Four major things to pay attention to when transforming variable frequency speed regulating motors

The 20th China International Motor Expo and Development Forum 2020 will continue to be held at the Shanghai New International Expo Center from November 16 to 18, 2020. The most influential professional event in the motor industry looks forward to your participation. Book a booth now and communicate face-to-face with high-quality buyers. Consultation hotline: 021-62277349

Variable frequency speed regulating motor, referred to as variable frequency motor, is a general term for motors driven by frequency converters. The advantage is that it has a starting function; it adopts electromagnetic design to reduce the resistance of the stator and rotor; it is adaptable to frequent speed changes under different working conditions; to a certain extent Save energy. It has become a mainstream speed regulation solution and can be widely used in continuously variable transmission in various industries.

1. Technical improvement of outlet circuit breaker control of high-voltage power supply system 

The high-voltage side of the transformer used in the variable frequency speed control equipment must be directly connected to the switch cabinet in the high-voltage system. However, the protection scope of the switch cabinet is only the short circuit between the power supply line and the low-voltage side of the transformer. The shortcomings of the frequency converter should be The own detection and protection system is completed. 
When a fault occurs in the inverter and a trip signal is issued, the circuit breaker should trip reliably. However, generally when the trip circuit is disconnected or the DC control power supply is missing inside the circuit breaker high-voltage switch cabinet, when the inverter happens to have a fault (requiring the circuit breaker to trip), the trip coil has lost power and the circuit breaker refuses to operate, thus causing the inverter to malfunction. The internal power device is damaged.
Therefore, a circuit breaker with an undervoltage trip coil was selected in the plan. Once the trip circuit is disconnected or the control power supply is missing, the circuit breaker will trip automatically first to protect the inverter. Equipment security.
2. Fundamental needs of the working environment of the inverter 
Since some high-voltage inverters use power devices such as high-voltage IGBTs for inversion, their opening and closing frequencies are greater than 100HZ. It is easy to form high-order harmonic current, causing the frequency conversion equipment to generate a certain amount of heat when working. Generally, there is an electric fan on the top of the frequency converter cabinet, which discharges the heat in the cabinet into the room. This causes the indoor ambient temperature to continue to rise, and ultimately affects the reliable operation of the components in the cabinet.
Therefore, in the water plant project plan, the frequency conversion speed regulation equipment is generally set up separately in the frequency conversion speed regulation room. There is necessary equipment and backup air conditioning equipment indoors to control the indoor ambient temperature within the range required by the frequency converter. , ventilation doors and windows are also provided, and special air ducts are used for forced ventilation and cooling when necessary. 
 
3. Key points in power cable selection and laying requirements 
Because the connection between the output end of the inverter and the motor adopts the cable attachment method, and each phase of the line exists Capacitance to ground, so the capacitance current on the line is not equal during operation. If the cable is installed at a long distance and there are high-order harmonic currents in the line, once a single-phase grounding occurs, the arc ignited by the defective capacitor current will stay for too long, which will cause the cable at this end to heat up and form non-defective insulation. . 
Therefore, in the variable frequency speed regulation renovation project, for the output power cable, the three-phase symmetry and shielding in the cable design should be considered, the cable cross-section should be appropriately increased, and the laying length should not exceed the limit value (100m) , if the original output power cable is unshielded or the cross-section current margin is less than 2, the power cable that meets the needs should be replaced. 
When laying equipment on site, the power cables, control cables and signal cables should be laid separately to prevent the magnetic field generated by the high-order harmonics in the power cables from interfering with other signals. 
4. Motor characteristic experiments and revision of technical standards 
When a general motor is powered by a variable frequency, the voltage and current at the output end of the frequency converter are harmonious. The wave component will increase the loss, reduce the power, and increase the temperature of the motor. The increase in loss caused by high-order harmonics is mainly reflected in the increase in copper loss, iron loss and additional loss of the stator and rotor. During this period, the rotor copper loss is more obvious. Since the asynchronous motor rotates with a slip close to 1, the rotor copper loss is very large. In general asynchronous motors, in order to improve the starting function of the motor, the skin effect of the rotor increases the actual impedance, thereby increasing the copper loss. 
On the other hand, due to the distributed capacitance between the motor coils, when high-order harmonic voltage is input, the voltage between the coils is uneven. This long-term repeated action causes the stator to The insulation of a certain part of the coil causes damage, resulting in coil aging, which is unacceptable in the insulation design of ordinary asynchronous motors. In addition, the electromagnetic circuit of the motor cannot be necessarily symmetrical, so the harmonic components contained in the output power of the inverter will interact with the inherent space harmonic components in the electromagnetic circuit to form various electromagnetic pulsations.
At the same time, because the motor is in a working state where the frequency is constantly adjusted, it is easy for mechanical resonance to occur with the mechanical parts of the motor, causing damage to the mechanical parts of the motor. 
Therefore, in the variable frequency speed regulation transformation project, in order to avoid the above problems when the frequency conversion speed regulation system is working, it is necessary to consider technical cooperation with the motor manufacturer during technical planning, and carry out technical planning on the relevant aspects of the motor. Conduct speed regulation experiments based on the characteristics of the motor and revise the technical standards of the original motor from scratch.