Description LOCKED-ROTOR MECHANISM FOR MOTOR Technical field The invention relates to a locked-rotor mechanism for motor, belonging to the technical field of motor detecting devices. Background art In order to detect current magnitude and other parameters of a motor with overloading, a load larger than maximum torque output by the motor is usually applied to the output end of the motor to stop the rotation of the motor so as to detect parameters of current and the like. There is no special locked-rotor mechanism for stopping the rotation of a motor in the prior art, and generally, the output end of a motor is bound with a device or apparatus together to cause the motor to be in a locked-rotor state, which, on one hand, is inconvenient in operation and has low working efficiency, and on the other hand, is unreliable on blocking the rotation of the motor, and therefore, it is hard to accurately detect current and other parameters of the motor with overloading. Summary of the invention The object of the invention is to provide a locked-rotor mechanism for motor which is convenient in operation, is capable of improving working efficiency, reliably locking a rotor of a motor and accurately testing current of the motor with overloading and solves the problems that the prior art has inconvenient operation, low working efficiency and unreliable rotation blocking and is hard to accurately detect current and other parameters of the motor with overloading. The above-mentioned technical object of the invention is realized by the following technical scheme: the locked-rotor mechanism comprises a bedplate, a motor positioning seat is arranged on the bedplate, the upper end of the motor positioning seat forms a V-shaped positioning groove, a motor pressing mechanism is disposed beside the motor positioning seat, the press head of the motor pressing mechanism is correspondingly positioned above the V-shaped positioning groove, and the part of the bedplate corresponding to one end of the V-shaped positioning groove is provided with a rotor locking seat, the rotor locking seat is provided with a rotor locking structure fitted with a transmission structure of the output shaft of a motor to be detected, and the rotor locking seat is fixed to the bedplate. In detection, the motor to be detected is arranged in the V-shaped positioning groove of the motor positioning seat, the output shaft of the motor is fitted with the rotor locking structure of the rotor locking seat, the press head of the pressing mechanism presses the motor to be detected tightly, wherein the V-shaped positioning groove of the motor positioning seat and the pressing mechanism are used for reliably positioning the motor to be detected, and the rotor locking structure of the rotor locking seat can reliably block the rotation of the motor. The locked-rotor mechanism has convenient operation, improves the working efficiency and can accurately detect current and other parameters of the motor in a locked-rotor state and with overloading. Preferably, the pressing mechanism of motor comprises a support fixed to the bedplate, the upper end of the support is fixed to a U-shaped support plate, one end of the U-shaped support plate closing to the motor positioning seat is articulated with one end of an L-shaped swinging member, the press head is fixed to the L-shaped swinging member, the corner of the L-shaped swinging is articulated with one end of a handle, the middle part of the handle is articulated with one end of a connecting plate, and the other end of the connecting plate is articulated with the U-shaped support plate at one end away from the motor positioning seat. When the handle is nrec'z'Pd dowxxnwzarde. thp ennnerina n1tp rontptc nroind thp hincre noint of thp connecting plate and the U-shaped support plate, the end part of the handle drives the corner of the L-shaped swinging member to rotate toward the lower inclined direction such that the press head of the end part of the L-shaped swinging member moves upwards to release the detected motor; and conversely, the motor to be detected can be pressed in the V-shaped positioning groove tightly. Preferably, the L-shaped swinging member is provided with a long-strip-shaped through slot, the press head is fixed to the end part of a screw rod, the screw rod penetrates through the long-strip-shaped through slot, and the upper side and the lower side of the screw rod corresponding to the long-strip-shaped through slot are respectively fitted with an upper screw nut and a lower screw nut. The transverse relative position between the press head and the V-shaped positioning groove can be adjusted by regulating the position of the screw rod in the length direction of the long-strip-shaped through slot, and the height of downward press can be adjusted by regulating the height position of the screw rod in the long-strip-shaped through slot, so that the requirements of pressing motors to be detected with different external diameters can be satisfied. Preferably, the rotor locking structure is a through hole fitted with the transmission structure on the output shaft of the motor to be detected, wherein the through hole can be a hexagonal hole, an internal-key-shaped hole or an internal-tooth-shaped hole according to different shapes of the transmission structure on the output shaft of the motor to be detected. Preferably, the press head is made of elastic rubber, wherein the press head made of elastic rubber can insure the press head to tightly press the motor to be detected without damage to the motor to be detected. Therefore, the invention has the advantages of convenient operation, improvement of working efficiency, reliable rotation blocking for a motor, accurate Brief description of the drawings Figure 1 is a front view of the structure of the invention; Figure 2 is a side view of the structure of the invention; and Figure 3 is a structural diagram of a motor to be detected. Detailed description of the embodiment The invention is further described in details by combining the following embodiments and figures. Embodiment 1: referring to figure 1 and figure 2, a motor positioning seat 2 is arranged on a bedplate 1, the upper end of the motor positioning seat 2 forms a V-shaped positioning groove 3, a motor pressing mechanism 4 is disposed beside the motor positioning seat 2, the press head 5 of the motor pressing mechanism 4 is correspondingly positioned above the V-shaped positioning groove 3, and the press head 5 is made of elastic rubber. The part of the bedplate corresponding to one end of the V-shaped positioning groove 3 is provided with a rotor locking seat 6, a rotor locking structure 8 fitted with a transmission structure on the output shaft of a motor 7 to be detected is configured on the rotor locking seat 6, the rotor locking structure 8 is a hexagonal through hole fitted with the transmission structure on the output shaft of the motor to be detected, and the rotor locking seat 6 is fixed to the bedplate 1. The motor pressing mechanism 4 comprises a support 9 fixed to the bedplate 1, the upper end of the support 9 is fixed to a U-shaped support plate 10, one end of the U-shaped support plate 10 closing to the motor positioning seat 2 is articulated with one end of an L-shaped swinging member 11, the corner of the swinging member 11 is articulated with one end of a handle 12, the middle part of the handle 12 is articulated with one end of a connecting plate 13, and the other end of the connecting plate 13 is articulated with the U-shaped support plate 10 at one end away from the motor positioning seat. The press head 5 is fixed to the L-shaped swinging member 11 which is provided with a long-strip-shaped through slot 14, the press head 5 is fixed on the end part of a screw rod 15, the screw rod 15 penetrates through the long-strip-shaped through slot 14, and the upper side and the lower side of the screw rod 15 corresponding to the long-strip-shaped through slot are respectively fitted with an upper screw nut 16 and a lower screw nut 17. Referring to figure 3, the output shaft of the motor 7 to be detected forms an external hexagonal transmission structure 18, in detection, as shown in figure 1 and figure 2, the motor 7 to be detected is placed in the V-shaped positioning groove 3 of the motor positioning seat and pressed by the pressing mechanism 4 tightly, the external hexagonal transmission structure 18 on the output shaft of the motor is fitted with the rotor locking structure 8 in the forms of an internal hexagonal through hole on the rotor locking seat 6 to stop the rotation of the output shaft of the motor. The locked-rotor mechanism has convenient operation, improves the working efficiency and can accurately detect current and other parameters of the motor in a locked-rotor state and with overloading.