AC motor question

My son was using a 1.5HP 120V AC motor to power his loam screening machine. He was using two 50ft # 12 and one 75 ft # 14 extension cords to power this up. After running a couple of hours he stopped the machine for awhile to do other things. When he attempted to get it going again about a half hour later he said it hummed loudly. He quickly unplugged it and noticed that there was a big spark at the plug end and the motor was quite hot. A while later he tried it again and it ran fine. I suggested that he check the ends of the cords and just like I had expected he said that they were all quite warm. We replaced this mess with one continuous length of # 12 with 20 amp ends and all seems fine now so I suspect low voltage or rather excessive voltage drops were his problem. But I have a couple of questions. No one has ever really explained to me why low voltage can damage an AC motor. I had thought that as per ohms law with less voltage there should be less of a current draw and therefore less heat. My other question concerns the initial motor failure. Wat kind of mechanism would cause a motor to shut down as it appeared to by drawing excessive current. It behaved just like a bad, (shorted) motor initially and then resumed working perfectly later on. Thanks. Lenny

 

[hrhofmann@sbcglobal.net]

The motor starts out running by using a starting winding that draws maybe 30% more current than normal. Typically, if the heavier current causes the voltage to be low, the motor cannot get up to speed to disconnect the starting winding as Jamie said.

Every plug/socket that is in the connection is a weak point for the overall system. Connections weaken with repeated heating and cooling, and may develope "crud" on the contacts that make the connection even worse. By disconnecting the plug your son may have cleaned up the contacts just enough that the next time he tried to start the motor the connection was just a little better, but enough better that the mtor was able to get up to speed. There may also have been a difference on the mechanical load on the motor when starting up, depending on how the loam screening is connected (or not) when the motor starts up.

 

[Maynard A. Philbrook Jr.]

In article <6f0d8f6b-01c7-405b-9cc2-c4124f93711a@googlegroups.com>,
captainvideo462009@gmail.com says...
> My son was using a 1.5HP 120V AC motor to power his loam screening machine. He was using two 50ft # 12 and one 75 ft # 14 extension cords to power this up. After running a couple of hours he stopped the machine for awhile to do other things. When he attempted to get it going again about a half hour later he said it hummed loudly. He quickly unplugged it and noticed that there was a big spark at the plug end and the motor was quite hot. A while later he tried it again
and it ran fine. I suggested that he check the ends of the cords and just like I had expected he said that they were all quite warm. We replaced this mess with one continuous length of # 12 with 20 amp ends and all seems fine now so I suspect low voltage or rather excessive voltage drops were his problem. But I have a couple of questions. No one has ever really explained to me why low voltage can damage an AC motor. I had thought that as per ohms law with less voltage
there should be less of a current draw and therefore less heat. My other question concerns the initial motor failure. Wat kind of mechanism would cause a motor to shut down as it appeared to by drawing excessive current. It behaved just like a bad, (shorted) motor initially and then resumed working perfectly later on. Thanks. Lenny

You're correct about the voltage drop, it's not good for an AC motor.

In your case however, you have a single phase motor and most likely a
centrifugal switch and the initial start using this switch to connect
a starting coil directly or via a capacitor, depending on the motor
type.
When it spins up the centre force will open the switch and disconnect
that start coil. This process draws more current than normal and the
switches are not that great to start with. The heat of the prior
operation could have cause it to get stuck and the starting process
didn't switch off, that causes a loud hum and excessive current.


As for running low voltage to a motor you need to be concerned about
having enough to not cause the switch to engage once running.

We operate 460V 3 phase motors with 230V 3 phase, the reason for this
is to have a low torque motor. When load is not against it it'll spin up
to speed otherwise it'll softly drop back with no damage and just stop
if need be. This is good for driving acme screws that pick up heavy
loads but you don't want the screw to get jammed. This seems to work
better than using a inverter for torque mode because it responds faster
to the load than the inverter can.

We run 3 phase motors as tension motors with a 3 phase variac where the
user can manually adjust the hold torque and the rotor does very little
turning at all.

The biggest problems with reduced voltage to AC motors like this is you
need cooling. These motors depend on air flowing through them otherwise
you need a motor with an open frame to let it vent or a fan on the
back. Those that drive the fan themselves depend on the motor being up
to speed.
Also effective induction decreases when the rotor slows down and is
normally due to load, this causes the current to go up in the motor.

Insufficient voltage to the motor can also cause the current to go up
because the rotor isn't cutting through fast enough to lower the effects
that causes counter force in the field. Most of the time this isn't
damaging, although it'll make the motor run a little hotter than normal
and it'll also cost you more to operate it..

Hope that did something for you.

Jamie

 

[Rheilly Phoull]

On 26/06/14 10:16, hrhofmann@sbcglobal.net wrote:

The motor starts out running by using a starting winding that draws maybe 30% more current than normal. Typically, if the heavier current causes the voltage to be low, the motor cannot get up to speed to disconnect the starting winding as Jamie said.

Every plug/socket that is in the connection is a weak point for the overall system. Connections weaken with repeated heating and cooling, and may develope "crud" on the contacts that make the connection even worse. By disconnecting the plug your son may have cleaned up the contacts just enough that the next time he tried to start the motor the connection was just a little better, but enough better that the mtor was able to get up to speed. There may also have been a difference on the mechanical load on the motor when starting up, depending on how the loam screening is connected (or not) when the motor starts up.

And there is usually a thermal device that opens when the motor gets too
hot which would explain the shutdown.

 

[dave]

On 06/26/2014 07:58 AM, bud-- wrote:

On 6/25/2014 5:50 PM, captainvideo462009@gmail.com wrote:
My son was using a 1.5HP 120V AC motor to power his loam screening
machine. He was using two 50ft # 12 and one 75 ft # 14 extension
cords to power this up. After running a couple of hours he stopped the
machine for awhile to do other things. When he attempted to get it
going again about a half hour later he said it hummed loudly. He
quickly unplugged it and noticed that there was a big spark at the
plug end and the motor was quite hot. A while later he tried it again
and it ran fine. I suggested that he check the ends of the cords and
just like I had expected he said that they were all quite warm. We
replaced this mess with one continuous length of # 12 with 20 amp ends
and all seems fine now so I suspect low voltage or rather excessive
voltage drops were his problem. But I have a couple of questions. No
one has ever really explained to me why low voltage can damage an AC
motor. I had thought that as per ohms law with less voltage there
should be less of a current draw and theref
ore less heat. My other question concerns the initial motor failure. Wat
kind of mechanism would cause a motor to shut down as it appeared to by
drawing excessive current. It behaved just like a bad, (shorted) motor
initially and then resumed working perfectly later on. Thanks. Lenny


The speed of an AC motor is basically set by the frequency. If the load
(HP) is the same, and the voltage is lower, the current has to be higher
to supply the same HP. Higher current produces higher heat. (But the
normal running temperature may be higher than you think.)


The NEC gives a running current for a 1.5 HP motor (at full mechanical
load) as 20A (which is likely somewhat high). Typical starting currents
for AC motors are about 6x the run current. Likely the motor did not
start because of voltage drop at the starting current. Pulling the plug
during a non-start you are interrupting something like 6x 20A. (The
current would have been significantly lower because of voltage drop.)

Is the motor an exact replacement? Are the RsPM the same? Otherwise you
may need a different pulley.

 

[bud--]

On 6/25/2014 5:50 PM, captainvideo462009@gmail.com wrote:

My son was using a 1.5HP 120V AC motor to power his loam screening machine. He was using two 50ft # 12 and one 75 ft # 14 extension cords to power this up. After running a couple of hours he stopped the machine for awhile to do other things. When he attempted to get it going again about a half hour later he said it hummed loudly. He quickly unplugged it and noticed that there was a big spark at the plug end and the motor was quite hot. A while later he tried it again and it ran fine. I suggested that he check the ends of the cords and just like I had expected he said that they were all quite warm. We replaced this mess with one continuous length of # 12 with 20 amp ends and all seems fine now so I suspect low voltage or rather excessive voltage drops were his problem. But I have a couple of questions. No one has ever really explained to me why low voltage can damage an AC motor. I had thought that as per ohms law with less voltage there should be less of a current draw and theref
ore less heat. My other question concerns the initial motor failure. Wat kind of mechanism would cause a motor to shut down as it appeared to by drawing excessive current. It behaved just like a bad, (shorted) motor initially and then resumed working perfectly later on. Thanks. Lenny

The speed of an AC motor is basically set by the frequency. If the load
(HP) is the same, and the voltage is lower, the current has to be higher
to supply the same HP. Higher current produces higher heat. (But the
normal running temperature may be higher than you think.)


The NEC gives a running current for a 1.5 HP motor (at full mechanical
load) as 20A (which is likely somewhat high). Typical starting currents
for AC motors are about 6x the run current. Likely the motor did not
start because of voltage drop at the starting current. Pulling the plug
during a non-start you are interrupting something like 6x 20A. (The
current would have been significantly lower because of voltage drop.)

 

On Wednesday, June 25, 2014 9:54:40 PM UTC-4, Maynard A. Philbrook Jr. wrote:

In article <6f0d8f6b-01c7-405b-9cc2-c4124f93711a@googlegroups.com>,

captainvideo462009@gmail.com says...

My son was using a 1.5HP 120V AC motor to power his loam screening machine. He was using two 50ft # 12 and one 75 ft # 14 extension cords to power this up. After running a couple of hours he stopped the machine for awhile to do other things. When he attempted to get it going again about a half hour later he said it hummed loudly. He quickly unplugged it and noticed that there was a big spark at the plug end and the motor was quite hot. A while later he tried it again

and it ran fine. I suggested that he check the ends of the cords and just like I had expected he said that they were all quite warm. We replaced this mess with one continuous length of # 12 with 20 amp ends and all seems fine now so I suspect low voltage or rather excessive voltage drops were his problem. But I have a couple of questions. No one has ever really explained to me why low voltage can damage an AC motor. I had thought that as per ohms law with less voltage

there should be less of a current draw and therefore less heat. My other question concerns the initial motor failure. Wat kind of mechanism would cause a motor to shut down as it appeared to by drawing excessive current. It behaved just like a bad, (shorted) motor initially and then resumed working perfectly later on. Thanks. Lenny



You're correct about the voltage drop, it's not good for an AC motor.



In your case however, you have a single phase motor and most likely a

centrifugal switch and the initial start using this switch to connect

a starting coil directly or via a capacitor, depending on the motor

type.

When it spins up the centre force will open the switch and disconnect

that start coil. This process draws more current than normal and the

switches are not that great to start with. The heat of the prior

operation could have cause it to get stuck and the starting process

didn't switch off, that causes a loud hum and excessive current.





As for running low voltage to a motor you need to be concerned about

having enough to not cause the switch to engage once running.



We operate 460V 3 phase motors with 230V 3 phase, the reason for this

is to have a low torque motor. When load is not against it it'll spin up

to speed otherwise it'll softly drop back with no damage and just stop

if need be. This is good for driving acme screws that pick up heavy

loads but you don't want the screw to get jammed. This seems to work

better than using a inverter for torque mode because it responds faster

to the load than the inverter can.



We run 3 phase motors as tension motors with a 3 phase variac where the

user can manually adjust the hold torque and the rotor does very little

turning at all.



The biggest problems with reduced voltage to AC motors like this is you

need cooling. These motors depend on air flowing through them otherwise

you need a motor with an open frame to let it vent or a fan on the

back. Those that drive the fan themselves depend on the motor being up

to speed.

Also effective induction decreases when the rotor slows down and is

normally due to load, this causes the current to go up in the motor.



Insufficient voltage to the motor can also cause the current to go up

because the rotor isn't cutting through fast enough to lower the effects

that causes counter force in the field. Most of the time this isn't

damaging, although it'll make the motor run a little hotter than normal

and it'll also cost you more to operate it..



Hope that did something for you.
I liked it anyways, but I know "squat" about motors.

George H.

Jamie

 

[Jon Elson]

captainvideo462009@gmail.com wrote:

No one has ever really explained to me why low voltage can

damage an AC motor. I had thought that as per ohms law with less voltage
there should be less of a current draw and therefore less heat.

Ohm's law does NOT APPLY to AC induction motors. The excitation of the
rotor shorting bars depends on applied magnetism from the stator coils.
As the stator magnetic field decreases, the induced field on the rotor
decreases, and the stator current must increase to provide the rotor
with field. So, as the voltage goes DOWN, the current goes UP!
Thus, the power input to the motor remains roughly constant.

My other
question concerns the initial motor failure. Wat kind of mechanism would
cause a motor to shut down as it appeared to by drawing excessive current.
It behaved just like a bad, (shorted) motor initially and then resumed
working perfectly later on. Thanks. Lenny

Many motors have a thermal protector, sometimes self-resetting, sometimes
manual reset. But, that should cut the motor completely off when
overheated. Possibly it is wired wrong, so it only cuts out the starting
circuit. Or, the windings and cord were so hot it couldn't draw enough
current to start under load.

Jon

 

On Wed, 25 Jun 2014 16:50:08 -0700 (PDT), captainvideo462009@gmail.com
wrote:

My son was using a 1.5HP 120V AC motor to power his loam screening machine. He was using two 50ft # 12 and one 75 ft # 14 extension cords to power this up. After running a couple of hours he stopped the machine for awhile to do other things. When he attempted to get it going again about a half hour later he said it hummed loudly. He quickly unplugged it and noticed that there was a big spark at the plug end and the motor was quite hot. A while later he tried it again and it ran fine. I suggested that he check the ends of the cords and just like I had expected he said that they were all quite warm. We replaced this mess with one continuous length of # 12 with 20 amp ends and all seems fine now so I suspect low voltage or rather excessive voltage drops were his problem. But I have a couple of questions. No one has ever really explained to me why low voltage can damage an AC motor. I had thought that as per ohms law with less voltage there should be less of a current draw and
therefore
less heat. My other question concerns the initial motor failure. Wat kind of mechanism would cause a motor to shut down as it appeared to by drawing excessive current. It behaved just like a bad, (shorted) motor initially and then resumed working perfectly later on. Thanks. Lenny

What Jon says about AC induction motors is correct. It is the nature
of these motors to draw more current as the voltage drops. And if the
voltage is low enough the motor will not be able to start spinning and
as a consequence will draw as much current as possible which can cook
the motor and/or the wires supplying the power. Universal motors, like
those in a vacuum cleaner, do not behave this way.
Eric

 

[Dave M]

bud-- wrote:

On 6/25/2014 5:50 PM, captainvideo462009@gmail.com wrote:
My son was using a 1.5HP 120V AC motor to power his loam screening
machine. He was using two 50ft # 12 and one 75 ft # 14 extension
cords to power this up. After running a couple of hours he stopped
the machine for awhile to do other things. When he attempted to get
it going again about a half hour later he said it hummed loudly. He
quickly unplugged it and noticed that there was a big spark at the
plug end and the motor was quite hot. A while later he tried it
again and it ran fine. I suggested that he check the ends of the
cords and just like I had expected he said that they were all quite
warm. We replaced this mess with one continuous length of # 12 with
20 amp ends and all seems fine now so I suspect low voltage or
rather excessive voltage drops were his problem. But I have a couple
of questions. No one has ever really explained to me why low voltage
can damage an AC motor. I had thought that as per ohms law with less
voltage there should be less of a current draw and theref
ore less heat. My other question concerns the initial motor failure.
Wat kind of mechanism would cause a motor to shut down as it appeared
to by drawing excessive current. It behaved just like a bad,
(shorted) motor initially and then resumed working perfectly later
on. Thanks. Lenny


The speed of an AC motor is basically set by the frequency. If the
load (HP) is the same, and the voltage is lower, the current has to
be higher to supply the same HP. Higher current produces higher heat. (But
the
normal running temperature may be higher than you think.)


The NEC gives a running current for a 1.5 HP motor (at full mechanical
load) as 20A (which is likely somewhat high). Typical starting
currents for AC motors are about 6x the run current. Likely the motor did
not
start because of voltage drop at the starting current. Pulling the
plug during a non-start you are interrupting something like 6x 20A. (The
current would have been significantly lower because of voltage drop.)

To make the numbers easy to handle, assuming that your whole extension cord
is a 14ga cord. Do the math on the extension cord; you'll find that a 175ft
14ga copper extension cord has a resistance of approximately 29 ohms (round
trip). That means that for a running motor current of 15A, the wire will
produce a voltage drop of about 13.25V. That's for a single conductor wire,
not enclosed in a jacket or in a bundle. For wiring enclosed in a jacket
and at elevated temperature, you must derate the ampacity of the wire by
about half. In other words, a 14ga extension cord could cause the voltage
at the motor to be reduced to about 100V. You could ease that loss by the
inclusion of the 12ga cords, but then you have to assume some additional
voltage drop caused by the plugs and sockets in the total length of the run.
Additionally, the heat generated by the wire in the extension cords can
cause the insulation to melt and cause a short circuit or fire.
For a 175 ft. run, the wire property charts say that for a 15A load, you
should be using a 6ga. extension cord. That would limit the voltage drop at
15A to about 4V, which the motor should be happy with.

If you measure the voltage that your motor is getting, you might be
surprised to see how low it goes at startup and after running for a period
of time. Low voltage to an induction motor can cause damage to the motor.
Overheating at a minimum, burned windings in the worst case.

Dave M

 

[bud--]

On 6/26/2014 7:47 PM, Dave M wrote:

bud-- wrote:
On 6/25/2014 5:50 PM, captainvideo462009@gmail.com wrote:
My son was using a 1.5HP 120V AC motor to power his loam screening
machine. He was using two 50ft # 12 and one 75 ft # 14 extension
cords to power this up. After running a couple of hours he stopped
the machine for awhile to do other things. When he attempted to get
it going again about a half hour later he said it hummed loudly. He
quickly unplugged it and noticed that there was a big spark at the
plug end and the motor was quite hot. A while later he tried it
again and it ran fine. I suggested that he check the ends of the
cords and just like I had expected he said that they were all quite
warm. We replaced this mess with one continuous length of # 12 with
20 amp ends and all seems fine now so I suspect low voltage or
rather excessive voltage drops were his problem. But I have a couple
of questions. No one has ever really explained to me why low voltage
can damage an AC motor. I had thought that as per ohms law with less
voltage there should be less of a current draw and theref
ore less heat. My other question concerns the initial motor failure.
Wat kind of mechanism would cause a motor to shut down as it appeared
to by drawing excessive current. It behaved just like a bad,
(shorted) motor initially and then resumed working perfectly later
on. Thanks. Lenny


The speed of an AC motor is basically set by the frequency. If the
load (HP) is the same, and the voltage is lower, the current has to
be higher to supply the same HP. Higher current produces higher heat. (But
the
normal running temperature may be higher than you think.)


The NEC gives a running current for a 1.5 HP motor (at full mechanical
load) as 20A (which is likely somewhat high). Typical starting
currents for AC motors are about 6x the run current. Likely the motor did
not
start because of voltage drop at the starting current. Pulling the
plug during a non-start you are interrupting something like 6x 20A. (The
current would have been significantly lower because of voltage drop.)


To make the numbers easy to handle, assuming that your whole extension cord
is a 14ga cord. Do the math on the extension cord; you'll find that a 175ft
14ga copper extension cord has a resistance of approximately 29 ohms (round
trip). That means that for a running motor current of 15A, the wire will
produce a voltage drop of about 13.25V.

What a radical idea - do a calculation.

The NEC table gives the resistance of #14 as 3.14 ohms per thousand ft.
For 350' round trip that should give about 1 ohm and a voltage drop of
15 volts. That is about what you came up with on voltage drop but not
resistance.

The NEC would like a max voltage drop of 5%, but it is not a
requirement. Motors often have a rating of 220V, which is the voltage at
the motor on a nominal 240V circuit. That would be about an 8% drop.

When starting, the current drop would be a lot higher, and quite
possible the motor wouldn't start.

Would be interesting what the actual current draw of the motor is.

That's for a single conductor wire,
not enclosed in a jacket or in a bundle. For wiring enclosed in a jacket
and at elevated temperature, you must derate the ampacity of the wire by
about half. In other words, a 14ga extension cord could cause the voltage
at the motor to be reduced to about 100V. You could ease that loss by the
inclusion of the 12ga cords, but then you have to assume some additional
voltage drop caused by the plugs and sockets in the total length of the run.
Additionally, the heat generated by the wire in the extension cords can
cause the insulation to melt and cause a short circuit or fire.

For most cords with 2 current carrying conductors the NEC gives a
current rating of 18A for #14 and 25A for #12. If run at their ratings I
would expect them to be "quite warm".

For a 175 ft. run, the wire property charts say that for a 15A load, you
should be using a 6ga. extension cord. That would limit the voltage drop at
15A to about 4V, which the motor should be happy with.

If you measure the voltage that your motor is getting, you might be
surprised to see how low it goes at startup and after running for a period
of time. Low voltage to an induction motor can cause damage to the motor.
Overheating at a minimum, burned windings in the worst case.

Dave M

 

[David Lesher]

Buy a Kill-a-Watt and you can see the voltage drop, current drawn, & watts.

<http://www.p3international.com/products/p4400.html>


--
A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

 

[josephkk]

On Wed, 25 Jun 2014 19:16:28 -0700 (PDT), "hrhofmann@sbcglobal.net"
<hrhofmann@sbcglobal.net> wrote:

The motor starts out running by using a starting winding that draws maybe 30% more current than normal. Typically, if the heavier current causes the voltage to be low, the motor cannot get up to speed to disconnect the starting winding as Jamie said.

30%, what a laugh! Induction motors pull up to 10X running current to
start. 6X is quite normal. About the only case where this is not true is
with reduced voltage starters (which are primarily used only with big
motors (100 kVA and up).

>Every plug/socket that is in the connection is a weak point for the overall system. Connections weaken with repeated heating and cooling, and may develope "crud" on the contacts that make the connection even worse. By disconnecting the plug your son may have cleaned up the contacts just enough that the next time he tried to start the motor the connection was just a little better, but enough better that the mtor was able to get up to speed. There may also have been a difference on the mechanical load on the motor when starting up, depending on how the loam screening is connected (or not) when the motor starts up.

No serious issues with that.

?-)

 

[Phoena J]

On 06/25/2014 11:50 PM, captainvideo462009@gmail.com wrote:
> No one has ever really explained to me why low voltage can damage an AC motor.

It goes something like this. Renee Everhart was using a vibrator and the
batteries were carbon zinc and they died rather quickly so
Renee took out the vibrator and stomped on it and broke the motor. I
know that batteries are DC and not AC but you get the picture.

 

[Maynard A. Philbrook Jr.]

In article <lpkcml$q96$1@dont-email.me>, phoena@happilychildfree.com
says...

On 06/25/2014 11:50 PM, captainvideo462009@gmail.com wrote:
No one has ever really explained to me why low voltage can damage an AC motor.

It goes something like this. Renee Everhart was using a vibrator and the
batteries were carbon zinc and they died rather quickly so
Renee took out the vibrator and stomped on it and broke the motor. I
know that batteries are DC and not AC but you get the picture.

That sounds like it needs to be past to the testicle department!

Jamie

 

[David Lesher]

captainvideo462009@gmail.com writes:

No one has =
ever really explained to me why low voltage can damage an AC motor.

The motor delivers power to its load. The motor tries hard to do
that, even if the voltage is off spec.

Power_out = Power_in - losses in the motor

Power_in = Volts * Amps * Power Factor.

If power out stays the same, than power in does as well.

If Volts goes down, current must go up to maintain the same power in.

When the current goes up, so do the current-related losses,
most importantly "I^2R losses" which [shockingly!] goes up as
the SQUARE of the current. Those losses present as heat in the
motor's windings.

IOW: something melts/burns outs.




--
A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

 

On Fri, 11 Jul 2014 22:04:36 +0000 (UTC), David Lesher
<wb8foz@panix.com> wrote:

captainvideo462009@gmail.com writes:

No one has =
ever really explained to me why low voltage can damage an AC motor.

The motor delivers power to its load. The motor tries hard to do
that, even if the voltage is off spec.

Power_out = Power_in - losses in the motor

Power_in = Volts * Amps * Power Factor.

If power out stays the same, than power in does as well.

If Volts goes down, current must go up to maintain the same power in.

When the current goes up, so do the current-related losses,
most importantly "I^2R losses" which [shockingly!] goes up as
the SQUARE of the current. Those losses present as heat in the
motor's windings.

IOW: something melts/burns outs.

Also, when induction motors slow down they will draw much more current
than when running at rated speed. A lower voltage will cause the
induction motor to slow, the back EMF will drop, and the current will
go way up. And since, as you point out, the heating from losses goes
up with the square, it really doesn't take long for the magic smoke to
escape. And if the speed drops low enough for the starting winding on
a single phase motor to turn on then things will get hot really,
really fast. Since corded drill motors, lawn mowers and weed whackers
use universal motors they just slow down with a voltage drop and don't
try to consume more current to get back up to speed.
Eric

 

[Maynard A. Philbrook Jr.]

In article <lppn1k$ih5$1@reader1.panix.com>, wb8foz@panix.com says...

If Volts goes down, current must go up to maintain the same power in.

When the current goes up, so do the current-related losses,
most importantly "I^2R losses" which [shockingly!] goes up as
the SQUARE of the current. Those losses present as heat in the
motor's windings.

IOW: something melts/burns outs.

You forgot the magic blue smoke.


Jamie

 

[Maynard A. Philbrook Jr.]

In article <9hq0s95gb360bb3r5t0orr3u6uo5f3kbll@4ax.com>,
etpm@whidbey.com says...

On Fri, 11 Jul 2014 22:04:36 +0000 (UTC), David Lesher
wb8foz@panix.com> wrote:

captainvideo462009@gmail.com writes:

No one has =
ever really explained to me why low voltage can damage an AC motor.

The motor delivers power to its load. The motor tries hard to do
that, even if the voltage is off spec.

Power_out = Power_in - losses in the motor

Power_in = Volts * Amps * Power Factor.

If power out stays the same, than power in does as well.

If Volts goes down, current must go up to maintain the same power in.

When the current goes up, so do the current-related losses,
most importantly "I^2R losses" which [shockingly!] goes up as
the SQUARE of the current. Those losses present as heat in the
motor's windings.

IOW: something melts/burns outs.
Also, when induction motors slow down they will draw much more current
than when running at rated speed. A lower voltage will cause the
induction motor to slow, the back EMF will drop, and the current will
go way up. And since, as you point out, the heating from losses goes
up with the square, it really doesn't take long for the magic smoke to
escape. And if the speed drops low enough for the starting winding on
a single phase motor to turn on then things will get hot really,
really fast. Since corded drill motors, lawn mowers and weed whackers
use universal motors they just slow down with a voltage drop and don't
try to consume more current to get back up to speed.
Eric

Keep in mind however, there are many applications of using induction
motors that do not get up to speed and may even just sit there for
tension operations.

So if the motor has not been sized and vented properly, you'll have
smoke!

We put 3 phase 480 motors in a 240 volt circuit so that we can get
speed with very little load and when load hits the wall, the motor
simple stalls. The motors have a self cooled frame so running them
like this does no harm and it saves on dealing with inverters.

Those that do not have self cooled frames will get on the warm side.

You don't want to do this with single phase starting aid type motors
unless you externally control that part of the circuit. with split
phase motors you can allow them to drag once they get turning and not
allow the start switch to engage. I am sure you've seen many single
phase motors with bad starters that you can spin start.

Jamie

 

[bud--]

On 7/11/2014 4:57 PM, etpm@whidbey.com wrote:

On Fri, 11 Jul 2014 22:04:36 +0000 (UTC), David Lesher
wb8foz@panix.com> wrote:

captainvideo462009@gmail.com writes:

No one has =
ever really explained to me why low voltage can damage an AC motor.

The motor delivers power to its load. The motor tries hard to do
that, even if the voltage is off spec.

Power_out = Power_in - losses in the motor

Power_in = Volts * Amps * Power Factor.

If power out stays the same, than power in does as well.

If Volts goes down, current must go up to maintain the same power in.

When the current goes up, so do the current-related losses,
most importantly "I^2R losses" which [shockingly!] goes up as
the SQUARE of the current. Those losses present as heat in the
motor's windings.

IOW: something melts/burns outs.

Also, when induction motors slow down they will draw much more current
than when running at rated speed.

When a higher load slows the motor down the current goes up - higher
power to the load results in higher power to the motor.

A lower voltage will cause the
induction motor to slow, the back EMF will drop, and the current will
go way up.

The motor speed is primarily set by the frequency in induction motors.
If the speed dropped significantly with a voltage drop it would likely
be advantageous - the motor would likely supply less power, so the
current may not increase with lower voltage. It is particularly true for
loads like fans, where the power is something like the 3rd power of the
RPMs.

The problem is the speed does not drop much, so same power out, and then
same power in, and higher current to compensate for the lower voltage.

And since, as you point out, the heating from losses goes
up with the square, it really doesn't take long for the magic smoke to
escape. And if the speed drops low enough for the starting winding on
a single phase motor to turn on then things will get hot really,
really fast.

Many motors are "thermally protected" (internal temperature sensor) so
it disconnects until it cools. Larger motors are likely to have a "motor
starter" that trips on high current.

Since corded drill motors, lawn mowers and weed whackers
use universal motors they just slow down with a voltage drop and don't
try to consume more current to get back up to speed.
Eric