Fuses in place of motor "heaters"? (induction motor protecti

[DaveC]

Looking to provide switching for 1-phase, 220 AC, 1/2 hp motor that doesn't
include use of a contactor. I appreciate the need for precise current limit
(LRA) that a branch breaker can't provide.

So, I choose to not have a contactor but instead, this, which provides the
power-fail protection:

http://www.amazon.com/Woodstock-D4160-110-Volt-Paddle-
Switch/dp/B005W17HYY/ref=pd_sim_hi_1/191-4569239-2528519

This has a relay (rated 1/2 hp) that drops out on power fail, and has
external connections for additional e-stop mushroom switch. But no provision
for motor protection.

Would 2 precisely-matched (ie, to the 1/4 amp), slow-blow HRC fuses be
sufficient to protect this motor?

Thanks.

 

[Mike Marlow]

DaveC wrote:

Looking to provide switching for 1-phase, 220 AC, 1/2 hp motor that
doesn't include use of a contactor. I appreciate the need for precise
current limit (LRA) that a branch breaker can't provide.

So, I choose to not have a contactor but instead, this, which
provides the power-fail protection:

http://www.amazon.com/Woodstock-D4160-110-Volt-Paddle-
Switch/dp/B005W17HYY/ref=pd_sim_hi_1/191-4569239-2528519

This has a relay (rated 1/2 hp) that drops out on power fail, and has
external connections for additional e-stop mushroom switch. But no
provision for motor protection.

Would 2 precisely-matched (ie, to the 1/4 amp), slow-blow HRC fuses be
sufficient to protect this motor?

Look at that switch again - it's a 110v switch, not a 220v switch. Hate to
see you order the wrong one. You could fuse the circuit behind the switch
but you'd have no way to ensure that both legs will drop out at the same
time the way that a breaker does. You would not want to find yourself in a
situation where one leg drops out but the other does not.

What is the application for this motor that requires so much more precision
than the circuit protection that a breaker would provide?

--

-Mike-
mmarlowREMOVE@windstream.net

 

[Jim Wilkins]

"DaveC" <invalid@invalid.net> wrote in message
news:0001HW.CF08B7560006C21DB01029BF@news.eternal-september.org...

Looking to provide switching for 1-phase, 220 AC, 1/2 hp motor that
doesn't
include use of a contactor. I appreciate the need for precise
current limit
(LRA) that a branch breaker can't provide.

So, I choose to not have a contactor but instead, this, which
provides the
power-fail protection:

http://www.amazon.com/Woodstock-D4160-110-Volt-Paddle-
Switch/dp/B005W17HYY/ref=pd_sim_hi_1/191-4569239-2528519

This has a relay (rated 1/2 hp) that drops out on power fail, and
has
external connections for additional e-stop mushroom switch. But no
provision
for motor protection.

Would 2 precisely-matched (ie, to the 1/4 amp), slow-blow HRC fuses
be
sufficient to protect this motor?

Thanks.

Why do you need two fuses for a single-phase motor?

Do you know the motor's actual i-squared-t overcurrent limit
requirement?

http://www.copleycontrols.com/motion/pdf/IsqT.pdf

When I ran a lab at Mitre an engineer kept asking me to buy him more
and more Polaroid film for his scope camera. I finally asked him why
and found that he was trying to discover the principles of how a fuse
blows. He looked really sheepish and subdued when I handed him a
Littelfuse pamphlet of current vs time curves.
http://www.littelfuse.com/~/media/files/littelfuse/technical%20resources/documents/product%20catalogs/autofuseology.pdf

jsw

 

[Mike Marlow]

Jim Wilkins wrote:

Why do you need two fuses for a single-phase motor?

He's running single phase 220v Jim.


--

-Mike-
mmarlowREMOVE@windstream.net

 

[William Sommerwerck]

"Jim Wilkins" wrote in message news:lc0deb$mat$1@dont-email.me...

When I ran a lab at Mitre an engineer kept asking me to buy him more and
more Polaroid film for his scope camera. I finally asked him why and found
that he was trying to discover the principles of how a fuse blows. He looked
really sheepish and subdued when I handed him a Littelfuse pamphlet of
current vs time curves.
http://www.littelfuse.com/~/media/files/littelfuse/technical%20resources/documents/product%20catalogs/autofuseology.pdf

When I worked one Summer at Bell Labs, one of the engineers told me he was
trying to figure out how much detail the eye could see in color. What he
didn't know (and I'd forgotten that I knew) was that this information was part
of the design of the NTSC system.

 

[Jim Wilkins]

"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:lc0ev7$u8p$1@dont-email.me...

"Jim Wilkins" wrote in message news:lc0deb$mat$1@dont-email.me...

When I ran a lab at Mitre an engineer kept asking me to buy him
more and more Polaroid film for his scope camera. I finally asked
him why and found that he was trying to discover the principles of
how a fuse blows. He looked really sheepish and subdued when I
handed him a Littelfuse pamphlet of current vs time curves.
http://www.littelfuse.com/~/media/files/littelfuse/technical%20resources/documents/product%20catalogs/autofuseology.pdf

When I worked one Summer at Bell Labs, one of the engineers told me
he was trying to figure out how much detail the eye could see in
color. What he didn't know (and I'd forgotten that I knew) was that
this information was part of the design of the NTSC system.

As a chemistry student I was expected to learn a lot of practical
detail and hands-on procedure that I later noticed new electrical (and
some mechanical) engineers often lacked.
jsw

 

[Jasen Betts]

qOn 2014-01-25, Jim Wilkins <muratlanne@gmail.com> wrote:

"DaveC" <invalid@invalid.net> wrote in message

Why do you need two fuses for a single-phase motor?

because it's got two live inputs, and either could develop a short to ground.


--
For a good time: install ntp

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

 

[Jim Wilkins]

"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:lc1923$74k$1@gonzo.reversiblemaps.ath.cx...

qOn 2014-01-25, Jim Wilkins <muratlanne@gmail.com> wrote:
"DaveC" <invalid@invalid.net> wrote in message

Why do you need two fuses for a single-phase motor?

because it's got two live inputs, and either could develop a short
to ground.

For a good time: install ntp
--- news://freenews.netfront.net/ - complaints:
news@netfront.net ---

The panel circuit breakers protect the wiring from that. The separate
fusing is for a motor overload condition.

 

[DoN. Nichols]

On 2014-01-25, Jim Wilkins <muratlanne@gmail.com> wrote:

"DaveC" <invalid@invalid.net> wrote in message
news:0001HW.CF08B7560006C21DB01029BF@news.eternal-september.org...
Looking to provide switching for 1-phase, 220 AC, 1/2 hp motor that
doesn't
include use of a contactor. I appreciate the need for precise
current limit
(LRA) that a branch breaker can't provide.

[ ... ]

Would 2 precisely-matched (ie, to the 1/4 amp), slow-blow HRC fuses
be
sufficient to protect this motor?

Thanks.

Why do you need two fuses for a single-phase motor?

In the UK, you don't, because 240 VAC comes with one side
grounded. However, in the USA, 240 VAC is normally supplied with a
grounded center tap, so if you have only one fuse -- or have two but
only one blows, you still have 120 VAC live in the motor's housing, and
potentially available for contact and personal zapping.

Do you know the motor's actual i-squared-t overcurrent limit
requirement?

http://www.copleycontrols.com/motion/pdf/IsqT.pdf

When I ran a lab at Mitre an engineer kept asking me to buy him more
and more Polaroid film for his scope camera. I finally asked him why
and found that he was trying to discover the principles of how a fuse
blows. He looked really sheepish and subdued when I handed him a
Littelfuse pamphlet of current vs time curves.

> http://www.littelfuse.com/~/media/files/littelfuse/technical%20resources/documents/product%20catalogs/autofuseology.pdf

And I'm not sure why he feels the need for precision current
limiting on the motor. Most motors will handle a fairly wide range of
time vs overcurrent exposures. (An exception is a permanent magnet DC
servo motor which can be partially demagnetized by a sort spike over the
rated current limit.

Enjoy,
DoN.

--
Remove oil spill source from e-mail
Email: <BPdnicholsBP@d-and-d.com> | (KV4PH) Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

 

[bud--]

On 1/25/2014 7:10 AM, Mike Marlow wrote:

DaveC wrote:
Looking to provide switching for 1-phase, 220 AC, 1/2 hp motor that
doesn't include use of a contactor. I appreciate the need for precise
current limit (LRA) that a branch breaker can't provide.

So, I choose to not have a contactor but instead, this, which
provides the power-fail protection:

http://www.amazon.com/Woodstock-D4160-110-Volt-Paddle-
Switch/dp/B005W17HYY/ref=pd_sim_hi_1/191-4569239-2528519

This has a relay (rated 1/2 hp) that drops out on power fail, and has
external connections for additional e-stop mushroom switch. But no
provision for motor protection.

Would 2 precisely-matched (ie, to the 1/4 amp), slow-blow HRC fuses be
sufficient to protect this motor?

May or may not, depending on the fuse characteristics. The fuse has to
be slow-blow enough to not open when the motor starts (starting current
is about 6x running current). But it has to open within the i-squared-t
rating of the motor (as in Jim's post).

The "overloads" in motor starters are matched to motor starting. And the
settings in are much finer steps (or are continuous) compared to fuses.

Many 1/2HP motors have internal overload protection ("thermally protected").

Look at that switch again - it's a 110v switch, not a 220v switch. Hate to
see you order the wrong one. You could fuse the circuit behind the switch
but you'd have no way to ensure that both legs will drop out at the same
time the way that a breaker does. You would not want to find yourself in a
situation where one leg drops out but the other does not.

In the US, using the NEC, fuses are allowed. And a "controller" only has
to open one of the supply wires. (You are right about the 110V rating on
the amazon switch.) A "disconnect" has to open all the live supply
conductors.

What is the application for this motor that requires so much more precision
than the circuit protection that a breaker would provide?

Application is important. But a breaker provides little overload
protection for a motor. For a hard-wired motor circuit (like central air
conditioning) the breaker does not usually provide overload protection
(that is provided at the motor). The breaker can be significantly larger
than the rating of the wires, and provides short circuit protection. A
breaker is large so it does not trip on the motor starting current.

 

[Ignoramus8074]

On 2014-01-25, Jim Wilkins <muratlanne@gmail.com> wrote:

"Jasen Betts" <jasen@xnet.co.nz> wrote in message
qOn 2014-01-25, Jim Wilkins <muratlanne@gmail.com> wrote:
"DaveC" <invalid@invalid.net> wrote in message
Why do you need two fuses for a single-phase motor?
because it's got two live inputs, and either could develop a short
to ground.

The panel circuit breakers protect the wiring from that. The
separate fusing is for a motor overload condition.

Fuses ARE heaters. They have thermal action and the slow ones are
meant for motors. I would say that fuses are "overload heaters for the
poor".

I spent some effort when replacnig electricals on my three phase
bandsaw, and finally found a contactor with the exact heaters that the
motor needed. Very happy about it.

i

 

[Phil Allison]

"Jim Wilkins"

Why do you need two fuses for a single-phase motor?

** Cos the AC supply is two phase.

The OP is an American.

They have spit phase power where 240VAC is split into a pair of 120V lines
with a common neutral.

While 120V appliances use one or the other, some ( usually high powered )
ones are rated at 220 /240 and connect across the pair.


...... Phil

 

[Phil Allison]

"bud--"

May or may not, depending on the fuse characteristics. The fuse has to be
slow-blow enough to not open when the motor starts (starting current is
about 6x running current). But it has to open within the i-squared-t
rating of the motor (as in Jim's post).

** Think you will have poor luck with a HRC fuse taking 6 time rated every
time the motor starts or stalls and not nuisance blowing.

A "motor start" circuit breaker is what is normally used.

Slow, thermal characteristic at up to 6 times and then a very fast magnetic
action at about 10 times.



..... Phil

 

[Jim Wilkins]

"DoN. Nichols" <BPdnicholsBP@d-and-d.com> wrote in message
news:slrnle97cu.5kc.BPdnicholsBP@Katana.d-and-d.com...

On 2014-01-25, Jim Wilkins <muratlanne@gmail.com> wrote:

Why do you need two fuses for a single-phase motor?

In the UK, you don't, because 240 VAC comes with one side
grounded. However, in the USA, 240 VAC is normally supplied with a
grounded center tap, so if you have only one fuse -- or have two but
only one blows, you still have 120 VAC live in the motor's housing,
and
potentially available for contact and personal zapping.

I assumed it was wired with a 20A double-pole breaker at the panel
protecting from shorts and a smaller fuse sized (how?) to blow before
the motor burned out.

jsw

 

[Bob Engelhardt]

On 1/26/2014 2:18 AM, bud-- wrote:

... The breaker can be significantly larger
than the rating of the wires, and provides short circuit protection.
....

I don't think so! The breaker is there to protect the wires. If there
is a load on the circuit greater than what the wires can handle, the
breaker opens. You do not want the wires overheating and it doesn't
take a short to do it.

Bob

 

[bud--]

On 1/26/2014 1:47 AM, Phil Allison wrote:

"bud--"

May or may not, depending on the fuse characteristics. The fuse has to be
slow-blow enough to not open when the motor starts (starting current is
about 6x running current). But it has to open within the i-squared-t
rating of the motor (as in Jim's post).

** Think you will have poor luck with a HRC fuse taking 6 time rated every
time the motor starts or stalls and not nuisance blowing.

Fuses are not usually used for motor overload protection in the US.
There aren't enough fuse ratings to match to the motor running current
except for relatively small motors.

A "motor start" circuit breaker is what is normally used.

Slow, thermal characteristic at up to 6 times and then a very fast magnetic
action at about 10 times.



.... Phil

Time delay breakers can be up to 175% of the motor running current in
the US. They often are used for short circuit protection of a motor
circuit, but not for overload protection. There are even fewer sizes of
breakers than fuses.

 

[Fred Abse]

On Sun, 26 Jan 2014 08:13:56 -0500, Jim Wilkins wrote:

"DoN. Nichols" <BPdnicholsBP@d-and-d.com> wrote in message
news:slrnle97cu.5kc.BPdnicholsBP@Katana.d-and-d.com...
On 2014-01-25, Jim Wilkins <muratlanne@gmail.com> wrote:

Why do you need two fuses for a single-phase motor?

In the UK, you don't, because 240 VAC comes with one side grounded.
However, in the USA, 240 VAC is normally supplied with a grounded center
tap, so if you have only one fuse -- or have two but only one blows, you
still have 120 VAC live in the motor's housing, and
potentially available for contact and personal zapping.


I assumed it was wired with a 20A double-pole breaker at the panel
protecting from shorts and a smaller fuse sized (how?) to blow before the
motor burned out.

Panel breakers are there to protect the upstream wiring from excessive
load current, not to protect the load.

Matching the I-squared-t of a breaker, or fuse, to startup and running
conditions of a motor is not trivial. Motors are subject to starting
inrush currents sometimes tens of times the rated full-load current. Be
guided by the data published by reputable manufacturers. There's plenty of
it.

--
"Design is the reverse of analysis"
(R.D. Middlebrook)

 

[bud--]

On 1/26/2014 9:27 AM, Bob Engelhardt wrote:

On 1/26/2014 2:18 AM, bud-- wrote:
... The breaker can be significantly larger
than the rating of the wires, and provides short circuit protection.
...

I don't think so! The breaker is there to protect the wires. If there
is a load on the circuit greater than what the wires can handle, the
breaker opens. You do not want the wires overheating and it doesn't
take a short to do it.

Bob

Under the US NEC the wires have to be 115% of the motor run current. A
time delay breaker can be 175% of the motor run current. (Breakers that
don't have a time delay can be even larger.) As stated, for a motor
circuit, the breaker provides _short circuit protection_. Breakers can
be large so they don't trip on normal motor starts. (I have seen an
example for something like a 1/2HP motor where the breaker was twice the
wire rating.)

_Overload protection_ is usually at the motor (at the load end of the
circuit). One common method is a motor starter with an 'overload' unit
that is closely matched to the run current. Another is an internal
thermal protector that disconnects the motor when it overheats. An
internal thermal protector may be wired to drop out a motor starter.


For welder circuits, the breaker can be even larger with respect to the
wire rating. That can happen if the welder has a low "duty cycle".

 

[Bob Engelhardt]

On 1/26/2014 12:15 PM, bud-- wrote:

On 1/26/2014 9:27 AM, Bob Engelhardt wrote:
On 1/26/2014 2:18 AM, bud-- wrote:
... The breaker can be significantly larger
than the rating of the wires, and provides short circuit protection.
...

I don't think so! The breaker is there to protect the wires. ...

... for a motor circuit, ...

Ah ... my mindset was residential branch circuit.

 

[Maynard A. Philbrook Jr.]

In article <lc45cf029t@news6.newsguy.com>, BobEngelhardt@comcast.net
says...

On 1/26/2014 12:15 PM, bud-- wrote:
On 1/26/2014 9:27 AM, Bob Engelhardt wrote:
On 1/26/2014 2:18 AM, bud-- wrote:
... The breaker can be significantly larger
than the rating of the wires, and provides short circuit protection.
...

I don't think so! The breaker is there to protect the wires. ...

... for a motor circuit, ...

Ah ... my mindset was residential branch circuit.

For readers that are not acquainted with the term "Motor Heaters"
in the subject line, this could be confusing..

Heaters in this context is referred to as the overload protection
device that drives the motor. Heater coils surround the release latches
and when they get hot enough, will allow the protection to activate.

These heater elements are electrically in series with each phase going
to the motor.

Inrush currents for non inverter driven AC motors when starting can be
quite high and the slow response from the heaters for the latches allow
for a delay trip.

It's customary to select the proper heater element so that it's close
to the full load amps of the motor. This way after a short period from
start up the coils will then remain warm and not too far from the trip
point. This means the motor should trip the overload quickly
if it experiences an excessive short inrush problem.

Have a good f'ing day

Jamie