Field [0001] The present invention relates to an electric rolling stock control device which controls a power converter for supplying power to an electric motor that drives an electric rolling stock without a sensor. Background [0002] Patent Literature 1 discloses an electric rolling stock control device including a car speed frequency converter which uses an output of a car speed sensor permanently installed in a vehicle of the electric rolling stock as a backup speed (definition of "backup speed" will be described later) and converts the output into a rotation frequency of an induction motor and a limiter which prevents an estimation value of a rotor rotation frequency of a rotor rotation frequency calculation unit from departing from a control range based on an output from the car speed frequency converter. [0003] According to the electric rolling stock control device disclosed in Patent Literature 1, the limiter can prevent the estimation value of the rotor rotation frequency from departing from the control range. Therefore, instability of control in a process from coasting of the electric rolling stock to restart is eliminated, and driving characteristics with high stability and high reliability can be obtained. Citation List Patent Literature [0004] Patent Literature 1: Japanese Patent Application Laid-Open No. 2003-324998 Summary Technical Problem [0005] However, the electric rolling stock control device disclosed in Patent Literature 1 has a system using the backup speed even though the electric rolling stock control device performs sensorless control. The electric rolling stock control device has had a problem in that when an error between the backup speed and an actual speed (referred to as "actual motor speed" below) of the electric motor for driving the electric rolling stock (appropriately referred to as "motor" below) increases, limit processing of an estimated speed originally performed to improve reliability of speed estimation prevents accurate speed estimation, and this causes deterioration in accuracy of the speed estimation. [0006] The present invention has been made in view of the above. An object of the present invention is to obtain an electric rolling stock control device capable of suppressing deterioration in accuracy of speed estimation even with a system using a backup speed. Solution to Problem [0007] To solve the above problem and achieve the object, an electric rolling stock control device for controlling a power converter that supplies power to an electric motor for driving an electric rolling stock without a sensor according to the present invention includes: a voltage controlling unit that controls an output voltage of the power converter; and a speed estimating unit that calculates a rotation speed estimation value of the electric motor. The speed estimating unit includes: an initial speed estimating unit that outputs an initial speed estimation value; a steady speed estimating unit that outputs a steady speed estimation value; a correction coefficient calculating unit that calculates a correction coefficient based on the steady speed estimation value and a backup speed that is speed information from outside that is stored in a set of electric rolling stocks; and a correction speed calculating unit that stores the correction coefficient in a storage unit and calculates a correction speed by multiplying the correction coefficient by the backup speed. Advantageous Effects of Invention [0008] According to the present invention, an effect to suppress deterioration in accuracy of speed estimation even with a system using a backup speed can be obtained. Brief Description of Drawings [0009] FIG. 1 is a configuration diagram of an overall system including an electric rolling stock control device according to the present embodiment. FIG. 2 is a block diagram illustrating a detailed configuration of a speed estimating unit according to the present embodiment. FIG. 3 is a diagram illustrating a transition of speed information used for control of an electric rolling stock or a display in the electric rolling stock. FIG. 4 is a diagram illustrating an operation profile according to the related art in a case where an error between the backup speed and an actual motor speed is caused. FIG. 5 is a diagram illustrating an operation profile according to the present embodiment in a case where a correction speed is calculated at a certain timing during a power running operation. FIG. 6 is a flowchart illustrating a processing flow of correction coefficient calculation in the electric rolling stock control device according to the present embodiment. FIG. 7 is a block diagram illustrating an example of a hardware configuration of the electric rolling stock control device according to the present embodiment. FIG. 8 is a block diagram illustrating another example of the hardware configuration of the electric rolling stock control device according to the present embodiment. Description of Embodiments [0010] First, before starting to describe the electric rolling stock control device according to the present embodiment, meanings of major terms used herein will be clarified. [0011] (Backup Speed) A backup speed is a speed obtained from a car speed sensor permanently installed in a vehicle of an electric rolling stock. As the car speed sensor, a Pulse Generator (PG) sensor attached to a non-driving wheel of the vehicle referred to as a trailing wheel is generally used. Speed information obtained by the PG sensor is stored by a train information managing system for managing train information as speed information of a set of electric rolling stocks, and the speed information is used for an operation or security of the train. From the viewpoint of a control device, the backup speed is positioned as speed information obtained from the outside. Since wheel diameters of the trailing wheel and a driving wheel which is a main driving wheel are different from each other, strictly speaking, the backup speed does not necessarily coincide with a rotation speed of the motor (appropriately referred to as "motor speed" below). Therefore, the backup speed is not sufficient to control the motor speed with high accuracy, and the motor is controlled by estimating a speed by additionally detecting a current flowing in the motor (appropriately referred to as "motor current" below). In some cases, the car speed sensor is attached to the driving wheel to directly detect the speed of the driving wheel. This method is referred to as a sensor control method. A method to which the present invention is applied is a method which does not directly detect the speed of the driving wheel and is referred to as a sensorless control method. [0012] (Steady Speed Estimation) In the sensorless control method, a steady speed estimation is a processing or a method of estimating the motor speed by using a voltage command of the motor and the motor current obtained from a current sensor. When an inverter for driving the motor is gating on and is continuously in a power running state or a regenerative state, an algorithm sequence for steady speed estimation is applied. The term "steady" is used to distinguish the term from "initial speed estimation" described below. [0013] (Initial Speed Estimation) When the electric rolling stock is in a coasting state, the inverter is in a gate-off state. When the inverter is restarted from this state, it is necessary to gate on the inverter while adjusting an output voltage frequency of the inverter to be equal to the motor speed. During coasting, since the motor is not excited and the speed cannot be estimated, an algorithm sequence for the initial speed estimation is prepared to restart the inverter so as not to generate overcurrent and the like. [0014] (Wheel Diameter Error) Since the driving wheel may idly rotate when power is transmitted to a rail, wear of the driving wheel is increased. However, the wear of the trailing wheel is less than that of the driving wheel. In addition, maintenance for cutting the wheels may be performed so as not to cause a difference between diameters of the wheels of the single vehicle. For these reasons, the difference between the diameters, that is, the wheel diameter error is caused in the wheels of the set of the vehicles. [0015] Hereinafter, the electric rolling stock control device according to the embodiment of the present invention will be described in detail with reference to the drawings. The present invention is not limited to the embodiment below. In the following description, a case where an electric motor is an induction motor will be described as an example. However, it goes without saying that main parts of the present invention can be applied to a synchronous electric motor. [0016] FIG. 1 is a configuration diagram of an overall system including the electric rolling stock control device according to the present embodiment. In FIG. 1, the electric rolling stock control device according to the present embodiment includes a voltage controlling unit 3, a gate driving circuit 8, and a speed estimating unit 20. The gate driving circuit 8 generates a gate driving signal for driving a switching element la of an inverter 1 which is a component of the electric rolling stock and outputs the signal to the inverter 1. The voltage controlling unit 3 generates a PWM signal for performing Pulse Width Modulation (PWM) control on the inverter 1, which is a power converter, and outputs the PWM signal to the gate driving circuit 8. The speed estimating unit 20 generates a speed estimation value oe which is a rotation speed estimation value of an electric motor 2 using a backup speed Ob and outputs the value to the voltage controlling unit 3. [0017] Next, the electric rolling stock will be described. A high-potential-side connection end of the inverter 1 is electrically connected to an overhead contact line 11 via a pantagraph 15, and a low-potential-side connection end of the inverter 1 is electrically connected to a rail 18 via a wheel 16. The inverter 1 is a power converter which converts a direct current to an alternate current with a variable voltage and a variable frequency. An AC-side of the inverter 1 is connected to the electric motor 2 which is an induction motor. The inverter 1 drives the electric motor 2. The electric motor 2 drives a driving wheel 17 coupled to the electric motor 2 via a gear 10 to apply a driving force to the electric rolling stock. A current detector 4 is provided between the inverter 1 and the electric motor 2. The current detector 4 detects motor currents iu, iv, and iw which are phase currents flowing in the electric motor 2. The phase currents iu, iv, and iw detected by the current detector 4 are input to the voltage controlling unit 3. In FIG. 1, a DC electric rolling stock is illustrated as an example. However, it goes without saying that the present invention can be applied to an AC electric rolling stock. [0018] A detailed configuration of the voltage controlling unit 3 is illustrated in FIG. 1. As illustrated in FIG. 1, the voltage controlling unit 3 according to the present embodiment includes a current command generating unit 31, a slip frequency calculating unit 32, a voltage command calculating unit 33, an integrator 34, a PWM controlling unit 35, and a coordinate converter 36. [0019] The coordinate converter 36 converts the motor currents iu, iv, and iw detected by the current detector 4 into two axes of a dq-axis rotational coordinate system and calculates a d-axis current id and a q-axis current iq. Here, the d-axis and the q-axis are respectively referred to as a magnetic flux axis and a torque axis, and both axes are orthogonal to each other in terms of vectors. [0020] The current command generating unit 31 calculates a q-axis current command iq* which is a torque axis current command and a d-axis current command id* which is a magnetic flux axis current command based on a magnetic flux command O* and a torque command Tm according to the following formulas (1) and (2). [0021] id*=*VM. . . (1) iq*=(L2xTm*)/(Mx