AMAZON multi-meters discounts AMAZON oscilloscope discounts
(cont. from part 1)
5. Ultra high speed motors
A concept of an ultra high speed PM synchronous motor rotating at 500, 000 is shown in FIG. 9. The rotor consists of a cylinder of rare-earth PM material magnetized radially and reinforced by a nonferromagnetic high tensile material, such as stainless steel, forming an external can (retaining sleeve).
The stator has three teeth (salient poles), three slots, and three or six coils.
In assessing the power limit at a particular speed it is important to establish
the optimum relationship between the rotor diameter and the outer stator diameter,
and the complex coupling of the mechanical bursting and whirling constraints
with the winding loss and power development. Optimum geometry prefers a small
rotor diameter in relation to the stator outer diameter.
FIG. 9. Cross section of high speed PM synchronous motor with cylindrical
PM and three stator slots: 1 - PM, 2 - stator core, 3 - stator winding.
FIG. 10 Ultra high speed 5 kW, 240,000 rpm PM brushless motor. All dimensions
are in millimeters. Courtesy of University of Nagasaki, Japan.
Small size ultra high speed two-pole PM brushless motor rated at 5 kW, 240,000 rpm and 4 kHz is shown in FIG. 10 The directly coupled motor is used to start up a gas turbine of a co-generation system. After the gas turbine is ignited and becomes the source of mechanical power, the motor operates as a generator. The two-pole rotor with surface PMs is equipped with a 2-mm thick metal retaining sleeve. The stator outer diameter is 60 mm, stator stack length is 40 mm and the rated voltage is 200 V. An oil circulation system is used for the shaft bearing and the cooling of the motor. This system consists of a filter, reserve tank and trochoid pump. The motor is circulated by the oil and the oil is cooled by the circulated water. The control electronics consists of a control unit with DSP TMS320C32. The power supply unit consists of a PWM controlled voltage source inverter (VSI).
6. Applications
6.1 High speed aerospace drives
High speed PM brushless motors are used in the following aerospace electromechanical drives:
• electric fuel pumps;
• electric actuation systems for flight controls;
• electric cabin air compressors;
• nitrogen generation systems;
• compartment refrigeration units;
• supplemental cooling units.
The concept of more electric aircraft (MEA), which removes hydraulic, pneumatic and gearbox driven subsystems in favor of electrical driven subsystems, has necessitated the development of high performance compact lightweight motor drives and starter/generator systems [306]. Table 9.3 identifies the numbers and power of motor drive systems necessary to support a generic MEA fighter.
Aerospace electromechanical drive systems require high power density (small size motors), high reliability, low EMI and RFI interference level, high efficiency, precise speed control, high starting torque, fast acceleration and linear torque-speed characteristics. Constructional design features of air craft electromechanical drives demand special optimized design and packaging, coolant passing through the motor and compatibility of materials with the coolant.
Table 3. Electrical power requirements for MEA fighter aircraft
FIG. 10 Cross section of the magnetic circuit of a PM d.c. brushless motor
with laminated rotor and axial channels for aerospace: (a) outline, (b) magnetic
flux distribution.
An exemplary magnetic circuit of a d.c. PM brushless motor used in air craft technology is shown in FIG. 11. To keep high magnetic flux density in the teeth, cobalt alloy laminations with saturation magnetic flux density close to 2.4 T are frequently used. There are symmetrical axial channels in the rotor stack. The number of channels is equal to the number of rotor poles. The laminated rotor with axial channels has several advantages, such as improved power-mass ratio, improved heat transfer and cooling system, lower moment of inertia (higher mechanical time constant) and lower losses in the rotor core.
In high speed motors the core losses form a large portion of the total losses. These losses can be minimized amongst others by designing laminated cores with uniform distribution of magnetic flux. FIG. 11 shows the magnetic flux distribution in the cross section area of the magnetic circuit. The magnetic flux in the rotor is uniformly distributed due to the axial channels.
To avoid saturation of stator teeth, wider teeth are required. For a PM brushless motor rated at 200 W efficiency of 90% is achievable, i.e., 9% higher than that of a motor with solid rotor core.
The PMs in the form of rectangular blocks or segments are secured on the laminated hub and embraced using a nonferromagnetic retaining sleeve (metal or carbon-graphite). The thickness of the sleeve is designed to be sufficient to protect PMs against centrifugal forces and provide the required air gap (mechanical clearance).
FIG. 12 Rotors of aerospace PM brushless motors with magnets arranged in
Halbach cylinder: (a) two-pole rotor; (b) four-pole rotor; (c) eight-pole rotor.
Other rotor constructions for aerospace PM brushless motors are shown in FIG. 12 Halbach cylinder adds from 3 to 5% to the air gap magnetic flux density. Rotors shown in FIG. 12 are sometimes equipped with an inner heat exchanger for intensive air cooling.
Most brushless d.c. motors in aircraft electromechanical drive systems use encoders or Hall sensors, but significant developments are coming up in favor of search coil techniques. Sensorless control methods increase electronics complexity, but improve reliability and allow the motor to operate at elevated temperature.
Aerospace brushless d.c. motors are easily integrated with their drive electronics in compact packages (integrated electromechanical drives). This reduces the number of sub-assemblies, connecting wires, production cost, space and mass. The motor is fully controllable and protected against faults. Life is limited by bearings only.
One of the main problems in designing electric motors for aerospace and defense is that the space envelope is often small and the motor must be very small. For example, an aerospace PM brushless motor rated at 90 kW, 32, 000 rpm, 95% efficiency, measures 125 mm in its outer stack diameter and 100 mm stack length (without winding overhangs).
6.2 High speed spindle drives
Owing to the improvements in power electronics and control techniques the electrical spindle drives have shown a significant evolution. At present time very high speed spindle drives acquire the speed range between 10,000 and 100,000 rpm. Moreover, there is demand to increase the speed limit up to 300,000 rpm for special applications. This evolution has been a consequence of the high speed metal cutting, milling and grinding machine tools used in manufacturing processes, e.g., light alloys for aerospace applications.
Spindle drives are used by commercial aircraft manufacturers in both drilling rivet holes and milling stringers, spars, and precision components. The main demands for high speed electrical spindles are:
• higher "power speed" product compared with the values for standard applications, particularly for milling and grinding machines;
• increased bearing robustness as a result of high mechanical stresses during the machining processes;
• position control at zero speed in order to allow for automatic changing of tools;
• high efficiency cooling system to reach the highest output power-to- volume ratio compatible with the actual magnetic and insulation material technology;
• suitable lubrication system in order to get high quality behavior and minor friction problems;
• capability to work in different positions;
• duty cycle requirements in proportion to the large size of the pieces to work.
FIG. 13. Rotor with embedded PMs for a brushless spindle motor.1-PM, 2 -
laminated core, 3 - inner sleeve. Photo courtesy of Mitsubishi, Nagoya, Japan.
PM brushless spindle motors for machine tools in comparison with their induction counterparts display much higher efficiency in the low-speed, high-torque range. This makes it possible to reduce the size of the spindle unit and to simplify the cooling structures, taking advantage of the lower heat generation. FIG. 13 shows a prototype of an embedded PM rotor. The brushless motor with embedded PM rotor can produce both synchronous and reluctance torque.
FIG. 14. Steady-state characteristics of a 12 kW, 500 to 20,000 rpm PM brushless
spindle motor (SD60124 type) manufactured by Fischer Precise, Racine, WI, U.S.A.
FIG. 14 shows output power-speed and torque-speed characteristics of a 12 kW, four-pole, 500 to 20,000 rpm, 65 A PM brushless spindle motor with liquid water cooling system. The length of the motor housing is 516.5 mm and outer diameter of housing is 119 mm. The length of the stator stack with winding overhangs is about one quarter of the housing length.
FIG. 15. Construction of a typical FES with PM brushless machine. 1 - fly
wheel, 2 - rotor of PM brushless motor/generator, 3 - stator of PM brushless
motor/generator, 4 - radial magnetic bearing, 5 - backup bearing, 6 - thrust
magnetic bearing, 7 - burst shield, 8 - vacuum containment, 9 - vacuum.
FIG. 16. Comparison of energy storage systems.
Table 4. Comparison of mechanical properties of metals and composite materials
6.3 Flywheel energy storage
A flywheel energy storage ( FES) system draws electrical energy from a primary source, such as the utility grid, and stores it in a high-density rotating flywheel. The flywheel system is actually a kinetic, or mechanical battery, spinning at very high speeds (> 20, 000 rpm) to store energy that is instantly available when needed. Upon power loss, the motor driving the flywheel acts as a generator. As the flywheel continues to rotate, this generator supplies power to the customer load.
Advanced FES systems have rotors made of high strength carbon-composite filaments that spin at speeds from 20,000 to over 50,000 rpm in a vacuum en closure and use magnetic bearings. Composites are desirable materials for flywheels due to their light weight and high strength. Lightness in high speed rotors is good from two standpoints: the ultra-low friction bearing assemblies are less costly and the inertial loading which causes stress in the material at high rotational speeds is minimized. High strength is needed to achieve maximum rotational speed.
Flywheel circumferential speed is higher than that given in Table 1. For example, for diameter D =0.5 m and 50,000 rpm the linear surface speed is 1308 m/s.
The kinetic energy stored in a rotating homogeneous disk with its outer diameter D, thickness t and density ? is [...]
The maximum energy density of a rotating mass m is dependent on only the failure stress sf of the material and its specific mass density ?. The best materials for high performance flywheels are those with high value of the failure stress-to-mass density ratio (material index). Composite materials such as, for example, glass fiber reinforced polymer (GFRP) or carbon fiber rein forced polymer (CFRP) are much better than metals or metal alloys (Table 4).
FIG. 15 shows a typical FES with PM brushless machine and magnetic bearings. To minimize tooth ripple losses in retaining sleeve and PMs, a slotless stator is sometimes used. When the mechanical energy is stored in the flywheel, the PM brushless machine operates as a motor. When the energy stored in the flywheel is utilized as electrical energy, the PM brushless machine operates in generating mode.
Flywheels have a much higher power density than batteries ( FIG. 16), typically by a factor of 5 to 10. While batteries can supply backup power for a significantly longer period than a flywheel and consume less standby power, most other characteristics favor a flywheel, i.e.,
• the design life for a flywheel is typically about 20 years, while most batteries in uninterruptible power supply (UPS) applications will last only 3 to 5 years;
• batteries must be kept within a narrow operating temperature range about room temperature, while flywheels tolerate normal outdoor ambient temperature conditions;
• frequent cycling has little impact on flywheel life, while frequent cycling significantly reduces battery life;
• flywheel reliability is 5 to 10 times greater than a single battery string or about equal to two battery strings operating in parallel;
• flywheels are more compact, using only about 10 to 20% of the space required to provide the same power output from batteries;
• flywheel maintenance is generally less frequent and less complicated than for batteries;
• flywheels avoid battery safety issues associated with chemical release.
6.4 Dental handpieces
Electric motor driven dental handpieces are rapidly replacing traditional air turbine driven handpieces. Air turbine driven high-speed handpieces typically develop speeds between 250,000 and 420,000 rpm and relatively low torque.
Electric motor driven handpieces have speeds up to 200,000 rpm and relatively high torque. This implies that air turbine driven handpieces are faster than electric handpieces. However, when a bur in an air turbine driven handpiece contacts material to be cut, the speed will drop by as much as 40% or more (depending on the hardness of the material) because the air pressure is insufficient to maintain the speed of the turbine under high load. An electric motor driven handpiece offers smooth, constant torque that does not vary as the bur meets resistance. Because of the absence of air, electric handpieces are quieter and the chance of air embolism in a surgical site is eliminated.
Electric motor driven handpieces offer cutting power from 33 to 45 W (greater than air turbine driven handpieces).
FIG. 17. Dental handpiece with PM brushless motor. Courtesy of Kavo Dental,
Lake Zurich, IL, U.S.A.
FIG. 18. Small 13-mm diameter, 73.6-mNm, 50-V d.c. BO512-050 PM brushless
motor (Table 9.5). Photo courtesy of Portescap, A Danaher Motion Company, West
Chester, PA, U.S.A.
Table 5. Small-diameter brushless slotted BO512-050 PM brushless motors for
dental and power surgical instruments manufactured by Portescap, A Danaher
Motion Company, West Chester, PA, U.S.A.
FIG. 19. Speed-torque characteristics of BO512-050 PM brushless motor (Table
5). Photo courtesy of Portescap, A Danaher Motion Company, West Chester, PA,
U.S.A.
In electric handpieces, the bur is connected through gears in the head of the handpiece to a central drive shaft that is physically turned by the motor ( FIG. 17). Electric handpieces have typically a step up 5:1 gear ratio. The Electro torque handpiece ( FIG. 17) from Kavo Dental, Lake Zurich, IL U.S.A. with PM brushless motor offers a speed range of 2,000 to 40,000 rpm at the motor shaft. When used in combination with their 25LPA handpiece (5:1 gear ratio), speeds can be increased to 200,000 rpm.
Small 13-mm diameter, four-pole, 73.6-mNm, 50-V d.c. slotted PM brush less motor for dental instruments and power surgical devices is shown in Fig. 18. It can withstand in excess of 1000 autoclave cycles. The speed-torque characteristic is shown in FIG. 19.
To reduce core losses and temperature of the stator, low loss ferromagnetic alloys are used for the magnetic circuit, e.g., Megaperm c 40 L with specific losses 0.2 W/kg at 1 T and 50 Hz or amorphous alloys.
Electric dental handpieces show the following advantages:
• high torque with very little stalling;
• quiet and smooth operation with a reduced potential for hearing damage and a less irritating sound for patients;
• low levels of vibration;
• precision cutting with high concentricity;
• one electric motor runs several handpiece attachments (high speed and low speed);
• low-speed attachments for the electric motor allow for easy cutting of dentures, temporary resin restorations, orthodontic appliances, occlusal splints, plaster or stone.
The disadvantages of high-speed electric dental handpieces include:
• higher price and weight compared with high-speed air-rotor handpieces;
• the heads on the contrangles are comparatively large;
• because of high torque, the dentist inadvertently may place excessive load on a tooth during cutting;
• infection control measures must be observed carefully to avoid damaging the handpiece with repeated sterilizations.
1. Autoclave is an apparatus (as for sterilizing) using superheated high-pressure steam cycles.
6.5 Sheep shearing handpieces
The force required to move a handpiece through wool can be reduced by limiting the number of teeth on the cutter and increasing the stroke of the cutter. The overall dimensions and temperature rise require a built in drive and motor efficiency of at least 96%. A feasibility study has indicated that a 2-pole PM brushless d.c. motor can meet these requirements. The most promising appeared to be a slotless geared drive with an amorphous stator core and unskewed winding ( FIG. 20). To minimize the motor losses the controller supplies sinusoidal voltages to match the back EMF waveforms and a position sensor has a phase-locked loop to allow the phase of the supply to be electronically adjusted.
FIG. 21. Cross section and magnetic flux distribution of a 100-kW, four-pole,
40,000-rpm high speed PM brushless motor.
FIG. 20. PM brushless motor for sheep shearing handpiece: (a) cross section;
(b) disassembled motor. 1 - two-pole NdFeB rotor, 2 - air gap (mechanical clearance),
3 - slotless armature winding, 4 - slotless amorphous laminations, 5 - housing.
Courtesy of CSIRO, Newcastle, New South Wales, Australia.
The stator winding losses can be reduced by applying a high magnetic flux density in the air gap (NdFeB PMs) and axial straight conductors rather than skewed conductors. The core losses can be minimized by reducing the flux change frequency leading to the choice of a two-pole rotor. Further reduction can be achieved by using an amorphous magnetic alloy instead of silicon steel or by eliminating the stator teeth and using a slotless core. The stray losses in the armature winding can be reduced by stranding the conductors. The eddy current losses in the frame can be reduced by ensuring the armature stack is radially not too thin, so that the external magnetic field is low.
The measured data of an experimental three-phase, two-pole, 13,300 rpm, 150-W brushless d.c. motor with NdFeB PMs and slotless amorphous stator core (Metglas 2605-S2) for sheep shearing are: full load phase current Ia = 0.347 A, total losses 6.25 W, phase winding resistance R1 =7 Ohm at 210C, mean magnetic flux density in the stator core 1.57 T and full load efficiency ? =0.96.