I have a question about sizing MCCB for motor. If a 22 kW motor has a full load current of 42A and a starting current of 250A, then what should the trip setting of the MCCB be? I can use an MCCB with a frame size of 100A, but not sure about the trip setting. Is there some general rule to select the trip setting (for example 1.4*full-load-current.)
Motor MCCB
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What sort of circuit breaker?
The really decent ones have different settings for thermal and magnetic trip currents. Set the thermal trip current to 42A and the magnetic trip to 250A
The really decent ones have different settings for thermal and magnetic trip currents. Set the thermal trip current to 42A and the magnetic trip to 250A
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That's the whole point. The thermal trip is the longer term average current, so you set that to match the FLC and it will withstand the start up current.
The magnetic trip is near instantaneous, so you set that as low as you can get away with without nuisance tripping at start up. If you say the start current is 250A than that's what it needs to be. Where did your 250A figure come from? measurement or guestimate?
The magnetic trip is near instantaneous, so you set that as low as you can get away with without nuisance tripping at start up. If you say the start current is 250A than that's what it needs to be. Where did your 250A figure come from? measurement or guestimate?
At 22kW SURELY you'd have that on a star/delta starter.
The start current & running current would then be proportional to the winding configuration.
Take the TM overloads used in starters, they will be sized differently for DOL & Y/D starters, so your MCCB selection & settings are not totaly B&W.
You could clamp the LRC of the motor to get an idea of the max current draw?
HTH
The start current & running current would then be proportional to the winding configuration.
Take the TM overloads used in starters, they will be sized differently for DOL & Y/D starters, so your MCCB selection & settings are not totaly B&W.
You could clamp the LRC of the motor to get an idea of the max current draw?
HTH
As above , probably star/delta so start load will be lower . An MCCB is not needed but type C or D . I would guess type C 50amp TP MCB .
Phone Crabtree Techical or similar , they're the experts.
I presume it has to be an MCB ?? HRC fuses are far better in industry ,IMHO.
Phone Crabtree Techical or similar , they're the experts.
I presume it has to be an MCB ?? HRC fuses are far better in industry ,IMHO.
Thinking about this my instinct would be a 50A or possibly a 63A C or D curve MCB.
As Deke says MCCB not necessary, but if you have a panel board that won't take an MCB you may have no choice.
the selection would depend on the mechanical inertia of the load.
A large mechanical inertia, thus a long transition time would need a different supply characteristic to a small mechanical load with a short transition time, and also, if the Y/D is set up correctly this would help.
HRC's can help, you may need gM's though.
To get Zs for gM's you need manuf' data oficially.
Another thing to think about is if it is Y/D is is open or closed transition?
This will have an affect on the change over current.
As Deke says MCCB not necessary, but if you have a panel board that won't take an MCB you may have no choice.
the selection would depend on the mechanical inertia of the load.
A large mechanical inertia, thus a long transition time would need a different supply characteristic to a small mechanical load with a short transition time, and also, if the Y/D is set up correctly this would help.
HRC's can help, you may need gM's though.
To get Zs for gM's you need manuf' data oficially.
Another thing to think about is if it is Y/D is is open or closed transition?
This will have an affect on the change over current.
3
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Guest
If its a star delta starter, the overload markings will relate to direct on line
So set it to motor current x 0.58
So set it to motor current x 0.58
300,
What if you have an O/L which has been supplied "downrated" for Y/D?
Your "formula" applies but, the O/L may not be rated at FLC.
What if you have an O/L which has been supplied "downrated" for Y/D?
Your "formula" applies but, the O/L may not be rated at FLC.
3
300TDI
Guest
Ok
For a star delta overload setting EVEN if the overload has been supplied downrated the answer will still be the same
ie a 40a motor x0.58 = 23.2 amps
so the overload would be set at around 23 amps
so if the overload has been supplied downrated - say 17-25 amps, you would still select 23 amps
If you have been supplied a 30-40a overload, then you cant select 23a
If you have been supplied a 12-18a overload then you cant select 23a
When i buy starters from the wholesaler they
1) select the starter, DOL,Reversing,Star Delta etc
2) select the overload
The overload is in effect the same item for each starter, however getting the correct one is simply the selection of the current rating for the application
They do not have DOL overloads on one shelf and Star Delta overloads on a different shelf - they simply have the one range to pick from
Correct selection is the answer
Hence the x0.58 advice
For a star delta overload setting EVEN if the overload has been supplied downrated the answer will still be the same
ie a 40a motor x0.58 = 23.2 amps
so the overload would be set at around 23 amps
so if the overload has been supplied downrated - say 17-25 amps, you would still select 23 amps
If you have been supplied a 30-40a overload, then you cant select 23a
If you have been supplied a 12-18a overload then you cant select 23a
When i buy starters from the wholesaler they
1) select the starter, DOL,Reversing,Star Delta etc
2) select the overload
The overload is in effect the same item for each starter, however getting the correct one is simply the selection of the current rating for the application
They do not have DOL overloads on one shelf and Star Delta overloads on a different shelf - they simply have the one range to pick from
Correct selection is the answer
Hence the x0.58 advice
300,
22kW FLC 42 - 45A (typically)
Y/D rating (@0.58) 24 - 26A (typically)
Thus my comment applies as I did not contradict your formula, I merely warned that the situation may arise where the O/L may not cover the correct range as one MAY order an O/L rated for DOL and fit to a Y/D starter!
Thus you have merely confirmed my point!
Selection of the O/L device must be correct for the starting mode.
BTW I'd never put a 22kW induction motor on DOL no matter what the external load really!
22kW FLC 42 - 45A (typically)
Y/D rating (@0.58) 24 - 26A (typically)
Thus my comment applies as I did not contradict your formula, I merely warned that the situation may arise where the O/L may not cover the correct range as one MAY order an O/L rated for DOL and fit to a Y/D starter!
Thus you have merely confirmed my point!
Selection of the O/L device must be correct for the starting mode.
BTW I'd never put a 22kW induction motor on DOL no matter what the external load really!
Let me re-phrase my problem:
To avoid confusion, let's take a large motor for which we can only use MCCB (not MCB). let's say 110 kW DOL motor. The Schneider's recommended combination is: magnetic only breaker (NSX250HMA220) plus contactor (LC1F225) plus overload (LRD05.)
The motor FLC is 200A, and let's assume the starting current is 1000A.
Now here is my problem:
How can I set the magnetic trip to 1000A, while the maximum breaker frame size is 250A?
To avoid confusion, let's take a large motor for which we can only use MCCB (not MCB). let's say 110 kW DOL motor. The Schneider's recommended combination is: magnetic only breaker (NSX250HMA220) plus contactor (LC1F225) plus overload (LRD05.)
The motor FLC is 200A, and let's assume the starting current is 1000A.
Now here is my problem:
How can I set the magnetic trip to 1000A, while the maximum breaker frame size is 250A?
Loyal,
First you'll never start a 110kW motor on a DOL starter!
Sorry the Q is flawed.
However, I'll have a go!
Why do you want to set the mag to 1kA?
Your starting current will be very short duration, so yes it will affect the M section rather than the T section of the MCCB.
You may have a max breaker frame of 250A, you wouls set the T & M dials to a proportion of the breaker rating, say 200A breaker you would set the "dials" according to the manuf' & designers specs for the system.
These would depend on many things, motor design specs, breaker design, cables, external motor load, supply characteristics etc. etc. etc.
HTH
First you'll never start a 110kW motor on a DOL starter!
Sorry the Q is flawed.
However, I'll have a go!
Why do you want to set the mag to 1kA?
Your starting current will be very short duration, so yes it will affect the M section rather than the T section of the MCCB.
You may have a max breaker frame of 250A, you wouls set the T & M dials to a proportion of the breaker rating, say 200A breaker you would set the "dials" according to the manuf' & designers specs for the system.
These would depend on many things, motor design specs, breaker design, cables, external motor load, supply characteristics etc. etc. etc.
HTH
3
300TDI
Guest
and that an LRD05 only fits contactors up to 32a, and has a setting of 0.63 - 1.0 A
I know its irrelevent really, just suprised to see that part number pop up in the combination of kit
:coat
I know its irrelevent really, just suprised to see that part number pop up in the combination of kit
:coat
I did think I recognised LRD05, I have one on the van!
Did not spot that though.
Me too, :coat
Did not spot that though.
Me too, :coat
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