Variation in voltage and frequency

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Thread Starter

Colvin, Chuck

Hello List Members, This question may seem strange to some you, but I need some insight. The utilization voltage here in the US is 460v 60Hz. On Brazil the Voltage is 440v 60Hz (please correct me if I am wrong). I know the importance of keeping the frequency as specified because of heat build up. I was told by some one who works for Baldor that you can operate an AC motor below name plate rating if you derate the Hp. I know this may seem dumb, but I'm asking because I am sending equipment to Brazil that was originally connected to a 380V system and Brazil's voltage is 440v. What are the consequences? Carlton Colvin [email protected]
 
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Hakan Ozevin

Your question is not strange and it is a good opportunity to explain the necessity for derating the motors when driven from a VSD. There are two aspects of your question: 1. For a constant torque load, a self cooled motor cannot be driven at low speeds for a long time, because the ventilation fan is directly connected to the shaft of the motor, and thus the motor will overheat (consider that the losses is nearly proportional to I2t, which is almost constant in constant torque loads). In such cases, you have to: i. increase the rated motor torque/load torque ratio by either using a bigger motor, or decrease the load torque, if possible. ii. apply forced ventilation to the motor. For loads which require variable torque (fans and some pumps) the above are usually not necessary, since load torque (thus current) decreases together with the speed. The Baldor guy had meant this, I think, and does not seem to be a problem for your case, or you didnt ask for it. 2. Your problem could be in the voltage level, but good news: Provided that VSD can work at 440 V as well, the current drawn from the motor will be lower as compared to the 380/400 V case. Therefore you can overload the VSD even more. The problem would be in a case like using a VSD designed for 380 V, but using it at a lower voltage, e.g. 3 x 230 V (with a transformer connected to the input of the VSD of course). In such cases look at the rated *current* of the VSD and compare it the with the current of the load (if you dont know the load, look at the motor current). You will not make a mistake then. 50/60 hz difference is not a problem in VSD's, because the output frequency is independent of the input. The only problem could be internal cooling fan of the VSD, but nowadays this fan is fed from the DC link. Best regards, Hakan Ozevin
 
Carlton, As you may remember electric motors are considered active components from an electrical point of view. Yes, if the voltage changes the motor will try to adjust most of the times this means the sacrifice of the same. In your case, connecting to higher voltages, well, the first thing that comes out to mind, is how many rings the stator may have, meaning if the relation of length of actual cooper on it will cause proper saturation in the metal, and yes, unproper voltage won't be in operating point for the hysteresis cycle, wich as you pointed out shows up in heating here and there, probably not much either cause making the saturation cycle area smaller. Other consideration to make is if the number of rings there will be enough to hold the fields, which means, if there isn't enough coper there, for the increased voltage, it may make the field to over heat and looses integrity, aka your engine will simply burn. It may happen slowly till the dielectric material in the cooper degrades due overheating, or as fast as what may happen on a voltage spike. If I remember right, specs for voltage and a certain equipment should not vary more than 10% of rated, double of what is allowed for the provider to vary. So, 440 to 380 comes out to be extra 15%, wich doesn't seem to be that much. The histeresis cycle, yes it will be steep, wich means just better operation, and smaller currents in the cooper, so one thing may compensate the other. I'd say, yes the engine will work just fine if proper protection is supplied and considering factor is the normal 1.5 american std. Hope this be of assistance, Doug.
 
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Vitor Finkel

At 19:08 23/03/01, Chuck wrote: >---------- Forwarded message ---------- >From: "Colvin, Chuck" <[email protected]films.com> >To: "'[email protected]'" <[email protected]> >Subject: ENGR: Variation in voltage and frequency > >Hello List Members, > This question may seem strange to some you, but I need some insight. >The utilization voltage here in the US is 460v 60Hz. On Brazil the Voltage >is 440v 60Hz Chuck, The frequency is 60Hz. The available motor voltage vary from place to place. I have already worked in places where 380/220 Wye/Delta was used, and I have also used 440V elsewhere. >(please correct me if I am wrong). I know the importance of keeping the >frequency as specified because of heat build up. I was told by some one who >works for Baldor that you can operate an AC motor below name plate rating if >you derate the Hp. I know this may seem dumb, but I'm asking because I am >sending equipment to Brazil that was originally connected to a 380V system >and Brazil's voltage is 440v. What are the consequences? A 380/220V motor, (unless if it's windings can be reconnected for 440V), may burn ( or be disconnected by thermal protection, etc ) when connected to 440V. The problem here is not to "operate below name plate rating", but submitting your windings to overvoltage. If the "iron" saturates, the current will raise in a disproportional way, and the motor may burn ( or disconnect due to it's installed protection devices) even running at no-load condition. An ammeter used during start-up will show if you are reaching this condition. Mind all mains supply voltage may have fluctuations, so in a moment you may be near magnetic saturation, in another moment/day you may reach it. Most motors have double windings that may be connected in series or parallel. If this is the case, reconnecting the (380) Wye to Delta and connecting the windings in series will change Delta nominal Voltage from 220 to 440V. If windings are properly marked, any good Industrial Electrician should know how to do it. If they don't know how to do it, ( else, why would you be sending this e-mail ? ) ask them to contact me. Vitor Finkel [email protected] >P.O. Box 16061 Tel (+55) 21 285-5641 >22221.971 Rio de Janeiro Brazil Fax (+55) 21 205-3339
 
Chuck, Great question . . . the important parameter relative to induction motors is flux density. This is the ratio of line volts over line frequency. V/Hz for a 480 Volt motor operating at 60Hz is 8 Volts/Hz and this is a very common operating voltage and frequency for motors all over the US market. But if you look at the nameplate of these motors, you will see that they are typically 460V rated and designed for a V/Hz ratio of 7.67 V/Hz. A 380V - 50Hz operating in Europe motor is very similar at 7.6 V/Hz. You are planning on sending motors designed for 380V / 50Hz in Europe to run on 440V / 60Hz in Brazil. Well, 60Hz x 7.6V/Hz = 456Volts . . . if you are running 440 Volts you will be operating the motor at about 3% overvoltage . . . well within spec of the motor design. Actual power from the motor will be about 440/380 x motor rating. Heating will not be a problem. Hope this helps . . . Ken Brown Applied Motion Systems, Inc. http://www.kinemation.com
 
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Bouchard, James [CPCCA]

380 volts is commonly used in Europe where the frequency is 50 HZ so I would ask myself about the equipment. Was it connected to 380 volts 60 HZ via a transformer from 460 volts 60 HZ in the US? Or is it equipment from Europe that was run in Europe at 380 volts 50 HZ. Generally you can go down in voltage if you derate the HP. If you try to go up in voltage you will probably generate more heat than was considered in the design and the motor will fail prematurely. Also the motor insulation is rated for 380 volts and running it on 440 volts may cause it to fail very quickly. A lot depends on the loading on the motor and the motor design. In Canada we operate at 600 volts and about 15 to 20 years ago we ran 460 volt motors at 600 volts ( after changing the overload relays and fuses and so on ) for over a year without any problems. These were older motors with more iron and copper than you see in some motors now. Some were not loaded much but at least one ran near full load. Just because it worked for us ( and we knew we were taking a calculate risk ) does not mean it will work for you. Also with used equipment you never know what it has suffered before you got it. James Bouchard
 
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Bruce Durdle

Carlton, As a first cut "idiot's guide" to motors, I like to remember that the voltage supplied to the motor must be matched by the back EMF drawn by the motor, which depends on flux and speed. If the speed is the same, the increase in voltage can only be met by an increase in flux, and the magnetising current component will go up. If speed changes, the flux and magnetising current will change in inverse proportion. This may not matter, but if the motor is specifically designed for 380 V I would say there is a fair risk of pushing the required flux level up to the point where you have gone well past the knee of the saturation curve and magnetising current becomes excessive. You can test this by applying a variable voltage source to the unloaded motor, and increase the voltage slowly while monitoring the current. Current should be approximately proportional to voltage - if it starts to increase faster than voltage, you have gone too far. The second law of motors is that the motor (whatever its rating) will develop whatever power is needed to make the load go at the motor running speed. If voltage is increased, the active component of current for the same mechanical load will be less, and if operated at rated current the motor will develop more power. If load is fixed, the current will be reduced in the same proportion as voltage is increased. If operating a motor at lower voltage than nameplate, the power that it can deliver without exceeding ratred current will be less. If frequency changes, the speed of the motor will change. This may affect the driven load - for fans, power is proportional to the cube of speed, so a small change in frequency can have a major effect on power requirements. You need to find out how the load is affected by a change in motor speed and make sure the system can handle it. Cheers, Bruce..
 
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Bouchard, James [CPCCA]

If your load includes fans, pumps or similar devices you may have the change the pulley ratio between the motor and the load if the design was originally done for 50 Hz because at 60 Hz they will turn faster and require more horsepower. They may also operate at a different part of their curve and not provide the required performance. ( We get a lot of equipment from Europe and this is often a problem even if the supplier has been advised of our concerns ) James Bouchard
 
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