Description Load Simulation Mechanism for Motor Technical Field The invention relates t a load simulation mechanism for a motor. Background of the Invention Nowadays, motors are used more and more widely, and mechanical equipment using motors as driving sources may be found everywhere. Various parameters of motors, such as rated power, rated current and service life, are major parameters for evaluation of a motor and also standards representing the performance of the motor. The performance of motors directly influences the related performances of various kinds of mechanical equipment driven by the motors. Therefore, measurement on various performance parameters of a motor also becomes very important. The operation situation of a motor under different loads is important information to be known. Summary of the Invention The purpose of the invention is to provide a load simulation mechanism for a motor, which can measure various parameters of a motor conveniently and has good universality. The technical purpose of the invention is mainly realized by the following technical solution: a load simulation mechanism for a motor is provided, comprising a soleplate, a motor supporting block is fixed on one end of the soleplate, and a load simulation device is fixed on the other end of the soleplate; the load simulation device comprises a support frame, the support frame is fixed on the soleplate and provided with a friction plate, a friction wheel which synchronously rotates along with an output shaft of a motor is disposed on one side of the support frame, and the friction wheel abuts against the friction plate; and the friction plate is connected with a hold-down 1 mechanism. After the motor is powered on, the motor drives the output shaft to rotate so as to drive the friction wheel to rotate as the output shaft is connected with the friction wheel. The friction wheel abuts against the friction plate, so that a friction force is generated between the friction plate and the friction wheel, that is, an external load is provided for the motor. The hold-down mechanism enables the friction wheel and the friction plate to be tightly connected to each other all the time, and meanwhile, the friction force between the friction plate and the friction wheel is adjusted by the hold-down mechanism. As the friction wheel rotates synchronously to the output shaft of the motor to be measured, the load on the friction wheel is changed, that is, the external load of the motor is changed. In this case, statistics on various performance parameters of the motor may be carried out according to demands of experimenter. The input voltage driving the motor is fixed, a current meter or a tachometer may be disposed on the external part of the simulation mechanism or a pressure gauge may be connected to the hold-down mechanism in accordance with different parameters to be measured. Different parameters of the motor under different load cases may be calculated conveniently by data on the meter, so the operation is simple and practicable. Moreover, the whole simulation mechanism has simple structure and low cost, and the used parts are standard parts which may be easily obtained by the user. The friction plate may be directly and movably connected onto the support frame, and may also be connected onto the support frame through other devices. Preferably, the friction plate is bonded to a friction plate fixing block. Brief Description of the Drawings Fig. 1 is a front view of a load simulation mechanism for a motor of the invention; Fig. 2 is a top view of Fig. 1; Fig. 3 is a three-dimensional diagram of Fig. 1; 2 Fig. 4 is a schematic diagram of connection between a friction wheel and a friction plate in Fig. 1. Detailed Description of the Invention The technical solutions of the invention will be further described in details as below by embodiments with reference to drawings. Embodiment 1 As shown in Fig. 1, Fig. 2 and Fig. 3, a load simulation mechanism for a motor is provided, comprising a soleplate 1. A motor supporting block 2 is fixed on one end of the soleplate 1, and a load simulation mechanism is disposed on the other end of the soleplate 1; a motor 3 to be measured is fixed on the motor supporting block 2; a sprocket wheel 4 is disposed on one end of an output shaft of the motor 3 to be measured, and the motor 3 to be measured is disposed on a center line A-A of the soleplate; the load simulation mechanism comprises a support frame 5 fixed on the soleplate 1; an upper end of the support frame 5 is hinged with a friction plate fixing block 6 by a hinge pin; a friction plate 7 is bonded to the friction plate fixing block 6 of which the lower end is connected with a hold-down mechanism. The hold-down mechanism comprises a hold-down rod 8 of which one end is connected with the fixed block of the friction plate 7 and the other end is sleeved with a compression spring 9, one end of the compression spring 9 abuts against a positioning block 10 fixed on the soleplate 1, and the other end of the compression spring 9 abuts against a hold-down nut 11. One side of the support frame 5 is provided with a friction wheel 12 which synchronously rotates along with the output shaft of the motor, the friction wheel 12 is sleeved with a transmission shaft 14 on a transmission frame 13, the transmission frame 13 is disposed on the center line A-A, two ends of the transmission shaft14 are respectively sleeved with a friction wheel 12, and each friction wheel 12 abuts against one friction plate 7, that is, two sets of hold-down mechanisms and simulation mechanisms are symmetrically distributed on two sides of the center line A-A. 3 The friction plate 7 is a circular surface of which the radian is the same as that of the friction wheel 12 to an outer circumferential surface, and the friction plate 7 is a canvas flat belt (see Fig. 4). A sprocket wheel 15 is disposed on the transmission shaft 14, hinges 16 are sleeved outside both of the sprocket wheel 15 on the transmission shaft 14 and the sprocket wheel 4 on the output shaft of the motor to be measured, the transmission shaft 14 and the output shaft are driven by the hinges 16 to do synchronous motion. 4