Title of Invention: ANGULAR ERROR CORRECTION DEVICE AND ANGULAR ERROR CORRECTION METHOD FOR POSITION SENSOR Technical Field [0001] The present invention relates to an angular error correction device and an angular error correction method for a position sensor that corrects an angular error in a position sensor including a periodic error uniquely determined in accordance with the rotational position of an electric motor, the position sensor being employed in, for instance, control devices of elevator traction machines, control devices of automotive electric motors, and control devices of electric motors in machine tools. Background Art [0002] Conventional angle detection devices are known wherein: an angle signal is detected from a signal detected by an angle detector, for instance a resolver; a position error is calculated by an angular error estimator by referring to the detected angle signal by relying on a feature whereby an error waveform of the resolver is formed from an n-th order component determined specifically for the resolver, and by exploiting the reproducibility of the error waveform; a speed error signal is calculated by differentiating the position error; a detection error for each frequency component is calculated through frequency analysis of the speed error signal, for instance on the basis of a Fourier transform; an estimated angular error signal is generated by combining the calculated detection errors; and the detected angle signal is corrected by an angle signal correction circuit, using the generated estimated angular error signal (see for instance PTL 1 ). Citation List Patent Literature [0003] [PTL 1] Japanese Patent Application Publication No. 2012-145371 Summary of Invention Technical Problem [0004] The following problems arise however in conventional art. In a speed detector of an angle detection device in a conventional resolver, the rotational speed of a motor is detected on the basis of an angle signal that is detected by an angle detector, and the angular error is estimated using this detected speed. When the angular error is estimated using the detected speed, the estimation precision of the angular error is determined by the speed resolution of the angle detector or the speed detector. This is problematic in that, as a result, a quantization error arises in angle detectors or speed detectors of low speed resolution, and sufficient estimation precision of angular error fails to be achieved. [0005] It is an object of the present invention, arrived at in order to solve the above problems, to provide an angular error correction device and an angular error correction method for a position sensor that allow estimating and correcting an angular error accurately. Solution to Problem [0006] The angular error correction device for a position sensor according to the present invention is an angular error correction device for a position sensor that detects a rotational position of an electric motor and corrects an angular error of the position sensor including a periodic error uniquely determined in accordance with the rotational position, the device being provided with: a current detection unit which detects current flowing in the electric motor; a frequency analysis unit which, using the rotational position of the electric motor, analyzes the frequency of the current detected by the current detection unit, and calculates an amplitude of a specific frequency component corresponding to the angular error; an angular error estimator which estimates, as an angular error estimated value, the angular error formed from the specific frequency component on the basis of the amplitude calculated by the frequency analysis unit and the rotational position of the electric motor; an angular error correction unit which, using the angular error estimated value, corrects the angular error for the rotational position of the electric motor detected by the position sensor; a speed computing unit which calculates the rotational speed of the rotating electrical machine on the basis of the rotational position of the electric motor for which the angular error has been corrected by the angular error correction unit; a speed command value generation unit which generates a speed command value for the electric motor on the basis of a speed command input from the outside; a speed controller which has a variable gain mechanism that varies a speed control gain from a preset value, and which generates a current command value for the electric motor on the basis of a speed deviation between the speed command value and the rotational speed of the rotating electrical machine; and a control state switching unit which switches an operation state of the electric motor between an angular error estimation operation and a normal operation, wherein the speed controller makes the speed control gain larger when the operation state of the electric motor is the angular error estimation operation than when the operation state of the electric motor is the normal operation. [0007] Further, an angular error correction method for a position sensor according to the present invention is a method for correcting an angular error of a position sensor, the method being executed by an angular error correction device for a position sensor that detects a rotational position of an electric motor and that corrects an angular error of the position sensor including a periodic error uniquely determined in accordance with the rotational position, the method including: a current detection step of detecting current flowing in the electric motor; a frequency analysis step of analyzing by using the rotational position of the electric motor the frequency of the current detected in the current detection step, and calculating an amplitude of a specific frequency component corresponding to the angular error; an angular error estimation step of estimating, as an angular error estimated value, the angular error formed from the specific frequency component, on the basis of the amplitude calculated in the frequency analysis step and the rotational position of the electric motor; an angular error correction step of correcting by using the angular error estimated value the angular error for the rotational position of the electric motor detected by the position sensor; a speed computation step of calculating the rotational speed of the rotating electrical machine on the basis of the rotational position of the electric motor for which the angular error has been corrected in the angular error correction step; a speed command value generation step of generating a speed command value for the electric motor on the basis of a speed command input from the outside; a speed control step of generating a current command value for the electric motor on the basis of a speed deviation between the speed command value and the rotational speed of the rotating electrical machine; and a control state switching step of switching an operation state of the electric motor between an angular error estimation operation and a normal operation; wherein in the speed control step the speed control gain is made larger when the operation state of the electric motor is the angular error estimation operation than when the operation state of the electric motor is the normal operation. Advantageous Effects of Invention [0008] In the angular error correction device and angular error correction method for a position sensor according to the present invention, a speed command value generation unit (step) generates a speed command value for an electric motor on the basis of a speed command input from the outside, a speed controller (step) generates a current command value for the electric motor, on the basis of a speed deviation between the speed command value and the rotational speed of the rotating electrical machine, and a control state switching unit (step) switches the operation state of the electric motor between an angle error estimation operation and a normal operation. The speed controller (step) makes the speed control gain larger when the operation state of the electric motor is the angular error estimation operation than when the operation state is the normal operation. As a result of an increase in the speed control gain, the current command value increases for a same speed deviation, and accordingly it becomes possible to increase the estimation precision of the angular error estimated value, being the output of the angular error estimator having current as an input, for a same resolution of the current sensor. Brief Description of Drawings [0009] Fig. 1 is a block diagram illustrating the overall configuration of a control device of an electric motor having an angular error correction device for a position sensor according to the present invention. Fig. 2 is a block diagram illustrating a control device of an electric motor in which there is used an angular error correction device for a position sensor according to Embodiment 1 of the present invention. Fig. 3 is a block diagram illustrating a control device of an electric motor in which there is used an angular error correction device for a position sensor according to Embodiment 1 of the present invention. Fig. 4 is a block diagram illustrating a control device of an electric motor in which there is used an angular error correction device for a position sensor according to Embodiment 1 of the present invention. Fig. 5 is a block diagram illustrating a control device of an electric motor in which there is used an angular error correction device for a position sensor according to Embodiment 1 of the present invention. Fig. 6 is a graph illustrating an example of detection error in a position sensor of the angular error correction device for a position sensor according to Embodiment 1 of the present invention. Fig. 7 is a block diagram illustrating an angular error estimation unit of the angular error correction device for a position sensor according to Embodiment 1 of the present invention. Fig. 8 is a block diagram illustrating a speed control unit of the angular error correction device for a position sensor according to Embodiment 1 of the present invention, depicted together with a control state switching unit. Fig. 9 is another block diagram illustrating the overall configuration of a control device of an electric motor having an angular error correction device for a position sensor according to the present invention. Description of Embodiments [0010] Preferred embodiments of the angular error correction device and angular error correction method for a position sensor according to the present invention will be explained next with reference to accompanying drawings. In the explanation that follows, identical or corresponding portions in the figures will be denoted with identical reference symbols. [0011] In the embodiments below there will be explained a method for increasing the precision of angular error estimation, through an increase in responsiveness by increasing a speed control gain of a speed controller during an angular error estimation operation in an angular error correction device for a position sensor which, on the basis of current amplitude, estimates and then corrects a position-dependent angular error included in the rotational position of an electric motor, being the output of a position sensor. [0012] Embodiment 1 Fig. 1 is a block diagram illustrating the overall configuration of a control device of an electric motor having an angular error correction device for a position sensor according to the present invention. Figs. 2 to 5 are block diagrams illustrating control devices of an electric motor in which there is used the angular error correction device for a position sensor according to Embodiment 1 of the present invention. [0013] In Figs. 1 to 5, the control device of an electric motor is provided with a speed command value generation unit 1, a speed controller 2, a current controller 3, an inverter 4, an electric motor 5, a position sensor 6, a current sensor (current detection unit) 7, a speed computing unit 8, a detected position correction unit 9, a position computing unit 11, a coordinate converter 12, an angular error estimation unit 20 and a control state switching unit 30. [0014] On the basis of a speed command input from the outside, the speed command value generation unit 1 generates and outputs a speed command value for the electric motor 5. Although not illustrated in the figures, the speed command value generation unit 1 may include a position control system. The present invention can also be used in a case where the speed command value generation unit 1 includes a position control system. [0015] The speed controller 2 has, as the input thereof, a speed deviation between a speed command value from the speed command value generation unit 1 and the rotational speed of the electric motor 5 calculated by the speed computing unit 8, and generates and outputs a current command value for the electric motor 5. The control state switching unit 30 switches the operation state of the electric motor 5 between an angular error estimation operation and a normal operation. The detailed function of the speed controller 2 and the control state switching unit 30 will be explained further on. [0016] The speed computing unit 8 calculates, and then outputs, the rotational speed of the electric motor 5 on the basis of angle information or position information resulting from correction, by the detected position correction unit 9, of the rotational position of the electric motor 5 being the output of the position sensor 6. Most simply, the speed computing unit 8 calculates the rotational speed through time differentiation of the position or the angle. [0017] The speed computing unit 8 may calculate the speed on the basis of position information (for instance number of pulses in an optical encoder) from the position sensor 6, as illustrated in Figs. 2 and 4, or on the basis of the angle information calculated by the position computing unit 11, as illustrated in Figs. 3 and 5. The speed computing unit 8 may include a configuration for measuring time. [0018] The current controller 3 has, as the input thereof, a difference between the current command value from the speed controller 2 and a phase current being the output of the current sensor 7 illustrated in Figs. 2 and 3, or an axial current of the electric motor 5 resulting from conversion of the phase current illustrated in Figs. 4 and 5 to for instance d-q axes, by the coordinate converter 12. The current controller 3 generates and outputs a voltage command value of the electric motor 5. [0019] The position computing unit 11 calculates, and then outputs, angle information of the electric motor 5 on the basis of the rotational position of the electric motor 5 being the output of the position sensor 6 or the position information corrected by the detected position correction unit 9. In the case of vector control of the electric motor 5, the coordinate converter 12 converts the phase current from the current sensor 7 to coordinates suitable for control, for instance oc-P axes, d-q axes or y-S axes. [0020] The detected position correction unit 9 adds/subtracts an angular error estimated value being the output of the angular error estimation unit 20, to/from the rotational position of the electric motor 5 being the output of the position sensor 6 or angle information resulting from conversion, in the position computing unit 11, of the rotational position from the position sensor 6, and outputs the corrected position information or angle information. [0021] The current sensor 7 measures the current in the electric motor 5. In a case where, for instance, the electric motor 5 is a three-phase electric motor, there are often measured phase currents of two phases, but also phase currents of three phases may be measured herein. In Figs. 1 to 5, the current sensor 7 measures output current of the inverter 4, but alternatively, the current sensor 7 may estimate respective phase currents through measurement of a bus current of the inverter 4, as in a current measurement scheme by a one-shunt resistor. This does not affect the present invention in any way. [0022] The inverter 4 converts the voltage of a power source, not shown, to desired variable voltage and variable frequency, on the basis of the voltage command value from the current controller 3. In the present invention, the inverter 4 denotes a variable-voltage variable-frequency power converter such as a power converter in which AC voltage is converted to DC voltage by a converter, and the DC voltage is thereafter converted to AC voltage by an inverter, for instance as in inverter devices that are available in the market, or alternatively, a power converter that converts AC voltage directly to AC variable voltage and variable frequency, as in matrix converters. [0023] The inverter 4 according to Embodiment 1 of the present invention may include the function of coordinate conversion, in addition to the function of the inverter 4 described above. In a case where the voltage command value is a voltage command value in the d-q axes, specifically, the term inverter 4 encompasses instances where the latter has also a coordinate conversion function of conversion to voltage according to an instructed voltage command value, through conversion of the voltage command value in the d-q axes to phase voltage or to line voltage. The present invention can also be used when there is provided a device or means, not shown, for correcting the dead time of the inverter 4. [0024] The position sensor 6, for instance an optical encoder, magnetic encoder, resolver or the like, detects the rotational position of the electric motor 5, as required to control the latter. As illustrated in Fig. 6, the rotational position information output by the position sensor 6 includes a periodic error uniquely determined in accordance with the rotational position of the electric motor 5. [0025] Herein, the periodic error that is uniquely determined in accordance with the rotational position of the electric motor 5 denotes for instance a detection error of the resolver described in PTL 1 (paragraphs 0020 and 0021), as well as errors having reproducibility in accordance with the rotational position, such as missing pulses or inter-pulse distance imbalance derived from slit defects in an optical encoder. [0026] The periodic error uniquely determined in accordance with the rotational position of the electric motor 5 will be expressed hereafter as an angular error 9err resulting from conversion of position information to an angle. The present invention can be used in a case where the position sensor 6 includes a periodic error uniquely determined in accordance with the rotational position of the electric motor 5, and a main component order of the angular error 0eiT is known. [0027] The periodic angular error 9err of the position sensor 6 can be approximated by a sine wave, as given in Expression (1) below. In Embodiment 1 of the present invention the notation has been unified in the form of a sine wave, since there is no substantial difference in notation by sine waves or cosine waves. [0028] [Math. 1] 6en. « Ax sin(W,0,„ +