VFD's and MOTOR STARTERS

[Kmansker]

hello everybody,


so to start this off i will explain my understanding of why someone would use a vfd vs. a motor starter. and i could be wrong but i think the whole thing boils down to efficiency. lower the cost of running a motor while improving control. I have had some technical schooling and was under the impression that before the VFD became marketable (size and cost effective) that the way you controlled the the output of a motor was with the use of a variable drive gearbox. the reason i'm giving my theories are because i thought the VFD was the replacement for the motor starter, and i'm finding out that not everyone thinks the same....... the place where i work uses the VFD in series with a motor starter??? Either up stream or down stream, which doesn't make any sense to me. does anyone out there have any input on this or a similar experience??

thank you, keith

 

[petersonra]

VFDs are often applied in lieu of variable gearboxes. It is just one application of VFDs. Electronic gearing is a lot simpler than variable gearboxes, and generally a lot more reliable.

They are also often applied where the mechanical system cannot tolerate the shock of a motor starting across the line.

They are also sometimes used to reduce the starting current.

 

[just the cowboy]

VFD in series

VFD in series

They put the starter in front of the VFD for safety, if drive runs away e stop will still stop it.

 

[petersonra]

They put the starter in front of the VFD for safety, if drive runs away e stop will still stop it.

These days there are VFDs with safety rated estop inputs that generally eliminate the need to have external contactors.

Even so, I have never heard of an AC drive that "ran away". That is a problem with DC drives if the field supply is lost. Most DC drives have a circuit to check for this and shutdown the drive.

I have run into a few cases where people have miswired an AC drive so that it would not stop under certain conditions. But that is not really a case where you need a safety circuit to deal with the problem. Just wire it right in the first place. The way I see it if they can wire the control circuit wrong to the point they can't stop the drive, what makes you think they are not going to screw up the estop circuit the same way?

 

[Besoeker]

hello everybody,


so to start this off i will explain my understanding of why someone would use a vfd vs. a motor starter. and i could be wrong but i think the whole thing boils down to efficiency. lower the cost of running a motor while improving control. I have had some technical schooling and was under the impression that before the VFD became marketable (size and cost effective) that the way you controlled the the output of a motor was with the use of a variable drive gearbox. the reason i'm giving my theories are because i thought the VFD was the replacement for the motor starter, and i'm finding out that not everyone thinks the same....... the place where i work uses the VFD in series with a motor starter??? Either up stream or down stream, which doesn't make any sense to me. does anyone out there have any input on this or a similar experience??

thank you, keith

My experience with variable speed drives stretches over four decades. Initially it was with various arrangements of DC drives - Ward-Leonard, Lineshaft with cone pulleys, mercury arc, carbon pile, and SCR to name but a few. And mostly in the paper industry where precise speed control is a must.

Life moves on and variable frequency drives became an economical option as larger and better semiconductors were developed.

The merit in terms of efficiency is often on centrifugal fans and pumps. Reduce the speed by 10%, and you reduce the power by 30%.

 

[Kmansker]

howdy all,


thanks for the feed back. i think i want to reword my question if i may. why would i ever use a motor starter (ac contactor with overload) with (in series) with VFD? i do have some applications that i need to control the motor and some where i don't. i know when to use which but both??? why would anyone ever do that? i'm going to rule out the safety post because the use of the overloads and there really isn't much mechanical load on the motor in question, but regardless, the VFD + Motor Starter is found everywhere in this plant. like it is common practice. my initial thought is to eliminate the motor starter but first i want to understand why it was done to begin with.

thank you, keith

 

[petersonra]

until fairly recently, there was a general acceptance of the idea that semiconductors could not be trusted so estops had to open a hard contact.

it was also true for a while that VFDs did not have UL listed overload protection built in so you had to add external overloads anyway. so it might have made sense at one time to have a motor starter and overload on a motor drive.

You still see safety rated contactors on VFDs where the required PL is very high, but they are not as common these days as most applications can be handled by safety rated inputs to drives.

 

[Kmansker]

ahhhhh..... thank you very much bob! awesome

 

[kwired]

hello everybody,


so to start this off i will explain my understanding of why someone would use a vfd vs. a motor starter. and i could be wrong but i think the whole thing boils down to efficiency. lower the cost of running a motor while improving control. I have had some technical schooling and was under the impression that before the VFD became marketable (size and cost effective) that the way you controlled the the output of a motor was with the use of a variable drive gearbox. the reason i'm giving my theories are because i thought the VFD was the replacement for the motor starter, and i'm finding out that not everyone thinks the same....... the place where i work uses the VFD in series with a motor starter??? Either up stream or down stream, which doesn't make any sense to me. does anyone out there have any input on this or a similar experience??

thank you, keith

Efficiency of the motor doesn't change, efficiency of energy used by certain certain driven loads can improve in particular as mentioned fans and pumps. If you don't need full output of a fan at certain times, then reducing the speed to output level needed does reduce energy used, but same fan will be nearly same inefficiency at full voltage and frequency as it is without a VFD. Power factor between the motor and the drive doesn't change either, at least not to any significant level, but power factor will be corrected on supply side of drive.

Other advantages is speed control, some applications you may be able to change gear reducer, pulley/sprocket sizes, etc. but others just need fine tuned for different application conditions and this can easily be done just turning a dial or even some automated method of detecting needed speed for the application. Some machines need to vary speed of a particular driven load as process conditions change, VFD's are able to change speed without any significant change in output torque.

They also can provide "soft starting" which can reduce voltage sags in the supply line or even on POCO distribution when it is a really large motor being controlled. Soft starting also can reduce mechanical stress on driven load as torque is gradually increased instead of hitting the machine with full torque all at once which is what happens with across the line starting.

VFD's also can be used as "phase converters" in some instances. Because of how they operate -rectifying incoming AC voltage into DC, then converting back to an AC with control over voltage and frequency you can have single phase input and three phase output, you must either have a drive designed specifically for this or derate the capacity of a drive with three phase input as part of the input structure is not used if only inputting single phase.

These drives can provide breaking and deceleration functions as well- depending on conditions they may require additional resistors to be added in which they dump some of the energy absorbed by the breaking action into and that energy is given up as heat.

VFD does essentially replace a motor starter in applications where they are used, there still can be control schemes where a contactor may be used to interrupt power - typically only for occasional shut down situations and not as a routine control method - VFD's don't play well with contactors in the circuit, total loss of supply power is not so much of a problem as long as you don't expect immediate restarting and does allow for use of a contactor for emergency stoppages or some master enable feature, but you should rely on the drive's internal controls for normal operations otherwise.

 

[Cmdr_Suds]

I often use VFDs in sewage lift station control panels where typically there are two or three pumps. When a pump is not required to be running, the drive input contactor is opened and is isolated from line voltage thus preventing lightning damage to the drive not being used if there is a strike close by. or at least that is the idea. Sometimes a bypass starter is requested and then an output contactor is required to prevent power being feed back into the drive.

 

[Jraef]

I often use VFDs in sewage lift station control panels where typically there are two or three pumps. When a pump is not required to be running, the drive input contactor is opened and is isolated from line voltage thus preventing lightning damage to the drive not being used if there is a strike close by. or at least that is the idea. Sometimes a bypass starter is requested and then an output contactor is required to prevent power being feed back into the drive.

You are likely creating more failures than you will prevent with this strategy. ALL VFDs have what is called a "pre-charge circuit" that prevents the inrush charging current of the DC bus capacitors from causing damage to the caps and the rectifier. It's usually a current limiting resistor in series below the rectifier, then a relay or contactor to short out the resistor after a second. Smaller drives use an NTC (Negative Temperature Coefficient of resistance) resistor that basically shorts itself out when it warms up a second or two after energizing. All of those components have a finite life, so every VFD mfr will tell you not to cycle line power to the drive unnecessarily. There are plenty of situations where it can't be avoided, but to do so on purpose is basically consuming any safety margins in component selection that may have gone into the design.

 

[Cmdr_Suds]

You are likely creating more failures than you will prevent with this strategy. ALL VFDs have what is called a "pre-charge circuit" that prevents the inrush charging current of the DC bus capacitors from causing damage to the caps and the rectifier. It's usually a current limiting resistor in series below the rectifier, then a relay or contactor to short out the resistor after a second. Smaller drives use an NTC (Negative Temperature Coefficient of resistance) resistor that basically shorts itself out when it warms up a second or two after energizing. All of those components have a finite life, so every VFD mfr will tell you not to cycle line power to the drive unnecessarily. There are plenty of situations where it can't be avoided, but to do so on purpose is basically consuming any safety margins in component selection that may have gone into the design.

I agree. I have had more problems with those drives than ones without input contractors. I was just stating one train of thought on the concept. When the consulting engineer tells you to build it that way via the plans and specs, I have no choice. they generally don't want to hear from us that their design isn't perfect.

 

[Jraef]

... When the consulting engineer tells you to build it that way via the plans and specs, I have no choice. they generally don't want to hear from us that their design isn't perfect.

No argument from me on that. Are you looking over my shoulder today?

I'm being told on one project that I have to "certify" that my VFD is "... approved by the NEC for installation into a system where IEEE-519 harmonics for a Dedicated System are allowed, per NEMA MG-1".

I'm not kidding, that is copied and pasted from the rejected submittal I just got back. So someone got PAID to vomit up a string of random buzz words like that which have no meaning or basis in reality whatsoever. I don't even know where to start with that. Do I tell the consultant he's a complete moron, then proceed to try to educate him about all of the misconceptions he has in that statement? Or do I just cobble together at least 30 pages of meaningless gobbledegook that it's obvious he won't understand anyway, and baffle him with BS?

 

[JRW 70]

Nonsense Specs.

Nonsense Specs.

When I had a similar situation that had meaningless
(or at least unrelated) requirements, I created an outline
of each of the requirements and why they had nothing
to do with each other. Then did a redesign on on the system
(pumps in this case) and I tried to be completely truthful
in my response, but did try to use the baffle w. BS method.
They were impressed with what I put together as a counter
proposal and I really tried to get every detail correct, while
making a simple project as complex as possible in response
to what we recieved. So make it factual, complete, and utterly
confusing all at the same time. I have only done this one time
but I wanted them to see what we expected on our project.
I did more research generating the specs. for that project than
probably any other three or four projects combined.

And I regret doing that because I try to look for the simplest
answer. But I couldn't pass that one up. It was a matter of
principle to me to look at garbage that we paid a sizeable
payment for that made no sense.

JR

 

[Aleman]

howdy all,


thanks for the feed back. i think i want to reword my question if i may. why would i ever use a motor starter (ac contactor with overload) with (in series) with VFD? i do have some applications that i need to control the motor and some where i don't. i know when to use which but both??? why would anyone ever do that? i'm going to rule out the safety post because the use of the overloads and there really isn't much mechanical load on the motor in question, but regardless, the VFD + Motor Starter is found everywhere in this plant. like it is common practice. my initial thought is to eliminate the motor starter but first i want to understand why it was done to begin with.

thank you, keith

It's not really a motor starter, it is part of a safety circuit. Someone presses an estop, the contactor removes power from the drive. If you remove the starter then you need some
other way of wiring a safety circuit. This can be a estop switch in series with the drive enable for example. The contactor approach is probably the safest. It's not meant to be switched out
as a means of control, it is only for safety.

 

[Cmdr_Suds]

No argument from me on that. Are you looking over my shoulder today?

Maybe , I think we have have driven the same road many times.

I have many stories where consultants have specified things that made no sense, rejected equipment that they specified down to the part number and even had a submittal rejected because the quality of the paper the submittal was printed on did not meet the requirements of the specs.

Sometimes they even get vindictive. I once had to build twelve drive panels where the engineer rejected the original submittal because I submitted on a different brand of drives then what the base design used. They did clearly indicate "or equal" in the specs. The engineer looked at the constant torque specs of the base drive and said they where superior to the one I was submitting and was not going to accept it. I had to point out to the engineer that these drives were controlling pumps and thus were a variable torque application and that the submitted drive clearly matched the variable torque ratings. So in order to punish me, he then required me to use machine tool relays in lieu of plug-in or "ice cube" relays. (12 drives x 5 relays/VFD @ $125 vs. $15 per relay) He did this through creative spec interpretation. The pump specs (under which these drives were being built), referred to the VFD spec which then referred to the MCC specs which required the use of machine tool relays (must have been about 30 years old at the time). The drives were not part of an MCC, they were stand alone but he still demanded they be built using machine tool relays. He conveniently ignored that the pump spec referred directly to to the I&C spec which required plug-in relays.

I once had a project where the stainless steel instrument tags were rejected because the the specs said they were to be "stamped" with the instrument number. The tags had been laser etched.

 

[Besoeker]

I have many stories where consultants have specified things that made no sense, rejected equipment that they specified down to the part number and even had a submittal rejected because the quality of the paper the submittal was printed on did not meet the requirements of the specs.

Consultants specs.........tell me about them.............we once bid a project for a number of fairly large variable speed drives for a project in the middle east.
The performance spec for the drives was about half a page. The spec for the wooden packing crates was forty pages long. I kid you not. 40. Four zero. One of the things that sticks in my mind about it was a section on the nails to be used and the "correct" angle for driving them in - 15 degrees it was.

We had another from a nuclear facility. The project engineer staggered into my office with a full box of A4 sheets. A4 is about your letter size and we get it in boxes of 5,000 sheets. Then there were other files taped to the side and top. All this after I had bid the project and after we got the order for it.

There was an entire section on how to write manuals. Them telling us how to write manuals for the equipment we are going design and manufacture???? It specified font size and style and even included a page on the correct use of the comma!! The delicious irony was that their own specifications didn't comply with their own instructions.

 

[JRW 70]

The people is Besoeker's example sound like the type
I was referring to above. They don't see the outside
of the office they work in and are in the document
generating buisness (which there is a place for, but
they need to think about the rest of us that have to
carry out what they put together)
It is absolutely amazing how complicated some of the
simplest things become.


JR

 

[Besoeker]

The people is Besoeker's example sound like the type
I was referring to above. They don't see the outside
of the office they work in and are in the document
generating buisness (which there is a place for, but
they need to think about the rest of us that have to
carry out what they put together)
It is absolutely amazing how complicated some of the
simplest things become.


JR

I also think they can be lazy or take the line of least resistance by using sections from past projects regardless of relevance to the job in hand.

 

[kwired]

Consultants specs.........tell me about them.............we once bid a project for a number of fairly large variable speed drives for a project in the middle east.
The performance spec for the drives was about half a page. The spec for the wooden packing crates was forty pages long. I kid you not. 40. Four zero. One of the things that sticks in my mind about it was a section on the nails to be used and the "correct" angle for driving them in - 15 degrees it was.

We had another from a nuclear facility. The project engineer staggered into my office with a full box of A4 sheets. A4 is about your letter size and we get it in boxes of 5,000 sheets. Then there were other files taped to the side and top. All this after I had bid the project and after we got the order for it.

There was an entire section on how to write manuals. Them telling us how to write manuals for the equipment we are going design and manufacture???? It specified font size and style and even included a page on the correct use of the comma!! The delicious irony was that their own specifications didn't comply with their own instructions.

I'd like to be able to set specifications on how to write manuals for a lot of equipment I have seen in the past 15 years or so:happyyes:

Quite frankly I think that listing agencies should specify at least some things when it comes to instructions, that apparently they don't require at this point.