Difference between VFD minimum frequency and starting frequency

[philly]

I'm trying to understand the difference between a VFD's minimum frequency and its starting frequency?

The way I understand it, is that the VFD's minimum frequency is the lowest frequency that the drive will run at during normal operation (exluding starting) and will not operate below this speed except for starting and stopping.

I believe the drives starting frequency is the frequcny that the drive will start at when staring a motor and ramping it up to base speed. For instance if I set a starting frequency of 8Hz the drive will start at 8Hz and start to ramp from there as opposed to starting at essentially 0Hz and ramping up to speed. Do I understand this correctly in regards to setting this starting frequency?

What would be a reason for setting this starting frequency to some value as opposed to starting from 0Hz?

 

[Jraef]

... Do I understand this correctly in regards to setting this starting frequency?

What would be a reason for setting this starting frequency to some value as opposed to starting from 0Hz?

yes you are essentially correct.

The reason to set the starting frequency different than the minimum speed is usually torque related. If for instance you are in Vector mode (any kind), you can get full torque at virtually any speed of course, but do you really want that? Is there any value in always starting at zero speed just because you can? What if, for example, you have a check valve in a pump and at some lower speed, your pump will not create enough flow to operate the check valve? Why waste accelerating time ramping to that point? And in many cases with pumps, that minimum speed for opening the check valve may be lower than the minimum speed you may want to run the pump at once it has established flow.

 

[philly]

yes you are essentially correct.

The reason to set the starting frequency different than the minimum speed is usually torque related. If for instance you are in Vector mode (any kind), you can get full torque at virtually any speed of course, but do you really want that? Is there any value in always starting at zero speed just because you can? What if, for example, you have a check valve in a pump and at some lower speed, your pump will not create enough flow to operate the check valve? Why waste accelerating time ramping to that point? And in many cases with pumps, that minimum speed for opening the check valve may be lower than the minimum speed you may want to run the pump at once it has established flow.

If the motor was started at some frequency as opposed to starting from zero wouldn't the starting current be greater due to the fact the slip would be greater during starting without the ramp? Is this higher current something to take into consideration when setting this number?

If there is no downside from ramping from zero for a particular application is it necessarily going to hurt anything.l

What about for the starting of a water pump? Any reason to not ramp from zero?

 

[Jraef]

If the motor was started at some frequency as opposed to starting from zero wouldn't the starting current be greater due to the fact the slip would be greater during starting without the ramp? Is this higher current something to take into consideration when setting this number?

Yes, you will get higher current, and that also means you will get higher torque / faster acceleration. Yes it is something to take into consideration, it is usually what falls into that "overload capacity" of the VFD, i.e. 150% current for 60 seconds (CT) or 120% for 10 sec. (VT) etc.

f there is no downside from ramping from zero for a particular application is it necessarily going to hurt anything.

Not, if your application can work with a ramp from absolute zero, by all means do it.

What about for the starting of a water pump? Any reason to not ramp from zero?

Yes, as I indicated above. For example; Franklin submersible pumps state that you MUST accelerate to full speed in 3 seconds or less, otherwise you risk damaging the bearings. That is a very common issue for submersibles. So if you only have 3 seconds, and you have a ramp time that is 5 seconds because you want a slower change in speed once it is running, you can essentially over ride your accel time by setting the starting frequency higher than zero. it will accelerate immediately, but afterward when you change speeds, it will do so slowly.

 

[ironsparks]

Yes you are absolutely correct! If the motor is starting from a dead stop, you will start from zero to what ever your programmed frequency is, at whatever ramp up speed you chose.
The drive itself, depending on the manufacturer will most likely be able to support running the motor at one single hertz if it wanted to, whether or not the motor can take running that slow without burning up is another question, and one that many electrtricians overlook.

So, again to answer your question, if you are starting the drive up from a dead start yes you have to run it from zero up to whatever your target frequency is. How long at each frequency depends on the ramp up time you give it to get to where your going

 

[philly]

Yes, as I indicated above. For example; Franklin submersible pumps state that you MUST accelerate to full speed in 3 seconds or less, otherwise you risk damaging the bearings. That is a very common issue for submersibles. So if you only have 3 seconds, and you have a ramp time that is 5 seconds because you want a slower change in speed once it is running, you can essentially over ride your accel time by setting the starting frequency higher than zero. it will accelerate immediately, but afterward when you change speeds, it will do so slowly.

What about for a centrifugal pump?

I know for most centrifugal pumps you dont want to run them below 40-50% speed because there will be minimal or no flow. For my specific pump the manufacturers minimum flow rating of the pump corrosponds to about 25-30% Hz which is about 40-50% speed. I guess below this speed there will be no flow and just increased heating on the pump.

For a centrifugal pump what what you say about a mimimum starting frequency? Can we ramp a centrifugal pump from zero during starting, or should we give it a starting frequency of the same 40-50% speed as mentioned above? If we ramped from zero, then there would be no flow until we got to mimimum flow speed.

The pumps speed will be controlled by a level in a tank based upon a level transmitter, and a PID loop in an external controller.