IET logo
 
IET
Decrease font size
Increase font size
Topic Title: TN-S Mobile test rig elecitrcal design - 3ph & SPh
Topic Summary: Design and installation of electrical supplies for a test rig
Created On: 25 April 2014 11:52 AM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 25 April 2014 11:52 AM
User is offline View Users Profile Print this message



ajaallen

Posts: 6
Joined: 18 March 2009

Hi all,

This is my first post (and first time designing one of these systems!) so please go easy on me!

The basic spec was to design an electrical system that would be fed from a 5 pole (3P+N+E) 63A, 400V socket.

It would feed two three phase motors (one18kW from a VFD, the other 2kW DOL). It would also feed 6 single phase circuits (lighting, heating, dehumidifier, 24V control circuit, 230V sockets, 110V socket)

Please check out my current schematic, sketched in word at:
Schematic

My primary questions are:

> Is a type ac RCD sufficient? (this is placed on the incomer as per 717.411.1)
> I have read some information which indicates separate neturals should exist in the dist. panel for SPh and 3Ph circuits (but this is typically with separate RCDs) Is a single neutral ok in this design?

If there are any other comments regarding 7671 compliance, I appreicate any help!
 25 April 2014 12:13 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19655
Joined: 23 March 2004

You don't state the tripping current of the 63A RCBO - if 30mA with no time delay then your sockets are OK - although I would question a 6A CPD on the 240V power ring - however you have a sensitive single point of failure

If the main RCBO is greater than 30mA and/or time delayed then you'll need a 30mA non delayed RCBO for the socket outlets specifically.

I don't know what you've read regarding neutrals, but you have shown a typical 4 pole pan assembly with a common neutral - which is perfectly acceptable (noting that you don't actually have any Three phase loads requiring a neutral)

You might want to look at the 6A CPD feeding the 230/110V TX - it may well trip when you energise due to the TX magnetising inrush

Ditto the VFD supply for the compressor motor - 40A protection against a 35A FLC gives you no scope at all for starting loads

In principle however - the schematic looks OK to me

Regards

OMS

-------------------------
Failure is always an option
 25 April 2014 02:52 PM
User is offline View Users Profile Print this message



ajaallen

Posts: 6
Joined: 18 March 2009

Hi,

Thanks for your comments.

1. I had originally planned to use a 30ma rated RCBO, but I wonder if there is any advantage to having a 100ma on the incomer and 30ma RCBOs on the two socket circuits? (I haven't found any guidance on this in the regualations...only the requriement of one on the incomer as above)

2. For the Variable Frequency Drive/Motor breaker, as it is a VFD, I wouldn't expect excessive starting currents - and a normal B type MCB does allow for short time overload (B type trips 3 - 5 times Full Load Current...C type [motor rated for DOL] 5 - 10 times FLC) so I hadn't anticiapated a problem as there is still a 12% margin for FLC?
Inrush for magnetising may be a bigger issue on the 230/110 circuit...

3. Are you able to also comment on wheather an A or B type RCD would be required ("where there are DC components")? I assume the VFD and AC/DC converter would not cause a significant problem (some harmonics?) here and "type ac" RCD would be acceptable?

Thank you!

Andrew
 25 April 2014 03:58 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19655
Joined: 23 March 2004

Originally posted by: ajaallen

Hi,

Thanks for your comments.
1. I had originally planned to use a 30ma rated RCBO, but I wonder if there is any advantage to having a 100ma on the incomer and 30ma RCBOs on the two socket circuits? (I haven't found any guidance on this in the regualations...only the requriement of one on the incomer as above)

OK - if you put a 30mA at the incoming, then you lose the lot for potentially quite low levels of natural leakage or in the event of any fault.

The socket outlets must have a 30mA non time delayed (for the 230 outlets - the 110V outlets are protected by the reduced voltage of 55V to earth (assuming a centre tapped transformer).

The incoming device would ideally be a 100mA time delayed unit to allow grading with the 30mA device for the sockets- and with a rating greater than 6A as I suggested - 32A/30mA or 20A/30mA would be usual



2. For the Variable Frequency Drive/Motor breaker, as it is a VFD, I wouldn't expect excessive starting currents - and a normal B type MCB does allow for short time overload (B type trips 3 - 5 times Full Load Current...C type [motor rated for DOL] 5 - 10 times FLC) so I hadn't anticiapated a problem as there is still a 12% margin for FLC?

Without specific knowledge of the motor and starter, then I would suggest your margin is too close - what does the motor and drive manufacturer have to say about it. It's quite easy to see currents exceeding 3 times FLC even on soft start devices - and lets not forget that the load is a compressor - so is quite aggressive to the electrical system unless you have unloading valves etc in conjunction with the drive.


Inrush for magnetising may be a bigger issue on the 230/110 circuit...

Indeed


3. Are you able to also comment on wheather an A or B type RCD would be required ("where there are DC components")? I assume the VFD and AC/DC converter would not cause a significant problem (some harmonics?) here and "type ac" RCD would be acceptable?

You aren't likley to see DC components injected via a good quality VFD - so a general purpose Type AC RCD should be be fine

Thank you!

Andrew


Regards

OMS

-------------------------
Failure is always an option
 25 April 2014 04:28 PM
User is offline View Users Profile Print this message



dp11

Posts: 49
Joined: 18 June 2011

Something you may wish to consider is some lights to show each phase is powered and possibly some lights to show correct phase rotation.
 28 April 2014 08:41 AM
User is offline View Users Profile Print this message



ajaallen

Posts: 6
Joined: 18 March 2009

Originally posted by: OMS

Without specific knowledge of the motor and starter, then I would suggest your margin is too close - what does the motor and drive manufacturer have to say about it. It's quite easy to see currents exceeding 3 times FLC even on soft start devices - and lets not forget that the load is a compressor - so is quite aggressive to the electrical system unless you have unloading valves etc in conjunction with the drive.

OMS



When I spoke to the drive manufacturer previsouly, I asked about this problem.

The recomended breaker for the device is a 50A MCB. In discussion, I made reference to the fact that I wanted to achieve discrimination between the motor MCB and the incomer - which would be tight, if not unachieveable, when using a 50A MCB. (although this statement is based on my understading of fuse discrimination where a 63A fuse will start to degrade before the 50A fuse blows...so even if the 63 doesn't blow, it's weakend...with MCBs there wouldn't be this problem - but would a 50A and a 63A be acceptable when discrimination is desired?

The manufcaturer said it was naturally risk averse in recomendations to avoid liability - and that it was likely that a 40A breaker would be sufficient.

I should also note that where on the diagram it says compressor motor, this may appear slighly misleading - the component under test is a sinlge row of compressor blades, running in a vacuum. It is hoped that this will minimise the loading on the motor.



Thanks all for your comments.

@dp11 I'll also bear in mind visual indicators of correct operation

Regards,

Andrew
 28 April 2014 11:17 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19655
Joined: 23 March 2004

It deoends on what you want from the discrimination study.

If you just want the 50A device to hold and the 63A device to hold under starting, then no problem

If however you want to clear a fault on the VFD supply cable with a 50A MCB and still not trip a 63A MCB then forget it - MCB's will almost certainly not grade with upstream MCB's - you would either need to use fuses or a MCCB with electronic (and adjustable) tripping characteristics - and even then you are at the mercy of any upstream protection supplying the connection socket outlet. I would disregard the requirements for a 50A MCB only by adequate in service testing of lower rated devices or a detailed study of the actual starting conditions for the motor/VFD combination.

That said, do you need discrimination. Presuming you have emergency lighting in the unit, then does it matter if a real fault on any circuit trips out the whole unit ?

Regards

OMS

-------------------------
Failure is always an option
 30 April 2014 12:36 AM
User is offline View Users Profile Print this message


Avatar for mapj1.
mapj1

Posts: 2761
Joined: 22 July 2004

If you want a Q & D phase rotation indicator, it can be done with a couple of resistors and single capacitor and a normal panel neon. (I tend to mis-use a bit of ceramic choc bloc for this and lose it in an adaptable box.. )
~VFDs rectify and then re-create the three phases, so you may not care, but I'd expect the DOL vacuum pump to be rotation sensitive.
regards Mike.

-------------------------
regards Mike
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.