Typically on a 2 speed motor for a fan, the low speed is 1/2 of the high speed. So 30Hz for the low speed, 60Hz for full speed.
With a VFD, you have a decision to make on how you are going to select the speed. Your choices are;
- Use the keypad on the VFD
- Use a Potentiometer (pot) on the VFD if there is one.
- Use an external pot wired to the analog input of the VFD.
- Use another external analog signal from a higher level control system.
- Use digital inputs in the VFD to select preset speeds that you program into the VFD.
For something this simple I would think #4 is out. #1 & #2 presume the VFD keypad is accessible and unless you have a fairly sophisticated enclosed version, it probably isn't. So most of the time in a situation like yours it will be #3 or #5. If all you want are two exact speeds, then #5 it is. You would set up the VFD to run at 60Hz unless the Digital Input that you select as "Low Speed" is closed, in which case it runs at 30Hz. Simple and straight forward. The alternative is to wire an external pot to the drive and tell it to follow the analog value of the pot input. The advantage to this is that the user can infinately adjust the speed to match conditions. The down side is that is sometimes too much decision making for some people.
:happyyes:
Side issue, you will also need something that tells the VFD to Start and Stop, and you need to decide if that is to be a 2 wire or 3 wire command. 2 wire means you just close a maintained switch, like your light switch, to one of the digital inputs and as long as that switch is closed, the VFD runs. If power fails, when it comes back on the VFD will immediately turn back on and run at whatever speed you had previously had it running. 3 wire control means you have a momentary Start push button and a momentary Stop push button. When you hit Start, the signal goes through a relay in the VFD that holds the command in until you hit the Stop button or the power fails. If the power does fail, the VFD will NOT come back on until someone tells it to by pushing the Start button again. This is a safety issue. Only you can decide which is best for your situation.
