introduction:
The fiber drawing industry in the chemical fiber sector and the finished product conveying industry in the glass manufacturing sector both require extremely high speed control precision. Permanent magnet synchronous motors (PMSMs) are typically used for these operations, with frequency converters providing open-loop speed regulation. Danfoss frequency converters have a long history of application in these two industries due to their stable performance and high reliability. To further improve the reliability of equipment operation and enhance the low-speed output performance of PMSMs, we analyze the basic characteristics of PMSMs and propose some beneficial solutions.
Structural characteristics of permanent magnet synchronous motors:
The typical permanent magnet synchronous motor mechanism is shown in Figure 1. The stator is made of ordinary silicon steel sheets with good magnetic permeability; the rotor surface is covered with permanent magnets, while the rotor interior is still made of iron with good magnetic permeability. The air gap between the rotor and stator is approximately 0.2 mm, similar to that of a typical asynchronous motor.
Figure 1. Structure of a permanent magnet synchronous motor
Modern mainstream permanent magnet synchronous motors mostly use rare-earth permanent magnet materials as permanent magnets. Because rare-earth permanent magnet materials have very high intrinsic coercivity, their magnetization characteristic curve is nearly rectangular, and below the coercivity point, their magnetic susceptibility is almost the same as air. Therefore, when performing magnetic circuit analysis, rare-earth permanent magnets can be treated as an air gap. Thus, the internal equivalent air gap of a permanent magnet synchronous motor is much larger than that of an asynchronous motor.
Based on the difference in electrical parameters between the d-axis inductance Xd and the q-axis inductance Xq, rare-earth permanent magnet synchronous motors can be classified into two types: salient pole synchronous motors and non-salient pole synchronous motors.
The main characteristic of a salient-pole permanent magnet synchronous motor is that the permanent magnets are attached to a uniform rotor. By modifying the surface shape of the permanent magnets, a sinusoidal magnetomotive force in the air gap is ensured. The air gaps along the d-axis and q-axis of the magnetic circuit are the same, therefore: Xd = Xq. The main characteristic of a salient-pole permanent magnet synchronous motor is that the actual air gap of the motor is uniform, but the air gap along the d-axis magnetic circuit is equal to the motor air gap plus the thickness of the permanent magnets, while the air gap along the q-axis magnetic circuit is equal to the motor air gap. Therefore, typically: Xd < Xq.
Precautions for using a frequency converter to drive a permanent magnet synchronous motor in open loop:
1. The Influence of Motor Ripple Current
Because the internal equivalent air gap of a permanent magnet synchronous motor is much larger than that of an asynchronous motor, the permanent magnet synchronous motor has a much smaller main inductance and electrical time constant than the asynchronous motor. This has the advantage of faster current response in control. However, frequency converters are usually designed to drive asynchronous motors with large inductance. If they are used to drive permanent magnet synchronous motors, the output current ripple of the frequency converter will be much larger than the current ripple when driving an asynchronous motor.
Under such operating conditions, many non-specially designed frequency converters will fail or have their lifespan significantly shortened. Danfoss' FC302 frequency converter is a drive specifically designed for driving permanent magnet synchronous and asynchronous servo motors. The converter's design takes into account the impact of high carrier ripple current on the converter's hardware and control software; it also specifically limits the voltage fluctuation rate of the output PWM square wave. Extensive testing in the factory and long-term practical application trials by numerous users have proven the long-term reliability of the FC302 drive for permanent magnet synchronous motors.
2. Motor control modes of frequency converters
When using frequency converters for open-loop control, there are generally several control modes, such as VVCplus control, flux vector control without speed feedback, and V/F control (scalar control). Among them, VVCplus control and flux vector control without speed feedback are control modes for asynchronous motors and cannot be used for synchronous motor control. Therefore, only V/F control can be used.
The Danfoss FC302 driver's V/F control mode allows users to customize 6 points (frequency, voltage), providing greater flexibility in application and making it particularly suitable for open-loop control of permanent magnet synchronous motors.
3. Driver Power Selection
When selecting the appropriate driver power for a permanent magnet synchronous motor (PMSM), the starting current of the PMSM is the primary consideration. This current is generally higher than the rated current, and the rated current of the driver should be greater than or equal to the starting current of the PMSM. When overloaded, a PMSM may generate a step-out current exceeding 10 times its rated current, far exceeding the 5-7 times stall current of an asynchronous motor. Furthermore, in open-loop control, the driver cannot provide effective torque limiting protection. Therefore, the user must be responsible for ensuring the load does not overload. In situations where tripping is not permitted, the driver power selection must have a sufficiently large margin. The open-loop automatic torque limiting and overvoltage limiting functions of general-purpose frequency converters are designed for asynchronous motors and are not well-suited for PMSMs. When driving a PMSM in open loop, these intelligent protection functions must be completely disabled.
Operating characteristics of permanent magnet synchronous motors:
After ignoring the stator resistance and leakage reactance, the vector diagram of the permanent magnet synchronous motor can be simplified as shown in Figure 6. Where: 0 is the induced electromotive force generated by the rotor permanent magnet, and the angle is strictly consistent with the rotor position. E0 = W•KW•Ф0•ω [1] W, number of winding turns; KW, winding coefficient; Ф0, magnetic flux generated by the permanent magnet. θ, is the angle between the applied space voltage vector and the permanent magnet rotor.
Power angle characteristics of permanent magnet synchronous motors :
◆ The power angle characteristic refers to the relationship curve of electromagnetic power PM as a function of power angle θ: PM = f(θ).
◆ The power angle characteristics of the salient pole permanent magnet synchronous motor [2] m, the number of phases Substituting formula [1] into E0, we can get [3] TM ∝ U•sinθ [4] [5] When unloaded, θ is 0, that is, the space voltage vector is consistent with the rotor angle; when the space voltage vector angle leads the rotor by 90°, the output torque of the permanent magnet synchronous motor reaches the maximum value. The load torque must not exceed TMmax at any time, otherwise it will cause loss of synchronism.
◆ Power angle characteristics of salient pole permanent magnet synchronous motor [6] Due to the influence of salient pole reaction torque, the power angle characteristics of salient pole permanent magnet synchronous motor deteriorate.
Methods to improve output performance when driving a permanent magnet synchronous motor in an open-loop manner:
1. Since the angle of attack characteristics of salient pole permanent magnet synchronous motors are worse than those of non-salient pole permanent magnet synchronous motors, non-salient pole permanent magnet synchronous motors are recommended for applications with high performance requirements.
2. For salient-pole permanent magnet synchronous motors, according to formula [4], TM ∝ U•sinθ, it can be seen that the load capacity of the salient-pole permanent magnet synchronous motor is proportional to the input voltage. Therefore, increasing the open-loop output voltage of the inverter can improve the load capacity of the permanent magnet synchronous motor and prevent loss of synchronism. However, if the open-loop output voltage of the inverter is increased too much, the output current power factor will decrease, and even the excitation current will be overcurrent. This requires adjustment according to the actual load conditions on site.
3. During open-loop control, the speed fluctuation caused by torque fluctuation. From formula [5], the torque of the step is obtained as follows: ∝ U•cosθ [7]. Therefore, it can be seen that when U is larger, the mechanical characteristics of the permanent magnet synchronous motor are also stiffer, and the angle (speed) fluctuation caused by the same torque fluctuation is smaller.
4. Adjustment method of V/F characteristic curve. At low frequency, increase the starting voltage, mainly considering voltage compensation for the resistance and leakage inductance of the stator and wires; at medium and low frequency, artificially increase the voltage to improve the open-loop operation performance of the synchronous motor; at high frequency, due to the inertial smoothing effect, the adjustment is relatively simple, just set 50Hz to 380V.
References:
[1] FC300 Design.MG.33.BB.02.Danfoss Company
[2] Principles and Design of Modern Permanent Magnet Motors. Tang Renyuan. Machinery Industry Press.
