Does anyone know if there is a way to assess the typical inrush current
for an induction motor?

I helped a mate install a new motor in a small saw table the other day.
The motor he had to hand was a monster 3kW jobbie that had been bought
for some other purpose but never used. I suggested this might be
overkill for the application, since we are only talking about a 7" blade
here!.

Anyway hooked it up via a DOL starter and set the max current trip on it
to something sustainable on a 13A plug. (the motors full load draw being
17A IIRC). Not unsurprisingly we could only spin it up on a circuit
protected by a 32A type B MCB - any circuits off 16A MCBs tripped
immediately. Load when running (but not cutting) was 7A (could not try
any cuts since we did not have a suitable belt and pulley).


--
Cheers,

John.

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\================================================= ================/

On Fri, 25 Jan 2008 21:57:18 +0000,it is alleged that John Rumm
spake thusly in uk.d-i-y:


Does anyone know if there is a way to assess the typical inrush current
for an induction motor?

I helped a mate install a new motor in a small saw table the other day.
The motor he had to hand was a monster 3kW jobbie that had been bought
for some other purpose but never used. I suggested this might be
overkill for the application, since we are only talking about a 7" blade
here!.

Anyway hooked it up via a DOL starter and set the max current trip on it
to something sustainable on a 13A plug. (the motors full load draw being
17A IIRC). Not unsurprisingly we could only spin it up on a circuit
protected by a 32A type B MCB - any circuits off 16A MCBs tripped
immediately. Load when running (but not cutting) was 7A (could not try
any cuts since we did not have a suitable belt and pulley).

Hi John, not sure how accurately you're wanting to assess it here, but
I was always told when training as an electrician, to assume 3x the
maximum run current as the inrush with induction motors, and
approximately 2x with universal motors. That was for BS88 (or BS1363
for smaller 1ph loads) fuselinks, which approximate (IIRC) a C-Curve
MCB.

In these H&S rich days I suspect that 'training as electrician' and
'assume' would be a bad mixture for the college/workplace, although
the late 80s/early 90s wasn't THAT long ago, it seems like it now.

--
_
( ) ASCII ribbon campaign against html e-mail
X and usenet posts
/ \

On Jan 25, 9:57*pm, John Rumm wrote:
Does anyone know if there is a way to assess the typical inrush current
for an induction motor?

I'd measure the resistance with a multimeter and divide it into the
supply voltage.

My 1.6kW pressure washer is 4 ohms and doesn't cause any problems with
a 16A breaker, but the supply is a loooong length of 1.5mm (100m!).

Why not try a long (uncoiled) length of 13A cable? Might just do the
trick.

Failing that try a google on 'inrush limiter', looks like RS and
Farnell/CPC do them.

cheers,
Pete.

In message , Chip
writes
On Fri, 25 Jan 2008 21:57:18 +0000,it is alleged that John Rumm
spake thusly in uk.d-i-y:


Does anyone know if there is a way to assess the typical inrush current
for an induction motor?

I helped a mate install a new motor in a small saw table the other day.
The motor he had to hand was a monster 3kW jobbie that had been bought
for some other purpose but never used. I suggested this might be
overkill for the application, since we are only talking about a 7" blade
here!.

Anyway hooked it up via a DOL starter and set the max current trip on it
to something sustainable on a 13A plug. (the motors full load draw being
17A IIRC). Not unsurprisingly we could only spin it up on a circuit
protected by a 32A type B MCB - any circuits off 16A MCBs tripped
immediately. Load when running (but not cutting) was 7A (could not try
any cuts since we did not have a suitable belt and pulley).

Hi John, not sure how accurately you're wanting to assess it here, but
I was always told when training as an electrician, to assume 3x the
maximum run current as the inrush with induction motors, and
approximately 2x with universal motors.


That's the rule of thumb figure I remember


--
geoff

On 25 Jan, 21:57, John Rumm wrote:
Does anyone know if there is a way to assess the typical inrush current
for an induction motor?

I helped a mate install a new motor in a small saw table the other day.
The motor he had to hand was a monster 3kW jobbie that had been bought
for some other purpose but never used. I suggested this might be
overkill for the application, since we are only talking about a 7" blade
here!.

Anyway hooked it up via a DOL starter and set the max current trip on it
to something sustainable on a 13A plug. (the motors full load draw being
17A IIRC). Not unsurprisingly we could only spin it up on a circuit
protected by a 32A type B MCB - any circuits off 16A MCBs tripped
immediately. Load when running (but not cutting) was 7A (could not try
any cuts since we did not have a suitable belt and pulley).

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Hi John
Been there, done that and have the ....., etc. I did a very similar
exercise for a 12" saw table some years ago, using a similar size of
motor off an old power washer, and found the anomaly that it would run
as a power washer off a 13A supply, but not the saw - must have been
something to do with the gearing of the belt drive I surmised while I
reorganised the whole workshop wiring off a 32A MCB in the house CU !

I even dismantled the electronic starter, reverse engineered it and
fiddled with the time constants, but it just continued to eat 13A
fuses, which must have been the initial inrush current as you are
suggesting.

Rob

In message , geoff
writes
In message ,
Chip writes
On Fri, 25 Jan 2008 21:57:18 +0000,it is alleged that John Rumm
spake thusly in uk.d-i-y:


Does anyone know if there is a way to assess the typical inrush current
for an induction motor?

I helped a mate install a new motor in a small saw table the other day.
The motor he had to hand was a monster 3kW jobbie that had been bought
for some other purpose but never used. I suggested this might be
overkill for the application, since we are only talking about a 7" blade
here!.

Anyway hooked it up via a DOL starter and set the max current trip on it
to something sustainable on a 13A plug. (the motors full load draw being
17A IIRC). Not unsurprisingly we could only spin it up on a circuit
protected by a 32A type B MCB - any circuits off 16A MCBs tripped
immediately. Load when running (but not cutting) was 7A (could not try
any cuts since we did not have a suitable belt and pulley).

Hi John, not sure how accurately you're wanting to assess it here, but
I was always told when training as an electrician, to assume 3x the
maximum run current as the inrush with induction motors, and
approximately 2x with universal motors.


That's the rule of thumb figure I remember

And as they get old and knackered, it goes up


--
geoff

On 2008-01-25 21:57:18 +0000, John Rumm said:


Does anyone know if there is a way to assess the typical inrush current
for an induction motor?

I helped a mate install a new motor in a small saw table the other day.
The motor he had to hand was a monster 3kW jobbie that had been bought
for some other purpose but never used. I suggested this might be
overkill for the application, since we are only talking about a 7"
blade here!.

Anyway hooked it up via a DOL starter and set the max current trip on
it to something sustainable on a 13A plug. (the motors full load draw
being 17A IIRC). Not unsurprisingly we could only spin it up on a
circuit protected by a 32A type B MCB - any circuits off 16A MCBs
tripped immediately. Load when running (but not cutting) was 7A (could
not try any cuts since we did not have a suitable belt and pulley).

I have a couple of related situations in my workshop.


The cyclone dust extractor has a 2kW single phase motor which is fired
up by a contactor, with that it turn operated by an impulse relay
controlled by an RF remote - i.e. I can have a little box on the table
saw and operate the extractor from there. Eventually I may do some
current detection stuff on the saw's motor and switch it that way.
At any rate, a 16A type B MCB was originally used for the dedicated
circuit for this. It had a tendency to trip, but didn't always. A
type C was fitted and resolved the problem.


My saw table has three 4kW three phase motors. These are
individually switched in for saw, spindle and planer operations as
required. The saw will take up to 315mm blades, so the power is a help.

There is also a VFD (variable frequency drive) essentially an
electronics module, which is useful in several ways.

Firstly it ramps up rather than hard on (as it were) and thus reduces
the inrush current.

Secondly, it provides single to three phase conversion.

Thirdly it does speed control.

Fourthly it does electronic braking.


I think that he should be concerned about the mechanical arrangements
to drive the blade etc. This much power on a small blade could pose
some safety problems with suitable belts, bearings, pullies etc.

Chip wrote:

Hi John, not sure how accurately you're wanting to assess it here, but
I was always told when training as an electrician, to assume 3x the
maximum run current as the inrush with induction motors, and
approximately 2x with universal motors. That was for BS88 (or BS1363
for smaller 1ph loads) fuselinks, which approximate (IIRC) a C-Curve
MCB.

Yup, that was the sort of assessment I was after...

The plate on the motor listed Imax as 17A so that would give 51A,
although if fig 3.4 of BS7671 is to be believed the instantaneous trip
region for a 16A type B ought to be 80A or more.

The 13A plug fuse also had no difficulty with repeated starts, so I
guess that would set an upper limit round about 90 - 100A

In these H&S rich days I suspect that 'training as electrician' and
'assume' would be a bad mixture for the college/workplace, although
the late 80s/early 90s wasn't THAT long ago, it seems like it now.

Plenty good enough for this application! ;-)



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Andy Hall wrote:

I have a couple of related situations in my workshop.


The cyclone dust extractor has a 2kW single phase motor which is fired
up by a contactor, with that it turn operated by an impulse relay
controlled by an RF remote - i.e. I can have a little box on the table
saw and operate the extractor from there. Eventually I may do some
current detection stuff on the saw's motor and switch it that way.
At any rate, a 16A type B MCB was originally used for the dedicated
circuit for this. It had a tendency to trip, but didn't always. A
type C was fitted and resolved the problem.

I have a similar issue in mine if I plug a 3kVA transformer in too near
to the CU - it can trip the 20A MCB. In the middle of the circuit or on
a short extension however it is fine.

My saw table has three 4kW three phase motors. These are
individually switched in for saw, spindle and planer operations as
required. The saw will take up to 315mm blades, so the power is a help.

There is also a VFD (variable frequency drive) essentially an
electronics module, which is useful in several ways.

Firstly it ramps up rather than hard on (as it were) and thus reduces
the inrush current.

This soft start for motors can make a big difference to inrush. I think
this is pretty much a requirement for motors much above 3kW IIUC.

I think that he should be concerned about the mechanical arrangements
to drive the blade etc. This much power on a small blade could pose
some safety problems with suitable belts, bearings, pullies etc.

I did suggest that not over egging it on the belt drive might be a good
plan so as to maintain a intentional weak link. I also set the current
limit on the starter to prevent the motor being able to ever develop
full output power.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 25 Jan, 21:57, John Rumm wrote:
Does anyone know if there is a way to assess the typical inrush current
for an induction motor?

Brook motors give inrush as 500-550% full load amps for single phase
motors and 300-650% for three phase motors.

wrote:

On 25 Jan, 21:57, John Rumm wrote:
Does anyone know if there is a way to assess the typical inrush current
for an induction motor?

Brook motors give inrush as 500-550% full load amps for single phase
motors and 300-650% for three phase motors.

This was single phase. The 500% of max rating would be 85A which would
tie in pretty closely with what I was seeing (i.e. 16A B MCB tripping,
but not the 13A fuse).

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:

Does anyone know if there is a way to assess the typical inrush current
for an induction motor?

I helped a mate install a new motor in a small saw table the other day.
The motor he had to hand was a monster 3kW jobbie that had been bought
for some other purpose but never used. I suggested this might be
overkill for the application, since we are only talking about a 7" blade
here!.

Anyway hooked it up via a DOL starter and set the max current trip on it
to something sustainable on a 13A plug. (the motors full load draw being
17A IIRC). Not unsurprisingly we could only spin it up on a circuit
protected by a 32A type B MCB - any circuits off 16A MCBs tripped
immediately. Load when running (but not cutting) was 7A (could not try
any cuts since we did not have a suitable belt and pulley).



Have a look he http://www.memonline.com/cb11.html

Dave Osborne wrote:
John Rumm wrote:

Does anyone know if there is a way to assess the typical inrush
current for an induction motor?

I helped a mate install a new motor in a small saw table the other
day. The motor he had to hand was a monster 3kW jobbie that had been
bought for some other purpose but never used. I suggested this might
be overkill for the application, since we are only talking about a 7"
blade here!.

Anyway hooked it up via a DOL starter and set the max current trip on
it to something sustainable on a 13A plug. (the motors full load draw
being 17A IIRC). Not unsurprisingly we could only spin it up on a
circuit protected by a 32A type B MCB - any circuits off 16A MCBs
tripped immediately. Load when running (but not cutting) was 7A (could
not try any cuts since we did not have a suitable belt and pulley).



Have a look he http://www.memonline.com/cb11.html

The figures they give for 3 ph seem ok, but I am not sure about the
single phase ones - the MCB ratings suggested seem excessive, especially
for type C devices.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Fri, 25 Jan 2008 21:57:18 +0000, John Rumm wrote:

Does anyone know if there is a way to assess the typical inrush current
for an induction motor?

I helped a mate install a new motor in a small saw table the other day.
The motor he had to hand was a monster 3kW jobbie that had been bought
for some other purpose but never used. I suggested this might be
overkill for the application, since we are only talking about a 7" blade
here!.

Anyway hooked it up via a DOL starter and set the max current trip on it
to something sustainable on a 13A plug. (the motors full load draw being
17A IIRC). Not unsurprisingly we could only spin it up on a circuit
protected by a 32A type B MCB - any circuits off 16A MCBs tripped
immediately. Load when running (but not cutting) was 7A (could not try
any cuts since we did not have a suitable belt and pulley).


Single phase DOL motors should be fused at about 6x full-load current,
using type gM or HRC fuses.

Alternatively you can use type C (5-10x FLC) or type D mcbs (10-20x FLC),
in which case you really need to check the tripping curves but you can
often get away with just picking the nearest size to your FLC.

Note that the size of protection used depends on the motor rating, how
long it takes to get up to speed and the load. A type C may be ok for a
pump, but put the same motor on a fan and you may need a type D.

Soft-starter units will allow motors to be started on much smaller fuses
or mcbs as they limit the available motor current during startup.

Hope this helps.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info http://mixpix.batcave.net