VFD question?

[rrc14]

Can a variable frequency drive (VFD) be used to replace a 3-phase motor starter? Seems like the VFD would need to provide overload protection for the motor as well.

Thanks,

 

[controlled]

Yes, part of the VFD parameters settings is the OL setting.

 

[raider1]

I agree, a VFD can replace a standard motor starter. You will still need motor overload protection but the VFD may have the capability to provide the required overload protection.

Chris

 

[tomatpton]

For smaller HP a Vfd may be more economical than a NEMA starter.

 

[Jraef]

Just a note of history, it's somewhat recent (10 years or so) that VFDs now all contain motor thermal OL protection, a side benefit of digital drive technology. In the days of analog drives, many did not include it and a separate external TOL was required downstream. However LOTS of people failed to understand that or read the manuals, so a lot of motors were installed behind VFDs that had no OL protection and burned up, leading to a general feeling of mistrust of VFDs that lingered on long after OL protection became standard.

 

[Besoeker]

Can a variable frequency drive (VFD) be used to replace a 3-phase motor starter? Seems like the VFD would need to provide overload protection for the motor as well.

Thanks,

The short answer is yes.
The longer question is why would you contemplate doing so?

 

[quinn77]

well, im no engineer, but just assuming that the nema starter could be replaced with a vfd would be premature right? certain things like length of conductors, amount of conductors in raceway, etc...like i said im no expert, i always consult an EE on these types of projects. its my experience with said EE that ive learned that standing waves generated by the switching of the vfd that can destroy a "standard duty" motor where an inverter duty would withstand the higher "spikes". and also might one need line side reactors and load side "chokes" depending on the application right. like i said, im no expert on this application, therefore i do what OP is doing. ask questions. the drive manufacturer could also help right?

 

[Besoeker]

well, im no engineer, but just assuming that the nema starter could be replaced with a vfd would be premature right? certain things like length of conductors, amount of conductors in raceway, etc...like i said im no expert, i always consult an EE on these types of projects. its my experience with said EE that ive learned that standing waves generated by the switching of the vfd that can destroy a "standard duty" motor where an inverter duty would withstand the higher "spikes". and also might one need line side reactors and load side "chokes" depending on the application right. like i said, im no expert on this application, therefore i do what OP is doing. ask questions. the drive manufacturer could also help right?

Fair points, quinn.
A bog-standard motor might be problematic without precautions being taken.
But, in principle, a VFD could be used as a starter.
In actual fact we have a few installations where we have used a VFD to start a motor then switched to direct on line once the motor was at speed. This was for applications where the supply didn't have enough capacity to supply motor starting current directly.

 

[petersonra]

VFDs are basically an electronic starter with built in OL protection and other features such as variable frequency output.

It is not unusual to see them installed in lieu of starters, especially on such things as conveyors. You can get a small VFD for not a whole lot more than a starter, and it is much cheaper to be able to change the speed from the VFD keypad than to have to change a gear ratio on the mechanical system. Plus you get a great deal of flexibility.

It would not surprise me at all if someone came out with a line of 1HP and smaller VFDs that was cheap enough that they became virtually the standard for small motors.

 

[Jraef]

VFDs are basically an electronic starter with built in OL protection and other features such as variable frequency output.

It is not unusual to see them installed in lieu of starters, especially on such things as conveyors. You can get a small VFD for not a whole lot more than a starter, and it is much cheaper to be able to change the speed from the VFD keypad than to have to change a gear ratio on the mechanical system. Plus you get a great deal of flexibility.

It would not surprise me at all if someone came out with a line of 1HP and smaller VFDs that was cheap enough that they became virtually the standard for small motors.

Nah, the cost to make across-the-line contactors is too low for that to happen. They will just drop the sell prices. That's already happening.

 

[petersonra]

Nah, the cost to make across-the-line contactors is too low for that to happen. They will just drop the sell prices. That's already happening.

I can already buy a 5 HP IEC contactor and overload for <$25. I doubt we will ever see VFDs in that price range for that size.

But, there are already VFDs that can be gotten for FHP motors that are < $100. It would not be that much of a stretch for someone to come up with a design for small motors like that for $25.

 

[Jraef]

I can already buy a 5 HP IEC contactor and overload for <$25. I doubt we will ever see VFDs in that price range for that size.

But, there are already VFDs that can be gotten for FHP motors that are < $100. It would not be that much of a stretch for someone to come up with a design for small motors like that for $25.

For sure if it needs to be reversing. But the way they are going for small loads like that now is ECMs (Electrically Commutated Motors) where they are essentially PMDC motors with the electronics built-in to the motor itself. They are now cheap enough to be seen on appliances like washers and dryers, the volume is already so high it's only a matter of time before we see them displacing FHP industrial motor applications. You'll have a 1 phase CB or fuse set feeding a motor, the motor control and protection will all be integral to the motor.

 

[gar]

110103-2107 EST

The following seems to be a fairly well written discussion on BLDC, ECM, etc. motors:
http://en.wikipedia.org/wiki/Brushless_DC_electric_motor
As stated at the beginning of the discussion these are basically AC synchronous motors with electronics added.

An important difference between brush and brushless DC motors is the current carrying coils move from the rotor to the stator thus providing much better heat transfer. So there should be more available power output from a brushless for a given package size and cooling method. Otherwise the theory and characteristics are quite similar.

.

 

[weressl]

Can a variable frequency drive (VFD) be used to replace a 3-phase motor starter? Seems like the VFD would need to provide overload protection for the motor as well.

Thanks,

An ASD CAN be a replacement for a contactor and overload relay, but there are other components within 430 that are required.