440V motor running on 415V? 1

[nifrabar]

Hi,

We are having a 60Hz mains on board a ship.
Management wants 60 Hz as this gives more power than 50Hz.
Now they want 3x415V mains which is no standard for 60Hz.
The idea is to have 440V-60Hz motors run on the 415-60 mains.
Meanwhile no additional transformers are required for lighting purpose as this might work well on 240V (phase-voltage).
I'm not happy with such a design.
Any comments from the professionals overhere?

TIA
Bart

 

[waross]

The standard 60 Hz motor voltage is 460 V.
At one time it was standard practice to run 230V rated motors on 208 Volts. Millions of motors ran for normal lifetimes under those conditions.
Times change. New design motors tend to fail when 230 V motors are used on 208 volts. The motors have less tolerance for under voltages and the loads tend to be more closely matched to the motors.
We now favor 200 Volt or 208 Volt rated motors for 208 Volt service.
Your 415/240 Volt system is the same ratio as 208/120 Volts and many of our 230 Volt motors are also rated for 460 Volts.
Bottom line:
Most of your motors will work happily, BUT.
Motors driving high inertia loads or loads that require high breakaway torques may either fail to start or eventually fail from starting stresses.
Any motors that are loaded up to 100% of their HP ratings will probably fail from overloading.
Both problems may be solved by over sizing the motors.
Management should be aware that although the system will probably work well, some motors may need to be up-sized and some equipment with integral motors that are not easily changed may have to have boost transformers added.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

"...Management wants 60 Hz as this gives more power than 50Hz...."

Well, that is a gross over simplification! Technically it may be true in some cases, but in some cases is will also INCREASE the amount of power that the load needs by MORE than the motor is capable of delivering! It just isn't that simple.*

In general though, your 415V motors will probably be fine with 480V 60Hz, but 440V may be too far out of tolerance. You need to ensure that the V/Hz ratio remains the same, +-10%. For NEMA design motors, the design voltage is 460V 60Hz which makes them OK for 440-480V no problem, so normally we don't worry about the distinction between 440V and 480V. But in this case, if the motor is truly designed for 415V 50Hz it needs a V/Hz ratio of 8.3:1 but if you give it 440V 60Hz, that is only 7.33:1, 12% low which is outside of a normal motor voltage tolerance. Add to that the possible increase in load on all your centrifugal loads and you could be in trouble.

Were you just being inaccurate when you said 440V and it really is 480V? Or are the motors actually designed for 380V 50Hz and you were just using them on 415V? Either of those situations would solve the tolerance issue, leaving you with just the centrifugal load problem.

* That problem works like this:
In centrifugal loads (quadratic loads), the power consumed, and thus REQUIRED by the load varies with the cube of the speed of the motor. So when going from 50Hz to 60Hz, the load will require 173% more power (120%³). But by increasing the speed from 50-60Hz, you only increase the motor power to 120% of what it was. You now have a 50+% deficit of motor power. So you either restrict the flow, add a VFD or use a bigger motor, none of which accomplishes the perceived "increase in power" stated as the reason for the frequency change.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[rhatcher]

"...Management wants 60 Hz as this gives more power than 50Hz...."Well, that is a gross over simplification!

Good answer (lps) jraef. Everything you said is absolutely correct.

I will take a simpler approach. It sounds like nifrabar is still in the design stage for a new vessel or for the refit of an older vessel. It is not clear what will be installed or replaced; new generators?, new motors?, new pumps?, new switchboard..etc

My answer for nifrabar is this:

Your management is looking for a 'magic' solution that does not exist. There is absolutely no benefit to be gained from mixing equipment of different frequency or voltage ratings to result in 'greater power output' from the same amount of power input.

If you do manage to somehow match the different ratings up, the result will be a very expensive design because the mismatched ratings will result in oversized motors, motor starters, motor conductors, circuit breakers, switchboards, etc. jraef explaind why this is true.

Finally, and I think most important: you will never find a port or docking facilty that can supply 415V-60hz power for a shore power connection.

If you cannot convince your management that there is no magic, perhaps they will reconsider when they realize that there is no port...

The proper design approach would be to pick a voltage and frequency based on the region that the vessel will be operating in. That way the shore power supply will be correct and any replacement parts that are ever needed (motors, starters, etc) can be supplied from local sources at the proper voltage/frequency rating.

 

[waross]

Did I read that backwards?
I read it as using 460 Volt motors on 415 Volts. 240 Volts single phase lighting circuits.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[nifrabar]

Hi to all,
Thanks for your time so far.
First I have to inform that I'm from Holland/Europe.
Here we have to deal with IEC-regulations (I uploaded a doc about that).
IEC states that for 60Hz only 230V or 440V is allowed.
What is happening now:
Management wants to buy motors/equipment for the (normalized) 440V-60Hz voltage.
They want to built new vessels with boardnet 415V-60Hz (non IEC standard). The vessels will sail everywhere around the world.
The 440V-60Hz equipment has to run on the 415-60 mains.
That way they skip the light-transformers as phase-voltage@415V = 240V. But indeed Volt / Hz will change and as far to my knowledge therefore I need to oversize motors as they will get to hot and also torque will be less. How do I calculate required kW of the motor (440V running on 415V) in this case?
In fact I am looking for technical reasons to change their mind.

-Bart

 

[jraef]

LOL Bill, I think I am the one who misread it!

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Amps will be 460 V/415 V or 110.8%
OR derate the motors 1/1.108 = 0.9 or 90%.
Note: Loads do not come in nice specific sizes to match standard motor sizes. Many motors will loaded to less than 90% and will work fine.
Then look at starting. Starting torque will be about 81% at the lower voltage. Most motors will start but some may need to be up-sized.
Consider a standard voltage and lighting transformers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ccjersey]

There must be some kind of system in common use to solve this problem. I would think lighting transformers would be a small issue compared to the other essential loads onboard.

What I see is the builder should pick either one frequency (50 or 60 hz and the standard 3 phase voltages that go with it to build the ship. When shore power is the wrong frequency, even if the motors are sized to account for undervoltage or volts/hz ratio is good, either things run too fast and overload or too slow and don't do the work required. At least for centrifugal pumps, throttling valves could be used and/or VFD's on individual motors.

Large motor/generator set?
Not use shore power if it's wrong frequency? (I think some ports would require use of shore power)