Ordinary asynchronous motors are designed according to the constant frequency and constant voltage operation plan. The narrow area near the rated operating point can operate efficiently as expected, and it is generally not suitable for wide-range frequency conversion speed regulation. Some customers, based on cost considerations, directly use ordinary asynchronous motors as variable frequency motors, resulting in frequent motor failures or shortened service life.
In fact, many motor companies have designed and developed broadband motors, which are specially used for wide-range frequency conversion and speed regulation occasions. So, the topic returns to the proposition of "ordinary asynchronous motors used as variable frequency motors". What are the potential hazards of this simple and rude application? Is it necessary to upgrade to a broadband motor? Today we start with theoretical analysis, clarify the internal mechanism of frequent failures, and briefly explain the problem.
Decrease in efficiency and increase in temperature
All frequency converters will produce different degrees of harmonic voltage and current during operation, making the motor run under non-sinusoidal voltage and current. The high-order harmonics of the frequency converter will cause the increase of the stator copper loss, rotor copper loss, iron loss and additional loss of the motor. The most significant is the rotor copper loss. These losses will cause the motor to generate additional heat, reduce the efficiency, and reduce the output power. If an ordinary three-phase asynchronous motor is used under the condition of a frequency converter, the most direct increase is the increase in temperature rise, especially for the IP23 series of motors.
The cooling effect becomes worse at low speed
First of all, the impedance of the asynchronous motor is not ideal. When the power supply frequency is lower, the loss caused by the higher harmonics in the power supply is larger. Secondly, when the rotation speed of an ordinary asynchronous motor decreases, the cooling air volume decreases in proportion to the third power of the rotation speed, which causes the low-speed cooling condition of the motor to deteriorate, and the temperature rise sharply increases, making it difficult to achieve constant torque output. When the power supply frequency is low, the loss caused by the higher harmonics in the power supply is larger. Secondly, when the speed of the ordinary asynchronous motor decreases, the cooling air volume decreases significantly, which makes the cooling condition of the motor worse. The direct result is a sharp increase in temperature rise and it is difficult to achieve constant torque output.
The insulation strength cannot be satisfied
The carrier frequency of the inverter is very high (approximately several thousand to ten kilohertz), which will make the stator winding of the motor bear a very high voltage, so that the inter-turn insulation of the motor can withstand more severe tests and pose a threat to the motor's ground insulation , The final result is that the motor is due to inter-turn, inter-phase and ground faults, and in severe cases, the windings are overloaded. Because the design margins of phase-to-phase insulation and ground-to-ground insulation are relatively large, the motor is more manifested as inter-turn and overload, but for the stator without interphase blocks processed by the automatic wire inserting machine, the relative failure will be relatively more.
Causes electromagnetic noise and vibration problems
When ordinary asynchronous motors are powered by frequency converters, the problems of vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complicated. The time harmonics in the variable frequency power supply interfere with the inherent spatial harmonics of the electromagnetic part of the motor to form various electromagnetic exciting forces. When the frequency of the electromagnetic force wave is consistent with or close to the natural vibration frequency of the motor body, resonance will occur, and severe electromagnetic high-frequency noise will be accompanied by a certain degree of vibration.
Structural fatigue and insulation aging
Powered by the frequency converter, the motor can be started at a very low frequency and voltage without inrush current, and can be quickly braked by using various braking methods provided by the frequency converter, which is created for the realization of frequent starting and braking. However, the mechanical system and electromagnetic system of the motor are under the action of cyclic alternating force, which brings fatigue and accelerated aging problems to the mechanical structure and the insulating structure.
The application of high-performance frequency converters has brought revolutionary changes to the motor industry. Most motor test equipment uses static adjustable frequency conversion power supplies to replace voltage regulators and frequency conversion units. However, most of the end customers power frequency power supply, which will inevitably bring a hidden danger that cannot be ignored: the motor can run normally when the static adjustable variable frequency power supply is powered, and the power frequency power supply may not start normally.
Motor's adaptability to frequent starting and braking
Since the inverter is used for power supply, the motor can be started at a very low frequency and voltage without inrush current, and various braking methods provided by the inverter can be used for rapid braking, in order to achieve frequent starting and braking. The conditions are created so that the mechanical system and electromagnetic system of the motor are under the action of cyclic alternating force, which brings fatigue and accelerated aging problems to the mechanical structure and the insulating structure.
In view of the above situation, the inverter motor adopts the following design:
1. Minimize the stator and rotor resistance and reduce the fundamental copper loss to compensate for the increase in copper loss caused by higher harmonics
2. The main magnetic field is not saturated design. First, consider that higher harmonics will deepen the saturation of the magnetic circuit. Second, consider that the output voltage of the inverter can be appropriately increased in order to increase the output torque at low frequencies.
3. The structural design is mainly to improve the insulation level; fully consider the vibration and noise of the motor; the cooling method adopts forced ventilation cooling, that is, the main motor cooling fan adopts an independent motor drive mode, and the function of the strong cooling fan is to ensure that the motor is in Cooling at low speeds.
4. The distributed capacitance of the coil of the variable frequency motor is smaller, and the resistance of the silicon steel sheet is larger, so that the influence of the high frequency pulse on the motor is smaller, and the inductance filtering effect of the motor is better.
5. Ordinary motors, power frequency motors, only need to consider the starting process and the working conditions of one point of the power frequency, and then design the motor; while variable frequency motors need to consider the starting process and the working conditions of all points within the frequency conversion range, and then design the motor.
6. In order to adapt to the PWM widened wave analog sine alternating current output by the frequency converter contains a lot of harmonics, the specially made frequency conversion motor can actually be understood as a reactor plus an ordinary motor.
How to distinguish between ordinary motors and variable frequency motors?
1. The difference in structure between ordinary motor and variable frequency motor
01. Higher insulation requirements
The insulation class of general frequency conversion motors is F or higher. Strengthen the insulation to the ground and the insulation strength of the turns, especially considering the ability of the insulation to withstand impulse voltage.
02. The vibration and noise requirements of variable frequency motors are higher
Inverter motor should fully consider the rigidity of the motor components and the whole (public number: pump housekeeper), and try to increase its natural frequency to avoid resonance with various force waves.
03. Different cooling methods for variable frequency motors
The frequency conversion motor generally adopts forced ventilation cooling, that is, the main motor cooling fan is driven by an independent motor.
04. Different protection measures
Bearing insulation measures should be adopted for variable frequency motors with a capacity exceeding 160KW. The main reason is that the asymmetry of the magnetic circuit is easy to occur, and the shaft current is also generated. When the currents generated by other high frequency components work together, the shaft current will greatly increase, which will cause the bearing to be damaged. Therefore, insulation measures are generally taken. For constant power variable frequency motors, when the speed exceeds 3000/min, special high-temperature-resistant grease should be used to compensate for the temperature rise of the bearing.