3Phase Motor cable calc.

Tuco88

I am looking for a little advise on this, its been a long time before I did any design work so I am a little unsure on the motor currents.

The setup is a 55KW 3Phase motor for a chilling process, The name place has been damaged a while back so we are awaiting more info at some stage from the factory on specs. It looks to be a PF of 0.85. The Run is 90m on ladder rack.

I worked out the Full load current to be = 55000/(400 x Squrt of 3 x 0.85) I got IL = 93.3 Amps, Is this amps per Phase?

excluding Eff and correction factors for now.

Does this go into the Mv/A/m as the Amps. I am worried because its a star delta motor, and if this is Star current I don't want it to be tripping out when running in Delta. ILine = IPhase in star?

Its SWA/XLPE multicore, was using method E on page 275 Regs.

Thanks for any info.

I am just looking for a ball park figure so I can plan out switch-gear and so forth.

Par1

You are correct with the calculation, i also make it 93.39 @ 400v but be careful because its 98.3 @ 380 so measure the line - line voltage first.

The 55 kw is over the 3 phases so its roughly 18.3A per phase at max torque so that should settle when running (delta) but expect it to pull that on startup (star) depending on weight/load etc

Dont forget star delta starting current is 3 x FLC so allow for that on fusing set-up (characteristics)

You will have volt drop starting (vs) and also running volt drop (vr)

I assume from above that your starter is located beside motor or very close if not that distance will have to be taken into account

Measure VL-VL as i said first then you can calculate current correctly

Ambient temperature will be a factor also so make sure you also get that whilst you are there

Best of luck with it

Mississippi.

Tuco88 wrote: »

I worked out the Full load current to be = 55000/(400 x Squrt of 3 x 0.85) I got IL = 93.3 Amps, Is this amps per Phase?

.

Yes that is amps per phase. Each phase will have a ballpark figure of 100 amps at F.L.C.

frankmul

Par1 wrote: »

The 55 kw is over the 3 phases so its roughly 18.3A per phase at max torque so that should settle when running (delta) but expect it to pull that on startup (star) depending on weight/load etc

Par1, could you explain this a bit more?

Par1

frankmul wrote: »

Par1, could you explain this a bit more?

55Kw/3 is 18.3Kw per phase...correct? I should of stated start-up torque as opposed to max torque but generally torque is at the higher end on start-up (unless additional weight is added further on of course)

Star Delta is usually referred to as 'Soft Starting' therefore it starts off in Star then when it gets close to normal running speed the motor connects to delta.

A major disadvantage to star - delta starting in my opinion is a reduction in starting torque. Therefore if a belt is unladen then torque is generally not an issue however add weight to the belt (for whatever reason) then more torque is needed especially on start-up. The reason for this is the phase voltage on the motor - delta connection is 380v (ish) whilst star is 220v (ish).

Star voltage VL = Sq root 3 x Vp
Delta voltage VL = Vp

Star current IL = Ip
Delta current lL = Sq root 3 x Ip


I have came across instances where a 'Stall' condition can exist (due to low torque) because the motor cannot accelerate because it does not have sufficient torque to overcome the load requirements. In this case the detection system set-up was actually able to detect the 'Stall' condition and trip the motor (avoiding damage).

I found that low torque can also increase Run up time which can be a factor also. I would always consider torque from the start because generally motors are not cheap so i avoid damaging them if i can

2011

Tuco88 wrote: »

Does this go into the Mv/A/m as the Amps.

mV per amp per meter is used to calculate the volt drop.

I am worried because its a star delta motor, and if this is Star current I don't want it to be tripping out when running in Delta.

Can you confirm that you are using a star delta starter?
The reason that I ask is that this is not used very much anymore.

If this has a star delta starter the loaction of the starter in relation to the motor is important beacuse both ends of the motor windings have to be wired to the starter (all 6 cores).

2011

Tuco88 wrote: »

The setup is a 55KW 3Phase motor

Be careful of this.
Does the motor have a mechanical output of 55kW at the shaft?
If it does then the electrical input will be about 10% more.
Contact the manufacturer for the efficiency and work backwards.

This motor selection guide link provides motor output in kW:


Do you know the make and model of the motor?
http://www.brookcrompton.com/pdf-files/20122E_Product Guide.pdf

Bruthal

Par1 wrote: »

You are correct with the calculation, i also make it 93.39 @ 400v but be careful because its 98.3 @ 380 so measure the line - line voltage first.

The 55 kw is over the 3 phases so its roughly 18.3A per phase

Its 93 amps per phase.

Par1 wrote: »

55Kw/3 is 18.3Kw per phase...correct?

It is 18kw per winding, but you have each line connecting into 2 windings in delta, not simply an 18kw winding connected to 2 phases.

Bruthal

2011 wrote: »

If this has a star delta starter the loaction of the starter in relation to the motor is important beacuse both ends of the motor windings have to be wired to the starter (all 6 cores).

Shouldnt make as much difference as would be expected if cable size is what you mean, as in delta, the 6 cores are doubled up compared to star. As in the 6 cores are in effect 2 sets of paralleled cables. In star, only 3 of the 6 cables are supplying the motor.

2011

Bruthal wrote: »

Shouldnt make as much difference as would be expected if cable size is what you mean, as in delta, the 6 cores are doubled up compared to star. As in the 6 cores are in effect 2 sets of paralleled cables. In star, only 3 of the 6 cables are supplying the motor.

Well the supply cable is 90m so the starter could be anything up to 90m from the motor. Therefore up to 180m of 3 core SWA may be required.

Bruthal

2011 wrote: »

Well the supply cable is 90m so the starter could be anything up to 90m from the motor. Therefore up to 180m of 3 core SWA may be required.

O yea you were talking about having the starter beside it to save cable.

2011

Bruthal wrote: »

O yea you were talking about having the starter beside it to save cable.

I'm just saying that the starter location is very important. Perhaps a cable to a starter local to the motor is an option. I don't know the arrangement. One way or the other the cabling will be expensive.

Bruthal

2011 wrote: »

I'm just saying that the starter location is very important. Perhaps a cable to a starter local to the motor is an option. I don't know the arrangement. One way or the other the cabling will be expensive.

Yea the closer the starter is to the motor, the less cable needed for the power side of things for sure.

Tuco88

Hi lads, thanks for all the Info and the fast respond. Its getting to be a bit painful now ha. The control panel will be between no more than 10m from the Motor its final location is to be decided tomorrow. Its 400v Line Voltage on the button, I have a feeling I might be looking at two runs of 35mm^2? I did not hear a word on the motor info yet there chasing down the previous owner of the plant id say its a waste of time.

55kw is down on the Input from what I can make out, PF looks to be 0.85, the made good use of a forklift when moving the motor to make life very hard for me. Its green in color id back money its of English origin. We tested all the winding and its in good shape.I am a still worried on the Delta current tho. I have 93 Amps FLC (lets say for now) its sure to say with all the correction factors and so on it will be greater. Do I take the 93 Amps for my Mv/A/M cable calc ? or do I need to look at Delta current of (I)L Sqrt 3 so 161Amps? Should I be sizing a cable for this or is this just Protection characteristics to be taking into consideration?

My understanding is than it will start on (soft start Phase voltage) Star 1/3 (I)L then by the use of a timer, change over to Delta receiving full Amps/High Torque output.

Its driving a big feck off cooling Fan by belt, It was driving this up til about 4 years ago (there is now a new switch room and plant layout)

What id do to go back to something with a VSD again!

Thanks again.

2011

Tuco88 wrote: »

The control panel will be between no more than 10m from the Motor

Does this contain the star delta starter ?
Or is it just the control station ?
Is it 55kW when connected in delta?
If so then the delta current is 93 amps under full load conditions.

Do I take the 93 Amps for my Mv/A/M cable calc ?

Yes, for the cable feeding the starter. After the starter it will be ½ of this per cable because two cables will be feeding the delta load.


By the way this is an example of a soft starter. It is very different from a star delta starter.

Par1

Is the 55Kw the rated power of the motor?.

Tuco88

Yes 55 kw is what it's rated. It's going to be mains out to the control panel and two legs to the motor. Am I correct in saying the the overload is to go under the main contactor of the "delta loop" set to around 55% of the f.l.c was my idea.

2011

Tuco88 wrote: »

Yes 55 kw is what it's rated.

For a delta connection or is it 55 for a star connection?
Your supply voltage is fixed so each configuration will provide a different power output.

If star provided the same output there would be no point in switching to delta.

Am I correct in saying the the overload is to go under the main contactor of the "delta loop" set to around 55% of the f.l.c was my idea.

From memory overloads are set at 0.58 of the full load current.

Hoagy

The overload is normally fitted below the delta contactor.
You will often find that they have done the 55% calc for you if it's a purpose made ASD starter, so the scale may already be FLC.
With a fan of that size there will be a long run up time in star and if a phase drops the starter won't protect it, so I'd be fitting a phase failure relay for peace of mind.

Hoagy

2011 wrote: »

For a delta connection or is it 55 for a star connection?
Your supply voltage is fixed so each configuration will provide a different power output.

If star provided the same output there would be no point in switching to delta.




From memory overloads are set at 0.58 of the full load current.

The motor rating is always for a delta connection. Running it in star just limits the currents while it's spinning up.

2011

Hoagy wrote: »

The motor rating is always for a delta connection.

Always good to check.
I have seen plenty of motors rated for the same output value in star & delta but for 2 different line voltages (for example 400 volts in star and 690 volts in delta).

2011

A very good link here. This states that the overloads should be set to 0.58 of full load current.

Hoagy

2011 wrote: »

A very good link here. This states that the overloads should be set to 0.58 of full load current.

Good link.

I take your point about the overload, also I see they are fitting it under the line contactor.
The difficulty I've had with that is tripping just after a long run-up.

Hoagy

double post.

2011

Hoagy wrote: »

also I see they are fitting it under the line contactor.
The difficulty I've had with that is tripping just after a long run-up.

What do you mean?

Hoagy

2011 wrote: »

What do you mean?

If the overload is fitted under the line contactor then it carries line current during the star period even though it is set for 0.58 of FLC.

If the run up is long enough it may trip while still in star, but more often it may trip just after the changeover into delta.

The simplest solution is fit the overload to the delta contactor so it doesn't carry the star current. but there is no protection during the star period.

I suppose the ideal solution is to have two overload relays, one on the line contactor set to FLC and one on the delta contactor set to 0.58 of FLC.

If there's room.

2011

Hoagy wrote: »

If the overload is fitted under the line contactor then it carries line current during the star period even though it is set for 0.58 of FLC.

If the run up is long enough it may trip while still in star, but more often it may trip just after the changeover into delta.

Due to the reduced current drawn in star configuration I don't see why the delta overload would trip. From the above link:

For normal star-delta starting, the controlgear must be sized in accordance with the following currents:
• Main contactor K1M 0.58 Ie
• Delta contactor K2M 0.58 Ie
• Star contactor K3M 0.34 Ie

I would expect to have to adjust overload settings during commissioning. The above are guidelines to ensure that overloads with the correct ranges are selected. A grip on ammeter would be invaluable. I think that overlad settings would be revised downwards as motors are generally oversized.

frankmul

Reduced starting current but it still is 2-3 times the delta running current.

2011

frankmul wrote: »

Reduced starting current but it still is 2-3 times the delta running current.

Properly selected overloads and fuses are designed to take account of starting currents.

frankmul

2011 wrote: »

Properly selected overloads and fuses are designed to take account of starting currents.

I agree, standard overloads can cause an issue on long run up period.

Bruthal

2011 wrote: »

Due to the reduced current drawn in star configuration I don't see why the delta overload would trip. From the above link:.

Star starting draws 1/3 of delta starting while rotor is stopped I.e full starting current at instant of starting.

If motor start current is 6 times running, then star starting might be 2 times delta fla.

Overload will have some time delay. But if the rotor is slow to get up to speed, the overland can trip. Even if doesn't while still in star, it still can just after changing to delta.