You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

---------->>>>>Get FREE PLC Programming Tips

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

PLC training tools sale

Reply
 
Thread Tools Display Modes
Old November 18th, 2002, 04:53 PM   #1
Tim
Member
United States

Tim is offline
 
Tim's Avatar
 
Join Date: May 2002
Location: Indiana
Posts: 291
Frequency Drive Question

I know this is a plc site, but I've got a question about a frequency drive.

First, let me say I have little knowledge on this matter.
Today at work we were having trouble with a particular machine that has a Mitsubishi frequency drive on it. It controls an AC motor that is mechanically "coupled" with a worm shaft. After opening up the control cabinet and watching a few machine cycles. A coworker had said to me that the display on the drive was showing that the motor was running at 80 htz. I said, "whats a matter with that". His concern was that the motor that it was driving was only rated at 60 htz and the drive was programmed at 80 htz.

I'm thinking with my little knowledge, that I know this is common because I've seen it done on many other applications. He was kind of confused on how the frequency could be bumped up higher than what the incoming supply voltage frequency was. I had him increase the htz up to 90 and then back down to 60 htz. There was a considerable speed difference and told him, well obviously you can run a motor at higher frequency then the motor rating.

Questions:
1. Is this a normal application?
2. Will it hurt the motor? "It doesn't run continuously"
3. Will a higher frequency cause more noise. "I'm only asking because I believe we have a noise problem with our plc".

Thanks in advance,
Tim
  Reply With Quote
Old November 18th, 2002, 05:32 PM   #2
Steve Bailey
Lifetime Supporting Member + Moderator
United States

Steve Bailey is offline
 
Steve Bailey's Avatar
 
Join Date: Apr 2002
Location: The boondocks of Western Massachusetts USA
Posts: 6,625
In a typical 3-phase motor, motor speed is proportional to frequency. The motor will develop almost constant torque up to its nameplate frequency. Beyond that, torque will drop off linearly with speed. If you plan to run the motor beyond 60 Hz, check to make sure that the motor bearings are rated for the higher RPM and that the rotor is balanced for the higher speed. Also check to make sure that the gearbox to which it's connected is rated for the speed. Assuming a 1750 RPM nameplate at 60 Hz, when you're running at 80 Hz, the motor is turning at almost 2400 RPM.

When you talk about noise, are you concerned about audible noise or electrical noise? There are adjustments you can make to the switching frequency of the drive to minimize the audible noise generated by the drive. I'm not sure if operating at 80 Hz generates substantially more electrical noise than operation at 60 Hz. I tend to doubt it.
  Reply With Quote
Old November 18th, 2002, 10:33 PM   #3
ndzied1
Lifetime Supporting Member
United States

ndzied1 is online now
 
ndzied1's Avatar
 
Join Date: Aug 2002
Location: Chicago, Illinois
Posts: 2,378
Quote:
1. Is this a normal application?
As long as the motor is rated for inverter duty and the motor wires/cable is rated & connected properly and considering Steve's answers then yes, perfectly normal. We regularly design systems able to go to 120 Hz with no problems.

Quote:
He was kind of confused on how the frequency could be bumped up higher than what the incoming supply voltage frequency was
... Well, the name IS Variable Frequency drive... Have him try to reverse the direction of the motor by swapping two wires before the drive... I'd try to explain it but I'm sure others here will do a much better job. The AC Tech manual s usually have a pretty good theory of operation section in them at least I remembe the QC series haveing someghing in it. Their web site is: http://www.actechdrives.com you have to sign up to get technical docs .
__________________
nOrM
======================
nOrM=Norman Dziedzic Jr.
"I decry the current tendency to seek patents on algorithms. There are better ways to earn a living than to prevent other people from making use of one's contributions to computer science." Donald Knuth
  Reply With Quote
Old November 19th, 2002, 07:30 AM   #4
Tom Jenkins
Lifetime Supporting Member
United States

Tom Jenkins is offline
 
Tom Jenkins's Avatar
 
Join Date: Apr 2002
Location: Milwaukee, WI
Posts: 5,920
The only question un-answered is how the drive gets a higher frequency out than is coming in. This is done by first converting the three phase AC into DC. This is at the same voltage as the incoming, so you can't get MORE than the incoming voltage out of the drive, but you can get less. Then the inveter simulates an AC wavefom by generating a series of very high speed pulses - several pulses for each half of the 60 Hz wave. The timing of the pulses is such that the output of the drive "looks" like an AC wave to the motor. Most VFDs will output up to 120 Hz.

Audible noise from the AC vave is highest at low frequency as a general rule. However, motor fans and bearings will be noisier at high speeds.
  Reply With Quote
Old November 19th, 2002, 07:37 AM   #5
Rick Densing
Member
United States

Rick Densing is offline
 
Join Date: Apr 2002
Location: Milwaukee, WI
Posts: 1,538
Most standard motors can do at least 90Hz. Many can do more. Like Steve said, it really is a function of the balance of the motor and bearings. Some inverter duty motors can do 6000 rpm with a base speed of 1750. ~200Hz. Contact the motor manufacturer.

Running the motor faster probably will not produce more noise, but will change the freqency of it. There are methods for reducing noise. (Line reactors, shielding your cables, etc.)
  Reply With Quote
Old November 19th, 2002, 08:19 AM   #6
DickDVgrl
Guest
 
Posts: n/a
A couple of detail items here. A motor-drive system can develop constant torque from zero or near zero speed up to the motor base speed which is the motor nameplate speed. Above that, the motor develops a maximum of constant horsepower which requires that the torque drop off as an inverse of the overspeed--not linearly. For example, at 70Hz the torque will be reduced by 60/70, at 80Hz by 60/80, and so on. Many ordinary motors begin to loose their constant hp characteristic above about 90Hz, so the torque drops off even faster.

Regarding the motors, any motor shop mechanic can tell you that the same bearings go into two pole (3600rpm) motors as in four pole and six pole motors. Therefore, certainly at any hp in the NEMA frame range, bearing life in overspeed is not an issue. Rotor balance can be, however, and more so as the frame gets bigger.

I've done this work for many years and follow this general rule. Motors 50hp and smaller can go to 90Hz without asking any questions. Motors from 50-250hp can go to 80Hz, and motors from 250-400hp can go to 75Hz. Above those speeds, better investigate with the motor mfgr. These rules apply to motors with base speeds 1800 and lower. For two pole motors, less overspeed is allowed.

Several good and bad things happen when you run a motor over its base speed. For good things, the motor cools better over its whole speed range since there is greater speed reduction in the power train, there is more available torque at lower speeds also because of the greater mechanical reduction ratio, and speed regulation is generally improved. For bad things, audible noise goes up a bit solely due to fan noise and there is a chance of lubricant foaming in a gearbox with excess input shaft speed (need to check with gearbox mfgr).

I've never seen EMI or RFI worsen with overspeed. If you've got those problems, you've likely got them at all speeds.

If you are dealing with a true inverter-duty motor, then you must deal with the motor design characteristics and forget the above rules. As mentioned, there are motors out there than can operate far into overspeed and there are also motors that are designed with higher base speeds. I just did a test cell job where the 100hp motor had a base speed of 16000rpm. That's 267Hz! That's 405 in-lbs torque constant from 0 to 16000 rpm! No overspeed required, fortunately!
  Reply With Quote
Old November 19th, 2002, 03:35 PM   #7
StillLearning
Member
United States

StillLearning is offline
 
StillLearning's Avatar
 
Join Date: Apr 2002
Posts: 8
I have a question regarding Steve Bailey's response. You stated that you can make adjustments to the drive settings to help with the noise. I noticed on AB drives you can change the PWM frequency. Is that what you are talking about? I always wondered what that setting was for but never thought it was one I should change. If that was not what you were talking about, could you please explain it to me?

Always learning some new from this site.
  Reply With Quote
Old November 19th, 2002, 03:53 PM   #8
Tim
Member
United States

Tim is offline
 
Tim's Avatar
 
Join Date: May 2002
Location: Indiana
Posts: 291
To everyone,
Thank You

This clears up a lot of things for me. Took this new info you gave to me and explained it to one of the electricians at work. It helped a great deal.

Great Web site!!
Tim
  Reply With Quote
Old November 19th, 2002, 03:54 PM   #9
Steve Bailey
Lifetime Supporting Member + Moderator
United States

Steve Bailey is offline
 
Steve Bailey's Avatar
 
Join Date: Apr 2002
Location: The boondocks of Western Massachusetts USA
Posts: 6,625
I don't have a manual for an AB drive. Different drive manufacturers will use different nomenclature, but that sounds like what I was talking about. Set up a drive on the bench and change the setting. You should hear a difference.
  Reply With Quote
Old November 19th, 2002, 04:04 PM   #10
Rick Densing
Member
United States

Rick Densing is offline
 
Join Date: Apr 2002
Location: Milwaukee, WI
Posts: 1,538
PWM stands for Pulse width modulation. That is the method used to create the chopped up pseudo-sinewave that the drive outputs. That is exactly what Steve was talking about.

You can usually hear that frequency in the motor. You can adjust it, but usually you have to derate the drive if you go higher than the default. My experience is only with small drives <15 Hp.
  Reply With Quote
Old November 19th, 2002, 07:45 PM   #11
DickDV
Member
United States

DickDV is offline
 
Join Date: May 2002
Location: Kalamazoo MI
Posts: 2,319
The rate at which the drive chops the DC bus to recreate AC for the motor is sometimes called the carrier frequency or PWM frequency.

The general rule is the lower carrier frequency results in more audible motor noise and higher frequencies reduce that audible noise somewhat.

The flip side is that lower frequencies are easier on motor insulation and can use the full drive rating. Some drives must derate their output if higher frequencies are chosen.

Note that the noise mentioned above is audible noise, not electrical noise. Changing the carrier frequency will rarely affect electrical noise but, if you are having a very lucky day, it might. Doesn't hurt to try.

Every mfgr seemss to have their own opinion about this but, in my personal view, carrier frequencies 4khz and below are "low" and above 4khz are "high". Now that should ignite a flurry of responses!
  Reply With Quote
Old November 19th, 2002, 08:03 PM   #12
harryg
Member
United States

harryg is offline
 
Join Date: Jul 2002
Location: Florida
Posts: 61
Quote:
Have him try to reverse the direction of the motor by swapping two wires before the drive...
This will not reverse the direction. It has to be done after the drive
  Reply With Quote
Old November 19th, 2002, 08:31 PM   #13
Steve Bailey
Lifetime Supporting Member + Moderator
United States

Steve Bailey is offline
 
Steve Bailey's Avatar
 
Join Date: Apr 2002
Location: The boondocks of Western Massachusetts USA
Posts: 6,625
Question for Dick DV

When I first had Variable Frequency drives explained to me, I was told that you can change the audible noise generated by the motor/drive by changing the carrier frequency as has been mentioned in this thread.

My question is: Is there any other reason to change the carrier frequency? It seems that every VF drive manufacturer includes the carrier frequency in its list of drive parameters, but it also seems that it is one of the least frequently changed parameters.

I've looked at a couple of manuals that I happen to have. GE (Fuji) drives default to the maximum carrier frequency. They label the parameter 'Motor sound'. Cutler Hammer AF91 drives default to 5 kHz in a range of 0.5 to 16.0 kHz. AB Bulletin 1305 drives default to 4 kHz in a range from 2 to 8 kHz. I've got a manual for an Omron model that defaults to 7.5 kHz in a range from 2.5 to 10 kHz. Any comments?
  Reply With Quote
Old November 20th, 2002, 12:07 AM   #14
Doug_Adam
Member
Australia

Doug_Adam is offline
 
Doug_Adam's Avatar
 
Join Date: Sep 2002
Location: Perth
Posts: 949
Carrier frequency generally has the following effects,

Low frequency gives high audiable noise, higher harmonics but higher 'efficiency' due to less switching transients.

High frequency generally gives lower audiable noise, due to moving much of the noise beyond the human hearing range. It also results in lower harmonic frequencys. Due to more switching transients, the efficiency goes down and the VFD will heat up more.

Why to change the carrier frequency?
Two reasons normally given are:
1, to remove annoying noises.
2, To prevent harmonic frequencys from damaging your equipment.

Hope this helps,

Doug
  Reply With Quote
Old November 20th, 2002, 07:05 AM   #15
ndzied1
Lifetime Supporting Member
United States

ndzied1 is online now
 
ndzied1's Avatar
 
Join Date: Aug 2002
Location: Chicago, Illinois
Posts: 2,378
The effect of carrier frequency may also be dependant on the length of the cable to the motor.

In the AB 1305 series drives, there is a parameter for cable length (can be set to short or long). When set to long, the carrier frequency is fixed at the low end (2kHz for this drive).
__________________
nOrM
======================
nOrM=Norman Dziedzic Jr.
"I decry the current tendency to seek patents on algorithms. There are better ways to earn a living than to prevent other people from making use of one's contributions to computer science." Donald Knuth
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
Abb Ac Drive rajendrad LIVE PLC Questions And Answers 13 November 5th, 2006 11:18 AM
current measure with frequency drive Combo LIVE PLC Questions And Answers 16 June 26th, 2006 12:01 AM
VFD training class was awesome! Question about Carrier Frequency... Russ LIVE PLC Questions And Answers 2 September 17th, 2004 08:28 AM
Drive control question? elevmike LIVE PLC Questions And Answers 16 March 10th, 2004 01:24 PM
Motor question, Old Frequency Converter RDay LIVE PLC Questions And Answers 16 October 24th, 2002 05:00 PM


All times are GMT -5. The time now is 11:11 AM.


.