Gingoog Electric Trading (G.E.T.) ™

March 5, 2009

What is a VFD? Part 2

Filed under: GET to know VFD's — Ralf @ 8:13 AM

What is a VFD and how does it work? Part 2

In order to provide our readers here the best possible information we are using a variety of resources such as manufacturer’s support materials, our extensive experience in this field but also online researches.

When we want to go more into applications in order to achieve the ultimate goal of enjoying all the benefits that come along with a Variable Frequency Drive we should also know what is a VFD and how does it work?

And to understand the VFD’s much better we found for you also the following very interesting material for your further review.

It explains in a very comprehensive way what a VFD actually is and how it works, it confirms what we have posted earlier in our category GET Savings that we can really safe a lot, it gives us an initial idea how we select a Drive and very important also how to size it up. We learn about the relevance of torque and that at times we do have to oversize the VFD for one step in order to reach the required torque. It touches also special applications where we have to watch out or should have at least a closer eye on and it tackles some important corresponding considerations.

It answers a lot of our questions and I would rate this article as another MUST READ, it is not the usual shop talk you might read on other blogs, we make it all easy digestible.

GET it easy, GET is easy, GET more today, with GET you GET it, GET it here and GET it now:

However, this is a larger entry which doesn’t upload in one piece, so we will do it in a few parts.

This is Part 2:

Key VFD specifications:

While there are many specifications associated with drives, the following are the most important.

Continuous run current rating:

This is the maximum RMS current the VFD can safely handle under all operating conditions at a fixed ambient temperature, usually 40 degrees Celsius. Motor ball load sine wave currents must be equal to or less than this rating.

Overload current rating:

This is an inverse time/current rating that is the maximum current the VFD can produce for a given time frame. Typical ratings are 110% to 150% overcurrent for 1 min., depending on the manufacturer. Higher current ratings can be obtained by oversizing the VFD. This rating is very important when sizing the VFD for the currents needed by the motor for break-away torque.

Line voltage:

As with any motor controller, an operating voltage must be specified. VFD’s are designed to operate at some nominal voltage such as 240VAC or 480VAC, with an allowable voltage variation of plus or minus 10%. Most motor starters will operate beyond this 10% variation, but VFDs will not and will go into a protective trip. A recorded voltage reading of line power deviations is highly recommended for each application.

Applications to watch out for:

If you answer any of the following questions with YES, be extra careful in your VFD selection and setup parameters of the VFD.

Will the VFD operate more than one motor?

The total peak currents of all motor loads under worst operating conditions must be calculated. The VFD must be sized based on this maximum current requirement. Additionally, individual motor protection must be provided here for each motor.

Will the load be spinning or coasting when the VFD is started?

This is very often the case with fan applications. When a VFD is first started, it begins to operate at a low frequency and voltage and gradually ramps up to a preset speed. If the load is already in motion, it will be out of sync with the VFD. The VFD will attempt to pull the motor down to the lower frequency, which may require high current levels, usually causing an overcurrent trip. Because of this, VFD manufacturers offer drives with an option for synchronization with a spinning load; this VFD ramps at a different frequency.

Will the power supply source be switched while the VFD is running?

This occurs in many buildings, such as hospitals, where loads are switched to standby generators in the event of a power outage. Some drives will ride through a brief power outage while others may not. If your application is of this type, it must be reviewed with the Drive manufacturer for a final determination of Drive capability.

Is the load considered hard to start?

These are the motors that dim the lights in the building when you hit the start button. Remember, the VFD is limited in the amount of overcurrent it can produce for a given period of time. These applications may require oversizing of the VFD for higher current demands.

Are starting or stopping times critical?

Some applications may require quick starting or emergency stopping of the load. In either case, high currents will be required of the Drive. Again, oversizing of the VFD may be required.

Are external motor disconnects required between the motor and the VFD?

Service disconnects at motor loads are very often used for maintenance purposes. Normally, removing a load from a VFD while operating does not pose a problem for the VFD. On the other hand, introducing a load to a VFD by closing a motor disconnect while the VFD is operational can be fatal to the VFD. When a motor is started at full voltage, as would happen in this case, high currents are generated, usually about six times the full load amps of the motor current. The VFD would see these high currents as being well beyond its capabilities and would go into a protective trip or fail altogether. A simple solution for this condition is to interlock the VFD run permissive circuit with the service disconnects via an auxiliary contact at the service disconnect. When the disconnecting contact is closed, a permissive run signal restarts the VFD at low voltage and frequency.

Are there power factor correction capacitors being switched or existing on the intended motor loads?

Switching of power factor capacitors usually generates power disturbances in the distribution system. Many VFD’s can and will be affected by this. Isolation transformers or line reactors may be required for these applications.

Power factor correction at VFD-powered motor loads is not necessary as the VFD itself does this by using DC internally and then inverting it into an AC output to the motor. All VFD manufacturers warn against installing capacitors at the VFD output.

Application considerations:

* Starting torque currents

* Running torque currents

* Peak loading currents

* Duty cycle

* Load type

* Speed precision required

* Performance (response)

* Line voltages (deviations)

* Altitude

* Ambient temperature

* Environment

* Motoring/regenerating load

* Stopping requirements

* Motor nameplate data

* Input signals required

* Output signals required

Ralf Wabersich

Gingoog Electric Trading (G.E.T.)


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