Low frequency operation risk of cold storage variable frequency compressor | Low speed oil return, winding overheating, minimum operating frequency setting
Variable frequency compressors rely on speed regulation to achieve stepless adjustment of cooling capacity. Low frequency is an energy-saving advantage range, but low speed can bring two core risks: oil return difference and insufficient motor heat dissipation.
Manufacturers of variable frequency semi enclosed compressors will set a minimum safe frequency, generally not lower than 20-25Hz. If the frequency is lower than this, the return pipe flow rate will be insufficient, and the refrigeration oil will be trapped in the evaporator.
The controller of the Xiteluo frequency converter model has a built-in frequency lower limit lock to prevent manual adjustment to a low frequency. At the same time, it sets a low-frequency continuous operation time limit and timed acceleration to flush back oil.
When running at low speed, the air volume of the built-in motor fan decreases, the cooling of the motor deteriorates, and the winding temperature rises; F-class insulated motors experience long-term low-frequency overload and accelerated insulation aging.
Many customers blindly pursue constant temperature, and the controller is set to extremely low frequency to stabilize the storage temperature for a long time. In the short term, the storage temperature is stable, but after 3-6 months, the evaporator accumulates oil and heat transfer decays.
The design logic of the return oil of the variable frequency unit is different from that of the fixed frequency. The diameter of the return air pipe cannot be directly copied from the fixed frequency. If the diameter is too large, it is more difficult to meet the low-speed flow rate standard. Therefore, it is necessary to match it during the selection stage.
Low frequency oil return solution: Regularly increase the speed and flush the oil program, run at high frequency for a few minutes at regular intervals, and bring the trapped refrigerant oil back to the crankcase.
In areas with unstable overseas voltage, voltage drops are more pronounced at low frequencies, motor current distortion, harmonic increase, frequency converter overheating, and fault rate increase.
The low-frequency exhaust temperature of the variable frequency unit is more likely to be high, and the low-speed compression ratio remains unchanged, resulting in poor cooling. It is necessary to cooperate with exhaust temperature protection and automatic frequency up protection for overheating.
The lower the frequency, the more energy-efficient the frequency conversion is. If it is lower than the safe frequency, the maintenance costs caused by oil return faults and motor overheating will far exceed the saved electricity bills.
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FAQ
1. What is the lower limit of the minimum safe frequency for variable frequency compressors? Mostly 20-25Hz.
2. What is the main risk of low-frequency operation? Insufficient flow rate in the return pipe and poor oil accumulation and return.
How to solve the low-frequency oil return problem? Regularly increase the speed and flush back the oil.
Why does the heat dissipation of low-frequency motors deteriorate? The motor comes with a cooling fan that synchronously reduces the speed.
5. Does the lower the frequency, the more energy-efficient it is? Below the safe frequency, the risk of malfunction increases significantly.
Can the diameter of the variable frequency return air pipe be directly copied from the fixed frequency? Not recommended, separate calculation of flow rate is required.
What are the problems with low-frequency operation in areas with unstable voltage? High harmonics make the frequency converter prone to overheating and damage.
