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issue: November 2008 APPLIANCE Magazine

Engineering: Interior Permanent Magnet Motors
IPM Motors: A New Solution for High-Performance Appliances


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Dan M. Ionel, engineering fellow, A.O. Smith Corporate Technology Center

Brushless (BL) permanent magnet (PM) motors have emerged in recent years as a very strong contender to replace induction motors used in electronically controlled variable-speed applications.

In most cases, BLPM motors can provide superior performance in terms of increased efficiency and reduced noise, while the total cost differential for the motor plus electronics is subject to relatively fast payback, especially considering the increasing cost of energy.

 

At the core of a BLPM motor are the PMs, which are placed in the rotor and provide the magnetizing flux. One immediate advantage is that in a typical design, there are virtually no rotor losses. Different magnet grades can be employed for rotor manufacturing, with ceramic-ferrites and rare earth‚Äîespecially neodymium-iron-boron (NdFeB)‚Äîbeing the common choices. In particular, the sintered NdFeB is a strong material with energy density higher by one order of magnitude than that of the lower-cost ferrites. Although NdFeB is a more expensive material, it can reduce overall motor size, increase energy efficiency, and, through careful design, can lead to an attractive motor solution.

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Figure 1. Schematic cross-section of a four-pole brushless (BL) motor with surface-mounted permanent magnets (PM; shown left) and of an interior PM motor (shown right). Magnets of
north and south polarity are shown in red and blue.

Why IPM?

BLPM motors can be classified into two major groups—motors with the PMs mounted on the surface of the rotor and motors with PMs placed in the interior of the rotor core (see Figure 1). The first group is commonly referred to as surface PM (SPM), and the typical manufacturing technology involves gluing arc magnets and/or securing them with special tape on the outer surface of a rotor core. While this technology may be cost-effective in conjunction with large ferrite magnets or with bonded magnet rings, it presents challenges for the sintered NdFeB designs. In this case, the solution is more complicated because in order to cope with thin magnets and to minimize eddy-current losses in the magnets, multiple smaller magnets are often used to make one pole.

On the other hand, interior PM (IPM) motors typically employ less-expensive rectangular blocks, which are placed inside slots made in the rotor-laminated core. Magnet retention is therefore enhanced and yields to simplifications in the manufacturing process. There is a large variety of designs for IPM rotors. Figure 1 shows the most conventional version with PM blocks magnetized parallel to the center pole radius.

Another major type of IPM rotor (not shown) employs magnets placed in a “spoke” arrangement along the rotor radius and magnetized tangentially. The spoke design has the intrinsic advantage of magnetic flux concentration, so that in high-polarity motors, the flux density in the motor air-gap is increased. This leads the way to further performance improvement and size reduction. Many combinations of the magnet shape, position, and number of magnets per pole are possible for IPM motors.

Better demagnetization withstand capability is another advantage of IPM motors. Unlike in the SPM, in which the PM is directly exposed to the magnetic field of the air-gap, PMs in IPM designs are shielded by the rotor steel that provides a leakage path for the armature reaction flux. Consequently, thinner magnets can be employed, potentially resulting in material cost savings.

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Figure 2. An example of electromagnetic torque components in an IPM motor. Drive electronics track rotor position and control the vector current in order to maximize torque production.

The synchronous electromagnetic torque in an IPM motor has two major components (see Figure 2). The main one is the alignment torque. It is proportional both with the flux linkage in the stator windings produced by the rotor magnets and with the vector component of the stator current in quadrature with the magnet flux. This current component is “active” only. This means it only produces torque and does not contribute to the magnetization of the motor magnetic circuit. In an SPM, this is the only synchronous component of the electromagnetic torque. In an IPM, due to the rotor variable magnetic reluctance, i.e., saliency, an additional torque component is developed. By means of electronic control, the torque angle, i.e., the angle between the magnet flux phasor and the current phasor, can be optimally set in order to increase the torque output for a given current magnitude. In the example of Figure 2, the optimal angle is approximately 115°.

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Figure 3. Stator and cutaway rotor for a two-pole line-fed IPM motor, which operates at synchronous speed without electronic controls. The rotor includes a squirrel cage and PMs.

This maximum torque per amp control procedure requires a demagnetizing current component that also reduces the magnetic circuit loading and core losses. As shown in Figure 2, an IPM motor has the potential of increased specific power or reduced size for the same rated power, as compared with an SPM machine. At reduced loads, both the active (quadrature) and demagnetizing current component are reduced, yielding a relatively flat efficiency curve. This is yet another advantage of BLPM machines over induction motors.

High Efficiency without Electronics

Despite recent advancements, for some applications the added cost of a variable-speed drive still remains prohibitive. In this case, the induction motor—the industry workhorse for the last century—appears to be irreplaceable. However, BLPM machines can also be made to operate directly from the mains and offer increased efficiency without electronics.

The solution, called a line-fed IPM motor, involves the use of a rotor that includes PMs and a squirrel cage, similar to that of an induction machine. In the cutaway rotor example shown in Figure 3, three magnets per pole are fitted in the rectangular slots. During the motor transient start-up, the rotor cage contributes to the production of an asynchronous torque that overcomes a PM transient braking torque and accelerates the rotor. In typical steady-state operation, the rotor moves in synchronism with the air-gap revolving magnetic field.

Boosting Performance with Electronics

From the point of view of electronic commutation, brushless PM motors can be driven with trapezoidal or sinusoidal current waveforms. The control of the first type, also referred to as BLDC, is typically simpler, especially when done sensorless (i.e., without a rotor position sensor). Nevertheless, the BLAC control with sinewave currents typically has superior performance both in terms of increased efficiency and reduced noise.

IPM motors are particularly beneficial when used in conjunction with vector control. In this case, the electronic controller tracks the rotor position with respect to the stator (armature) field and injects the current to optimize torque production and efficiency, as shown in Figure 2. The salient rotor structure of the IPMs lends itself to robust sensorless applications.

In principle, both SPM and IPM rotor types can be mated with the same stator design. However, the design should be carefully completed to ensure that, among other characteristics, a sinusoidal back emf is achieved so that the electromagnetic torque ripple is reduced. In this respect, a low harmonic content of the air-gap magnetic field is preferable and is also beneficial for reducing core losses.

A distributed stator winding—e.g., concentric or lap, typical for induction machines—can also be used with a lamination having more than one slot per pole and phase (see Figures 1 and 3). For fractional slot designs, a concentrated winding with coils wound around a tooth can be employed. This second choice may result, depending on the actual application requirements, in reduced copper losses that could boost efficiency, especially at low speed or torque. A concentrated winding design can be more sensitive than its distributed winding counterpart in terms of the influence of tolerances, particularly those of the air-gap, on the motor unbalanced magnetic pull.

The example in Figure 4 is from an electromagnetic finite-element model analyzed with the PC-FEA software produced by the SPEED Laboratory, University of Glasgow, UK. This detailed method of motor performance simulation is preferred because of the strong nonlinearity present in the magnetic circuit of a high-performance IPM design.

Applications

Some of the most popular IPM applications, possibly not familiar enough to the general audience, are the electric motors/generators of hybrid or all-electric vehicles. In the servomotor world, more and more designs are shifting away from SPM to IPM to take advantage of the inherent advantages previously discussed. In principle, there are no size limitations to IPM designs, and these can be developed from small fractional horsepower (hp) to large (hundreds of hp ratings).

Compressors, including those of smaller ratings for residential unitary air-conditioning applications, are potential candidate applications for IPM designs. For example, the use of a line-fed IPM, as the one exemplified in Figure 3, can increase the motor-rated efficiency by up to four points above the already high efficiency of a top-of-the-line induction motor. Such improvement is possible while maintaining the starting torque requirements of a demanding application, such as a reciprocating compressor. The addition of electronics makes possible true variable-speed operation and enables further energy savings achieved by running the driven mechanism for longer time, but at substantially reduced speed and torque.

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Figure 4. Magnetic field in the cross-section of a three-phase, nine-slot, six-pole electronically controlled IPM motor. Coils are wound around every tooth and interconnected to produce a concentrated-type winding.

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