Brushless Interior Permanent Magnet (IPM) Motors A New Solution for High Performance Appliances

By Dan M. Ionel, Ph.D.,Engineering Fellow, A. O. Smith Corporate Technology Center, Milwaukee, WI.

Permanent magnet (PM) brushless (BL) 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 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, especially neodymium-iron-boron (NdFeB), being 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 enables overall motor size reduction and/or energy increase and, through careful design, can lead to an attractive motor solution.

Fig. 2 Electromagnetic torque components in an example IPM motor. Drive electronics tracks rotor position and controls the vector current in order to maximize torque production.

Fig. 1 Schematic cross-section of a 4-pole brushless (BL) motor with surface mounted permanent magnets (PM; left) and of an interior PM motor (right). Magnets of North and South polarity are shown in red and blue.

BLPM motors can be classified in 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 (Fig.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.

magnetic field of the air-gap, in an IPM the PMs are shielded by the rotor steel that provides a leakage path for the arma-ture reaction flux. As a consequence, thinner magnets can be employed, potentially resulting in material cost savings.

The synchronous electromagnetic torque in an IPM motor has two major components (Fig. 2). The main component is the alignment torque, which is proportional with the flux linkage in the stator windings produced by the rotor magnets and with the vector component of the stator current that is in quadra-ture with the magnet flux. This current component is active only, i.e. 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 Fig. 2 the optimal angle is approximately 115 degrees.

Why is an IPM design so attractive?

On the other hand, IPM motors typically employ less expensive rectangular blocks, which are placed inside slots made in the laminated rotor core. Magnet retention is therefore enhanced and yields to simplifications in the manufacturing process. There are a large variety of designs for IPM rotors; shown in Fig.1 is just 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/or size reduction. Many combinations of the magnet shape, position and number of magnets per pole is possible for IPM motors. In recent years, A. O. Smith Corporation has developed proprietary solutions to optimize designs for different applications.

Better demagnetization withstand is another advantage of IPM motors. Unlike in the SPM, in which the PM is directly exposed to the

This maximum torque per amp control procedure requires a demagnetizing current component that also reduces the mag-netic circuit loading and core losses. As shown in the graph, 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 (quadra-ture) and demagnetizing current component are reduced yielding a relatively flat efficiency curve, yet another advantage of BLPM machines over induction motors.

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 Fig. 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 non-linearity present in the magnetic circuit of a high-performance IPM design.

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

The solution, called a line-fed IPM motor involves the use of a rotor that includes, apart from the PMs, also a squirrel cage, similar to that of an induction machine. In the cut-away rotor example of Fig. 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.

Fig. 3 Stator and cut-away rotor for a 2-pole line-fed IPM motor, which operates at synchronous speed without electronic controls. The rotor includes a squirrel cage and PMs.

Fig 4. Magnetic field in the cross-section of a 3-phase 9-slot 6-pole electronically controlled IPM motor. Coils are wound around every tooth and inter-connected to produce a concentrated type winding.

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 sensor-less, 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 particularly shine 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 such as to optimize torque production and efficiency, as explained with reference to Fig. 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 care-fully completed in order to ensure that, among other char-acteristics, a sinusoidal back EMF is achieved so that the elec-tromagnetic 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 (e.g. Figs. 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 require-ments, in reduced copper losses that could boost efficiency,

High efficiency without electronics

Electronic control boosts motor and drive performance

A. O. Smith Corporate Technology Center12100 West Park PlaceP. O. Box 245012Milwaukee, WI 53224414-359-4200www.aosmith.com 2008 A. O. Smith Corporation All Rights Reserved

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Dr. Dan M. Ionel is the Electromagnetics Engineering Fellow of A. O. Smith Corp., Milwaukee, WI. He has 20+ years of experience with electric motors and earlier in his career worked in the U.K. for Invensys Brook Crompton Co. and for the SPEED Laboratory, University of Glasgow.

ApplicationsSome of the most popular IPM applications, possibly not familiar enough to the wide general audience, are the electric motors/generators of hybrid or all-electric vehicles. In the servo motor 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 Fig. 3, can increase the motor rated efficiency by up to 4 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 Fig.5 AOS ECM (Electronically Commutated Motor) - motor and electronics platform.

running the driven mechanism for longer time but at substantially reduced speed and torque. For relatively small ratings, the power electronics hardware for IPMs is based on the AOS ECM platform, which is currently in production (Fig. 5), and a new controller incorporates proprietary vector algorithms that maximize motor performance.