1 Characteristics of variable frequency motors
1.1 Electromagnetic Design
For ordinary asynchronous motors, the main performance parameters considered when designing variable frequency motors are overload capacity, starting performance, efficiency and power factor. As for variable frequency motors, since the critical slip rate is inversely proportional to the power supply frequency, it can be started directly when the critical slip rate is close to 1. Therefore, overload capacity and starting performance no longer need to be considered too much. The key issue to be solved is how to improve the adaptability of the motor to non-sinusoidal power supplies. First, reduce the stator and rotor resistance as much as possible.
Reducing stator resistance lowers fundamental-frequency copper losses to compensate for the additional copper losses caused by higher harmonics. Second, motor inductance should be increased appropriately to suppress higher harmonics in the current. However, greater rotor-slot leakage reactance also increases the skin effect and higher-harmonic copper losses. Motor leakage reactance must therefore be chosen with suitable impedance matching across the entire speed control range in mind. In addition, the main magnetic circuit of a variable-frequency motor is generally designed to remain unsaturated. One reason is that higher harmonics increase magnetic circuit saturation; another is that, at low frequencies, the inverter output voltage is increased appropriately to improve output torque.
1.2 structural design
When designing the structure, the impact of non-sinusoidal power supply characteristics on the insulation structure, vibration, noise cooling method, etc. of the variable frequency motor is mainly considered. First of all, in terms of insulation level, which is generally F level or higher, the ground insulation and wire turn insulation strength are strengthened, and the ability of the insulation to withstand impulse voltages is particularly considered. Regarding the vibration and noise problems of the motor, we must fully consider the rigidity of the motor components and the entire body, and try our best to increase its natural frequency to avoid resonance with each force wave.
Forced ventilation cooling is generally used, with the main motor's cooling fan driven by an independent motor. Motors rated above 160KW should have bearing insulation, primarily because magnetic-circuit asymmetry can readily occur and produce shaft currents. When combined with currents generated by other high-frequency components, shaft currents increase substantially and can damage bearings, so insulation measures are generally required. In addition, for constant-power variable-frequency motors running above 3000/min, special high-temperature-resistant grease should be used to accommodate the rise in bearing temperature.
2 Frequency conversion motor common fault diagnosis
2.1 inter-turn short circuit, partial discharge
Turn-to-turn short circuit and partial discharge are currently common forms of insulation faults in variable frequency motors. Turn-to-turn short circuit generally manifests as large-area damage to a coil in the motor, while partial discharge is concentrated in motor coils that have a good appearance, but the insulation resistance has reached zero. At this time, the damage to the motor insulation system is not only caused by a single factor, but also caused by factors such as partial discharge and local dielectric heating.
Partial discharge: At present, during the operation of small and medium-capacity inverters, the pulse width modulation technology of IGBT power devices is generally chosen. The PWM speed regulation device formed by these components can provide towering peaks. The wave peaks have steep front characteristics. At the same time, its modulation frequency is high, so the harmful impact on insulation is more serious.
Local dielectric heating: If the electric field strength E in the motor has significantly exceeded the insulation critical value, then the dielectric loss will become more and more serious. Especially as the frequency continues to rise, partial discharge will also increase, thereby generating heat. This heat will inevitably lead to more serious problems such as leakage current. Over time, not only will the loss per unit volume increase, but the temperature rise of the motor will also continue to rise, which will inadvertently cause the insulation to age faster and faster.
Cyclic alternating stress: Through the PWM variable frequency power supply mode, when the variable frequency motor is put into formal use, it can be directly braked through various methods provided by the frequency converter. Under the influence of cyclic variable stress, the entire insulation of the motor insulation will age faster and faster. Since the electrical and mechanical integrity are not comprehensively considered in the early design process, the aging process of the motor will continue to increase.
2.2 Bearing damage and excessive vibration
In practice, bearing damage in PWM inverter-driven motor systems can become increasingly severe, with recurring bearing damage and excessive vibration. At a high-speed wire-rod plant, a 690kW inverter-fed motor developed severe vibration just 3 months after entering service. During diagnosis and repair, the motor was removed and dismantled. Numerous clear burn marks were found on the bearing surfaces. Shaft currents were identified as the cause of the severe motor-bearing damage.
2.3 current oscillation
In one case analyzed, a 250kW/400V/430A variable-frequency motor system at a cold-rolling plant had repeatedly suffered burnt-out components during operation. Before inspecting and repairing the frequency converter, the motor was subjected to a no-load test under V/F control. The test showed abnormal current in the 7–30Hz range. More importantly, the amplitudes of the three-phase currents oscillated markedly, with peak oscillating current amplitudes reaching 700A.
After this fault occurred, maintenance personnel immediately tested existing motors, inverters, and other equipment of the same type. The results showed that motors and inverters exhibited instability and other problems within the same frequency range [2]. Motor operation was relatively stable near the power supply frequency. However, at a frequency of 40 Hz, especially in the range of 20 to 30 Hz, motor current oscillated at approximately 10 to 20 Hz. If the peak value became too high, overall motor operation was severely affected.
For an asynchronous motor, operation at zero slip can introduce instability into positive and negative changes in transient torque. More significantly, inverter-driven torque pulsations and transient V/F changes can cause marked torque fluctuations. These may develop into vibration, including sustained vibration. Such torque pulsations are related to harmonic currents and other factors. If an inverter-fed motor operates unstably, do not simply assume that either the motor or inverter is faulty. Analyze them together using the parameters of both units to make a sound fault diagnosis.
3 Maintenance measures for variable frequency motor faults
The application of variable frequency motors is becoming more and more widespread. For the maintenance of variable frequency motors, effective measures need to be taken according to the characteristics of the variable frequency motors to ensure the normal operation of the variable frequency motors.
3.1 Frequency conversion speed motor maintenance requirements
Variable-frequency speed-controlled motors generally use 4-pole motors, with the base-frequency operating point designed at 50Hz. The motor operates at constant torque over 0–50Hz (0–1480r/min) and at constant power over 50–100Hz (1480–2800r/min). The overall speed-control range is 0–2800r/min, essentially meeting the requirements of ordinary driven equipment. Its operating characteristics are the same as those of a DC speed-controlled motor, with smooth and stable speed adjustment.
If higher output torque is required within the constant-torque speed-control range, a 6-pole or 8-pole motor can also be selected, although its overall size will be somewhat larger. Flexible CAD software used in the electromagnetic design of variable-frequency motors allows the base-frequency design point to be adjusted as needed. A computer can accurately simulate motor operating characteristics at different base frequencies, thereby expanding the constant-torque speed-control range. According to the motor's actual operating conditions, greater power can be achieved within the same frame size, or greater output torque can be obtained using the same frequency converter, allowing the motor to be designed and manufactured for optimal performance under various operating conditions.
Variable-frequency speed-control motors can be fitted with an optional additional speed encoder, providing high-precision speed and position control and rapid dynamic response. A dedicated DC (or AC) motor brake can also be fitted for fast, effective, safe, and reliable braking. Because the base frequency of these motors is designed to be adjustable, we can also manufacture various high-speed motors that maintain constant torque during high-speed operation. They can replace conventional medium-frequency motors to a certain extent and are inexpensive.
The variable frequency speed motor is a three-phase AC synchronous or asynchronous motor. According to the output power of the frequency converter, there are three-phase 380V or three-phase 220V, so the motor power supply also has three-phase 380V or three-phase 220V. Different differences, generally only inverters below 4KW have three-phase 220V. Since the variable frequency motor is divided into different constant power speed regulation areas and constant torque speed regulation areas based on the motor's base frequency point (or inflection point), the settings of the frequency converter base frequency point and the variable frequency motor base frequency point are very important.
3.2 improves insulation performance
Proper use of corona-resistant enameled wire helps increase the shielding enamel film layer appropriately. Applying quantum chemistry technology allows the shielding chemicals to participate directly in the polycondensation reaction of the enamel-base polymers as primary constituents of the enamel film. This ensures that high-frequency impulse voltage is promptly dispersed and dissipated, improving the overall corona resistance of the enamel film. Slot insulation materials are currently made using combinations of different materials, including NHN and Class F DMD. These materials have strongly organic characteristics and therefore do not possess corona resistance.
Based on this, a new slot insulation containing mica was chosen. The addition of mica is conducive to improving corona resistance. In terms of interphase insulation, you should choose a product type with polyester fleece on the surface. Compared with other materials, this type of product has obvious advantages in absorbing resin, which is conducive to effective bonding with wires.
The impregnation process has always been one of the most important processes in the maintenance of variable frequency motors. The most important point is to prevent resin from flowing out. Usually VPI is used for treatment, or after VPI treatment, the impregnation process can be appropriately added, which will help to eliminate air bubbles in time and continuously fill the air gaps in the winding. It can also improve the electrical and mechanical strength of the winding to ensure that its own heat resistance and pollution resistance are strengthened.
If conditions permit, it can be processed by ultraviolet heating current drying method, which can achieve good results.
In addition, it should be noted that during the entire frequency conversion motor maintenance process, it is important to avoid short circuits and other problems, ensure that the assembly of various components such as motor bearings can meet basic accuracy requirements, and try to avoid serious local heating and other problems caused by eddy current losses, otherwise it will inevitably affect the insulation performance of the motor.
3.3 eliminates the influence of shaft current
In order to ensure that the shaft current can be reduced to a harmless level, it is usually necessary to ensure that the shaft current is controlled below 0.4A/mm2 or 0.35mV. Based on this, targeted countermeasures should be taken based on the specific use environment and type characteristics of the motor to eliminate the adverse effects caused by shaft current.
Suppress power supply harmonics: In order to eliminate the impact of shaft current, through the reasonable application of the inverter power supply speed regulation system, you can directly add a filter to it, or use a matching frequency conversion speed regulation device, which will help reduce harmonics, and can also reduce adverse effects such as shaft current and vibration.
Bearing insulation measures: Taking targeted insulation measures for bearings can also eliminate the adverse effects caused by shaft current in a timely manner. At present, the common methods are to ground the load-side bearing of the motor and insulate the non-load-side bearing. When using the rolling bearing structure, you can choose to use the insulated bearing as the main bearing form, or use the ion spraying method to evenly spray an insulating layer from 50 to 100mm on the surface of the inner ring and outer ring of the bearing.
In addition, depending on the actual situation, you can also directly add a sleeve to the end cover bearing chamber, add an insulation layer between the sleeve and the end cover, and tighten the inner and outer cover bearings. When using a sliding bearing structure, you can directly add an epoxy glass cloth plate to the fixed bearing position, or add insulating pipe joints to the oil inlet and outlet pipes. These methods can effectively eliminate the adverse effects of shaft current.
When eliminating shaft current, in addition to using the above methods, you can also choose to use monitoring lines to enhance insulation performance and improve the motor operating environment. All in all, no matter which method you choose to use, you must start from multiple angles based on the characteristics and requirements of the actual situation to achieve good results.
3.4 improves current oscillation problem
Long-term testing and analysis indicate that current oscillation and instability can be addressed by increasing motor rotational inertia or operating with a load. Increasing the DC-side capacitance of a voltage-source inverter can also reduce voltage fluctuations. For PWM-controlled inverters, using faster switching components or lowering the PWM modulation frequency helps reduce output-voltage fluctuations caused by dead time.
In order to improve the current oscillation problem, motors with high slip can also be used, and current feedback and other methods can be used to ensure that the vector control situation of the circuit can be fed back in time, thereby ensuring the improvement of the operating stability of the variable frequency motor.
(Source: Motor Transformer Maintenance Technology Exchange)