AMAZON multi-meters discounts AMAZON oscilloscope discounts
(cont. from part 1)
9. Construction of motors with disk rotors
Stators of large axial flux PM brushless motors with disk type rotors usually have three basic parts:
• aluminum cold plate
• bolted ferromagnetic core
• polyphase winding
The cold plate is a part of the frame and transfers heat from the stator to the heat exchange surface. The slots are machined into a laminated core wound in a continuous spiral in the circumferential direction. The copper winding, frequently a Litz wire, is placed in slots and then impregnated with a potting compound. The construction of a double disk motor developed by Kaman Aerospace EDC, Hudson, MA, U.S.A. is shown in FIG. 16. Specifications of large axial flux motors manufactured by Kaman are given in Table 4.
FIG. 17. PM transverse flux motor: (a) single sided, (b) double sided.1-PM,
2 - stator core, 3 - stator winding, 4 - stator current, 5 - rotor yoke, 6
- mild steel poles shoes, 7 - magnetic flux.
FIG. 18. Three-phase TFM consisting of three single phase units with: (a)
internal stator, (b) external stator.
10. Transverse flux motors
10.1 Principle of operation
In a transverse flux motor (TFM) the electromagnetic force vector is perpendicular to the magnetic flux lines. In all standard or longitudinal flux motors the electromagnetic force vector is parallel to the magnetic flux lines. The TFM can be designed as a single-sided ( FIG. 17a) or double-sided machine (FIG. 17b). Single-sided machines are easier to manufacture and have better prospects in practical applications.
The stator consists of a toroidal single-phase winding embraced by U shaped cores. The magnetic flux in U-shaped cores is perpendicular to the stator conductors and direction of rotation. The rotor consists of surface or buried PMs and a laminated or solid core. A three-phase machine can be built of three of the same single phase units as shown in FIG. 18. The magnetic circuits of either stator or rotor of each single-phase unit should be shifted by 3600/(pm1) mechanical degrees where p is the number of the rotor pole pairs and m1 is the number of phases. A TFM with internal stator ( FIG. 18a) has a smaller external diameter. It is also easier to assemble the winding and internal stator cores. On the other hand, the heat transfer conditions are worse for internal than for external stator.
If the number of the rotor PM poles is 2p, the number of the stator U shaped cores is equal to p, i.e., the number of the stator U-shaped cores is equal to the number of the rotor pole pairs p. Each of the U-shaped cores creates one pole pair with two poles in axial direction. The more the poles, the better utilization and smoother operation of the machine. The power factor also increases with the number of poles. TFMs have usually from 2p =24to72 poles. The input frequency is higher than power frequency 50 or 60 Hz and the speed at an increased frequency is low. For example, a TFM with 2p = 36 fed with 180 Hz input frequency operates at the speed ns = f/p = 180/18 = 10 rev/s = 600 rpm.
Specifications of small two-phase and three-phase TFMs manufactured by Landert-Motoren AG, Bulach, Switzerland are shown in Table 5.
The peak value of the line current density of a single phase is
Am =
v2IaN1 2t
= p v2IaN1 pDg (eqn.18)
where Ia is the stator (armature) rms current, N1 is the number of turns per phase, t is the stator pole pitch and Dg is the average air gap diameter.
At constant ampere turns-to-diameter ratio the line current density can be increased by increasing the number of pole pairs. Since the force density (shear stress) is proportional to the product AmBmg, the electromagnetic torque of the TFM is proportional to the number of pole pairs. The higher the number of poles, the higher the torque density of a TFM.
Since at large number of poles and increased frequency the speed is low and the electromagnetic torque is high, TFMs are inherently well-suited propulsion machines to gearless electromechanical drives. Possible designs of magnetic circuits of single-sided TFMs are shown in FIG. 19. In both designs the air gap magnetic flux density is almost the same. However, in the TFM with magnetic shunts the rotor can be laminated radially .
Table 5. TFMs manufactured by Landert-Motoren AG, Bulach, Switzerland.
FIG. 19. Practical single-sided TFMs: (a) with magnetic shunts and surface
PMs, (b) with twisted stator cores and surface PMs.
FIG. 20. Dimensions of U-shaped stator core and coil.
10.2 EMF and electromagnetic torque
According to eqn the first harmonic of the magnetic flux per pole pair per phase excited by the PM rotor of a TFM is Ff1 = 2 p tlpBmg1 (eqn. 19)
where t = pDg/(2p) is the pole pitch (in the direction of rotation), lp is the axial length of the stator pole shoe ( FIG. 20) and Bmg1 is the first harmonic of the air gap peak magnetic flux density. With the rotor spinning at constant speed ns = f/p, the fundamental harmonic of the magnetic flux is [...]
10.4 Armature reaction and leakage reactance
The mutual reactance corresponding to the armature reaction reactance in a synchronous machine can analytically be calculated in an approximate way.
One U-shaped core (pole pair) of the stator can be regarded as an a.c. electromagnet with N1 turn coil which, when fed with the sinusoidal current Ia, produces peak MMF equal to v2IaN1. The equivalent d-axis field MMF per pole pair per phase which produces the same magnetic flux density as the armature reaction MMF is [...]
10.4 Armature reaction and leakage reactance
The mutual reactance corresponding to the armature reaction reactance in a synchronous machine can analytically be calculated in an approximate way.
One U-shaped core (pole pair) of the stator can be regarded as an a.c. electromagnet with N1 turn coil which, when fed with the sinusoidal current Ia, produces peak MMF equal to v2IaN1. The equivalent d-axis field MMF per pole pair per phase which produces the same magnetic flux density as the armature reaction MMF is [...]
10.6 Advantages and disadvantages
The TFM has several advantages over a standard PM brushless motor, i.e., (a) at low rotor speed the frequency in the stator (armature) winding is high (large number of poles), i.e., a low speed machine behaves as a high speed machine, which is the cause of better utilization of active materials than in standard (longitudinal flux) PM brushless motors for the same cooling system, i.e., higher torque density or higher power density;
(b) less winding and ferromagnetic core materials for the same torque;
(c) simple stator winding consisting of a single ring-shaped coil (cost effective stator winding, no end connection);
(d) unity winding factor (kw1 = 1);
(e) the more the poles, the higher the torque density, higher power factor and less the torque ripple;
(f) a three-phase motor can be made of three (or multiples of three) identical single-phase units;
(g) a three-phase TFM can be fed from a standard three-phase inverter for PM brushless motors using a standard encoder;
(h) the machine can operate as a low speed generator with high frequency output current.
Although the stator winding is simple, the motor consists of a large number of poles (2p = 24). There is a double saliency (the stator and rotor) and each salient pole has a separate "transverse flux" magnetic circuit. Careful attention must be given to the following problems:
(a) to avoid a large number of components, it is necessary to use radial laminations (perpendicular to the magnetic flux paths in some portions of the magnetic circuit), sintered powders or hybrid magnetic circuits (laminations and sintered powders);
(b) the motor external diameter is smaller in the so-called "reversed design," i.e., with external PM rotor and internal stator;
(c) the TFM uses more PM material than an equivalent standard PM brush less motor;
(d) the power factor decreases as the load increases and special measures must be taken to improve the power factor;
(e) as each stator pole faces the rotor pole and the number of stator and rotor pole pairs is the same, special measures must be taken to minimize the cogging torque.
FIG. 21. PM brushless motor rated at 36.5 MW and 127 rpm for advanced ship
propulsion. Photo courtesy of DRS Technologies, Parsippany, NJ, U.S.A.
FIG. 22. Pod propulsor with electric motor.
11. Applications
11.1 Ship propulsion
The ship electric propulsion scheme shown in Fig. 25 requires large electric motors. For example, a 90,000 gt cruise ship employs two 19.5 MW synchronous motors. Low speed PM brushless motors offer significant savings in mass (up to 50%) and efficiency (2 to 4% at full load and 15 to 30% at partial load) as compared to high speed synchronous motors with electromagnetic excitation and reduction gears. FIG. 21 shows the most powerful PM brush less motor in the world for advanced ship propulsion rated at 36.5 MW and 127 rpm.
An optimum undisturbed water inflow to the propeller and consequently reduced propeller pressure pulses (causing vibration and noise) and increased propulsion efficiency can be achieved with the aid of pod propulsor (FIG. 22).
Reduction of vibration and noise considerably enhances passenger comfort.
The propeller acts as a tractor unit located in front of the pod. The pod can be rotated through 3600 to provide the required thrust in any direction. This eliminates the requirement for stern tunnel thrusters and ensures that ships can maneuver into ports without tug assistance.
FIG. 23. 20-MW TFM for electric ship propulsion: (a) construction; (b) longitudinal
section. 1 - U-shaped twisted core, 2 - stator winding, 3 - PM, 4 - mild steel
pole. Courtesy of Rolls-Royce, Derby, U.K.
Table 6. Specification data of a 20 MW TFM prototype for naval ship propulsion.
Courtesy of Rolls-Royce, Derby, U.K.
Rolls-Royce, Derby, U.K. has been developing a 20 MW TFM for naval electric ship propulsion ( FIG. 23). The TFM has been considered to be the optimum propulsion motor to meet the high efficiency and power density requirements. Each phase consists of two rotor rims with buried PMs and two armature coils (outer phase and inner phase). An 8-phase TFM emits a low noise and has significant reverse mode capability as all eight phases are truly independent from one another. The stator components are supported by a water-cooled aluminum frame. The specifications of the TFM are shown in Table 6.
To date, TFMs are predicted primarily for a conventional (inboard) fit for a surface ship or submarine. The use of the TFM for podded electromechanical drives is highly attractive due to the low mass of the motor in comparison with other PM brushless motors.
FIG. 24. Diesel-electric submarine: 1 - forward battery cells, 2 - snorkel
(air intake), 3 - after battery cells, 4 - exhaust, 5 - diesel engines (compartment
1), 6 - generators (compartment 1), 7 - diesel engines (compartment 2), 8 -
generators (compartment 2), 9 - electric propulsion motors, 10 - reduction
gear, 11 - propulsion shafts, 12 - propeller.
11.2 Submarine propulsion
A typical "fleet" type submarine with four diesel engines and four electric motors is shown in FIG. 24. The diesel engines are coupled to electric generators which supply power to large electric propulsion motors which turn the propellers and charge batteries. Approximately, a 100m long submarine uses four 1.2 MW diesel engines and 4 MW total power of electric propulsion motors. Diesel engines are divided between two compartments separated by a watertight bulkhead. If one room becomes flooded, the other two engines can still be operated.
When the submarine submerges below periscope depth, the diesel engines are shut down and the electric motors continue to turn the propellers being fed from batteries. The snorkel system permits a submarine to use its diesel engines for propulsion or charge batteries, while operating submerged at the periscope level. Both the air intake and exhaust are designed as masts which are raised to the position above the surface of the water.
FIG. 25. Large PM synchronous motor Permasyn for submarine propulsion. Courtesy
of Siemens AG, Hamburg, Germany.
A large PM synchronous motor Permasyn with low noise level, high efficiency and small mass and volume specially designed for submarine propulsion at Siemens AG, Hamburg, Germany is shown in FIG. 25.
FIG. 26. A hybrid bus drive system with a.c. motors and reduction gears integrated
into each of its four driven wheels.
11.3 Hybrid electric transit bus
Modern transit buses should, amongst other things, emit reduced pollution and be designed with low floors for easy access to people with physical problems. A hybrid electric bus with electric motors integrated in each of its four driven wheels can meet these requirements.
Table 7. Hybrid transit bus performance
The components included in the hybrid transit bus ( FIG. 26) are brushless motors (induction, PM or switched reluctance) to supply or accept power from the wheels, power electronics converters, a battery for energy storage, and the auxiliary power unit consisting of a diesel engine, alternator, rectifier, and associated control. The bus is a series hybrid with four independent brushless motors - one integrated into each driven wheel.
To minimize emissions and maximize fuel economy the electric drives are powered by a battery-assisted auxiliary power unit consisting of a down-sized combustion engine coupled to an alternator operating under closed-loop control. Each electromechanical drive consists of a 75-kW brushless oil-cooled motor integrated with a transmission into a compact wheel motor unit. Since the transmission is a simple single-speed device, and the integrated wheel motor units are located right at the drive wheels, torque is transmitted to the four rear drive wheels very efficiently and the need for an expensive and heavy multi-speed transmission and a bulky rear axle-differential assembly are eliminated. The output torque from each propulsion motor is controlled by a microprocessor-based electronics package containing both a d.c. - a.c. inverter and the actual motor-control circuitry. The auxiliary power unit sup plies electric power to each a.c. integrated wheel motor in addition to charging the propulsion batteries and supplying power to operate electric accessories.
In fact, the auxiliary power unit supplies average power to the drives, while the propulsion batteries fill in the demand peaks during vehicle acceleration and also receive power during regenerative braking. The auxiliary power unit is rated at 100 kW with controls to operate over a d.c. voltage range from 250 to 400 V. A battery with a capacity of 80 Ah and a total energy storage of about 25 kWh is required.
Table 8. Comparison of different 75 kW brushless motors for electrical vehicles.
Courtesy of Voith Turbo GmbH & Co. KG, Heidenheim, Germany.
The hybrid bus uses its electronic drive controls to operate the a.c. motors as generators during deceleration or while maintaining speed on a downhill grade to recover a portion of its kinetic energy. The recovered energy is routed to the propulsion battery, thus lowering fuel consumption and emissions.
The performance of a 12.2-m long hybrid electric transit bus with a seated load mass of 15, 909 kg and gross vehicle mass of 20, 045 kg are shown in Table 7.
Table 8 shows a comparison of different 75-kW brushless electric motors for propulsion of hybrid electric buses.
The induction motor (IM) has a rotor cage winding. The switched reluctance motor (SRM) has 6 poles in the stator (2 poles per phase) and 4 poles in the rotor.
The hybrid synchronous motor (HSM) has both PM and electromagnetic excitation. The PMs are located on the rotor tooth faces and the d.c. excitation winding is located in rotor slots. The electromagnetic excitation is used to boost the starting and allows a real field weakening in the upper speed range.
The PM synchronous motor (PMSM) has been designed with surface configuration of rotor PMs.
The polyphase PM modular synchronous motor (PMMSM) has the stator coil span equal to only one tooth instead of full pole pitch; i.e., it has concentrated non-overlapping coils (FIG. 8). Thus, the end connections are extremely short and the winding power losses are substantially reduced. The simple coil shape is easy to manufacture using automated techniques and allows for a high slot fill factor.
The double-sided TFM has U-shaped stator cores made of powder materials, buried magnets and external rotor.
FIG. 27. Gearless motor wheel for a light rail vehicle with PM brushless
motor:
1-stator, 2-external rotor with PMs, 3-axle of the wheel, 4-rotor enclosure, 5 - terminal board, 6 - rim of the wheel, 7 - brake.
FIG. 28. Comparison of direct wheel electromechanical drives with brushless
motors. 1 - PM TFM, 2 - induction motor, 3 - clutch.
11.4 Light rail system
The most important advantages of a direct electromechanical drive with gear less PM brushless motor over a geared motor are:
• the gravity center of the bogie can be lowered;
• the wheel diameter can be reduced as motors are removed from the trolleys and gearboxes eliminated;
• it is easy to design a steerable bogie for negotiating sharp curves;
• electromechanical drives with gearless motors require limited maintenance (no oil);
• the noise is reduced.
Typical full load mass of a street car is 37 t and wheel diameter is 0.68 m.
Four gearless motors with rated torque of 1150 Nm can replace the traditional propulsion system. The motor-wheel is shown in FIG. 27. The motor is built in the wheel in order to minimize the overall dimensions and to improve the heat transfer. To achieve a speed of 80 km/h the rated power of the motor should be about 75 kW. The external rotor with PMs is integral with the wheel and the stator (armature) is integral with the spindle. Representatively, the armature diameter is 0.45 m, the external motor diameter is 0.5 m, the armature stack length is 0.155 m, the air gap magnetic flux density is 0.8 T and the armature line current density is 45 kA/m. The motor is located externally while the disk brake is located internally.
The size of the direct wheel drive with PM TFM as compared with an equally rated induction motor is shown in FIG. 28.
In the 1990s the Railway Technical Research Institute (RTRI) in Tokyo (Kokubunji), Japan, carried out research in traction PM brushless motors for narrow gauge express train to reach a maximum speed of 250 km/h. Japan has about 27,000 route km of narrow 1067 mm track and over 2000 route km of standard 1435 mm Shinkansen track. The target speed increase on narrow gauge tracks in Japan is over 160 km/h at the beginning of the 21st century (a speed record of 256 km/h on 1067 mm track was achieved in 1973 in the Republic of South Africa).
Several motor designs and drive configurations have been studied. The specifications of the RMT1A motor with surface PMs rated at 80 kW, 580 V, 1480 rpm motor are given in Table 9 and steady state load characteristics are plotted in FIG. 29. The gearless wheel-mounted motor with internal stator (armature) is shown in FIG. 30.
FIG. 29. Steady-state characteristics of RMT1A PM brushless motor at constant
speed n = 1480 rpm.
FIG. 30. RMT1A traction PM brushless motor: 1 - surface PMs,2-internal stator,
3 - position sensors, 4 - wheel. Courtesy of K. Matsuoka and K. Kondou, Railway
Technical Research Institute, Kokubunji, Japan.
Table 9. Specification data of RMT1 PM brushless motor for narrow gauge ex
press trains developed by K. Matsuoka and K. Kondou, Railway Technical Research
Institute, Kokubunji, Japan.
Numerical examples
Numerical example 1
The speed of a large, three-phase, 24-pole, brushless motor is 2000 rpm. The full pitch armature winding has been designed from AWG copper conductors with square cross section. The height of a single conductor is hc =3.264 mm, the number of conductors per slot arranged above each other in two layers is msl = 8 and the conductivity of copper at 750Cis s1 =47 × 10^6 S/m. The ratio of the end connection-to-effective core length is l1e/Li =1.75. Find the coefficient of skin effect.
Solution
The frequency of current in the armature conductors is ...
The coefficient of skin effect is too high and either stranded round conductors or rectangular conductors with transpositions must be used. The winding has been incorrectly designed since at 400 Hz the winding losses exceed 1.961 (1200 square wave) and 2.486 (1800 square wave) times the losses calculated for d.c. current.
Numerical example 2
A three-phase, 50-kW, 6000-rpm, 200-Hz, 346-V (line-to-line), Y-connected PM synchronous motor has stator winding impedance Z1 = R1 + jX1 = 0.027 + j0.026 Ohm, inner stator diameter D1in =0.18 m, effective length of stator core Li =0.125 m, number of stator turns per phase N1 = 32, stator winding factor kw1 =0.96 and air gap magnetic flux density Bmg =0.7 T. The PMs are uniformly distributed on the rotor surface. The total air gap (PMs, damper, bandage, mechanical clearance) multiplied by the Carter's coefficient kC and saturation factor ksat is gt = gkCksat+hM/µrrec = 15 mm. The stator core losses are equal to 2% of the output power, the additional losses are equal to 1% of the output power and the rotational losses are equal to 436 W. Find the output power, efficiency and power factor at load angle d =14.40.
Table 10. Design data of a single-sided three-phase TFM with external rotor
Numerical example 4
The design data of a single-sided three phase TFM with external rotor and surface PMs are given in Table 10. The stator windings are Y-connected and the stator magnetic circuit is composed of U-shaped cores with I-shaped magnetic flux shunts between them (FIG. 19a). Both U-shaped and I-shaped cores are made of cold-rolled electrotechnical steel. The rotor PMs are glued to laminated rings (two rings per phase) of the same axial width as PMs.
Find the steady state performance of the TFM for Iad = 0. The saturation of the magnetic circuit and influence of the armature reaction flux on the rotor excitation flux can be neglected.
FIG. 31. Steady state characteristics at ns = 480 rpm: (a) input and output
power versus torque, (b) torque and phase voltage versus current, (c) efficiency
and power factor versus torque.
Although the power factor increases, both the shaft torque and efficiency slightly decrease.