VFD vs motor starter

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kwired

kwired

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NE Nebraska
controlled acceleration and deceleration, ability of controller to interface with other electronics for many reasons.
 
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Haldor

Member
Makes the motor emit an audible, annoying whine between 2KHz to 4KHz which is right in the middle of the most sensitive portion of human hearing. This whine seems to be worse the slower you run the motor (or perhaps the noise emitted by everything else is lower so it is easier to hear). I have gotten flack over this in conveyor applications. If wonder if motor manufacturers are considering changing how they make windings in order to reduce this?
 
Jraef

Jraef

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Haldor said:
Makes the motor emit an audible, annoying whine between 2KHz to 4KHz which is right in the middle of the most sensitive portion of human hearing. This whine seems to be worse the slower you run the motor (or perhaps the noise emitted by everything else is lower so it is easier to hear). I have gotten flack over this in conveyor applications. If wonder if motor manufacturers are considering changing how they make windings in order to reduce this?
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You can fix that now. If your VFD has an adjustable carrier frequency (most new ones do now) the audible whine will move out of the range of human hearing at around 8-10kHz (depending on how young the listener is). The VFD's efficiency will drop a little because of the higher switching losses in the transistors, but not much on most modern drives now.Older and cheaper designs sometimes require a slight de-rating of the drive because of this however, so check your manual to see if this may be an issue.
 
Jraef

Jraef

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RETRAINDAILY said:
will a noise filter control the harmonics? would yo use a line reactor with a drive that has a noise filter?
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The biggest down side of a VFD if you are not concerned about speed control is that the VFD is not 100% efficient. Even the best will present a 2-3% loss of circuit throughput energy in the form of heat in the drive. If you add filters with inductors (reactors), you will increase those heat losses in the circuit. Most of the time if the VFD is running at reduced speed, the losses are proportionally reduced and you are most likely saving some energy anyway. The problem with losses only is a significant concern if you are spending all of your time at full speed and compare that to running across the line, where there are virtually no circuit losses.

The "unity power factor" issue is sort of not true. The displacement power factor is improved to around .95, but the distortion power factor, that which is associated with the harmonics, is actually much worse. Fortunately, utility PF meters don't seem to see much effect from the distortion power factor so you still can help avoid PF penalties by using VFDs. I only bring it up because if you have your own transformers or generators feeding equipment, you still have to account for the distortion PF in the sizing of your equipment with regards to heating / de-rating etc.
 
PowerQualityDoctor

PowerQualityDoctor

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Israel
Many utilities do measure TPF (True Power Factor - including harmonics), so VFD reduces your PF issue.

As Jraef said, the main issue with VFD is their losses. 2-3% is excellent, but usually the real value is much worse (as said above - usually you increase the switching frequency to reduce noise and harmonics and increase losses).

To summarize - if you can reduce speed to save energy (for at least 10-20%), or need to control the process, you VFD. Otherwise, use motor starter with bypass. For motor starter - it is better to select harmonics-free starter rather than phase controlling.
 
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rcwilson

Senior Member
Location
Redmond, WA
On retrofit applications, a VFD's ability to limit torque can provide benefits.

For example, addition of pollution control equipment in the exhaust stack at a power plant increased the back pressure so larger FD & ID fans were required. Larger fans meant more starting torque on the motors and foundations. Using VFD's to limit torque within the original design parameters allowed the existing foundations to be reused, saving a lot of expense and space. A soft start might have been able to do the same but other benefits made the VFD cost effective.
 
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sgunsel

Senior Member
For many installations the cost of a VFD is comparable to a soft starter alone. But you get the soft starts plus speed control, if needed.
 
PowerQualityDoctor

PowerQualityDoctor

Senior Member
Location
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sgunsel said:
For many installations the cost of a VFD is comparable to a soft starter alone. But you get the soft starts plus speed control, if needed.
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I know that I am a minority being the opposition to VFDs, but the truth must be said.

As being said, VFD can provide speed control if needed. If not needed - VFD is just waste of money, CO2 emission and many potential problems. When no speed control is needed, the overall cost of VFD is way more than soft starter as it induces at least 10% of lossses (internal losses, harmonics related losses plus additional motor losses).

On the other hand, my job is solving harmonics problems, so please use VFDs as much as possible ;)
 
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