Variable frequency speed control of Roots blowers
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Is there anyone who has studied the variable-frequency operation characteristics of Roots blowers? The issue is as follows: according to general theory, a Roots blower is a positive-displacement blower, and its flow rate is proportional to its speed. With the addition of an inverter, the speed becomes proportional to the frequency; therefore, when the frequency decreases, the power of the blower should decrease (assuming the outlet back pressure remains constant). However, in several projects we have been working on at present (blowing air for aeration, maintaining a constant water level in tanks), we have observed that when the frequency is reduced using an inverter, the current drawn by the Roots blower increases as well, and its heating also increases. Sometimes the current becomes so high that the inverter triggers its protection mechanism. We are not sure what the reason for this is; we would appreciate some guidance from experts.
It’s possible that the load on your fan is too high, similar to trying to pull a heavy load with a weak horse. Another possibility is that the slow speed of the motor causes the fan connected to it to rotate slowly as well, preventing heat from being dissipated. However, since you mentioned that the current is actually higher, it’s likely that the issue lies in the high load on the motor. If everything was working fine before but has started to malfunction recently, it’s advisable to check the bearings to see if there are any problems
The motor is an inverter-driven motor, and the cooling fan is wired separately. The fan outlet pressure display is also normal. The fan manufacturers and motor manufacturers have gone to the site together to address the issue, but the cause has not yet been identified. The electrician who handles frequency converters is on his way there now; it’s not clear whether he’ll be able to solve the problem.
Is this device being used for the first time, or have you been using it before? If it’s being used for the first time, check whether the motor selected is too small. If you’ve been using it before and there are no issues, then it’s likely that the motor is under too much load. It’s recommended to turn the device by hand to see if there is any sticking
The selection process may not have taken your frequency conversion requirements into full consideration. Moreover, the motor chosen by the manufacturer is somewhat undersized; they tend to adjust the parameters accordingly. When the frequency of the motor is adjusted below 50 Hz, it operates in constant-torque mode, resulting in a decrease in the motor’s actual power. The shaft power required by your fan is roughly equal to, or even higher than, the motor’s actual power under these conditions. It’s lucky if no damage occurs; the frequency cannot be reduced any further. The fan heats up because rotary vane fans have a minimum speed limit – below this speed, heat cannot be dissipated properly, which can lead to jamming. I specialize in imported rotary vane fans; trust me on this
Why do problems arise when the frequency decreases (the speed drops)? This doesn’t conform to theory!
You’re not reading my replies, man. For example, with a 75-kilowatt motor, at 50HZ it can deliver 75 kilowatts of power. If you reduce the frequency to 25HZ, the power the motor can produce will be less than half of 75 kilowatts. If the shaft power required by the fan approaches the actual capacity of the motor, then you need to be careful – I have already damaged a motor because of this issue in the past
The theory you’re referring to is the theory regarding fan power; that’s fine. The problem is that the motor power has changed – the star remains the same star, but the moon is no longer the same moon
This post was last edited by wyf600 on 2015-6-11 at 13:09. The frequency decreases, the rotational speed drops, the voltage increase remains unchanged, the air volume reduces, and the shaft power should decrease accordingly. As the frequency decreases, it is said that the voltage output by the inverter also decreases. Could the increase in current be caused by this decrease in voltage?
The shaft power has decreased, and so has the power of your motor, as the frequency is lower. As a result, the current shaft power is higher than the motor’s current power, resulting in an overload
So, if a Roots blower requires variable frequency control, does that mean it needs to be equipped with a motor with higher power?