. The main variables of the magnetizing coil are the number of turns and its size. The general rule for a magnetizing coil of fixed size is that when the turns increase, the conductor size decreases, the peak field decreases, and the pulse duration increases.

The design of magnetizing coils and fixtures requires careful consideration of conductive materials that may be in the magnetic circuit. Conductive materials, the size of the part to be magnetized, and its shape should also be taken into account when designing magnetizing fixtures and coils. All coils and fixtures should be impregnated with epoxy to ensure that conductors cannot move during the magnetic field pulse.

In some magnetizing applications, it’s advantageous to use steel poles to increase the magnetic field produced by the magnetizing coil. Care must be taken when using steel pole pieces. The steel pole piece will increase the inductance of the magnetizing coils; introducing steel pole pieces into a magnetizing process may, therefore, require some change in the magnetizer capacitance. Since steel pole pieces are a source of eddy current loss, stacked steel laminations are recommended instead of solid pole pieces. Stacked laminations reduce the eddy current losses and allow larger fields to be produced.

Multiple poles can be magnetized with the proper magnetizing fixture design.

Magnetizing coils can be wound around two pole pieces. The magnet to be magnetized completes the magnetic circuit. When the current pulse passes through the magnetizing coils, north and south poles are created in the magnet. Multiple poles can be created on a single magnet by repeating this design along the length or circumference of the magnet. Ill. 125 illustrates a simple example of pole creation at the ends of a magnet.

In general, the fields generated by the magnetizing coils should be oriented in the optimal direction to magnetize the magnet. This optimal direction is determined by the magnet design. If the magnetizing fields don’t magnetize the magnet in the orientation specified in the design, the motor may very likely not perform as designed.

A poorly conceived magnetizing coil design can rob a motor of performance.

ILL. 125 A simple example of magnetizing poles along the length of a magnet. The horizontal magnet completes a magnetic circuit between the two vertical magnetizing pole pieces.