introduction
Permanent magnet AC servo motors are widely used in automation and other industries due to their characteristics such as fast response, high power density, high efficiency, and stable operation. However, many engineers are confused about the following: how to determine the phase sequence of the permanent magnet AC servo motor windings and the encoder signal phase sequence; the meaning of the key motor parameters kt and ke and how to effectively utilize these two parameters in engineering calculations; how to understand motor power; and how to use these parameters in engineering applications. Therefore, this article provides a detailed explanation of these issues from an engineering application perspective for readers' reference.
Basic principle of permanent magnet AC servo motor
The stator winding structure of a PMSM is a three-phase symmetrical winding, and the rotor structure is a surface-mount permanent magnet structure or an embedded permanent magnet structure. Its winding back electromotive force is sinusoidal. When a three-phase symmetrical sinusoidal current is applied to the stator winding, the motor will generate continuous electromagnetic torque. The rotor permanent magnet of the PMSM generates a near-sinusoidal magnetic field in the working air gap, so a near-sinusoidal back electromotive force is induced in the armature winding when the rotor rotates. The three-phase armature winding of the PMSM is connected to a 180° conduction half-bridge inverter circuit, and the drive voltage is a pulse voltage modulated by space vector pulse width modulation. When the motor is running, the three-phase armature windings are simultaneously energized, generating a "continuous" circular rotating magnetic field in the working air gap. To achieve servo control, the position sensor of the PMSM can be a rotary transformer or a photoelectric encoder; currently, incremental photoelectric encoders are more commonly used in industrial applications. Servo-driven encoders generally require two sets of signals:
1) a, b, z signals, where the a and b pulses have a 90° phase difference, which makes it easy to determine the direction of the motor rotation; the z signal outputs one pulse per revolution and is used for reference point positioning (currently, Leadsai servo does not require the z signal).
2) u, v, w signals: These three pulse signals are 120° out of phase with each other, and the number of pulses emitted per revolution is the same as the number of pole pairs of the motor. The current position of the motor rotor can be determined based on the high and low level relationship of the u, v, w signals.
Before the motor starts, the current position of the motor's magnetic poles can be estimated based on the high and low levels of the three pulse signals u, v, and w. Once the motor starts rotating, the a and b signals can accurately detect the rotor's position angle.
Most mainstream PMSM servo drives currently use vector control technology, and the system block diagram is shown in Figure 1.
How to define or determine the phase sequence of a permanent magnet AC servo motor?
In the production process of PMSM servo motors, there is an important assembly and debugging process to ensure the proper phase relationship between the back electromotive force of the motor's three-phase windings and the encoder signals u, v, w, as shown in Figures 2, 3, 4, and 5.
kt and ke of permanent magnet AC servo motor
1) The back electromotive force constant ke of a permanent magnet motor
As long as a motor is rotating, coils will inevitably cut magnetic lines of force, thus generating a back electromotive force (EMF). For a specific motor model, the faster its rotational speed, the higher the back EMF voltage. In other words, the back EMF voltage is directly proportional to the motor's rotational speed. The back EMF constant ke is used to represent this proportional relationship.
ke = e/n (where e is the back electromotive force in V; n is the motor speed in krpm).
Example: A motor is driven to rotate at 3000 rpm using a reverse-drive method. The voltage between phase a and phase b of the motor is measured to be 30V. The calculation method for its ke is as follows:
ke = e/n = 30v/3krpm = 10v/krpm
2) Torque constant kt of permanent magnet DC motor
For a specific model of motor, the greater the current flowing through the motor windings, the greater the torque generated by the motor shaft. In other words, the motor torque is directly proportional to the motor winding current. The torque constant kt is used to represent the ratio of the motor torque to the current flowing through the motor windings.
kt=t/i
(In the formula, t is torque: nm, and i is current: a)
Example: The output torque of a motor shaft is measured to be 0.35 nm, and the winding current is measured to be 5 A. The calculation method for kt is as follows:
kt = t/i = 0.35/5 = 0.07 nm/a
3) The relationship between kt and ke in a permanent magnet AC servo motor
The relationship between the back electromotive force constant ke and the torque constant kt of a permanent magnet DC motor:
When calculating in units of angular velocity (1/s):
ke = w × p × φ / π = kt
In the above formula: w — total number of conductors calculated per pole
b—Calculate the magnetic field strength on the diameter cylinder surface
p—Polar pair
φ—Magnetic flux per pole…………φ=lτb
Therefore, you should know that for a specific motor, if you know either its back electromotive force constant ke or its torque constant kt, then you know the other one. An important formula:
kt = ke ÷ 104.7 (where kt is in nm/a and ke is in v/krpm)
For example: A motor has a ke value of 9.39 V/krpm. What is the kt value of this motor? A simplified engineering calculation method is as follows:
kt = ke ÷ 104.7
=9.39 ÷ 104.7
=0.0897nm/a
Rotor inertia of permanent magnet AC servo motor
The requirement for the load inertia of a servo motor in a servo driver is that the applicable load inertia should be less than five times the motor's rotor inertia. Therefore, a larger motor rotor inertia can handle a larger load inertia, but the motor's mechanical time constant will increase, and the motor's speed response will decrease. Conversely, a smaller motor rotor inertia results in a smaller mechanical time constant and a faster speed response, but the permissible load inertia cannot be large.
Torque and power of permanent magnet AC servo motor
The rated torque of a permanent magnet AC servo motor refers to the torque that the motor can continuously and safely output. At an ambient temperature of 25℃, this will cause the motor winding temperature to reach its maximum permissible value. The rated power of a motor refers to the mechanical power output by the motor at its rated speed and rated torque. The formula for calculating motor power is:
p = n × t ÷ 9549.3
(Where p: power, in kW; n: rated speed of the motor, in rpm; t: rated torque, in nm).
For example: A motor has a power of 200W and a rated speed of 3000rpm. What is its rated torque?
From the above formula, we get:
t = (9549.3 × p) ÷ n
= (9549.3 × 0.2) ÷ 3000
=0.64nm
Because servo motors operate under closed-loop control, in id=0 control mode (vector control), the motor torque is directly proportional to the current supplied to the motor. The motor's torque output varies with the load; when the load is constant, the servo motor's torque output is also constant. Servo motors can operate under overload for short periods; the overload factor is determined by the overload current output by the driver, and the overload time is determined by the driver's capacity and the servo motor's temperature rise. A typical overload factor is 3 times.
Insulation class of permanent magnet AC servo motor
Motor insulation class: indicates the insulation materials and insulation structure used in the motor, and the allowable operating temperature.
The heat resistance rating of motor insulation structure is divided into 5 grades, as shown in Table 1:
Table 1 Heat Resistance Rating of Motor Insulation Structure
Heat resistance rating: aebfh
Maximum permissible temperature (°C): 105, 120, 130, 155, 180
The insulation class of PMSM servo motors is generally B or F.
Protection rating of permanent magnet AC servo motor
Protection rating: The protection rating of a motor is indicated by IP followed by two digits.
The first digit indicates the degree of protection the motor provides against solid objects.
The second digit indicates the degree of protection the motor provides against liquids.
The meanings of the two digits are shown in Tables 2 and 3.
Conclusion
This article addresses several key questions that engineers often have about permanent magnet AC servo motors: determining the phase sequence of the motor windings and the encoder signal; and key motor parameters.
