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nvrsrinivas
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What's a VFD Rated Motor?

01/09/2020 3:54 AM

What is the difference between Normal motor and VFD rated motor.
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PWSlack
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#1

Re: VFD rated motor

01/09/2020 5:19 AM

What an excellent question for:

  • an internet search engine, or
  • a tame local motor distributor; telephone?
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JRaef
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#2

Re: What's a VFD Rated Motor?

01/09/2020 11:45 AM

As mentioned there are many many sites that will explain the difference in more detail, but here is a grossly condensed version.

Using a VFD creates the possibility of having new types of stresses impressed on the motor itself. First and foremost is the winding insulation. VFDs can create what is called a “reflected wave phenomenon” in the conductors going from the drive to the motor, to where spikes of voltage that can exceed 300% of the rated voltage may be seen (under worst case scenarios) in the motor winding insulation. Normal motor insulation is typically rated for 150% of nominal voltage, so running a standard motor on a VFD can result in fast degradation of the windings, sometimes in a matter of weeks. VFD rated motors typically have higher voltage rated insulation and winding techniques that mitigate that risk, 1600V or even 2000V insulation is often used.

Another problem is cooling; standard motors often rely on fans to move air across the frame to cool the motor, but if you slow the motor down, the fans slow down as well and don’t cool as well, so the motor overheats. VFD motors are designed with this in mind.

Thirdly is the motor bearings. The way a VFD works can create a capacitive effect between the stator and rotor, creating a voltage differential. That voltage will build up on the rotor until it exceeds the dielectric strength of the bearing grease and will then seek ground by discharging through the bearings and races. That then causes microscopic damage to the races and bearings similar to Electric Discharge Machining (EDM); think of a plasma cutter. As time goes on that gets worse and worse until your bearing races look like washboards and fail. Good quality (but not all) VFD rated motors will either have a “shaft grounding bushing” making contact of the rotor shaft to the frame to give that voltage a safer path to ground, or they will use insulated bearing races (typically ceramic) to prevent any discharge. But that last method may just move the problem to the next grounded part of the machine, so you must be careful.

Hope that helped.

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Brave Sir Robin
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#3
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Re: What's a VFD Rated Motor?

01/09/2020 1:18 PM

Bingo.
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SolarEagle
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#4
In reply to #2

Re: What's a VFD Rated Motor?

01/09/2020 6:35 PM

In a nutshell...

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HiTekRedNek
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#5
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Re: What's a VFD Rated Motor?

01/10/2020 7:34 AM

"When I read that you are what you eat,I knew I was nuts!"

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7anoter4
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#9
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Re: What's a VFD Rated Motor?

01/11/2020 4:38 AM

Very,very good answer, indeed.

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jmueller
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#10
In reply to #2

Re: What's a VFD Rated Motor?

01/11/2020 6:03 PM

Regarding the very real problem of over-voltage on the winding's insulation, if the motor is a dual rated motor (such as 480/240 volts) and is being operated at the lower voltage, then the over-voltage generally does not cause winding insulation failure.

Additional points that JRaef intentionally and appropriately ignored would include various ways to mitigate the harm to the non VFD-rated motor, such as load reactors, sine wave filters, or impedance matching networks near the motor. Also careful attention has to be paid to the distance from the VFD to the motor. All-in-all the topic merits considerable care and attention.
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Rakesh Mishra
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#11
In reply to #2

Re: What's a VFD Rated Motor?

01/12/2020 2:35 AM

Agree with JRaef comments. However would like to add some practical advice

1. If motor rating is small less than 30 hp, you already have a standard motor available with you, and you are not running it continuously at lower speeds, you can run run it with vfd without any problems. Problem is there if you run it low speeds for longer time.
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daveoliver
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#15
In reply to #2

Re: What's a VFD Rated Motor?

01/30/2020 10:45 AM

This is a great explanation for the most part. I have to add that due to the way that VFDs operate they all create Shaft voltages and the damage to the bearing races that you described is called Fluting. The addition of grounding bushings can add extra cost to the motor if they are specified and many VFD rated motors do not have them. These Ground rings can be installed after the fact by either drilling and tapping the motor end bell or by using a conductive adhesive. Aegis offers some really great products for this.

It would seem that you have attended some of the VFD trainings that I have with your detailed explanation.

As a rule of thumb, I do not worry about these ground rings below 100-150Hp. especially since most places consider motors that are 50Hp and below as expendable and the added cost of these rings doesn't prove to be economical in most cases and the reason for failure is often something else.

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Anonymous Poster #1
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Re: What's a VFD Rated Motor?

01/10/2020 11:40 AM

Why would you try shut down a great question?

I learned a lot (more) about VFD's from the great answer below your worthless contribution.
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Brave Sir Robin
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#7
In reply to #6

Re: What's a VFD Rated Motor?

01/10/2020 4:20 PM

Your response might have been more effective if you knew how to reply to the post that offended you.

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#8
In reply to #7

Re: What's a VFD Rated Motor?

01/11/2020 2:55 AM

Not beyond the realms of possibility that he is referring to No 1's standard response to any question on this site these days.
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NotassmartasLynn
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#12
In reply to #8

Re: What's a VFD Rated Motor?

01/13/2020 1:48 PM

I sure know what reply he is referring to. Why discourage us from asking a simple Q.
Here’s my question: have you noticed there are few questions on the site anymore (Lynn?)
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SolarEagle
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#13
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Re: What's a VFD Rated Motor?

01/14/2020 10:11 PM

This is a public site, you are not always going to like all the answers, or posters...no need to fixate on the answers you don't like and attack them...take what you get, and go in peace, happy that you have perhaps gained some insight or knowledge...

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vimal76
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#14

Re: What's a VFD Rated Motor?

01/22/2020 8:26 AM

The VFD rated motor has following features.

1. To withstand the standing wave voltage which is higher than the motor rated voltage and the standing wave voltage appears at the first turns of the motor. To withstand higher PWM voltage the higher insulation of winding is done.

2. VPI process is done in inverter grade motor.

3. NDE side bearing is insulated to prevent the circulation current.

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why VF ratio is kept constant in VFD?
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Users who posted comments:

7anoter4 (1); Anonymous Poster (2); Brave Sir Robin (2); daveoliver (1); HiTekRedNek (1); jmueller (1); JRaef (1); NotassmartasLynn (1); PWSlack (1); Rakesh Mishra (1); SolarEagle (2); vimal76 (1)

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