Can multiple motors share 1 mcb.

[Apex13]

Hi,
I am installing 24 No 2.2Kw 3Ph motors that will drive cooling fans. The plant room for the install is not what you would call spacious so trying to keep everything as tight as possible within reason.
The motors are all controlled through an automated system which feeds back to an enclosure which will essentially be full of contractors, mcb's, etc.
I am wondering if multiple motors can be supplied from 1 mcb, eg 1 x 20A Mcb supplying 3 motors/contractors with overloads, to allow me to only need space for 8 mcb's instead of 24, or does each individual motor require its own mcb?
Thanks.

 

[pc1966]

It would critically depend on the sequencing of start-up and the type (smooth VFD, star-delta, or DOL). Typically a 2.2kW 400V motor would need something like 16A C-curve or 20A B-curve MCB for start-up, even though running current is of the order of 4.4A, so it has some capacity for two or three motors sharing once running. But if you had the power briefly cycle with all motors running it would probably trip.

Personally I would steer clear of it, it might save some DB space but you are asking for future operational and fault-finding problems.

 

[Apex13]

Hi pc1966
Thanks for your reply.
The motors are all DOL start and can start at any time depending on what the temp and humidity is and if the automated side requires them to start cooling/drying, so unlikely but not impossible for more than 1 to start at the exact same time.

Can I ask how you calculate the running current at 4.4A as I have it at 5.5A and this would change the size of O/L I am looking at.

Can I also ask why choose a B or C curve mcb over a D which is what I would normally use in most motor applications?

I do agree that it would be preferable to have each motor on an individual mcb but just wondered if it was an option, if the size of the cabinet becomes an issue when required to house 24 mcb's.
Thanks

 

[pc1966]

The motors are all DOL start and can start at any time depending on what the temp and humidity is and if the automated side requires them to start cooling/drying, so unlikely but not impossible for more than 1 to start at the exact same time.

OK, that is a touch brutal.

Can I ask how you calculate the running current at 4.4A as I have it at 5.5A and this would change the size of O/L I am looking at.

Just the typical figure from the Hager commercial catalogue for running currents (pages 112 and 113 of current edition):

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I think it works out at about a PF of 0.72 for 2.2kW (3.2A resistive), however, if you have the real values for the specified motors you should use them instead.

Can I also ask why choose a B or C curve mcb over a D which is what I would normally use in most motor applications?

Again, just examples given in the catalogue recommendations. I would not normally go for B-curve as you say, but if you were looking to have higher average for a given max start/. trip for I2t, etc, limits then it is an option to consider.

I do agree that it would be preferable to have each motor on an individual mcb but just wondered if it was an option, if the size of the cabinet becomes an issue when required to house 24 mcb's.

I don't think there is any regulation that says it can't be done, providing fault protection is met and they all have independent isolation switches, etc, so nobody is tempted to work live instead of locking off more than one motor on its MCB. Electrically it comes down to juggling the start surges and running currents with the protection limits for the motors. Overload protection should be down to the starters, so really it is about faults and you might be OK to bo a bit higher on the MCB rating to meet operational needs.

 

[Apex13]

OK, that is a touch brutal.

Just the typical figure from the Hager commercial catalogue for running currents (pages 112 and 113 of current edition):

Hager Brochures, Catalogues & Documentations | Hager UK

Discover all the product documentation you need in our brochures & catalogues! Download what you need in one click straight from Hager Official website!

hager.com

I think it works out at about a PF of 0.72 for 2.2kW (3.2A resistive), however, if you have the real values for the specified motors you should use them instead.

Again, just examples given in the catalogue recommendations. I would not normally go for B-curve as you say, but if you were looking to have higher average for a given max start/. trip for I2t, etc, limits then it is an option to consider.


I don't think there is any regulation that says it can't be done, providing fault protection is met and they all have independent isolation switches, etc, so nobody is tempted to work live instead of locking off more than one motor on its MCB. Electrically it comes down to juggling the start surges and running currents with the protection limits for the motors. Overload protection should be down to the starters, so really it is about faults and you might be OK to bo a bit higher on the MCB rating to meet operational needs.

Thank you for taking the time to reply to all that, much appreciated.

The automatic/random starting of the motors is definitely a bit brutal.

I have just spoke to someone that has suggested i have a look at motor circuit breakers (Schneider GV2ME10) which I believe may cover both bases and also help slightly with the space issue compared to individual mcb and contactor with O/L as I would only need a contactor and motor circuit breaker. These are not something I have used before so just looking into them now but hopefully they do what I need.
Thanks again.

 

[pc1966]

I had a quick look for the GV2ME10 and (as usual) the "datasheet" covered damn-all about it, though Schneider generally do provide good information and support somewhere.

They are only rated to break 3kA so probably would need to be used with a MCB to allow breaking high enough for your panel's PFC/PSCC, but here that might be something like a 50A C-curve per 8 motor relays or similar as that MCB is not needed for motor fault protection, just to save the GV2ME10. For cascaded OCPD there usually is a rating for the breaking capacity of the two cascaded, and usually for that you need the upstream one to be opening on the fault as well (so not "total selectivity" up to the lower one's breaking limit, such as the 3kA here). Schneider do have some on-line tools for their range of MCCB/MCB so maybe this part is also in the choices you can analyse.

It is not obvious if they fit in place of a MCB in the TPN panel, if so it might be acceptable to have a 160A MCCB as the incomer to achieve a high enough breaking limit for the cascade, but that would be a bit worrying if the cascaded is selective to > 3kA (as it might well be with a MCCB of that rating).

 

[ipf]

Seems like a control panel designed by specialists is required. There are plenty of companies who provide such services. Too many 'unknowns' involved, I feel, to comment further.

 

[timbobelfast]

Thank you for taking the time to reply to all that, much appreciated.

The automatic/random starting of the motors is definitely a bit brutal.

I have just spoke to someone that has suggested i have a look at motor circuit breakers (Schneider GV2ME10) which I believe may cover both bases and also help slightly with the space issue compared to individual mcb and contactor with O/L as I would only need a contactor and motor circuit breaker. These are not something I have used before so just looking into them now but hopefully they do what I need.
Thanks again.

I would have separate TP MCBs or fuses for these lot. A bigger panel required, yes. Siemens or Schneider make the combined mccb / overload units.

You would really need to study the automation logic to get a picture of motor running sequence. Likely to be a PLC with Analog inputs closed loop starting and stopping the coils.

With this amount of control gear inside there would also be the need for adequate cooling within the panel as well. Best to consult a panel builder who uses Eplan or similar.

 

[WallaceP]

I would take 1 power supply from your db to a motor control panel. Then have the motor overloads in this panel with the contactors. Ideally leaving that design for someone else.
As far as starting and stopping motors the automation engineer should factor this into his control logic.

 

[Darkwood]

A small plc would suit ideally, simple program and we even have 1 modular sized micro plc's that could effectively progressively start 3 motors from 1 signal command, incorporating all the motors into a single control box would be more efficient utilising a budget plc to monitor each sensor and start the relevent motors as needed with timing solutions to stop mcb trips.

 

[pc1966]

As @Darkwood suggests if you can incorporate some overall sequencing logic that prevents multiple motors from starting at once then most of the protection issues are easier to deal with.

I'm guessing if the are all DOL then there is no expectation of rapid sequencing or very fine control and so the logic could almost be a second-by-second polling of the sensors and decision on motor action so worst case you get all sensors lighting at one and followed by a 2 second period as the 3 motors (for example, might be a few more per PLC depending on motor overload and MCB decisions) go in turn. Of course, that sort of information should be in the specifications already!