Motor rated frequency???

[Mr_B]

If u check on a AC motor name plate, u get certain information like Motor Rated Current, Supply Voltage, frequency etc.

My question 2 u is...What exactly does the frequency tell u?

I mean,

does it mean u HAVE 2 supply that frequency

or

simply that only if u apply that frequency, the rpm rated will be right.

Reason I'm asking, is that our drives can be adjusted to 400Hz max.

So what wud happen if u run a 50Hz motor at, say, 100Hz?

Ya ya I know it will go faster but can u hurt the motor?

 

[Steve Etter]

Ya ya I know it will go faster but can u hurt the motor?

Yes, you can.

There have been many posts on this site regarding motors and variable frequency drives. If you do a search you should find answers to most of your VFD questions.

Steve

 

[31415327]

Frequency

I am no drive expert but wouldn't the base frequency
of the motor be determined by location? U.S. is
60 hertz , Europe 50 hertz, etc...
The maximum rpm you can safely speed up the motor
should be on the nameplate or data sheet.

 

[Mr_B]

that was my understanding as well. But we have 50Hz motors running @ 79Hz
constantly for over a year now hence my confusement.

tx steve, will do a search.

 

[rsdoran]

Here are some links that may help:
http://www.patchn.com/what_is_a_drive.html

http://www.patchn.com/mtrwhtpaper.htm

 

[Mr_B]

Did the search and read through alota stuff. So I guess only stuff that u need 2 worry abt when upping the frequency wud be loss of torgue and bearings which aren't really an issue.

 

[JRW]

In my experience it tells the drive the volts per hertz curve.

460/60= 7.66 volt/per 1 hertz

or

380/50= 7.6

 

[Tom Jenkins]

Did the search and read through alota stuff. So I guess only stuff that u need 2 worry abt when upping the frequency wud be loss of torgue and bearings which aren't really an issue.

Not so.

The nominal frequency of a motor is the motor power frequency (Hz) for which the motor will produce the rated power at the rated speed when getting the rated voltage and drawing the rated current.

Most motors are designed for a specific ratio of volts to Hz (frequency). A 50 Hz motor is often rated for 380 Volts, and a 60 Hz motor is rated for 460 Volts. Both have a V/Hz ratio of 7.67. Most VFDs maintain that V/Hz ratio up to the nominal output frequency and the max input voltage. An expert like Dick Dv can tell you what happens if you stray too far from this ratio.

In addition to bearings and bearing lubrication issues (which are NOT MINOR ISSUES in my book) you can run into problems with centrifugal forces (do you want to see the motor fly apart?) and cooling and audible noise.

A standard 4 pole motor (3,600 rpm @ 60 Hz) can usually go up to 75 Hz (25% overspeed). For any motor, and especially for a two pole motor, you should definitely contact the motor manufacturer and get the maximum allowable speed.

The idea of running at 400 Hz (or even 100 Hz) is very scary!

 

[Leadfoot]

Not so.

The nominal frequency of a motor is the motor power frequency (Hz) for which the motor will produce the rated power at the rated speed when getting the rated voltage and drawing the rated current.

Most motors are designed for a specific ratio of volts to Hz (frequency). A 50 Hz motor is often rated for 380 Volts, and a 60 Hz motor is rated for 460 Volts. Both have a V/Hz ratio of 7.67. Most VFDs maintain that V/Hz ratio up to the nominal output frequency and the max input voltage. An expert like Dick Dv can tell you what happens if you stray too far from this ratio.

In addition to bearings and bearing lubrication issues (which are NOT MINOR ISSUES in my book) you can run into problems with centrifugal forces (do you want to see the motor fly apart?) and cooling and audible noise.

A standard 4 pole motor (3,600 rpm @ 60 Hz) can usually go up to 75 Hz (25% overspeed). For any motor, and especially for a two pole motor, you should definitely contact the motor manufacturer and get the maximum allowable speed.

The idea of running at 400 Hz (or even 100 Hz) is very scary!

Well said, however a 4 pole motor has a synchronous speed of 1800 rpm at 60 hertz. A 2 pole motor runs 3600. The name plate rpm, typically between 1735 and 1765 rpm on most INVERTER duty motors is the full load rpm after slip.

If you have ever been familiarized with the NEC rules on motors and generators, the code usually requires motors to be able to sustain an overload/overspeed of usually 10% for about 1 hour without degredation. Many a plant manager/production supervisor wants the motors to run in that "ALLOWABLE" region continuously.

My company makes them sign a statement taking total responsibility for any and all happenings that results if that is what they want, before we will consider doing that.

Having seen many motors run at 10% to 25% over speed, they do not last as long. Many times the additional production revenue offsets the additional maintenance costs.

If you look in the VFD manuals, there are usually several V/HZ patterns available to satisfy the application.

Just setting a drive to 400 hertz and powering a standard 50 or 60 hertz motor to see what happens is a disaster waiting to happen.

The rotor will swell to a point where it can grab the stator windings to give it the necessary leverage to launch itself thru the roof or walls.

 

[Tom Jenkins]

a 4 pole motor has a synchronous speed of 1800 rpm at 60 hertz

D'oh!!!! I knew that. Once again a victim of diarrhea of the keyboard.

 

[manmeetvirdi]

Go to Electrical group of www.eng-tips.com

 

[Bruce99]

Motor speed

OK, name-plate is the rated HP at a specific voltage running at a certian frequency. If the name plate says 30 HP @ 460v 1800 rpm at 60 hz this is what you get at these conditions. Today, in the sawmill industry, 60 Hz is slow. Most of our machines run at 90Hz. The name plate is for standard across the line starters that run at line frequency, what ever that may be. If you want to run above name plate frequency, better buy a high efficiency (Like toshiba) motor. The rotor tends to get very hot. In a standard motor the heat will turn the grease to water and sieze the bearings. We have motors running at 110Hz. The application must be designed with speed, Hp and torque considered. I dont give a rats *** about the math, give me the HP with the max speed and Ill select the frequency drive and get the silly thing running. Single motor, two or four or more, does not matter. There are charts that supply all the info regarding the speed, torque, and HP. Ive never had to use them. The design people do that. I just have to get it to work. Im just a dumb electrician, in the middle of nowhere doing the best I can to keep the mill running. At times people, this is very hard to do. I have to work in 7 hrs. Got to run.. Have a good weekend
Bruce

 

[rsdoran]

I could post a bunch of links like I normally do but not sure anyone reads them.

Simple fact is that Inverter rated motors are usually designed to run above base speed. In many cases the nameplate will state 1800rpm base speed but 3600rpm max speed for a 4 pole motor with a base frequency of 60HZ. This means they can be run up to 120HZ. There are spec sheets showing torque/HP curves when used above base frequency.

Technology changes designs and capabilities everyday for many devices used in industrial applications. Induction motors and VFDs have easily exceeded the 75HZ range for a motor with base frequency of 60HZ, its very common to see applications running at 90-100HZ.

Maybe DickDV or Don will provide more detailed info.

 

[monkeyhead]

I could post a bunch of links like I normally do but not sure anyone reads them.

Your links have helped me quite a bit when I run into problems and searched the board. Especially when my search turned up only a slightly relevant topic.

 

[David Butler]

One might also want to consider that an inverter duty rated motor is good to have if one is planning to run under 60 hz. The slower speed results in less cooling as the cooling fan fins are not turning as fast and therefore not providing as much cooling. An inverter rated motor is designed to handle this decreased cooling, as I understand them.