IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 1, JANUARY 2005
505
Design Characteristics of the Induction Motor Used for Hybrid Electric Vehicle Tiecheng Wang, Ping Zheng, Qianfan Zhang, and Shukang Cheng
Abstract—Design characteristics of the induction motor used for hybrid electric vehicle (HEV) are discussed. Equivalent circuits corresponding to the starting, operating performance, and harmonics are given. The selecting rules of the starting voltage and frequency are got. The relations between the iron core and copper loss versus frequency are analyzed and compared. The influence of harmonics on the induction motor is analyzed. The design principles of the induction motor used for HEV are given considering the impacts of the starting, operating performance, and harmonics. Index Terms—Design, harmonics, induction motor, operating performance, starting performance.
I. INTRODUCTION
I
NDUCTION motors were used in hybrid electric vehicles (HEVs) very early because of its simple structure, low price, reliable operation, and mature control technique. Before the 1990s, people paid much attention to the inverter and control strategy, whereas the motors were not redesigned for the using of the inverter. So the induction motor system normally had the problem of high loss, low efficiency, low power factor, and low usage factor of the inverter, which is more serious for the high-speed, large-power motor. After the 1990s, this problem was taken into consideration, and people did much research on the design principle of the induction motors used for HEV [1]–[6]. The design theory of the induction motors used for HEV is discussed in the paper.
Fig. 1. Typical mechanical characteristic of the induction motor used for HEV.
II. DESIGN THEORY OF THE INDUCTION MOTOR USED FOR HEV BASED ON THE STARTING PERFORMANCE
Fig. 2. Equivalent circuit of the induction motor at the variable-frequency starting.
The starting performance of the induction motor is one of the most important performances that should be considered seriously. In the traditional induction motor supplied with industrial frequency power, deep-slot or double-cage rotor is often adopted to decrease the starting current and increase the starting torque. The induction motor used for HEV is supplied by inverter, which is different from the case of traditional motor. The typical mechanical characteristic of the induction motor used for HEV is the base frequency. Below the is shown in Fig. 1, where
base frequency, the motor has a constant torque characteristic, and above the base frequency, it has a constant power characteristic. Normally at the base frequency, the voltage supplied to the motor is rated or near rated one. The equivalent circuit of the induction motor at the variablefrequency starting is shown in Fig. 2. If the starting frequency , the coefficient in Fig. 2 is is
(1)
Manuscript received December 19, 2003. This work was supported in part by Chinese 863 project Jiefang brand hybrid electric city bus motor and control system 2001AA501522. The authors are with the Harbin Institute of Technology, Harbin 150001, China (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]). Digital Object Identifier 10.1109/TMAG.2004.838967
Since the alternating frequency of the magnetic field is very low at starting, the iron core loss is little. So the iron core loss resistance can be neglected. Thus, Fig. 2 can be simplified and are the equivalent parallel resisas Fig. 3, where tance and reactance of the magnetizing and rotor branch circuit respectively.
0018-9464/$20.00 © 2005 IEEE
506
IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 1, JANUARY 2005
Fig. 3. Simplified equivalent circuit of the induction motor at the variable-frequency starting.
From Fig. 3, the starting current of the induction motor at the variable-frequency starting is (2) The starting torque is (3) where is the phase number of the motor. Normally, the starting current is controlled as the rated value, and the starting torque is controlled as the maximum one. The starting frequency can be deduced by differentiating (3)
Fig. 4.
Equivalent circuit at variable-frequency operating.
wide speed scope, with the operating frequency varying from zero to several hundred hertz. The operating frequency is wide. Because of the high operating frequency, the iron core loss cannot be neglected, and the equivalent circuit is shown in Fig. 4. With the increase of the operating frequency, the skin effect is and the equivmore and more serious. The stator resistance increase, the copper losses of the stator alent rotor resistance and rotor increase. The iron core loss corresponding to the operating frequency is (6) where iron core loss constant.
(4) (7)
The starting voltage can be obtained from (2) (5) From the above analysis, we can see that the low-startingcurrent high-starting-torque objective can be obtained for the inverter fed induction motor used for HEV by applying suitable starting voltage and frequency, and the rotor resistance is not determining factor any more, so we do not need to increase the rotor resistance to improve the starting performance. The deeper the rotor slot is, the more the leakage flux is, the more the rotor leakage reactance is, which will decrease the power factor of the starting and low-speed operating. Therefore, from the angle of improving the starting performance, the advantage of deep-slot and double-cage induction motor does not exist. Shallow and wide slot motor is more suitable for the inverter drive. III. DESIGN THEORY OF THE INDUCTION MOTOR USED FOR HEV BASED ON THE OPERATING PERFORMANCE Traditional induction motor operates at the speed determined by the industrial frequency, i.e., rated speed, so the motor design normally focuses on this speed. The performance around rated speed is mainly considered, whereas that at other speeds can be neglected. The induction motor used for HEV is quite different from the traditional one. It needs to operate at variable loads and
where iron core loss at rated frequency; stator back electromotive force (EMF) at rated frequency. The iron core loss resistance is (8) where the ratio of stator frequencies, ; iron core loss resistance at rated frequency, . Since the operating characteristic of the induction motor used for HEV consists of constant-torque and constant-power stages, different control strategies should be used for different stages. At constant-torque stage, the electromagnetic torque can be expressed as (9) where torque coefficient; air-gap fundamental flux per pole; rotor phase current;
WANG et al.: DESIGN CHARACTERISTICS OF THE INDUCTION MOTOR USED FOR HEV
507
rotor copper loss does not change. During the whole operating period, the copper loss varies slightly with the changing of the frequency. It can be seen that the iron core and copper losses have different varying characteristics with the change of the operating frequency. When we design a motor, the maximum efficiency can be gained by making the iron core and copper losses close. In order to improve the efficiency at the whole operating region, we can make the iron core loss more than the copper loss near base speed, and make the copper loss more than the iron core loss at low and high speeds. The maximum torque of the motor is Fig. 5. Curve of the iron core loss versus frequency.
(12) phase difference that the rotor current lags the rotor EMF; active component of rotor phase current. To assure the constant torque with the changing of the inverter and should be constant. To make confrequency, should be constant. To make constant, rotor stant, should be constant. frequency At constant-power stage, the electromagnetic power of the motor can be approximately expressed as (10) If we assume
is equal to
, (10) can be expressed as (11)
In (11), is constant, and changes slightly. To assure constant-power performance, the supplied voltage and slip should be constant. The design principle of the induction motor used for HEV should be adjusted according to the above characteristics. With the increase of the frequency, the resistances of the stator and rotor both increase because of the skin effect, so multi turns of thin enameled wires in parallel can be adopted for the stator winding, and shallow slot can be adopted for the rotor to weaken the skin effect. With the increase of the operating frequency, the iron increases. At constant-torque stage, core loss resistance , so the iron core loss increases with the increase of the frequency, i.e., at base speed the iron core loss is not reaches its maximum. At constant-power stage, since can be considered as unchanged, and therefore the changed, iron core loss decreases with the increase of the frequency, i.e., it also reaches its maximum at base speed. The curve of the iron core loss versus frequency is shown in Fig. 5. constant, the current At constant-torque stage, to assure should be kept constant, and meanwhile the rotor current should be kept constant. So the stator current remains constant, and the copper losses of the stator and rotor change slightly. At remains unchanged, the stator constant-power stage, since current is approximately unchanged, so the stator copper loss is approximately unchanged too. Since the slip is fixed, the
where . As can be seen from (12), with the increase of the operating frequency, the resistance of the stator and the leakage reactance of the stator and rotor increase, which makes the maximum torque decrease, and the overload ability of the motor decrease. Thus, when the motor is designed, some methods should be found to decrease the resistance of the stator and the leakage reactance of the stator and rotor. The formerly mentioned method of multiturns of thin enameled wires in parallel can be adopted to decrease the stator resistance. The decrease of the rotor leakage reactance can be gained by adopting shallow and wide rotor slot on the basis of assuring the rotor teeth flux density. It can be seen from the equivalent circuit that the decrease of the leakage reactance can improve the power factor of the motor. IV. IMPACT OF HARMONICS ON INDUCTION MOTOR DESIGN When the induction motor is fed by inverter, the current is not absolutely sinusoidal, but contains some harmonics. The resultant three-phase fundamental magnetomotive force (MMF) can be expressed as
(13) Some discussions are made to (13). 1) When ( is integer), (14) 2) When
, (15)
508
IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 1, JANUARY 2005
magnetic slot wedge can be used. On the one hand, the stator leakage reactance can be increased, and on the other hand, highorder air-gap harmonic magnetic fields can be decreased. For the rotor, closed slots or half closed slots can be adopted to increase the rotor leakage reactance and decrease the high-order air-gap harmonic magnetic fields. In addition, for the slot combination, the numbers of stator and rotor slots should be increased and be close with the number of rotor slots less than that of stator slots to decrease the influence of harmonics. V. CONCLUSION
Fig. 6. Harmonic equivalent circuit at variable-frequency operating.
This MMF is a spatial backward fundamental MMF that is sinusoidal distribution, and has synchronous speed and pole pair . The slip is ( is the rotor speed). The produced torque is a braking torque. , 3) When (16) This MMF is a spatial forward fundamental MMF that is siand pole nusoidal distribution, and has synchronous speed . The produced torque is pair . The slip is a driving torque. The harmonic equivalent circuit at variable-frequency operating is shown in Fig. 6. The asynchronous additional torque produced by the th harmonic voltage is (17) where is the rotor speed. When , is a braking torque; when , is a driving torque. Because some methods have been used to weaken the low order harmonics when the inverter is designed, the existing harmonics are high order ones whose synchronous speed is much higher than that of the fundamental magnetic field. Therefore, serious torque ripple will not occur when the driving and braking asynchronous additional torque is added to the fundamental asynchronous torque. The braking asynchronous additional torque will produce braking loss, and the harmonic current will produce copper loss in the stator and rotor windings. These two losses form the harmonic loss. In addition, the harmonic magnetic fields of different orders interact in the space to produce ripple torque whose magnitude is in direct proportion to the magnitude product of these harmonic MMFs. It can be seen that the existence of harmonic currents is disadvantageous to induction motor, which should be overcome. When the induction motor is fed by voltage-type inverter, the amplitude of high-order harmonic current is in inverse proportion to the leakage reactance of the stator and rotor, so the leakage reactance of the stator and rotor should be increased from the angle of restraining harmonic current. For the stator,
1) Since the low-starting-current high-starting-torque objective can be obtained for the inverter fed induction motor by applying suitable starting voltage and frequency, the advantage of deep-slot and double-cage induction motor does not exist. Shallow and wide rotor slot induction motor is more suitable for HEV. 2) With the increase of the operating frequency, the resistances of the stator and rotor both increase because of the skin effect, so multi turns of thin enameled wires in parallel can be adopted for the stator winding, and shallow slot can be adopted for the rotor to weaken the skin effect. 3) With the increase of the operating frequency, the iron core loss increases first, and then decreases. At base speed, the iron core loss reaches its maximum value. The copper loss varies slightly with the changing of the frequency. The iron core loss should be more than the copper loss near base speed and the copper loss should be more than the iron core loss at low and high speeds to improve the efficiency at the whole operating region. 4) Suitable leakage reactance should be chosen to consider the influences of variable-frequency and harmonics simultaneously. Magnetic slot wedge used for the stator and closed or half closed slots used for the rotor can restrain the influence of harmonics. 5) For the slot combination, the numbers of stator and rotor slots should be increased and be close with the number of rotor slots less than that of stator slots to decrease the influence of harmonics. REFERENCES [1] S.-b. Park, H.-b. Lee, and S.-y. Hahn, “Stator slot shape design of induction motors for iron loss reduction,” IEEE Trans. Magn., vol. 31, no. 3, pp. 2004–2007, May 1995. [2] D.-H. Cho, H.-K. Jung, and C.-G. Lee, “Induction motor design for electric vehicle using a niching genetic algorithm,” IEEE Trans. Ind. Appl., vol. 37, no. 4, pp. 994–999, Jul.-Aug. 2001. [3] T. Stefanski and S. Karys, “Loss minimisation control of induction motor drive for electrical vehicle,” Proc. IEEE Int. Symp. Industrial Electronics, pp. 952–957, Jun. 1996. [4] K. S. Smith and L. Ran, “A time domain equivalent circuit for the inverter-fed induction motor,” in Proc. 9th Int. Conf. Electrical Machines and Drives, 1999, pp. 1–5. [5] T. Pham-Dinh and E. Levi, “Core loss in direct torque controlled induction motor drives: Detuning and compensation,” Proc. IEEE 32nd Annu. Power Electronics Specialists Conf., pp. 1429–1434, Jun. 2001. [6] M.-K. Kim, C.-G. Lee, and H.-K. Jung, “Multiobjective optimal design of three-phase induction motor using improved evolution strategy,” IEEE Trans. Magn., vol. 34, no. 5, pp. 2980–2983, Sep. 1998.
