Meaning of minimum stable speed of hydraulic motor

On the meaning of the minimum stable speed of hydraulic motor, on the basis of full study and research of previous research results, combined with the research of engineering topics, the meaning of the minimum stable speed is described as follows:

(1) Any hydraulic system with a hydraulic motor as a power actuator has a minimum stable working speed r/min determined by the internal and external working conditions of the hydraulic system.

(2) The minimum stable speed is the average minimum limit speed under the known hydraulic system parameters, which can maintain a basically stable low-speed operation for a long time without "crawling" phenomenon.

(3) The so-called basic stable speed can be marked by the δn size of the speed fluctuation rate.

Formula:

ωmax - maximum pulsating angular speed of hydraulic motor;

ωmin - minimum pulsating angular speed of hydraulic motor;

ωmean - Average angular speed of hydraulic motor.

The speed fluctuation rate δn can be 10% or more, and its value should be determined by the technical requirements of the engineering application, but it is not allowed to occur any short period of zero (stop) speed phenomenon, that is, the "crawling" phenomenon. As the name suggests, the minimum stable speed is basically stable, any angular speed and angular acceleration are equal to zero stop speed, that is, "crawling" phenomenon, it is unstable, does not conform to the definition, and will cause damage to the hydraulic motor, the hydraulic system itself and the controlled load object.

(4) The minimum stable speed r/min is the rule and limit value of the minimum stable working speed of the hydraulic motor in the low speed region of the hydraulic system (relative to the rated speed nr). The low speed working area of the general hydraulic motor is less than 5 ~ 15% of the rated working speed nr of the motor, which varies with the type and specifications of the hydraulic motor. Whether it is high speed, medium speed or low speed motors (for nr), in some engineering applications, it is generally necessary to consider the minimum stable speed problem, not only for low speed high torque hydraulic motors.

(5) The low speed region below 5 ~ 15%nr is the low efficiency region, which is rarely studied at present; Conventional hydraulic motor performance test only provides experimental study of various efficiency characteristics of the rated speed of more than 25% of the speed. In fact, in the low speed zone, the leakage characteristics and torque loss characteristics of the hydraulic motor are completely different from those in the speed zone above 25%nr. For example, MZD90 line star medium speed hydraulic motor (nr=800r/min) produced by Rexroth Company, the working characteristic curve, leakage flow Qmc and torque loss Tmf characteristic curve drawn according to its sample data are shown in FIG. 1, FIG. 2 and FIG. 3 respectively.

The characteristic curves in FIG. 1, FIG. 2 and FIG. 3 show that the characteristic law in the low speed region (below 15%nr) is completely different from that in the speed region above 25%nr. In the low speed region, Qmc does not decrease with the decrease of motor speed nm, and is almost constant: Tmf does not decrease with the decrease of nm, but increases, and increases faster when nm≤50r/min, that is, "negative damping" is stronger. Therefore, the Qmc and Tmf characteristics in the low speed region of the motor are definitely one of the main factors affecting the nmmin of the hydraulic motor.

(6) Since the minimum stable speed of the hydraulic motor nmmin refers to the continuous and stable low-speed   operation for a long time, only allowing relatively small speed fluctuations, then the mathematical equation for studying the minimum stable speed should be based on the static speed regulation equation, and the external load should also be the static load torque Tm, because the inertial load can not be added under the low-speed constant rotation.   (Author: Peng Xiwei, Wang Yu, Department of Automatic Control, Beijing Institute of Technology)

Figure 1 MZD90 motor operating characteristic curve

Figure 2 Leakage flow characteristic curve of MZD90 motor in low speed zone

Figure3 Torque loss curve of MZD90 motor in low speed zone