MOTOR DRIVING DEVICE AND VEHICLE DRIVING SYSTEM Field [0001] The present invention relates to a motor driving device that performs driving control of a synchronous motor and a vehicle driving system in which the motor driving device is mounted on a railroad vehicle. Background [0002] To accurately control the rotational operation of a synchronous motor, rotor magnetic pole position information of the synchronous motor and information concerning an electric current flowing to the synchronous motor are necessary. In a conventional general synchronous motor, the rotor magnetic pole position information is obtained by separately attaching a rotating position sensor to the synchronous motor. However, there are significant disadvantages in separately providing the rotating position sensor from viewpoints of a cost reduction, space saving, and improvement of reliability. Therefore, recently, there has been an increasing demand for a configuration in which the rotating position sensor is not separately provided, that is, elimination of the rotating position sensor. [0003] As a control method for the elimination of the rotating position sensor in the synchronous motor, there are a method of estimating a rotor magnetic pole position of the synchronous motor mainly from an induced voltage of the synchronous motor and a method of estimating a rotor magnetic pole position of the synchronous motor using saliency of the synchronous motor, in other words, position dependency of inductance. [0004] The former method is a method of estimating a rotor magnetic pole position of the synchronous motor using a characteristic that the magnitude of the induced voltage is proportional to the speed of the synchronous motor. However, the magnitude of the induced voltage is small in a zero-speed or a low-speed region and an S/N ratio is deteriorated. Therefore, it is difficult to accurately estimate the rotor magnetic pole position of the synchronous motor. [0005] On the other hand, the latter method of estimating a rotor magnetic pole position of the synchronous motor using saliency is a method of applying, to the synchronous motor, a voltage command for position estimation having a frequency different from a driving frequency of the synchronous motor and higher than the driving frequency, detecting a synchronous motor current flowing to the synchronous motor according to the voltage command for position estimation, and performing position estimation making use of the fact that the magnitude of the synchronous motor current changes depending on the rotor magnetic pole position according to the saliency. [0006] As explained above, when the saliency is used, the voltage command for position estimation for estimating the rotor magnetic pole position of the synchronous motor has to be injected into the synchronous motor. However, there is an advantage that it is possible to estimate the rotor magnetic pole position without depending on the rotating speed of the synchronous motor. Therefore, in particular, in the zero-speed or low-speed region, a method of performing position sensorless control using the saliency is used. [0007] However, in such a method of performing position sensorless control using the saliency, it is necessary to apply a high-frequency voltage command for position estimation to the synchronous motor. Therefore, noise occurs according to the application of the voltage command for position estimation. The method gives displeasure to a human. [0008] Therefore, as measures for reducing the displeasure due to the noise caused according to the application of the voltage command for position estimation to the synchronous motor, a method of reducing the amplitude of the voltage command for position estimation applied to the synchronous motor to reduce the magnitude itself of the noise that occurs from the synchronous motor, a method of improving the sound quality of the noise, and the like have already been proposed. [0009] In the former method of reducing the amplitude of the voltage command for position estimation applied to the synchronous motor to reduce the magnitude itself of the noise that occurs from the synchronous motor, the amplitude of the voltage command for position estimation decreases. Therefore, it is difficult to accurately estimate a rotor magnetic pole position of the synchronous motor. [0010] On the other hand, as the latter method of improving the sound quality of the noise, for example, there is a method described in Patent Literature 1 below. In the method described in Patent Literature 1, because there is a characteristic that, when a specific frequency component stands out in sound audible to the human, the human feels the sound unpleasant, the frequency of the voltage command for position estimation applied to the synchronous motor is deliberately changed at random to prevent the sound of the specific frequency component from standing out to thereby reduce the displeasure felt by the human. Citation List Patent Literature [0011] Patent Literature 1: Japanese Patent Application Laid-Open No. 2004-343833 Summary Technical Problem [0012] However, when the frequency component of the voltage command for position estimation is changed at random to prevent the sound of the specific frequency component from standing out, the frequency of an electric current necessary for estimating the rotor magnetic pole position of the synchronous motor also changes at random according to the change in the frequency component. Therefore, it is difficult to detect an electric current necessary for the estimation of the rotor magnetic pole position from the synchronous motor current. As a result, possibility that the position estimation accuracy is deteriorated and the position estimation cannot be performed is recognized. [0013] The present invention has been devised in view of the above and an object of the present invention is to obtain a motor driving device that can accurately estimate a rotor magnetic pole position and effectively reduce displeasure caused by noise occurrence involved in application of a voltage command for position estimation to a synchronous motor. Solution to Problem [0014] To solve the problem and achieve the object, the present invention provides a motor driving device that drives a synchronous motor, the motor driving device including: a modulated-wave generating unit to output a modulated wave; a carrier-wave generating unit to output a carrier wave; a switching-signal generating unit to compare the carrier wave and the modulated wave to output a switching signal; and a power converting unit including a switching element that operates according to the switching signal, the power converting unit supplying electric power to the synchronous motor, wherein the power converting unit has a high-frequency superimposition sensorless mode for applying a voltage for position estimation having a frequency higher than a frequency of a fundamental wave to the synchronous motor to estimate a magnetic pole position of the synchronous motor, in the high-frequency superimposition sensorless mode, the modulated-wave generating unit generates the fundamental wave and a voltage command for position estimation having a frequency higher than the frequency of the fundamental wave and outputs, as the modulated wave, a signal obtained by superimposing the voltage command for position estimation on the fundamental wave, and in the high-frequency superimposition sensorless mode, the carrier-wave generating unit changes a frequency of the carrier wave independently from the voltage command for position estimation. Advantageous Effects of Invention [0015] According to the present invention, there is an effect that it is possible to accurately estimate a rotor magnetic pole position and effectively reduce displeasure caused by noise occurrence involved in application of the voltage command for position estimation to the synchronous motor. Brief Description of Drawings [0016] FIG. 1 is a block diagram showing the configuration of a motor driving device according to a first embodiment. FIG. 2 is a block diagram showing an example of a specific configuration of a position estimating unit according to the first embodiment. FIG. 3 is a waveform chart of current amplitude for position estimation obtained by current amplitude calculators shown in FIG. 2. FIG. 4 is a diagram served for operation explanation of a position calculator shown in FIG. 2. FIG. 5 is a diagram showing an example of a specific configuration of a modulated-wave generating unit shown in FIG. 1. FIG. 6 is a waveform chart of a voltage command for position estimation generated by a voltage-for-position-estimation generating unit shown in FIG. 5. FIG. 7 is a waveform chart of a voltage command for position estimation different from the voltage command for position estimation shown in FIG. 6. FIG. 8 is a diagram showing a specific configuration of a carrier-wave generating unit shown in FIG. 1. FIG. 9 is a block diagram showing a modification of a modulated-wave generating unit. FIG. 10 is a diagram showing an example of a loudness curve. FIG. 11 is a graph showing a frequency analysis result of synchronous motor driving sound corresponding to presence or absence of random modulation. FIG. 12 is a graph (FC=750[Hz], without voltage command for position estimation) showing an analysis result of a noise spectrum. FIG. 13 is a graph (FC=750[Hz], WH=250[Hz]) showing an analysis result of a noise spectrum. FIG. 14 is a graph (FC=750[Hz], WH=166[Hz]) showing an analysis result of a noise spectrum. FIG. 15 is a graph (FC=1000[Hz], WH=166[Hz]) showing an analysis result of a noise spectrum. FIG. 16 is a graph (FC=750[Hz], without voltage command for position estimation, random modulation) showing an analysis result of a noise spectrum. FIG. 17 is a graph (FC=750[Hz], WH=250 [Hz], random modulation) showing an analysis result of a noise spectrum. FIG. 18 is a graph (FC=750[Hz], WH=166[Hz], random modulation) showing an analysis result of a noise spectrum. FIG. 19 is a graph (FC=1000[Hz], WH=166[Hz], random modulation) showing an analysis result of a noise spectrum. FIG. 20 is a diagram for explaining operation at the time when a synchronous motor is accelerated using a motor driving device in a second embodiment. FIG. 21 is a diagram showing the configuration of a vehicle driving system according to a third embodiment. Description of Embodiments [0017] Driving devices for synchronous motors (hereinafter referred to as "motor driving devices") according to embodiments of the present invention are explained below with reference to the accompanying drawings. Note that the present invention is not limited by the embodiments explained below. In the following explanation, a control method for applying, to a synchronous motor, a voltage obtained by superimposing, on a fundamental wave, a voltage for position estimation having a frequency higher than the frequency of the fundamental wave to estimate a magnetic pole position of the synchronous motor and driving the synchronous motor is referred to as "high-frequency superimposition sensorless control". A state in which a power converting unit explained below operates according to the high-frequency superimposition sensorless control is referred to as "high-frequency superimposition sensorless mode". Further, a control method for estimating a magnetic pole position of a synchronous motor using an induced voltage generated in the synchronous motor and driving the synchronous motor is referred to as "induced voltage use sensorless control". A state in which the power converting unit operates according to the induced voltage use sensorless control is referred to as "induced voltage use sensorless mode". [0018] First Embodiment. FIG. 1 is a block diagram showing the configuration of a motor driving device according to a first embodiment. As shown in FIG. 1, a motor driving device 1 according to the first embodiment includes, as components for driving a synchronous motor 50, a power converting unit 2, a control unit 3, current detectors 9a and 9b, and a voltage detector 10. The power converting unit 2 has a function of converting direct-current power supplied from a direct-current power source 60 into alternating-current power having a variable voltage and a variable frequency and supplying the alternating-current power to the synchronous motor 50. The control unit 3 includes a carrier-wave generating unit 5, a modulated-wave generating unit 6, a switching-signal generating unit 7, and a position estimating unit 8. [0019] The voltage detector 10 is a detector that detects a direct-current voltage EFC applied to the power converting unit 2 by the direct-current power source 60. The direct-current voltage EFC is used for, for example, calculation of modulated waves ecu, av, and aw explained below. The direct-current voltage EFC detected by the voltage detector 10 is input to the modulated-wave generating unit 6. The current detectors 9a and 9b are detectors that detect electric currents for two phases in a three-phase electric current flowing into the synchronous motor 50 from the power converting unit 2. The electric currents detected by the current detectors 9a and 9b are input to the position estimating unit 8. [0020] Note that, in FIG. 1, the current detector 9a is disposed in a U phase and the current detector 9b is disposed in a W phase. However, the current detectors 9a and 9b can be disposed in the U phase and a V phase or can be disposed in the V phase and the W phase. Current detectors can be disposed in all of the U phase, the V phase, and the W phase to detect electric currents for three phases. [0021] The power converting operation in the power converting unit 2 is performed by driving a plurality of semiconductor switch elements configuring the power converting unit 2 according to switching signals SWu, SWv, and SWw generated by the switching-signal generating unit 7. Note that, for a detailed configuration of the power converting unit 2, please refer to FIG. 21 explained below. [0022] The carrier-wave generating unit 5 generates, on the basis of a triangular wave, a carrier wave (referred to as "carrier" as well) Ca having a frequency higher than the frequency of a fundamental wave of a modulated wave. The frequency of the carrier wave Ca is basically a switching frequency of the power converting unit 2. [0023] Note that, as a frequency range of a general carrier wave, a frequency range that can be used is sometimes limited by a power capacity of the power converting unit used in an application to which the frequency range is applied. For example, in an electric railroad application, the frequency range is approximately 500 hertz to 2000 hertz. In an electric automobile application, the frequency range is approximately 5000 hertz to 20000 hertz. In this embodiment, a frequency range of a carrier wave is set to 500 hertz to 2000 hertz. [0024] The modulated-wave generating unit 6 generates fundamental waves of modulated waves of the U phase, the V phase, and the W phase on the basis of a q-axis voltage command Vq*, a d-axis voltage command Vd*, a modulation factor PMF, and an estimated phase angle 6e of the synchronous motor estimated by the position estimating unit 8. In the high-frequency superimposition sensorless control, the modulated-wave generating unit 6 generates, as the modulated waves au, av, and aw of the U phase, the V phase, and the W phase, signals obtained by superimposing a signal (referred to as voltage command for position estimation as well) having a frequency higher than the fundamental wave frequency on the fundamental waves of the phases. [0025] Note that there are limiting conditions explained below concerning a frequency range of a general voltage command for position estimation. For example, in an electric railroad application, the frequency range is approximately several hundred hertz to 500 hertz. In a general industrial application, the frequency range is 1000 hertz or less. In this embodiment, a frequency range of a voltage command for position estimation is set to 100 hertz to 500 hertz. [0026] The modulated waves au, av, and aw generated by the modulated-wave generating unit 6 and the carrier wave Ca generated by the carrier-wave generating unit 5 are input to the switching-signal generating unit 7. The switching-signal generating unit 7 compares signal values of the modulated waves cm, av, and aw and a signal value of the carrier wave Ca, which change at every moment. PWM modulation (Pulse Width Modulation: hereinafter written as "PWM modulation") for generating the switching signals SWu, SWv, and SWw is performed on the basis of a magnitude relation among the signal values. [0027] An example of switching signals is explained. When the power converting unit 2 is a two-level inverter, signals described below corresponding to a magnitude relation of the modulated waves au, av, and aw and the carrier wave Ca are generated as the switching signals SWu, SWv, and SWw output to the power converting unit 2. Note that a direct-current voltage applied to the power converting unit 2 is referred to as a direct-current voltage input. [0028] (a) A period in which the modulated wave>the carrier wave A signal for selecting high-order side potential of the direct-current voltage input (b) A period in which the modulated wave