380 Volt 50 Hz 3Ph. motor operating at 460V, 60 Hz 4

[oldnukeet]

I know that I have seen somewhere recently a fairly simple mathematical model that shows why running a 380 / 50 motor at 460 / 60 is OK. Obviously, it is a matter of maintaining the proper ratio of volts to hertz to avoid magnetic saturation and the resulting eddy currents and overheating problems. But it seems to me that I have seen a simple one page dissertation with all the proper caveats as to what motors are suited to this activity per the nameplate data and most importantly WHY this is electrically equivalent energy flow from the stand point of the motor. Anyone seen the sheet I am referring to or have one themselves ready made? Cruising the Web gets old quick...Thanks!!

 

[davidbeach]

I would be much more inclined to say that you can go the other direction. In terms of iron saturation, 460V@60Hz and 380V@50Hz will have very similar results. On the other hand, a motor designed for 380V@50Hz will be operated at 20% higher voltage and 20% higher speed. Does that work? Probably for most motors, but you will have less margin than you would with a motor designed for 460V.

Going the other direction, a 460V@60Hz motor running at 380V@50Hz is not subjected to these overvoltage/overspeed conditions.

 

[DickDV]

As a general rule, you can operate at 460/60 but you must not increase the motor load above the kw rating at 380volts.

This means that the torque on the motor must be reduced by 5/6 since the speed has been increased by 6/5. Failure to do this will result in a motor that runs too hot.

 

[Marke]

Hi DickDV

My understanding has always been that you must not exceed the rated torque and that means that the power will increase by 20%.
If you increase the frequency with out increasing the voltage, as in overspeeding on the output of a drive, then the flux will reduce due to voltage limiting, then you must not operate above rated power. I think that there may be some confusion here.

oldnukeet, you can certainly operat a 380 volt 50Hz motor at 460V 60Hz, but ensure that the insulation of the motor will withstand the higher voltage and be aware that many loads increase with the speed squared so you may need to control the load to prevent the motor being overloaded.
The important points are the V/Hz ratio and the insulation voltage rating, and the maximum shaft torque.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[DickDV]

Marke, you make my point exactly when you say that the motor power would increase by 20% if the torque is constant up to 60hz. Since the motor continuous rating is essentially a thermal limit, the motor will not somehow become 20% more powerful without damaging overheating effects. For that reason, while the motor will continue to produce constant torque up to 60Hz, you must not ask for that much torque from the motor. Or, to say it a different way, once the motor reaches its nameplate KW at 50hz, that is all it should be expected to deliver, even in the range up to 60Hz.

In order to do that you will need to reduce the load torque once you exceed 50Hz.

 

[oldnukeet]

Thanks to everyone who has responded. This application were discussing is in fact motors that will be operated at 380/50 but were orginally 460/60 so all things being equal (i.e., the workload of the individual motors) there really shouldn't be a problem. The equipment in question is woodworking machines (industrial size) so the work load as the machines function is essentially constant. It appears, Dick, from what you are saying that in fact, going to the lower voltage and frequency as my customer is - that he will actually gain up to 20% torque availability - which is good news.

 

[DickDV]

The torque behavior of an induction motor is determined by the volts/hz ratio. In the case of a 460/60 motor operating at 380/50, the ratio is nearly the same so you can expect about the same torque.

Since that means that the kw output will be reduced to 5/6, I suppose that the motor heat would be somewhat less too. If you can stand the increase in slip speed, you could probably overload the motor a little and not exceed its thermal limits but that is not usually done.

As was discussed earlier, going from 380/50 up to 460/60 guarantees thermal overload at constant torque and is not acceptable. The torque has to be reduced to control motor temperature.

 

[oldnukeet]

OK - Now I've got it. Thanks again for the input. This site has been very helpful. I'll be back!

 

[Marke]

Hello DickDV

I agree that the rating is thermal and have a problem understanding the power limitation at the higher frequency.
The thermal limitation is related to the power losses.
If we keep the V/Hz constant, then the flux in the iron will be the same at 50 Hz and 60Hz, so the iron loss will be equal. The current will be the same, so the copper loss will also be equal for the same torque output.
Therefore the power dissipated in the motor will be the same at 50Hz and at 60Hz for the same output torque. So if the motor is "torque limmited", there should be no need to apply a power limitation that I can see. The effective power capacity of the motor will be higher at 60Hz. At 60Hz the cooling of the motor will be much higher, so in theory, the motor can safely dissipate more power than at 50Hz without excessive temperature rise.

Can you please elaborate on the power limitation at 60 Hz for me.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[edison123]

I agree with Marke.

Higher the speed, higher is the power output from the same m/c (subject of course of mechanical stress limits). The basic output of any m/c (electrical or mechanical) is proportional to product of volume and speed.

 

[DickDV]

Marke, I follow your analysis and am tempted to agree but the motor manufacturers that I have contacts with require the torque derate. So I am simply repeating what I have been told without understanding precisely why.

I guess, if a 380/50 motor could really be operated at full torque up to 460/60, that would give every drive application engineer in Europe an opportunity to increase their motor kw by 20%.

If that is so, then maybe we here in the US could do a similar thing by increasing our 460/60 motors to 600/75. I don't really believe this but you get my point.

I'd like to hear some comments from Europeans on this subject.

 

[oldnukeet]

Dick, yes exactly right. And it is European equipment that I am largely talking about in this case. Almost all industrial Woodworking equipment comes from Italy or Germany, some minor amounts from Austria and Spain. This is in fact a common thing that they do presenting equipment with the same motors but marketed here with ~20% better ratings in the literature under the "60Hz" column of specs.

 

[Marke]

Hello oldnukeet

I think that the clue is in your post. European motors designed for 50Hz 380 volts are capable of a 20% increase in rated output power without thermally overloading the motor. This is because, as previously explained, the governing parameters are the power dissipated in the motor and the ability of the motor to dissipate that power determine the temperature rise. Proivided that the V/Hz remains the same, the iron loss will be constant and at rated torque output, the copper loss will also not change. Result, same power dissipated in motor, higher speed fan gives better thermal resistance and a lower temperature rise. Same torque, 20% higher speed is 20% higher output power. The major limiting factors are the effect of increased speed on the bearings and the higher voltage on the insulation.

I have not seen any limitations put on by motor manufacturers other than on the output of VSDs due to the voltage limiting of the VSDs reducing the V/Hz above line frequency.

I believe that your equipment manufacturers are indeed correct.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Skogsgurra]

Amen to that, Mark.

Gunnar Englund

http://www.gke.org

 

[jraef]

I must agree with Marke and Skogsgurra, that is the way I have always done it and never had any probelms, as long as you watch out for the pitfalls that Marke has already mentioned. Here in the US, motor manufacturers do it this way all the time, only the other way around.

If you work out the math, it shows up plainly. For the benefit of all you non-North Americans I'll show it in your crazy terms ;-)

A 15kW motor running at 380V 50 Hz, 1450RPM is 98.8 N-m (15kW x 9550 / 1450RPM = 98.8 N-m). We already know that if the V/Hz ratio remains constant, torque remains the same. If the voltage is increased to 460V so that torque remains constant when the motor is running 20% faster at 60Hz (1740 RPM), you get 98.8 * 1740/9550 = 18kW.

Can motor manufacturers then "cheat" and get more power from their motors? Absolutely! But of course, they need to have a higher voltage to feed to them. If you only have 380V available, a VFD cannot create 460V potential that is not available at the line terminals. So in order to "cheat" like that, one would need to add a boost transformer, and that would cost more than just buying a larger motor. On the other hand, motors sent to the US where the application can handle the increased speed are frequently re-labeled at a higher power rating. For instance, we see a lot of OEM equipment with odd sizes such as 24HP motors. 24HP * .746 kW/HP = 17.9kW! Imagine that.

Why do some manufacturers state that you must limit the power rating to that of a 380/50 application? Either they don't know how to work out the math so they defer to what they believe is safe, they want to sell you a bigger motor, or they are just plain lazy and don't want to have to explain it! In all fairness, it may also be a result of having to deal with the "unwashed masses" who would see a 15kW nameplate on a motor and think it is undersized for an 18kW application at a higher voltage and frequency. Nameplate information can be tricky because it involves regulating authorities who are often not engineers (slightly washed masses I suppose), who are leary to allow anything like that to be listed on a nameplate unless someone shows them in black-and-white.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[DickDV]

Kinda makes you wonder just how much of that we could get away with here in North America!!

Basically, I agree with you Marke but at least here I'll stay with mfgr's recommendations unless I'm in a real jam.

 

[Skogsgurra]

Dick,

It is not at all a question of "getting away" with anything. I think that you have misunderstood the difference between running above base speed and running a motor at a higher voltage and frequency.

Take a little 400 Hz motor (airplane application) as an example. Let's say it can produce 1 HP. It does this in a surprisingly small package - all as a result of the high frequency and corresponding high speed. That's why 400 Hz is used on airplanes - everything electric (except heaters and incandescent lamps) can be built much smaller. The power density increases as frequency goes up. That is well known and need not be debated.

The relation between power density and frequency is monotone and close to linear. That means that a motor that is built for 50 Hz can deliver 20 percent more power at 60 Hz. Actually a bit more since cooling gets better at higher speed if the motor is ventilated with a shaft driven fan. Which is the rule with standard motors for machinery.

It would be a good thing if you admit that your advise to oldnukeet is wrong. He has drawn the wrong conclusion: "It appears, Dick, from what you are saying that in fact, going to the lower voltage and frequency as my customer is - that he will actually gain up to 20% torque availability - which is good news."

If we shall keep up the standard of Eng-Tips, we shall not let our advise lead fellow engineers in the wrong directions. In this case, oldnukeet has a problem with motor output when running the motor at 50 Hz. He is worried about it and asks the forum. He leaves the forum with the conviction that he will not have a problem at all - he thinks that he gains 20 percent output from the motor - while the truth is that he loses 20 percet power. Imagine the problems oldnukeet will be having with his overseas customers when it turns out that he didn't gain - but lose - 20 percent output power.

I know that it will hurt. Just take the pill. You will feel a lot better afterwards.

Gunnar Englund

http://www.gke.org

 

[DickDV]

skogsgurra, my post immediately following oldkuneet's statement concerning +20% more torque says "you can expect the same torque", not +20% which oldkuneet acknowledges in the next post. But to be sure:

oldkuneet, the available continuous torque from a motor is constant from base speed down to the speed where cooling is no longer effective. As has been discussed, this constant torque level may even be available above base speed up to the insulation limits of the windings even tho most manufacturers recommend not using overvoltage to acheive it.

My apologies for any misunderstanding on this subject.

There, skogs, does that meet the forum's "engineering standards"?

 

[oldnukeet]

Understood - My problem is that because I don't work with the equations every day I tend to get torque and output power confused. So yes - it is understood that we may not exceed the thermal limits of the motor's ability to dissipate heat caused by the increased "power density" (due to increased Frequency and Voltage) even though, theoretically, we have improved cooling via the increased fan speed on a TEFC (Totally Enclosed Fan Cooled) motor. So, basically, if the OEM of the motor made allowances in the insulation winding resistance and the overall heat capacity of the rotor/stator/frame design then the 380/50 can be safely operated at 460/60. I think that covers it!

 

[Skogsgurra]

Dick,

I am not to judge the standards in these fora. But I think that it is a good thing to let go when there is a need for it. I appreciate your correction a lot more than you probably think.

Gunnar Englund

http://www.gke.org