IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Motor with no rating plate
Topic Summary:
Created On: 21 February 2014 02:04 PM
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.
 21 February 2014 02:04 PM
User is offline View Users Profile Print this message


Avatar for leckie.
leckie

Posts: 2021
Joined: 21 November 2008

Hi

Got a motor with no rating plate to a compressor.

Its wired via a star delta starter. Its wired in 1.5sq.mm swa from a 63A TPN BS88 switch fuse at the moment!

I want to check the approximate power of the motor and check what size supply and overload device should be fitted.

Im a bit rusty with the calcs so perhaps some of you motor experts can check my sums. I was going to measure the line current when its running in delta, so does this make sense.

Assume I measure 20A (haven't measured it yet), so
Power Kva = 1.73 x 400 x 20 x 0.85 x 0.85 = approx. 10Kva

So in star line I will be 20/1.73 = approx. 11.6A

So starting current would be approx. 6 x 11.6 = 69.6A

So something like a 32A type C mcb in a bit of 4.00sq.mm SWA would be about right for this example.

Is that correct? Thanks in advance
 21 February 2014 02:18 PM
User is offline View Users Profile Print this message



robuck

Posts: 49
Joined: 21 May 2002

Should it not be a type D mcb for motors.
 21 February 2014 03:04 PM
User is offline View Users Profile Print this message


Avatar for leckie.
leckie

Posts: 2021
Joined: 21 November 2008

I thought it could be any device that had a suitable time current characteristic
 21 February 2014 03:38 PM
User is offline View Users Profile Print this message


Avatar for davezawadi.
davezawadi

Posts: 2816
Joined: 26 June 2002

Not really sufficient here.
Are you sure the compressor is that large?
If it is in a busy garage or spray shop there will be a lot of starting and stopping which may well affect your cable size.
A 10kW output motor would be fitted to a compressor of about 40 - 60 CFM, would probably have 3 or 4 B section V belts in parallel, and a 25mm or larger pipe between the pump aftercooler and tank. Compressor pumps of this size are about a metre tall or a slightly lower V formation.
I expect it is quite old not having a plate, and old motors are comparatively large for the rating.
Anyway measure the line current, running just before the pressure switch operates (doing the most work here), and we can work from there. You need to forget the power factor when you set the overload current and breaker and cable size, they all need to cope with the actual running current! If there are more than 6 starts per hour then you need to uprate the cable to at least the next larger size to cope with the additional heating from the start current.
However if this 1.5 has survived so far and does not show signs of melting, it cannot be too far from adequate. Check if the cores are brittle or discoloured as another clue to the rating.
Correct overload setting is vital to protect the motor from phase loss, do not set it high for "good luck", many burned motors suggest this is a bad idea!
Realistically 1.5 is not much small for 10kVA in a reasonable ambient temperature, but 63A fuses (I assume thats what are fitted) don't provide much overload protection do they? Remember that the breaker only provides short circuit protection, the overload protection for the circuit is provided by the motor overload so do not fit too close a breaker to the FLC. You may well need a type D if the compressor takes more than a second or two to get to speed, despite the star delta starter.

Your equation should only have one 0.85, you are thinking of the power output equation where one is the PF and the other the per unit efficiency, so your phase current will be about 15A for 10kVA input.

-------------------------
David
CEng etc, don't ask, its a result not a question!
 21 February 2014 04:40 PM
User is offline View Users Profile Print this message


Avatar for leckie.
leckie

Posts: 2021
Joined: 21 November 2008

Thanks for the reply David

I don't have a clue what rating it is I was just using 20A as a example to check the sums.

Its only a smallish workshop and they don't often use this compressor, there is only one operative and he normally uses another newer compressor, so not stop start usage. The motor is very old and is physically quite large, its about 300 - 350mm diameter.

I know the overload protects against overload for the motor but I thought the cable still had to be sufficient for the running current, and rated within the protective device rating so the 63A would be too high.
The 1.5sq.mm is clipped direct, if its XLPE it would be rated at 22A so I am assuming that the MCB would need a max current rating of 20A. A C type MCB.

So when I measure the current it will be in delta configuration, so am I right in thinking that during the start in star, the current will be approx the measured running current (as near to full load as I can measure it), divided by 1.73 x approx 6? So for a 10Kva input current is approx 15A as you said, therefore starting current would be approx 15/1.73 x 6 = 52A?

If that is right, a 20A type C should able to take about 60A for about 40 seconds (if I am reading the graph correctly!), so should be OK?
 21 February 2014 05:14 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

If you measure the current in the supply to the starter when the motor is running in delta then that is it - you have line current - it's already accounting for power factor and efficiency.

You can select the starter overload from that and set it to 58% FLC (assuming that the OL is in the winding at the delta contactor).

Keep in mind that 1.5mm2 will be good for circa 17A

Regards

OMS

-------------------------
Failure is always an option
 21 February 2014 05:19 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

know the overload protects against overload for the motor but I thought the cable still had to be sufficient for the running current, and rated within the protective device rating so the 63A would be too high.


Nope - a 63A BS 88 on a 1.5mm2 cable feeding a 11KW motor is an almost standard design (FLC = 21A)

The fuse provideds short circuit/earth fault protection - the overload provides just that. Basically you most certainly don't want the OL to not coordinate with the fuse or CB - overloads are not designed to open high fault currents - fuses or circuit breakers are.

Regards

OMS

-------------------------
Failure is always an option
 22 February 2014 08:23 PM
User is offline View Users Profile Print this message


Avatar for zeeper.
zeeper

Posts: 1427
Joined: 11 July 2008

a 63A BS 88 on a 1.5mm2 cable feeding a 11KW motor is an almost standard design (FLC = 21A)


I think I would be worried about volt drop on start up. and how can 1.5mmsq be ok if you dont know the PFC.
 23 February 2014 11:36 PM
User is offline View Users Profile Print this message


Avatar for davezawadi.
davezawadi

Posts: 2816
Joined: 26 June 2002

What has the PFC got to do with it?

-------------------------
David
CEng etc, don't ask, its a result not a question!
 24 February 2014 09:59 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

I think Zeeper is commenting on adequacy of fault level to allow the 1.5mm2 conductor to be thermally protected under short circuit and earth faults

ie sufficiently high to avoid thermal damage to conductors and sufficiently low to achieve same if using devices with definite minimum time operation (circuit breakers rather than fuses for example)

Regards

OMS

-------------------------
Failure is always an option
 24 February 2014 03:02 PM
User is offline View Users Profile Print this message


Avatar for davezawadi.
davezawadi

Posts: 2816
Joined: 26 June 2002

I thought so too OMS and hoped he would elucidate the point fully, as a nice example of design over "standard circuits" and the inevitable negative reactions so far, but obviously he didn't get the point and perhaps you thought I didn't either.......

As this point would be totally lost on many of the readers of the forum, it would be nice to see an explanation from "the floor".

-------------------------
David
CEng etc, don't ask, its a result not a question!
 24 February 2014 10:04 PM
User is offline View Users Profile Print this message


Avatar for leckie.
leckie

Posts: 2021
Joined: 21 November 2008

Well I'm off to the site tomorrow so I will do a bit of accurate measuring.

I've been having a bit of a refresher / re-read on all this. Blimey I realise through lack of use I have forgotten loads! I will repost in a day or two. Thanks for the replies, much appreciated.
 25 February 2014 07:26 AM
User is offline View Users Profile Print this message


Avatar for zeeper.
zeeper

Posts: 1427
Joined: 11 July 2008

but obviously he didn't get the point and perhaps you thought I didn't either.......


In fairness to myself, I have been very busy this last year. So only pop in from time to time.

However I think it is good to leave a little to the imagination, so people think a bit before getting the answer.
 25 February 2014 06:36 PM
User is offline View Users Profile Print this message


Avatar for leckie.
leckie

Posts: 2021
Joined: 21 November 2008

Well I think it's sorted anyway.

I thought the swa was 1.5 at a glance. Had a bit better look and it was 2.5.

The motor overloads were set to 13.5A.

I did a calc on the cable for fault current and it passed at 2.5 but was failing it at 1.5; said the Isq x t for the device was higher than
the Ssq x Ksq of the cable I think, not got it next to me but I think that's what it said. But 2.5 was ok.

When I tried the 1.5 it failed as noted above for 24m, if I dropped the length about 15m it passed, but I haven't worked out why yet! I'm going to have a good read through the IET electrical calculations guide book tomorrow as I have a short day, and see if I can remember how all that works. I know there are some motor calc examples in there.
 25 February 2014 07:35 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

When I tried the 1.5 it failed as noted above for 24m, if I dropped the length about 15m it passed, but I haven't worked out why yet!


Well, what links cable length with fault current - resistance maybe ?

So for a long circuit, Zs increases, PSCC goes down and fuses take longer to blow - thus thermally damaging the cable. A shorter run has less resistance, more fault current flows, fuses operate faster and the cable is less thermally stressed.

Basically what you are looking for is the energy let through by the fuse (the I2t) being less than the energy absorption by the cable (the K2S2) - happy days

So a bit more "I" squared means a lot less time for fuses - so happy cables - which is why the odd 10m or so of small cable which has relatively high resistance makes a world of difference to compliant circuit design or not

Regards

OMS

-------------------------
Failure is always an option
 25 February 2014 09:25 PM
User is offline View Users Profile Print this message


Avatar for leckie.
leckie

Posts: 2021
Joined: 21 November 2008

See, I told you I had forgot everything! Use it or lose it!
 26 February 2014 07:52 AM
User is offline View Users Profile Print this message


Avatar for zeeper.
zeeper

Posts: 1427
Joined: 11 July 2008

Assume I measure 20A (haven't measured it yet), so
Power Kva = 1.73 x 400 x 20 x 0.85 x 0.85 = approx. 10Kva


It looks like you have calculate Power, W not KVA. The reason I say this is because it looks like you have a PF of 0.85

so maybe a 11KW motor.



So in star line I will be 20/1.73 = approx. 11.6A



Delta 11000/1.72*400*0.85=13.6A (I find it more accurate to use the current from ID plate on motors)



So starting current would be approx. 6 x 11.6 = 69.6A


Maximum starting current in Star 3 x 13.6A = 40A

Star to delta change over set at 80% of full speed.

would not a 20m36 have saved all the drama. however I suspect a 20A c type would be ok for a short duration startup

(from the floor)
Statistics

See Also:



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