Economical selection of 90KW Motor Starter Components 3

[luispereiraenvc]

Dear All,

I'm new in this forum, but I would like to have your opinions related to the following Motor Starter components selection.

Usually, when we need to dimension the contactors, circuit breakers, cables and so on, for a certain Pump, we consider the nominal power of the electric motor of such pump.
Let's see an example; for a Sea Water pump of 100m3/h, the manufacture provided the following information:

Pump Data
Capacity: 100m3/h
Power Consumption: 50,43KW
Non-Overload Power: 69,43KW

Motor Data
Power Supply: 3x440V/60Hz
Nominal Power: 90KW

So, we take the 90KW, as the manufacture didn't provided the Nominal Current we calculate it based on cos(phi)=0,90, so 130Amp.
Then we select the Overload Protection to be in the range of 130Amp.
The Contactors also (depending on the starting method), the cables from the starter to the motor, and from the Distribution Switchboard to the starter also consider 130Amp, and the Circuit Breaker on the Distribution Switchboard also!

Now with Economical concern on mind, my question is; the power consumption imposed by the pump will be in normal operation 50KW (and in the worst case 70KW if we double the Flow to 200m3/h), can we consider for the Calculations 70KW instead of 90KW? 20KW in cables can be a considerable amount of money! It can also allow the reduction of contactors and Circuit breakers range.
You can ask why the motor is 90KW instead of something close to 70KW. Well... the pump manufacture have some fixed range of Electric motors for that frame of pumps... and sometimes that can lead to such differences...

My first oppinion is that if we consider the cables to 70KW, the protections will have to be set to 70KW also, and an abnormal fault on the pump (bearings for example) will be protected by the Overload protection of 70KW. However if we set it to 90KW, some mechanical problem in the pump will be supported by the 20KW gap. That could be a problem!?!

What's your oppinion?
Did you know any regulation that can support this or other criteria?

Thank you for your support!
Luis Pereira

 

[DRWeig]

Hi luispereiraenvc,

Where in the world will this be applied?

In the US, you have to base everything on the full-load current of the motor. Estimating based on load KW and power factor is not allowed. There is even a table in the NEC from which full-load currents can be picked. You can't reduce conductor size based on expected load, either.

Let us know the location, perhaps someone here knows the relevant codes for you.

Good on ya,

Goober Dave

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[jraef]

As Goober Dave said, you cannot size the control and protection components for the expected load on the motor, you must size them for the motor maximum capabilities. This is because of two things:

1) an AC motor is a "dumb animal" so to speak. It has no knowledge of what it is supposed to do, it will try to do what it is capable of doing and destroy itself in the process. So let's take for example your pump; the pump impeller and pipe and head design is such that you will only use 69.43kW worst case. But then let's say that your pipe breaks and suddenly there is NO head and NO pipe restriction to the flow, so your 90kW motor is going to pump out 90kW (or more). If you are lucky, your overload relay will trip in time to prevent the 70kW contactor from welding, but if not, then the relay trips but the contactor fails to open and the motor catches on fire.

2) there are a number of factors that go into the design of the conductive elements in that contactor (and breaker) which will not be directly dependent on the motor load but rather on the motor design. So if you have a 90kW rated motor, you are required to use a contactor that is rated for 90kW, even if the load is only going to use 70kW. Otherwise if you used a smaller contactor, the inrush current of the 90kW motor may be so high as to weld the contacts and you again can eventually cause a fire.

There is no economy in designing for destruction.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[luispereiraenvc]

Thanks DRweig and jraef.

I’m on Europe. Portugal, to be more specific.
This pump is from a Vessel we are building to Venezuela, so 440V/60Hz instead of 400V/50Hz.
Usually we base our calculations in the Nominal Power of the Motor. When the Pump manufacturer don’t provide the Full Load Current, we calculate it from Nominal Power. For this motor size, cos(phi) will be close to 0,9 instead of 0,85 as I previous said.
P = sqr(3).U.I.cos(phi)
There’s any problem with that?
In this case the manufacturer already provided the current (128Amp). That's cos(phi) = 0.92. If we have used 0,9 we would be with 131Amp instead of 128Amp. No big deal, don’t you agree?

My curiosity about this case was triggered by the fact that we request a pump for 100m3/h, 12bar, and the manufacturer provided a pump that will need 50KW for that, but if we break the pipes (“NO head and NO pipe restriction to the flow”) it will need 70KW for 210m3/h, 4bar. So an oversized pump, with an oversized motor of 90KW.
I ask them why they don’t provide an Motor close to 70KW, and they said that only exist 65KW and 90KW motors for this IEC frame sizes, 440V/60Hz, 2 poles, 3500rpm! Quite strange for me…

Jraef, in your answer can you please clarify the following;

1) an AC motor is a "dumb animal" so to speak. It has no knowledge of what it is supposed to do, it will try to do what it is capable of doing and destroy itself in the process. So let's take for example your pump; the pump impeller and pipe and head design is such that you will only use 69.43kW worst case. But then let's say that your pipe breaks and suddenly there is NO head and NO pipe restriction to the flow, so your 90kW motor is going to pump out 90kW (or more). If you are lucky, your overload relay will trip in time to prevent the 70kW contactor from welding, but if not, then the relay trips but the contactor fails to open and the motor catches on fire.

According Pump Manufacturer, “for NO head and NO pipe restriction to the flow” the pump will need 70KW. I will try to attach the diagram. I think that the only way the Motor can be loaded above 70KW is due to mechanical failures in the motor of the pump itself. Bearings and so one. Protection of such failures in an initial state can’t be a good thing?
Usually on the starters we have the overload protection and also a Circuit Breaker on the input. Together with the CB from the distribution switchboard didn’t you think will be enough?

2) there are a number of factors that go into the design of the conductive elements in that contactor (and breaker) which will not be directly dependent on the motor load but rather on the motor design. So if you have a 90kW rated motor, you are required to use a contactor that is rated for 90kW, even if the load is only going to use 70kW. Otherwise if you used a smaller contactor, the inrush current of the 90kW motor may be so high as to weld the contacts and you again can eventually cause a fire.

It is wrong to assume that if the motor will be loaded with 50 or 70KW, it will have almost the same behavior as an 70KW motor at full load (concerning inrush currents)?
Anyway, even if we size the Contactors for 90KW, and circuit Breaker for 90KW but adjusted to 70KW, can’t we save money in the cables?

Thank you all!

Pump Characteristics

 

[waross]

True story. An engineer with a PhD in engineering spec'ed a 200 HP starter for a 400 HP motor based on his incomplete testing of a 200 HP starter. He was running a wound rotor motor with low starting current. One test that he neglected to perform was interrupting running current at full load on a humid day.
When the smoke cleared and the shrapnel stopped flying we had a 400 HP motor with a bent shaft.
We had a large fan with a bent shaft.
The coupling was distributed in pieces throughout the machine room.
When the contactor could not interrupt the current, the arcs joined up and shorted the motor terminals. This shorted the back EMF similar to a short on a generator. The motor de-cellerated so fast that the coupling exploded.
In the US, you would use the motor current published in the NEC. In Canada, you would use the nameplate motor current.

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

 

[DRWeig]

luispereiraenvc,

If the pump manufacturer won't provide you with the nameplate FLA of the motor, and you don't have enough info about the motor to select its full load current from a table in your local electrical code, you can't specify the motor circuit. You must have that information. Calculating is NOT a good idea, especially when your result is a load less than the motor's full-load capability. You're asking for a fire.

Get the data first. If it were me, and the manufacturer refused to supply the data on the motor's nameplate, I'd simply wait until the motor showed up on site and read it.


Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[LionelHutz]

I agree with the others. You need to follow the electrical codes and electrical equipment standards that apply for your region. These codes and standards weren't arbitrarily created and there are good reasons for the rules they spell out.

In North America the standards followed are typically CSA, UL and ANSI but can also be local state or provincial or city electrical codes. In Europe, I would expect you have some CE standards as well as local electrical codes you should be following.

You really should not be doing that kind of work if you are not familiar with the standards the equipment needs to meet.

 

[luispereiraenvc]

Hi,

I’m not sure if it is clear to everybody, but I’m just raising questions triggered in my mind by this particular case with such big differences in pump and motor selection. I’m not planning to proceed with it!

First objective is to know the reference publication in Europe concerning this matter! An IEC for sure! Probably “60034-1” or “60034-12”, but I would like someone to confirm that.

Second objective, is to obtain your opinion in the failures I expect with the “20KW gap” between maximum pump load in defective conditions, and motor nominal power. Won't it be better for the pump if we for example adjust the thermal relay for 70KW?!?

Meanwhile some other concerns; the calculation I show above give me a very precise value of current consumption (for this squirrel cage induction motor).
P = sqr(3).U.I.cos(phi)
P – Nominal Power
I – Full Load Current

I don’t know the NEC tables you are indicating for US, but is a table more precise that the above formula?!?

DRWeig, Yes, usually when we ask the manufacturer provide the full load current, as it did for this case.
But concerning the “20KW gap”, did you experience some situation similar to this?

Kind Regards

 

[waross]

Take a look at the Cowern papers.

http://www.baldor.com/support/literature_load.asp?LitNumber=PR2525

This will help you with some of the basic motor information that is missing from your personal information base.
#1 There are a lot of good reasons to size protection and control gear to the motor rating regardless of the expected load.
#2 Motor current is not linear with motor loading due to the reactive components of the motor currents. Setting the overloads below the manufacturers recommended setting may result in nuisance trips when starting. These trips may only happen during periods of lower than normal voltage making trouble shooting challenging.
#3 Don't confuse precision with accuracy. You are probably not aware that the PF changes with loading, but not linearly. The wrong answer taken to four significant figures may be precise but it is still wrong. As well as an incorrect assumption of the power factor, I don't see motor efficiency mentioned in your equation. (But I was not motivated to look too diligently).

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

 

[Drivesrock]

There may have been other reasons for rating the motor at 90kW - such as altitude / height above sea-level (probably not an issue with this being installed in a vessel!) and ambient temperature (ventilation) in the pump room. Perhaps even voltage drops - there will be a tolerance from the generating system to consider and this motor with, say -10% voltage, would perhaps still be required deliver what is expected by the pump.
Also, is this pump for something critical?

Perhaps your cable lengths are not so long to make much difference in the cost? Remember also that cable sizes are determined by how they are laid - for their heat dissipation. So bigger section cables may be required anyway.

I didn't see any motor efficiency used in the motor FLA calcs?

I don't see any problem setting the motor overload protection for the real load.

 

[DRWeig]

I think that setting the overloads lower than required might be valuable if your pumping process needs to be shut down in the case of overload for quality reasons, but it's really hard to do --- waross discussed this above pretty thoroughly. It's very iffy to use the thermals to detect, for example, a pumping overflow condition that would cause your 70KW to become something higher. Remember, too, that thermal overloads include a significant time delay.

If a downstream pipe breaks open, your pump power will increase. However, there are better and more reliable ways to sense this occurence and deal with it quickly. A pressure switch, flow monitor, motor power monitor, or other device to detect a process problem is preferable in my opinion.

Remember, the overloads are to protect the motor and its feeder or branch circuit, not the pump and process. You'll find that the requirement for overloads is in a fire safety standard, not a process safety standard. If you want to limit your pumping power with the overloads, I suppose it can be done. I just wouldn't do it myself.





Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[luispereiraenvc]

Hummm, it looks that my classification of (Marine/Ocean) is not well accepted in this forum!
In fact it should be Electrical, I didn’t realized that when I subscribed this forum.

Thanks for the documentation, waross. And for the example you described in the 1st post. It makes me remember a supplier we had in the past. N@i@d Dyn@mics, former VT Maritime Dyn@mics. Fortunately nothing exploded because we made changes on the equipment supplied.
Sorry if I violated some forum rules…
#1 – I agree.
#2 – I agree.
#3 – I’m perfectly aware that PF changes with the load, but I’m applying the formula for Full Load/Nominal Power. That’s why I’m not concerned with efficiency. For new Squirrel Cage Induction Motors, the PF is more less the same with any manufacturer. Probably we are using different terms for the same thing?!? Or still something missing from my side.

Thanks Drivesrock,
The manufacturer already stated that they selected 90KW simply because that’s the next step they have above 70KW for that frame size… :/
The pump is installed almost at sea level, it is for a simple service. Cleaning the tanks with Sea Water when in harbor!
Voltage drop is not a concern because it according regulations it must be below 5%. And the pump is for an unessential service.

Concerning efficiency, that “P – Nominal Power” is Electrical Power Consumption! Not horse power or mechanical Power in KW! The formula I used is pure Electrical. It’s the Power formula for 3 phase electrical systems. No mechanical to electrical conversion! If the motor is consuming 90KW at full load, the current in a 3phase system of 440V, the current will be 130Amp for this motor that have PF=0,9 (at full Load). For this motor size the PF varies 0,01 to 0,02 between different manufactures.
Maybe that’s why I’m raising so many confusions.

I don't see any problem setting the motor overload protection for the real load.

I agree, but unfortunately we know that real load will be 50KW and the motor is of 90KW. 70KW is the maximum that the pump will be able to take from the motor under ANY circunstanc, even with broken pipes.

DRWeig, thanks for your answer too. I must agree with your arguments. Economical concerns is the only argument it’s still possible, but that’s better to put pressure in the manufacturer to downsize the motor instead of adapt the rest of the system!

 

[LionelHutz]

Why are you making a big deal about your classification?

You should learn a little about overload relays. An overload relay is designed to protect an electric motor against being overloaded. An overload relay has a trip curve. The trip time varies a lot depending on the level of overload. You can't set-up a hard limit in an overload, such as expecting it to trip immediately if the current goes above 110A of current. Simply put, there are better pieces of equipment to use to protect a process.

You're not raising any confusions about your calculations. Everyone that has posted answers knows how to calculate the current of a motor. You calculations are incomplete which makes them wrong. If you post wrong info or the wrong calculation at this site you will be told it is wrong.

That 90kW motor running at full load produces 90kW of mechanical power at the shaft. It does not consume 90kW of electrical power at the terminals. You must use voltage, efficiency and power factor to calculate the rated motor current. So, your explanation of why or how you used 90kW to calculate the the current does not magically make the calculation correct. It's still wrong.

You calculation can be no more than a rough estimate of the current. You must either be given the rated current or be given the rated voltage, efficency and power factor of the motor to calculate the rated current. Any calculation where you make an assumption or guess about any of these values will not give you the real rated current.

 

[luispereiraenvc]

Hi LionelHutz,

That’s because some answers are to explain me how overload relays, circuit breakers or contactors work.
Indeed, what I need to learn is how pump manufacturer provide info about its pumps and attached Motors!
Previously what I was looking for was:

First objective is to know the reference publication in Europe concerning this matter! An IEC for sure! Probably "60034-1" or "60034-12", but I would like someone to confirm that.
Second objective, is to obtain your opinion in the failures I expect with the "20KW gap" between maximum pump load in defective conditions, and motor nominal power. Won't it be better for the pump if we for example adjust the thermal relay for 70KW?!?

Anyway, I was looking in previous constructions, and fortunately just in some rare cases they didn’t provided Rated current, but only “Electrical Power”. That’s why I was insisting in Electrical Power formula!
I read somewhere that in US usually the motors are rated in horse-power instead of KW. With horse-power it is clear power on the shaft, and you in US clearly distinguish power on the shaft from electrical Power consumption.
But that’s not so clear for us (EU) when we are talking about KW and when a manufacturer only provides “Electrical Power”. And that was probably the reason of previous discussion!

Thanks to everyone.
I already have the answer to second point, but still need some support to know the correct IEC in force about this matter.

 

[ScottyUK]

From IEC 60034-1,

"The rated output is the mechanical power available at the shaft and shall be expressed in watts (W)."

Not sure if that answers your question?


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Drivesrock]

And of course the pump supplier could be hedging his bets by over rating the motor just in case his calcs are wrong and he needs to change his pump or the impellor. That is an internal cost to him and he doesn't need to buy in a bigger motor, change the baseplate get a higher rated starter, cables etc etc etc.

 

[LionelHutz]

You should look at the details of your mechanical system (pump and piping and other related parts) and the overload trip curve and then you decide if setting the overload to a lower value will provide any extra usable protection for the mechanical system.

No one here knows anything about your pump system beyond what you have already posted. The little detail posted is not enough for anyone to know if setting the overloads lower will give any extra useable protection for your system.

Maybe you want a general answer. I wouldn't bother setting the overload lower than the rated current because I doubt it would do anything useful.

As for the IEC standards, I have no idea off hand because I don't deal with that part of the world very much.

 

[luispereiraenvc]

ScottyUK, is that IEC 60034-1 related only to Electrical Motors or also to Starters, Starting methods, etc.?!?

LionelHutz,
Yes, I was expecting a general answer. This pump was just an example that triggered my curiosity because of such big diferences between predicted Power consumption (50KW), Max Power Consumption (70KW), and Motor Nominal Power (90KW).

 

[LionelHutz]

http://lmgtfy.com/?q=IEC+60034-1

 

[luispereiraenvc]

ah ah!
IEC's are well paid publications! You are only able to see a preview.
Sometimes you can found complete publications but they are replaced/withdraw versions.
That's why I'm asking that. Expecting some one is using it and that let me know.
By the way, 60034-1 is only related to motors construction.
Initial question is still unanswered!