Permanent magnet motors: Introduction (part 2)

(cont. from part 1)

4. Classification of permanent magnet electric motors

In general, rotary PM motors for continuous operation are classified into:

• DC brush commutator motors

• DC brushless motors

• AC synchronous motors

The construction of a PM DC commutator motor is similar to a DC mo tor with the electromagnetic excitation system replaced by PMs. PM DC brushless and AC synchronous motor designs are practically the same: with a polyphase stator and PMs located on the rotor. The only difference is in the control and shape of the excitation voltage: an AC synchronous motor is fed with more or less sinusoidal waveforms which in turn produce a rotating magnetic field. In PM DC brushless motors the armature current has a shape of a square (trapezoidal) waveform, only two phase windings (for Y connection) conduct the current at the same time, and the switching pattern is synchronized with the rotor angular position (electronic commutation).

FIG. 11. Comparison of PM brushless and PM DC commutator motors.

FIG. 12. Machine tool for milling grooves across a steel bar stock: 1 - PM brushless servo motor for indexing the stock, 2 - stepping motor for controlling the mill stroke. Courtesy of Parker Corporation, Rohnert Park, CA, U.S.A.

FIG. 13. An industrial robot (M - electric motor).

The armature current of synchronous and DC brushless motors is not transmitted through brushes, which are subject to wear and require maintenance. Another advantage of the brushless motor is the fact that the power losses occur in the stator, where heat transfer conditions are good. Consequently the power density can be increased as compared with a DC commutator motor. In addition, considerable improvements in dynamics can be achieved because the air gap magnetic flux density is high, the rotor has a lower inertia and there are no speed-dependent current limitations. Thus, the volume of a brushless PM motor can be reduced by 40 to 50% while still keeping the same rating as that of a PM commutator motor (FIG. 11).

The following constructions of PM DC commutator motors have been developed:

• motors with conventional slotted rotors

• motors with slotless (surface-wound) rotors

• motors with moving coil rotors:

(a) outside field type

- cylindrical

- wound disk rotor

- printed circuit disk rotor (b) inside field type with cylindrical rotor

- honeycomb armature winding

- rhombic armature winding

- bell armature winding

- ball armature winding

The PM AC synchronous and DC brushless motors (moving magnet rotor) are designed as:

• motors with conventional slotted stators,

• motors with slotless (surface-wound) stators,

• cylindrical type:

- surface magnet rotor (uniform thickness PMs, bread loaf PMs)

- inset magnet rotor

- interior magnet rotor (single layer PMs, double layer PMs)

- rotor with buried magnets symmetrically distributed

- rotor with buried magnets asymmetrically distributed

• disk type:

(a) single-sided (b) double-sided

- with internal rotor

- with internal stator (armature)

The stator (armature) winding of PM brushless motors can be made of coils distributed in slots, concentrated non-overlapping coils or slotless coils.

5. Trends in permanent magnet motors and drives industry

The electromechanical drives market analysis shows that the DC commutator motor drive sales increase only slightly each year while the demand for AC motor drives increases substantially. A similar tendency is seen in the PM DC commutator motor drives and PM brushless motor drives.

FIG. 14. Centrifugal chiller compressor with high speed PM brushless motor and magnetic bearings: 1 - rotor with surface PMs and nonmagnetic can, 2 - stator, 3 - radial magnetic bearing, 4 - axial magnetic bearing, 5 - impeller, 6 - shaft.

FIG. 15. Automatic labeling systems with a PM brushless motor: 1 - PM brush less motor servo drive, 2 - controller, 3 - spool of self-adhesive labels, 4 - registration mark sensor, 5 - box position sensor, 6 - conveyor speed encoder. Courtesy of Parker Corporation, Rohnert Park, CA, U.S.A.

FIG. 16. Bar code scanner: 1 - laser, 2 - photodecoder converting laser beam into electric signal,3-PM brushless motor, 4 - holographic three-layer disk, 5 - mirror, 6 - bar code, 7 - scanned object, 8 - scan window, 9 - housing.

FIG. 17. Electric shaver with a PM brushless motor: 1 - PM brushless motor, 2 - position sensor, 3 - printed circuit board, 4 - cam-shaft, 5 - twizzer head, 6 - platinum-coated shaving foil, 7 - rechargeable battery, 8 - input terminals 110/220 V.

Small PM motors are especially demanded by manufacturers of computer hardware, automobiles, office equipment, medical equipment, instrumentation for measurements and control, robots, and handling systems. The 2002 world production of PM motors was estimated to be 4.68 billion units with a total value of U.S.$ 38.9 billion. Commutator motors account for 74.8% (3,500 million units), brushless motors account for 11.5% (540 million units) and stepping motors account for 13.7% (640 million units). From today's perspective, the Far East ( Japan, China, and South Korea), America and Europe will remain the largest market area.

Advances in electronics and PM quality have outpaced similar improvements in associated mechanical transmission systems, making ball lead screws and gearing the limiting factors in motion control. For the small motor business, a substantially higher integration of motor components will increasingly help to bridge this gap in the future. However, there is always the question of cost analysis, which ultimately is the key factor for specific customer needs.

FIG. 18. Toy space shuttle: 1 - PM DC commutator motor, 2 - transmission, 3 - on-off switch, 4 - 1.5-V battery, 5 - driven wheels.

FIG. 19. Cassette deck:1-PM motor, 2 - capstan,3-belt, 4 - flywheel, 5 - pressure roller, 6 - rec/play head, 7 - erase head, 8 - tape, 9 - supply reel table, 10 - take-up reel table, 11 - cassette.

6. Applications of permanent magnet motors

PM motors are used in a broad power range from mWs to hundreds kWs.

There are also attempts to apply PMs to large motors rated at minimum 1 MW. Thus, PM motors cover a wide variety of application fields, from step ping motors for wrist watches, through industrial drives for machine tools to large PM synchronous motors for ship propulsion (navy frigates, cruise ships, medium size cargo vessels and ice breakers). The application of PM electric motors includes:

• Industry (Figs 12, 14 13 and 15):

- industrial drives, e.g., pumps, fans, blowers, compressors (FIG. 14), centrifuges, mills, hoists, handling systems, etc.

- machine tools

- servo drives

- automation processes

- internal transportation systems

- robots

• Public life:

- heating, ventilating and air conditioning (HVAC) systems

- catering equipment

- coin laundry machines

- autobank machines

- automatic vending machines

- money changing machines

- ticketing machines

- bar code scanners at supermarkets (FIG. 16)

- environmental control systems

- clocks

- amusement park equipment

• Domestic life (FIG. 17, 18 and 19):

- kitchen equipment (refrigerators, microwave ovens, in-sink garbage disposers, dishwashers, mixers, grills, etc.)

- bathroom equipment (shavers, hair dryers, tooth brushes, massage apparatus)

- washing machines and clothes dryers

- HVAC systems, humidifiers and dehumidifiers

- vacuum cleaners

- lawn mowers

- pumps (wells, swimming pools, jacuzzi whirlpool tubs)

-toys

- vision and sound equipment

- cameras

- cellular phones

- security systems (automatic garage doors, automatic gates)

• Information and office equipment (Figs. 20 and 21):

- computers

- printers

- plotters

- scanners

- facsimile machines

- photocopiers

- audiovisual aids

• Automobiles with combustion engines (FIG. 22);

• Transportation (Figs. 23, 24, 25 and 27):

- elevators and escalators

- people movers

- light railways and streetcars (trams)

- electric road vehicles

- aircraft flight control surface actuation

- electric ships

- electric boats

- electric aircrafts (FIG. 27)

• Defense forces (FIG. 26):

- tanks

- missiles

- radar systems

- submarines

- torpedoes

• Aerospace:

- rockets

- space shuttles

- satellites

• Medical and healthcare equipment:

- dental handpieces (dentist's drills)

- electric wheelchairs

- air compressors

- trotters

- rehabilitation equipment

- artificial heart motors

• Power tools (FIG. 28):

- drills

- hammers

- screwdrivers

- grinders

- polishers

-saws

- sanders

- sheep shearing handpieces

• Renewable energy systems (FIG. 29)

• Research and exploration equipment (FIG. 30)

FIG. 20. Computer hard disk drive (HDD) with a PM brushless motor: 1-in-hub PM brushless motor mounted in 2.5-inch disk, 2 - integrated interface/drive controller, 3-balanced moving-coil rotary actuator, 4-actuator PM, 5-read/write heads, 6 - read/write preamplifier, 7 - 44-pin connector.

FIG. 21. Laser beam printer: 1 - stepping motor, 2 - scanner mirror, 3 - semiconductor laser, 4 - collimator lens, 5 - cylindrical lens, 6 - focusing lenses, 7 - photosensitive drum, 8 - mirror, 9 - beam detect mirror, 10 - optical fiber.

FIG. 22. PM motors installed in a car.

FIG. 23. Power train of Toyota Prius hybrid electric vehicle (HEV):1 - gasoline engine,2-PM brushless generator/starter (GS),3-power split device (PSD), 4 - silent chain, 5 - PM brushless motor/generator (MG), 6 - GS solid state converter, 7 - battery, 8 - MG solid state converter, 9 - reduction gears, 10 - differential, 11 - front wheels.

FIG. 24. Electromechanical actuator for night control surfaces: 1 - brushless motor, 2 - gearbox, 3 - ball lead screw, 4 - clevis or spherical joint end.

FIG. 25. Ship propulsion system with a PM brushless motor: 1-diesel engine and synchronous generator, 2 - converter, 3 - large PM brushless motor, 4 - propeller shaft, 5 - propeller.

FIG. 26. Stealth torpedo: 1 - guiding system, 2 - conformal acoustic arrays, 3 - advanced rechargeable batteries,4-integrated PM brushless motor propulsor, 5 - active and passive noise control, 6 - synergic drag reduction.

FIG. 27. Taurus Electro two-seat self launching glider with a 30 kW, 1800 rpm, 15.8 kg PM brushless motor.

FIG. 28. Cordless electric screwdriver:1-PM DC commutator motor 3.6 V/240 mA,2-speed reducer,3-lockerformanual screwing, 4 - screwdriver bit, 5 - forward-reverse rotation switch, 6 - rechargeable Ni-Cd battery,7-bit compartment.

FIG. 29. Water pumping system for a remote population center:1-solar panels, 2 - inverter, 3 - submersible PM brushless motor-pump unit, 4 - well, 5 - water storage tank.

The automotive industry is the biggest user of PM DC commutator motors.

The number of auxiliary DC PM commutator motors can vary from a few in an inexpensive car to about one hundred in a luxury car.

Small PM brushless motors are first of all used in computer hard disk drives (HDDs) and cooling fans. The 2002 worldwide production of computers is estimated to be 200 million units and production of HDDs approximately 250 million units.

FIG. 30. Underwater robotic vehicle for remote inspection of hydro power plants: 1 - two thruster sets for forward and reverse (200-W DC brushless motors), 2 - two thruster sets for lateral and vertical (200-W DC brushless motors), 3 - buoyancy material, 4 - transponder, 5 - food light, 6 - still camera, 7 - video camera, 8 - electric flash, 9 - cover. Courtesy of Mitsui Ocean Development and Eng. Co., Tokyo, Japan.

PM brushless motors rated from 50 to 100 kW seem to be the best propulsion motors for electric and hybrid road vehicles.

Given below are some typical applications of PM motors in industry, manufacturing processes, factory automation systems, domestic life, computers, transportation and clinical engineering:

• Industrial robots and x, y-axis coordinate machines: PM brushless motors

• Indexing rotary tables: PM stepping motors

• X-Y tables, e.g. for milling grooves across steel bars: PM brushless servo motors

• Linear actuators with ball or roller screws: PM brushless and stepping motors

• Transfer machines for drilling a number of holes: ball lead screw drives with PM brushless motors

• Monofilament nylon winders: PM DC commutator motor as a torque mo tor and PM brushless motor as a traverse motor (ball screw drive)

• Mobile phones: PM DC commutator or brushless vibration motors

• Bathroom equipment : PM commutator or brushless motors

• Toys: PM DC commutator motors

• Computer hard disk drives (HDD): PM brushless motors

• Computer printers: PM stepping motors

• Cooling fans for computers and instruments: PM brushless motors

• Auxiliary motors for automobiles: PM DC commutator and PM brushless motors

• Gearless elevators: PM brushless motors

• Electric and hybrid electric vehicles (EV and HEV): PM brushless motors of cylindrical or disk type

• Ship propulsion: large PM brushless motors or transverse flux motors (above 1 MW)

• Submarine periscope drives: direct-drive PM DC brushless torque motors

• More electric aircraft (MEE): PM brushless motors

• Dental and surgical handpieces: slotless PM brushless motors

• Implantable blood pumps: PM brushless motors integrated with impellers.

7. Mechatronics

A new technology called mechatronics emerged in the late 1970s. Mechatronics is the intelligent integration of mechanical engineering with microelectronics and computer control in product design and manufacture to give improved performance and cost saving. Applications of mechatronics can be found in the aerospace and defense industries, in intelligent machines such as industrial robots, automatic guided vehicles, computer-controlled manufacturing machines and in consumer products such as computer hard disk drives (HDD), video cassette players and recorders, cameras, CD players and quartz watches.

FIG. 31. Gear train design: (a) conventional system, (b) mechatronics system.

A typical example of a novel mechatronics application is in the control of multi-shaft motion. A gear train has traditionally been employed with the performance, i.e. speed, torque and direction of rotation determined by the motor and gear rated parameters as shown in FIG. 31a. Such a configuration is acceptable for constant speed of each shaft but where variable speeds are required, a different set of gears is needed for each gear ratio. In the mechatronics solution (FIG. 31b) each shaft is driven by an electronically controlled motor, e.g. a PM brushless motor with feedback which provides more flexibility than can be obtained from mechanical gear trains. By adding a microprocessor or microcomputer, any required motion of the mechanism can be programmed by software. The common term for this type of control system is mechatronics control system or mechatronics controller . The "electronic gearbox" is more flexible, versatile and reliable than the mechanical gearbox. It also reduces acoustic noise and does not require maintenance.

8. Fundamentals of mechanics of machines

8.1 Torque and power

The shaft torque T as a function of mechanical power P is expressed as

[...]

Table 1. Basic formulae for linear and rotational motions

8.2 Simple gear trains

In the simple trains shown in FIG. 32, let n1, n2 = speeds of 1 and 2, z1 and z2 = numbers of teeth on 1 and 2, D1, D2 = pitch circle diameters of 1 and 2.

FIG. 32. Simple trains.

The negative sign signifies that 1 and 2 rotate in opposite directions. The idler, 3, FIG. 32b, does not affect the velocity ratio of 1 to 2 but decides on the directions of 2. The ratio ? = z2/z1 is called the gear ratio.

8.3 Efficiency of a gear train

Allowing for friction, the efficiency of a gear train is [...]

8.4 Equivalent moment of inertia

In the simple trains shown in FIG. 32a, let J1, J2 = moments of inertia of rotating masses of 1 and 2, O1 and O2 = angular speed of 1 and 2, D1, D2 = pitch circle diameters of 1 and 2, 0.5J1O2 1 ,0.5J2O2 2 = kinetic energy of 1 and 2, respectively.

The net energy supplied to a system in unit time is equal to the rate of change of its kinetic energy Ek (Table 1.1), i.e. [...]

The equivalent moment of inertia is equal to the moment of inertia of each wheel in the train being multiplied by the square of its gear ratio relative to the reference wheel.

8.5 Rotor dynamics

All spinning shafts, even in the absence of external load, defect during rotation. FIG. 33 shows a shaft with two rotating masses m1 and m2. The mass m1 can represent a cylindrical rotor of an electric machine while the mass m2 can represent a load. The mass of the shaft is msh. The combined mass of the rotor, load and shaft can cause defection of the shaft that will create resonant vibration at a certain speed called whirling or critical speed. The frequency when the shaft reaches its critical speed can be found by calculating the frequency at which transverse vibration occurs. The critical speed in rev/s of the ith rotating mass can be found as […]

FIG. 33. Solid cylindrical shaft loaded with two masses m1 and m2.

The shaft is also considered a rotor with mass msh concentrated at 0.5L where L is the length of the shaft (bearing-to-bearing).

Dunkerley's empirical method uses the frequencies that each individual load creates when each load acts alone and then combines them to give an approximation for the whole system. Thus eqn (13) is an approximation to the first natural frequency of vibration of the system which is assumed nearly equal to the critical speed of rotation. Rayleigh's method is based on the fact that the maximum kinetic energy must be equal to maximum potential energy for a conservative system under free vibration.

8.6 Mechanical characteristics of machines

In general, the mechanical characteristic T = f(O) of a machine driven by an electric motor can be described by the following equation:

...where Tr is the resisting torque of the machine at rated angular speed Or, ß = 0 for hoists, belt conveyors, rotating machines and vehicles (constant torque machines), ß = 1 for mills, callanders, paper machines and textile machines, ß = 2 for rotary pumps, fans, turbocompressors and blowers.

9. Torque balance equation

An electromechanical system can simply be described with the aid of the following torque balance equation, ...

10. Evaluation of cost of a permanent magnet motor

The cost of an electrical machine is a function of a large number of variables.

The cost can be evaluated only approximately because it depends on:

• number of electrical machines of the same type manufactured per year

• manufacturing equipment (how modern is the equipment, level of automation, production capacity per year, necessary investment, etc.)

• organization of production process (engineering staff-to-administrative and supporting staff ratio, qualification and experience of technical management, overhead costs, productivity of employees, small company or large corporation, company culture, etc.)

• cost of labor (low in third world countries, high in North America, Europe and Japan)

• quality of materials (good quality materials cost more) and many other aspects It is impossible to take into account all these factors in a general mathematical model of costs. A logical approach is to select the most important components of the total cost and express them as functions of dimensions of the machine.

The most important costs of an electrical machine can be expressed by the following approximate equation:

[...]

FIG. 34. Electric motor driven rolling mill. Numerical example 1 .

FIG. 35. Torque profile of the electric motor. Numerical example 2 .

FIG. 36. Speed and torque profiles of a servo drive. Numerical example 3.