18-12-2012, 06:04 PM
Design and Study of a Permanent Magnet Synchronous Motor for an Electric Compressor
Design and Study of a Permanent Magnet Synchronous.pdf (Size: 813.52 KB / Downloads: 93)
INTRODUCTION
Nowadays, electrical vehicles (EV) present a technological solution to reduce gas emissions. However
the success of EV is facing with some drawbacks, notably the sensitivity of the battery to climatic
conditions. In hard conditions, the autonomy of the battery may be reduced within 100 kms instead
of 150{200 kms; in addition the thermal comfort of EV is required. Consequently air conditioning
compressors are embedded in EV. Mechanical air compressors are now substituted by electrical
devices in order to improve e±ciency, compactness and °exibility. An example of an electric
compressor is given in Figure 1.
The electrical motor in such devices is subject to the following constraints. The necessary
electrical power is about 6kW and the maximal torque and speed are respectively equal to 6Nm
and 10000 rpm. The space required for the electrical motor is limited the raison why active length
and outer diameter were reduced. The mass of the motor, must be lesser than 2 kg. Furthermore
torque oscillations must not exceed 3% of the average torque to reduce the electromagnetic noise.
The supply voltage depends on the battery level (200 to 410V DC). Due to the restricted space,
the cooling gas °ows through the motor (stator and airgap).
In this paper, the design and the study of a permanent magnet synchronous motor able to satisfy
the previous constraints is proposed. The ¯rst part of the paper deals with motor structures. The
second part is dedicated to an analytical model (reluctance networks) used to pre-design the motor.
In order to have more accurate results, the third part concerns a 2D ¯nite element model. The
features of the proposed motor are ¯nally given.
PROPOSED ELECTRICAL MOTOR
Dierent types of electrical motor can be used in air compressor. Synchronous reluctance ma-
chine [1], wounded rotor induction motor [2] or synchronous permanent magnet motor with con-
centrated °ux [3] can be found. The main drawback of almost con¯gurations is the torque ripple [1]
and some con¯gurations present a low e±ciency.
ANALYTICAL MODEL
The analytical model is presented under the form of a 2D reluctance network [7]. Only half part
of the motor is taken into account due to the magnetic symmetry, see Figure 2(b). In this model,
the stator yoke and teeth reluctance are represented by Rs and Rt respectively. The °ux leakage
between the teeth is modelled by RL. The reluctance Ra represents the airgap and magnet thickness
and is calculated using Carter factor. The magnetomotive force due to the \n" coil is given by fn
and the one due to the \p" magnet is given by fmp. The magnetomotive forces due to the magnets
are function of the rotor displacement (sinusoidal waveform). All the reluctance values are constant.
The following procedure has been applied to size the motor. Optimal tooth and magnet openings
giving low ripples are used (®t = 0:45 £ ¸t and ®m = 0:4 £ ¸m). The thicknesses of the stator and
rotor yokes have been chosen equal to 6mm to avoid magnetic saturation. The diameter of the
conductors was ¯xed to 1mm and the ¯lling factor equal to 0.4. The motor length is ¯xed (do not
exceed 50mm) and also the airgap (do not exceed 1 mm), thus only two parameters de¯ne entirely
the geometry: ra (radius in the middle of the airgap) and lm (magnet thickness). The variations
of theses parameters give the maximal electromotive force value (emf) for one phase and torque
ranges in Figure 3. The model was built around initial values (ra = 19:2mm and lm = 2:9 mm).
FINITE ELEMENT STUDY
The model is established with the software Femm [8]. The \weighted stress tensor" method is
used to obtain the torque. The movement is simulated either via a complete remeshing at each
rotation step or via a method equivalent to the locked step approach (based on the use of periodical
conditions). This last permits to work with a constant number of elements. One example of mesh
is given in Figure 4(a).
Stator and rotor yokes are composed of laminated steels (thickness of 0.35 mm, length of 41 mm).
The steel grade is M330-35A (silicon iron, equivalent to former standard AISI M36). Rare earth
magnets (NdFeB) are used with a coercivity of 955 kA/m. The wire diameter and the ¯lling factor
are equal to 1.4mm and 0.4 respectively. The tooth and magnet openings respect the previous
constraints on purpose to minimize the no-load torque ripples. The geometry is built again with
only the variables ra and lm. The average torque and emf curves depending on the variations of
theses parameters are represented on Figures 5(a) and (b).