lock roto & voltage drop

[D]

Hello I have a question on induction scenarios.
I read that an electric motor will pull more amps
at start up (lock rotor state). I have not as yet fully
visualized why more current flows thru the windings
when they are not spinning and/or spinning as fast.
But this brings another curious thought to mind. Does
the same theory apply to say like ceiling fans? I.e. when
the fan is on a rheostat or different speed switches, doesn't
this drop the voltage so the fan rotates slower and does
this cause more current to flow thru the windings also??
And one other question, if the correct wire size for the
amp rating on a motor is close to maxed out
(say like #12 copper and the pump pulls 16.5amps normal
running speed) but I need to run more than 100 ft ( say like
140 ft ) of wire to get to the motor, do I need to use the
next higher (#10 copper) wire to keep the voltage from
dropping too low over the length of the conductor??

 

[Rich Grise]

on Thursday 01 July 2004 05:59 pm, D wrote:

Hello I have a question on induction scenarios.
I read that an electric motor will pull more amps
at start up (lock rotor state). I have not as yet fully
visualized why more current flows thru the windings
when they are not spinning and/or spinning as fast.

google "back emf".

But this brings another curious thought to mind. Does
the same theory apply to say like ceiling fans? I.e. when
the fan is on a rheostat or different speed switches, doesn't
this drop the voltage so the fan rotates slower and does
this cause more current to flow thru the windings also??

The slower rotation causes lower back emf, yes, but there's
a lower supply voltage, which predominates.

And one other question, if the correct wire size for the
amp rating on a motor is close to maxed out
(say like #12 copper and the pump pulls 16.5amps normal
running speed) but I need to run more than 100 ft ( say like
140 ft ) of wire to get to the motor, do I need to use the
next higher (#10 copper) wire to keep the voltage from
dropping too low over the length of the conductor??

I'd use the next higher size for _any_ run, and go up to
#8 for 100'; and I'd still keep an eye on the motor (or
thermometer) for a few days to feel sure that it's working
OK.

--
Cheers!
Rich

 

[Rheilly Phoull]

"Rich Grise" <null@example.net> wrote in message
news:1430257.4y6xWq8G62@entheos.thunderbird.ops.dsl-verizon.net...

on Thursday 01 July 2004 05:59 pm, D wrote:

Hello I have a question on induction scenarios.
I read that an electric motor will pull more amps
at start up (lock rotor state). I have not as yet fully
visualized why more current flows thru the windings
when they are not spinning and/or spinning as fast.

google "back emf".

But this brings another curious thought to mind. Does
the same theory apply to say like ceiling fans? I.e. when
the fan is on a rheostat or different speed switches, doesn't
this drop the voltage so the fan rotates slower and does
this cause more current to flow thru the windings also??

The slower rotation causes lower back emf, yes, but there's
a lower supply voltage, which predominates.

And one other question, if the correct wire size for the
amp rating on a motor is close to maxed out
(say like #12 copper and the pump pulls 16.5amps normal
running speed) but I need to run more than 100 ft ( say like
140 ft ) of wire to get to the motor, do I need to use the
next higher (#10 copper) wire to keep the voltage from
dropping too low over the length of the conductor??

I'd use the next higher size for _any_ run, and go up to
#8 for 100'; and I'd still keep an eye on the motor (or
thermometer) for a few days to feel sure that it's working
OK.

--
Cheers!
Rich

Also the fan design and windings are different and will withstand much more

abuse.


--
Regards ........... Rheilly Phoull

 

[John Popelish]

D wrote:

Hello I have a question on induction scenarios.
I read that an electric motor will pull more amps
at start up (lock rotor state).

In general, the motor draws the least current when the rotor spins in
lock step with the AC frequency, and the current rises smoothly as the
rotor slip increases. This implies that at maximum possible slip
(locked rotor,) the current is highest.

I have not as yet fully
visualized why more current flows thru the windings
when they are not spinning and/or spinning as fast.

In induction motor can be visualized as a transformer that has half of
the core rotating and with a short circuited secondary. In an
ordinary transformer, the lowest current draw occurs when the
secondary is open circuited, and hits maximum draw if the secondary is
shorted. The induction motor does not draw high current into its
shorted secondary, if the core and secondary can flip over each half
cycle.

But this brings another curious thought to mind. Does
the same theory apply to say like ceiling fans? I.e. when
the fan is on a rheostat or different speed switches, doesn't
this drop the voltage so the fan rotates slower and does
this cause more current to flow thru the windings also??

If the fan was slowed by an increased torque load, then, yes, the
current would increase as it slowed. But if the slowing is caused by
a lowered applied voltage (and the torque load actually falls as the
fan slows) then the current may drop as the fan slows. The details
involve the design of the rotor and how resistive the rotor shorting
bars are, and how deeply they are buried in the rotor iron.

And one other question, if the correct wire size for the
amp rating on a motor is close to maxed out
(say like #12 copper and the pump pulls 16.5amps normal
running speed) but I need to run more than 100 ft ( say like
140 ft ) of wire to get to the motor, do I need to use the
next higher (#10 copper) wire to keep the voltage from
dropping too low over the length of the conductor??

This is a good idea if the motor is starting under a significant
torque load. The temporary voltage sag may not allow enough locked
rotor current to get it moving so that the speed can rise quickly
enough to bring the current down to rated. In that case, the fuse is
likely to blow, to protect the wire.

--
John Popelish

 

[Don Kelly]

"Rich Grise" <null@example.net> wrote in message
news:1430257.4y6xWq8G62@entheos.thunderbird.ops.dsl-verizon.net...

on Thursday 01 July 2004 05:59 pm, D wrote:

Hello I have a question on induction scenarios.
I read that an electric motor will pull more amps
at start up (lock rotor state). I have not as yet fully
visualized why more current flows thru the windings
when they are not spinning and/or spinning as fast.

google "back emf".
-

You are unlikely to find references on "back emf" (a much misused term) of
much help when considering induction motors.
John Popelish has it correct -minimum input impedance at standstill
(assuming the motor is not rotating in the reverse direction at the time it
is energised. )
--------------------------

But this brings another curious thought to mind. Does
the same theory apply to say like ceiling fans? I.e. when
the fan is on a rheostat or different speed switches, doesn't
this drop the voltage so the fan rotates slower and does
this cause more current to flow thru the windings also??

The slower rotation causes lower back emf, yes, but there's
a lower supply voltage, which predominates.
------

John has this one right as well.
--
Don Kelly
dhky@peeshaw.ca
remove the urine to answer

-----------

And one other question, if the correct wire size for the
amp rating on a motor is close to maxed out
(say like #12 copper and the pump pulls 16.5amps normal
running speed) but I need to run more than 100 ft ( say like
140 ft ) of wire to get to the motor, do I need to use the
next higher (#10 copper) wire to keep the voltage from
dropping too low over the length of the conductor??

I'd use the next higher size for _any_ run, and go up to
#8 for 100'; and I'd still keep an eye on the motor (or
thermometer) for a few days to feel sure that it's working
OK.

--
Cheers!
Rich