Rotary transformer, abbreviated as rotary transformer, is divided into various series such as reluctance type, brushed rotor winding type, brushless rotor winding type, dual channel type, etc. It can be simply understood as: rotary transformer is an analog output angle sensor that needs to be combined with rotary transformer decoding to obtain the required measured angle.
The reason why permanent magnet servo motors use rotary angle sensors is to perform vector control, so that the electromagnetic field generated by the stator winding is always orthogonal to the rotor permanent magnetic field, thereby achieving the best output effect. To achieve this characteristic, it is necessary to accurately obtain the current rotor position status of the motor.
To achieve this goal, there are two very important parameter indicators that need to be controlled: 1. The deviation angle between the zero position of the rotating transformer angle and the zero position of the motor rotor; 2. The angle deviation after the installation of the rotary transformer, also known as the "comprehensive electrical error" of the rotary transformer.
For example, if the deviation between the zero position of the rotary transformer and the zero position of the motor is+30 ', the electrical error of the rotary transformer is ± 25 minutes, and the error of the rotary transformer decoding board is ± 10', then within the range of 0-360 degrees, the deviation expressed by the rotary transformer on the actual position of the motor rotor is -5 '~+65'.
There are currently three main methods for measuring zero position deviation:
Static measurement of zero position deviation is the most widely used method in China, which only requires a DC power supply and a rotary converter calculation device to zero. The usual practice is to first apply a low-voltage direct current to the motor winding, with the U phase being positive and the V phase or VW phase being negative. At this point, the motor rotor will be pulled down to a fixed position. For example, when using UVW connection, the theoretical electrical angle of the rotor is 0 °, and reading the calculated angle value of the rotary transformer at this time is the zero position deviation between the rotary transformer and the motor.
The equipment is simple and cost-effective, requiring only a low-voltage constant current source to power the motor and a rotary decoding device to display the rotary angle value for zeroing.
The operating conditions are static, safe, continuously adjustable, and the operation is simple.
Disadvantages:
Static measurement assumes that the three-phase windings of the motor are balanced and in a fixed position after being powered on. In fact, due to factors such as cogging torque, friction, and imbalance of two-phase currents, the rotor of the motor may deviate from the zero position, and this deviation is difficult to measure and perceive. This results in a low accuracy of the actual zero position deviation test for static measurements, with a maximum deviation between 0.5 ° and 2 °.
The zero position static test deviation mainly consists of two parts:
1. Random error: Due to the presence of friction torque and rotor inertia, there is a certain degree of randomness in the final stopping position of the rotor when energized from different rotor positions. This random error can be reduced by increasing the DC power supply current to the motor. However, the motor cooling device often does not work when zeroing, so it is necessary to consider the current value that the motor can withstand in this state to avoid motor burnout.
2. Fixed deviation: If the current flowing through the motor UV and UW is unbalanced, it will cause a fixed deviation in the stopping position of the motor rotor. This deviation is not only caused by the imbalance of the three-phase windings of the motor, but also by the inconsistent DC resistance of the power cables in the U and W phases and the inconsistent contact resistance of the test clamp. Because the DC resistance of the motor winding is very small, even a deviation of a few milliohms in contact resistance can cause significant deviations in the current flowing through the UV and UW phases. At present, zero adjustment systems have paid attention to this issue. For example, in the 2019 model of the Resolver Analyzer Pro, a current monitoring function has been added, which can effectively avoid measurement errors caused by current imbalance in static testing.
Dynamic measurement is based on the principle that the back electromotive force waveform can accurately reflect the position and state of the motor rotor. It measures the angle deviation between the motor rotor zero position and the rotation state when the tested motor is rotating and generating electricity. There are usually two methods: inertia method and drag method.
1. Inertia method: It is to drive the test motor to a certain speed, then remove the driving voltage, and use the inertia of the motor to complete the measurement.
2. Drag method: It is to use another motor to drive the test motor to generate electricity and measure the back electromotive force voltage.
The measurement accuracy of the motor rotor position is high, so the zero position deviation measurement accuracy is one order of magnitude higher than that of static, and can reach different levels.
Dynamic measurement requires the tested motor to be in a generating state.
Self learning is the function of the controller to have the ability to self learn the rotation zero position. This function requires the support of controller software and hardware, and there are three mainstream solutions: one is static measurement, which determines the position and status of the motor rotor by supplying DC power to the motor winding; The second is the principle of dynamic measurement, also known as back electromotive force measurement. The third is to inject high-frequency voltage or current into the motor winding to measure the state of the motor rotor.
Due to the first technology requiring the motor to be in a free axis state and the second technology requiring the motor to operate with inertia, zeroing can often only be completed when not connected to the load. The third technology can be used for sensorless control of motors, which can complete zero position measurement in any assembly state.
Self learning is usually used in conjunction with software compensation to complete the zeroing process without adjusting the rotary stator.
Zero position measurement can be completed without the need for external zeroing equipment.
Specific controllers are required, and methods one and two require specific operating conditions, often only after separating the motor from the load.