Permanent Magnet Motor: High-Speed Motor (part 1)

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High-speed motors that develop rotational speeds in excess of 5000 rpm are necessary for centrifugal and screw compressors, grinding machines, mixers, pumps, machine tools, textile machines, drills, handpieces, aerospace, flywheel energy storages, etc. The actual trend in high-speed electromechanical drives technology is to use PM brushless motors, solid rotor induction motors or switched reluctance motors. The highest efficiency and highest power density is achieved with PM brushless motors.

1. Why high-speed motors?

The output equation for a PM synchronous machine indicates that the developed power is basically proportional to the speed and the volume of the rotor, i.e., the output coefficient itself is dependent upon both the rotor geometry and speed. Thus, equation can be rewritten to obtain the output power per rotor volume of a synchronous machine proportional to the synchronous speed ns, peak value of the air gap magnetic flux density Bmg and peak value of the line current density Am of the armature winding, i.e.,


The higher the speed ns the higher the power density of the machine. Increase in speed ns (frequency) at the same output power Pout results in smaller volume and mass of the machine. Eqn (eqn. 1) also says that the power density can be increased by applying high magnetic flux density Bmg in the air gap and high line current density Am of the armature winding. High magnetic flux density Bmg can be achieved by using magnetic materials with high saturation magnetic flux density, e.g., cobalt alloys. High line current density Am can be achieved by using intensive cooling systems, e.g., liquid cooling system.

Iron-cobalt (Fe-Co-V) alloys with Co contents ranging from 15 to 50% have the highest known saturation magnetic flux density, up to 2.4 T at room temperature. Fe-Co-V alloys are the natural choice for applications such as aerospace, where mass and space saving are of prime importance. Addition ally, the Fe-Co-V alloys have the highest Curie temperature of any ferromagnetic alloy family and have found use in elevated temperature applications.

The nominal composition, e.g., for Hiperco 50 from Carpenter, PA, U.S.A. is 49% Fe, 48.75% Co, 1.9% V, 0.05% Mn, 0.05% Nb and 0.05% Si. Similar to Hyperco 50 is Vacoflux 50 (50% Co) and Vacodur 50 cobalt-iron alloy from Vacuumschmelze, Hanau, Germany.

2. Mechanical requirements

The rotor diameter is limited by the bursting stress at the design speed. The rotor axial length is limited by its stiffness and the first critical (whirling) speed.

Since the centrifugal force acting on a rotating mass is proportional to the linear velocity squared and inversely proportional to its radius of rotation, the rotor must be designed with a small diameter and must have a very high mechanical integrity. The surface linear speed (tip speed) of the rotor:

(eqn. 2)

… is an engineering measure of mechanical stresses on the rotor under action of the centrifugal forces. The maximum permissible surface linear speed depends on the rotor construction and materials.

FIG. 1. Single mass flexible rotor with residual unbalance and possible modes of oscillations: (a) 1st mode; (b) 2nd mode; (c) 3rd mode. O - center of rotation, G - center of gravity, P - geometric center.

When the shaft rotates, centrifugal force will cause it to bend out. For a single rotating mass m ( FIG. 1a) the first critical (whirling) rotational speed [...]

FIG. 2. Maximum rotor outer diameters for metal and carbon-graphite sleeves as functions of design speeds.

FIG. 3. Rotor of a 100 kW, 70,000 rpm PM brushless motor for an oil free compressor. 1 - PM rotor with a retaining sleeve, 2 - foil bearing journal sleeve. Photo courtesy of Mohawk Innovative Technology, Albany, NY, U.S.A.

Table 1. Characteristics of retaining sleeves for high speed PM brushless machines [Quantity non-magnetic metal sleeve non-metallic wound sleeve]

3. Construction of high speed PM brushless motors

Design guidelines for high speed PM brushless motors include:

• compact design and high power density;

• minimum number of components;

• high efficiency over the whole range of variable speed;

• power factor close to unity over the whole range of speed and load;

• ability of the PM rotor to withstand high temperature;

• optimal cost-to-efficiency ratio to minimize the system cost-output power ratio;

• high reliability (failure rate less than 5% within 80,000 h);

• low total harmonics distortion (THD).

Given below are the fundamental issues, which are essential in electromagnetic, mechanical and thermal design of high speed PM brushless motors:

• Volume and mass: the higher the speed, the higher the power density;

• Power losses and efficiency: special attention must be given to windage and core losses;

• Laminations: cobalt alloy, non-oriented silicon steel or amorphous alloy laminations;

• Stator conductors: small diameter stranded conductors or Litz wires;

• Higher harmonics generated by the solid state converter: parasitical effect as losses, vibration and noise depend on the harmonic content;

• Cooling system: intensive air or oil cooling system;

• PM excitation system: if the temperature of the hot end of the shaft exceeds 150 C, SmCo PMs must be used;

• Rotor tensile hoop stresses: properly selected rotor diameter, rotor diameter-to-length ratio and rotor retaining sleeve (material and thickness);

• Thermal compatibility of rotor materials: thermal expansions of the rotor retaining sleeve and rotor core produce compressing stresses on PMs,

fluctuating with the temperature;

• Rotor dynamics: the first critical speed of the rotor should be much higher or much lower than the rated speed.

The stator core is stacked of slotted or slotless laminations. For input frequencies 400 Hz and lower, 0.2-mm thick laminations are used. For higher frequencies, 0.1-mm laminations are necessary. Vacuum impregnated coils made of stranded conductors are inserted into slots. To minimize the space harmonics, the stator winding is made as a double layer winding with shorted coils. For very high speeds and low voltages, when the EMF induced in single turn stator coils is too high, small number of coils, single layer winding or parallel paths must be used. Hollow conductors and direct water cooling are too expensive for machines rated below 200 kW. The stator volume is affected by winding losses and heat dissipation.

PM rotor designs include bread loaf, surface-type, inset-type or interior-type rotors. All surface-type PM rotors are characterized by minimal leakage flux. Bread loaf surface-type PM rotors provide, in addition, the highest magnetic flux density in the air gap (large volume of PM material). All surface-type, including bread loaf and inset-type PM rotors, can be used only with an external rotor retaining sleeve (can). In the case of an interior-type PM rotors the retaining sleeve is not necessary, but the ferromagnetic bridge in the rotor core between neighboring PMs must be very carefully sized. From an electromagnetic point of view, this bridge should be very narrow to obtain full saturation, preventing the circulation of leakage flux between neighboring rotor poles. From a mechanical point of view, this bridge cannot be too narrow to withstand high mechanical stresses. In practice, interior-type PM rotors without retaining sleeves can be used at speeds not exceeding 6000 rpm.

Good materials for retaining sleeves are nonferromagnetic and have high permissible stresses, low specific density and good thermal conductivity. If the magnetic saturation effect is used effectively, a thin steel sleeve in low power machines can sometimes be better than a sleeve made of nonferromagnetic material. Typical materials, maximum operating temperatures and maximum surface linear speeds are given in Table 1. Maximum rotor outer diameters for metal and carbon-graphite sleeves as functions of design speeds are plotted in FIG. 2. A PM rotor with a metal retaining sleeve for a 110 kW, 70,000 rpm brushless motor is shown in FIG. 3. Reinforced plastics and brass can also be used for retaining sleeves.

To increase the electromagnetic coupling between the magnets and the stator, the air gap should be made as small as mechanically possible. However, the use of a small air gap increases the tooth ripple losses in the retaining sleeve, if the sleeve is made of current-conducting material.

To minimize the losses in the retaining sleeve and PMs, torque ripple and acoustic noise, the stator slots should have very narrow slot openings or be closed. In the case of closed stator slots, the slot closing bridge should be highly saturated under normal operating conditions.

Active radial and axial magnetic bearings or air bearings are frequently used. High speed PM brushless motors integrated with magnetic bearings and solid state devices are used in gas compressors providing a true oil free system, reduced maintenance and high efficiency.

FIG. 4. Typical rotor of a four-pole high speed PM brushless motor with bread loaf PMs and retaining sleeve: 1 - PM, 2 - rotor ferromagnetic core, 3 - shaft, 4 - nonmagnetic retaining sleeve, 5 - nonmagnetic material.

Typical rotor of a high speed PM brushless motor has bread loaf PMs secured by a metal or carbon-graphite retaining sleeve. A four-pole rotor with bread loaf PM is shown in FIG. 4. The 2D magnetic field distribution in the cross section of a four-pole PM brushless motor is plotted in FIG. 5. A six-pole rotor is shown in FIG. 6.

A PM rotor with retaining sleeve proposed by SatCon Technology Corp., Cambridge, MA, U.S.A., is shown in FIG. 7. The rotor is divided into segments. Within each segment a retaining sleeve holds the plastic bonded NdFeB PM magnetized radially. Although bonded NdFeB exhibits only half the remanent magnetic flux density of the sintered NdFeB, the lower electric conductivity limits the rotor eddy current losses at high speeds. A high speed motor developed by SatCon for centrifugal compressors rated at 21 kW, 47,000 rpm, 1567 Hz has the rotor diameter 46 mm, efficiency 93 to 95% and power factor cos f ˜ 0.91. The predicted reliability is 30,000 h lifetime and cost 13.4$/kW (10 $/hp) for rotor and stator set (assembly separate).

Specifications of small two-pole high speed motors with slotless windings manufactured by Koford Engineering, Winchester, OH, U.S.A. are given in Table 2. Slotless design reduces tooth ripple losses in the metal retaining sleeve and PMs and allows for reducing the stator outer diameter. These motors are designed either with Hall sensors or for sensorless controllers. High speed slotless motors listed in Table 2 can be used in laboratory pumps, aerospace applications, unmanned aircrafts, military robots, handpieces and medical instruments.

FIG. 5. Magnetic flux distribution in a cross section of a four-pole high speed PM brushless motor with ferromagnetic shaft.

FIG. 6. Rotor of a six-pole high speed PM brushless motor with metal retaining sleeve. Photo courtesy of Electron Energy Corporation, Landisville, PA, U.S.A.

FIG. 7. PM rotor of segmented construction: (a) single segment, (b) three-segment rotor. 1 - PM, 2 - nonferromagnetic material, 3 - retaining sleeve, 4 - steel sleeve, 5 - complete rotor consisting of three segments. Courtesy of SatCon, Cambridge, MA, U.S.A.

Table 2. Specifications of small high speed slotless PM brushless motors manufactured by Koford Engineering, Winchester, OH, U.S.A.

4. Design of high speed PM brushless motors

The objective function is generally the maximum output power available from a particular high speed motor at given speed. The power is limited by the thermal and mechanical constraints.

In the design of high speed PM brushless motors the following aspects should be considered:

(a) Mechanical design constraints are important due to the high cyclic stress placed on the rotor components. Materials with high fatigue life are favored. Materials with low melting points, such as aluminum, should be avoided or restricted.

(b) Capital and operational costs are generally directly linked. The use of magnetic bearings over traditional rolling element bearings or oil lubricated bearings is a very important consideration. The capital cost of magnetic bearings is high, but the operational costs are less since the rotational loss and power consumption are reduced and there is no maintenance.

(c) Dynamic analysis of the rotor assembly, including shaft, core stack and bearing sleeves should be carried out with great detail using the 3D FEM simulation.

(d) Static and dynamic unbalance. Even a very small unbalance can produce high vibration. For example, a static unbalance of 0.05 N at a speed of 100,000 rpm produces an additional centrifugal force of more than 600 N.

Unbalance occurs when the center of gravity of a rotating object is not aligned with its center of rotation. Static unbalance is where the rotor mass center (principal inertia axis) is displaced parallel to the rotor geometric spin axis.

Dynamic unbalance is where the rotor mass center is not coincidental with the rotational axis.

It is generally not difficult to design a high speed PM brushless motor rated at a few kWs and speed 7000 to 20,000 rpm with efficiency about 93 to 95%. The efficiency of high speed PM brushless motors rated above 80 kW and 70,000 to 90,000 rpm is over 96%. Core losses, windage losses and metal sleeve losses are high. Slotless stator, amorphous cores and foil bearings can increase the efficiency up to 98%.

FIG. 8a shows a 5-kW, 150, 000 rpm motor with surface PMs and nonferromagnetic stainless steel retaining sleeve. The input voltage is V1 = 200 V, input frequency is f = 2500 Hz, number of poles 2p = 2, the effective air gap is 6 mm, outer stator diameter is 90 mm, thickness of stator laminations is 0.1 mm, stator winding d.c. resistance per phase is R1 =0.093 Ohm, and stator winding leakage inductance is L1 =0.09 mH. The slot ripple losses can be substantially reduced by expanding the air gap. High energy NdFeB magnets are therefore required.

At the rated speed of 150,000 rpm, the rotor surface speed will reach nearly 200 m/s and the resultant stress is calculated as high as 200 N/mm^2.

Because this far exceeds the allowable stress of the magnets (80 N/mm^2), to prevent the magnets from exfoliating, initially, a nonferromagnetic stainless steel sleeve was shrunk on the PMs to retain them. Although the stainless steel has low electric conductivity, the losses occurred in a relatively thick can were still quite large at the speeds over 100,000 rpm. Nonconductive fiber reinforced plastic was then used.

To provide a high frequency (f = 2500 Hz) and realize a compact power circuit, a quasi-current source inverter (CSI) has been employed. This inverter consists of a diode rectifier, a current-controlling d.c. chopper and a voltage type inverter (FIG. 8b). To improve the input power factor, the large electrolytic capacitor Cd of the filter has been replaced with a substantially reduced film capacitor, which is enabled by the appropriate current control of the chopper.

FIG. 8. High speed, 5-kW, 150, 000 rpm PM brushless motor: (a) longitudinal section, (b) circuit configuration of the quasi-current source inverter.1-PMs, 2- retaining sleeve, 3 - stator core, 4 - stator winding, 5 - shaft; Dp on = inverter operates as a VSI, Dp off = inverter operates as a CSI.

(cont. to part 2)

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