IPM Designs Improve Brushless Servos

Permanent magnet synchronous motors—brushless servo motors for short—rely on a rotating magnet rotor to generate the magnetic field needed for efficient torque production. Most often, the magnets reside on the rotor's outside periphery for the sake of design simplicity, providing good dynamic performance for many applications at lower cost than alternatives.

10/01/2005


AT A GLANCE

 

  • Place magnets within rotor

  • Get wider torque-speed range

  • Brushless servo motors

  • Improve motor efficiency

  • Surface vs. embedded magnets

Permanent magnet synchronous motors—brushless servo motors for short—rely on a rotating magnet rotor to generate the magnetic field needed for efficient torque production. Most often, the magnets reside on the rotor's outside periphery for the sake of design simplicity, providing good dynamic performance for many applications at lower cost than alternatives. Hence, the so-called surface permanent magnet (SPM) design.

For higher power brushless servo motors, alternative rotor designs exist under such names as interior permanent magnet (IPM), internal PM, embedded PM, or buried PM. Whatever the naming, the idea is to locate magnets within the rotor structure to increase motor torque-speed performance and derive other benefits. The first terminology of IPM is used here.

An IPM rotor's shape and salient magnetic structure favor development of a reluctance torque component and help increase flux density. This extra torque component can be harnessed to increase output, but requires a more sophisticated servo amplifier and control algorithms. Rotor saliency also simplifies use of sensorless feedback, where it's appropriate for the application.

One obvious benefit of embedding magnets is smaller rotor diameter and lower inertia. Paul Webster, servo product manager at GE Fanuc Automation Americas Inc., notes high-speed and high-acceleration capability of IPM servo motors as a result of using rare-earth interior magnets, such as neodymium-ferric-boron (Nd-Fe-B). 'Embedding high-density magnets into the rotor structure allows for the arc of the yoke to be optimized so that magnetic flux distribution is as sinusoidal as possible,' says Webster. 'Embedded magnets also allow for a low magnetic saturation due to the armature reaction.'

Other IPM design benefits include a mechanically robust, well-balanced rotor. Embedded magnets will not become detached or damaged, allowing high-speed rotation without vibration or fear of rotor and/or bearing failure, explains Webster. 'In fact, mean time between failure [MTBF] has been improved to an impressive 1.4 million-plus hours for the current line of GE Fanuc Alpha i servo motor,' he states.

Cogging torque has been reduced to 0.05% of rated torque, on average, via an IPM design, which is slightly better than with surface-mounted magnet design, according to Webster. A prime application of these IPM motors is the feed axis for CNC machine tools, where high accuracy at high feed rates is key. Low cogging torque improves machining accuracy.

Surface-mount or buried

Motors, drives, motion control

GE Fanuc servo motors used for CNC machining apply these IPM designs to reduce cogging torque and increase feed-axis smoothness. Skewed, circumferential design of exposed ferrite magnets (A) is used where inertia matching is important for stability. More compact, lobed-rotor shaping (design B) applies rare-earth magnets to obtain higher speed and acceleration.

Lee Stephens, systems engineer at Danaher Motion, notes that brushless servo motors come almost exclusively with rotating permanent magnets—either surface-mounted or buried in the rotor. 'Interior permanent magnets offer a magnetic density whose geometry fits with high power motors,' he says.

Surface permanent magnet designs permit simpler construction, explains Stephens, allowing preformed magnets simply to be 'glued' around the armature OD to supply the motor's magnetic-flux source. SPM designs are cost-effective, especially for lower power systems typically found in NEMA 34 frame or smaller motors. In contrast, IPM designs make the magnet a part of the rotor structure. 'Buried magnets are where the magnet and rotor are nearly one,' says Stephens. However, there is tradeoff between power and flux density.

'Surface-mount magnets provide a high-speed magnetic field that translates to high motor speeds, while an IPM design yields high magnetic flux density and torque at the expense of different time constants for the magnetic-field generation,' he states. While magnetic force takes finite time to develop, it doesn't hinder high-speed capability of IPM motors. IPM motors tend to be physically large, but a significant overlap exists in size and power capability of IPM and SPM designs.

Rare-earth magnets can be the most significant cost of the motor. This favors SPM design, where quantity of magnetic material used tends to be less, especially for smaller motors 'With less material and simple construction, it will be significantly more cost-effective than attempting a buried magnet design,' remarks Stephens, '[but] costs and benefits must be weighed against requirements and compromises are often the result.' For example, IPM designs use easier to manufacture flat-shaped magnets versus curved shapes for SPM designs.

Stephens agrees about benefits of installing magnets within the rotor. The resulting smaller diameter means less rotor inertia. Reduction is by the square of the radius as given by the formula for inertia of a cylindrical rotor, 1/2mr , where m is rotor mass and r is its radius. 'This yields a respectable tradeoff in power density—one of many figures of merit for a motor—especially with large frame motors,' he says. For high-power units, SPM rotor inertia could become too high, with significant power consumed just to accelerate the motor.

Field-weakening, novel motor configuration

One of the main benefits of IPM synchronous motors, notes Steffen Winkler—head of product management for Drive Systems, Bosch Rexroth AG—is the possibility for field-weakening control (up to 6:1 or even 10:1). This method helps extend the operating speed range, similar to induction motors.

Indicative of wide design approaches for electric motors, Bosch Rexroth offers an innovative, direct-drive (hollow-shaft) PM synchronous motor with rotor-internal magnets. IndraDyn H reportedly offers the highest torque density compared to other designs and has rated power up to 57 kW (76 hp). A smaller model reaches maximum speed of 30,000 rpm. Winkler calls IndraDyn H 'the first kit motor with a complete closed-cooling circuit, which reduces the amount of work for the mechanical engineer and increases the cooling efficiency.' Water cooling built into the stator is standard for best efficiency, but other cooling fluids (oil, air) and natural convection are options at reduced continuous power operation, he explains.

Yaskawa Electric uses IPM technology on high-power servo motors, for example, its Large Capacity SGMBH motor, offered as a standard product up to 55 kW output. Chris Knudsen, product marketing manager at Yaskawa Electric America, adds that these motors have been applied up to 90 kW. SGMBH servo motor's smaller, lighter-weight rotor reduces centrifugal forces to allow higher acceleration and deceleration for thermoforming, electric injection molding, extrusion, metal forming, material converting, automated packaging, and other applications. 'At higher speeds, larger diameter rotors cause high centrifugal forces on surface-mounted permanent magnets,' says Knudsen. 'By embedding the magnets in the rotor, these forces, as well as the strong magnetic forces caused by the motor current, can easily be accommodated.'

Using IPM design was appropriate for the large capacity motor based on production costs and the application, notes Knudsen. For smaller Yaskawa servo motors, surface-mounted PM design has proven to be cost-effective, reliable, and offering high performance for demanding applications, such as machine tools.

Cost, complexity, controls

Implementing an IPM servo motor design comes at significant added cost and complexity. GE Fanuc's Webster cites use of finite-element method (FEM) magnetic analysis, costly high-density rare-earth magnets, and high-resolution encoders as necessary to realize performance and size improvements.

One major design challenge is IPM rotor yoke shaping. Embedded magnet location and smaller rotor size limit the freedom to shape the rotor yoke periphery. 'FEM analysis of the magnetic structure is used to find proper balance in the shaping of the rotor yokes between torque and cogging force. IPM design represents a balance between an extremely smooth and extremely powerful motor,' continues Webster. Progressive dies are needed to form the complex shape required. Further complicating rotor yoke manufacture is a laminated rotor, used to reduce surface heating losses. In contrast, 'a surface-mounted magnet design can use a solid-steel rotor with simple, glued-on magnets,' he adds.

An IPM motor typically requires a faster, more advanced control system, according to Webster. Stable operation with reduced inertia of a smaller, lighter rotor calls for a high-resolution motor-mounted encoder and quick servo-loop response control. Encoders with up to 16 million ppr are an option to increase the amount of feedback data available to the control system for closing velocity and position loops. Inertia matching becomes important to stable operation, since an IPM motor, using rare-earth magnets, can reduce rotor inertia by several times compared to a SPM design, he explains.

Servo-loop response time also must be faster for improved stability and feed smoothness at higher inertia ratios. Webster mentions high-response vector (HRV) control—an advanced form of field-oriented control—that can deliver a current loop of 32.25 microseconds (and velocity loop of 62.5

Bosch Rexroth's Winkler concurs that IPM motors require higher control effort than conventional permanent-magnet motors. 'To ensure best possible performance, control of the motors should be tuned with special motor parameters, especially in the field-weakening area,' he says. 'This means that a system solution motor/controller from one supplier is recommended but not absolutely required.'

Technology differences between SPM and IPM designs require a level of commutation control substantially more refined than sine commutation, explains Danaher Motion's Stephens. Moreover, some higher power IPM motors need added control functions to obtain increased speed with little or no loss in torque. Known as 'torque-angle advance,' this feature allows for timing of signals to begin commutation ahead of rotor position—not unlike the advance in an automobile distributor. The feature overcomes a time delay in generating the magnetic field caused by armature inductance, the reluctance path, and physical shape of the armature and teeth. 'We need to get the full magnetic field to ensure full torque of the motor at speed,' he says. 'With advancement of DSP architecture, this is accomplished quite accurately with either look-up tables or calculations within the drive.'

Torque-angle advance algorithms required to obtain the speed-torque performance of an IPM design may not be needed with SPM motors. It's one example why control systems for IPM servo motors are typically more complicated. However, more complex control does not mean that reliability must decline, notes Stephens. 'Reliability requirements in the industry are always increasing and being met with drives that can outperform their analog counterparts of yesterday,' he concludes.

Overall, IPM motors can deliver enhanced performance over their counterparts with surface-mount magnet rotors and other motor technologies.






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