Induction motor anti-loading

D

David Lesher

Guest
Take a stock 3-ph 7.5HP motor driving a load. At normal load, speed
is 1725rpm, as you have slip, and it draws X amps. Everyone is happy.

Oh, it's in a escalator.

What happens when the LOAD is dragging the motor along; so the shaft
speed is 1850-1900 RPM? This because the escalator is going DOWN,
and full of football players.

I'm thinking the motor will expend power to reduce the slip, trying
to get DOWN to sync speed - the mirror of what it does when it's
too slow. But I can't recall anything from 2 quarters of Machines
that covered that.

[Yea, there's LOTS of drag in the innards, so much so I doubt it could
freewheel; but I still wonder...]




--
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
 
David Lesher wrote:
Take a stock 3-ph 7.5HP motor driving a load. At normal load, speed
is 1725rpm, as you have slip, and it draws X amps. Everyone is happy.

Oh, it's in a escalator.

What happens when the LOAD is dragging the motor along; so the shaft
speed is 1850-1900 RPM? This because the escalator is going DOWN,
and full of football players.

I'm thinking the motor will expend power to reduce the slip, trying
to get DOWN to sync speed - the mirror of what it does when it's
too slow. But I can't recall anything from 2 quarters of Machines
that covered that.
Click to expand...
Yes, it generates. I recall from the power machines module (the only
time I've touched on them) that induction generators are popular in
remote hydroelectric schemes because they have a leading power factor,
and compensate for the lag in the long lines to civilisation.


Paul Burke
 
Paul Burke wrote:

David Lesher wrote:

Take a stock 3-ph 7.5HP motor driving a load. At normal load, speed
is 1725rpm, as you have slip, and it draws X amps. Everyone is happy.

Oh, it's in a escalator.

What happens when the LOAD is dragging the motor along; so the shaft
speed is 1850-1900 RPM? This because the escalator is going DOWN,
and full of football players.

I'm thinking the motor will expend power to reduce the slip, trying
to get DOWN to sync speed - the mirror of what it does when it's
too slow. But I can't recall anything from 2 quarters of Machines
that covered that.


Yes, it generates. I recall from the power machines module (the only
time I've touched on them) that induction generators are popular in
remote hydroelectric schemes because they have a leading power factor,
and compensate for the lag in the long lines to civilisation.


Paul Burke

They're also good for various opportunistic power generation methods
Click to expand...
because the shaft does not have to be synchronized to the power line --
they generate some inductive VARs (that leading power factor that you
talked about), but the shaft speed doesn't have to be well controlled at
all.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
"David Lesher" <wb8foz@panix.com> wrote in message
news:clkia9$7fh$1@reader1.panix.com...
Several of you seem to have my PoV reversed.

I do NOT think an oversped induction motor generates; I believe the
opposite. Why? Shaft power from an induction motor is a function of
slip, the delta between sync. speed [1800 or 3600 RPM for the most
case in 60 Hertzland] and rated speed [1725/3450 RPM]. The closer
to sync speed the motor runs, the less voltage induced into the
cage, ergo the less lines of force to interact with the rotating
stator field. In the limit, if the motor were at sync speed; it
would produce no shaft power...and draw only I^2R losses & other minor
ones.

My question is: what is different if the shaft speed is ABOVE the
sync speed? You still have slip, you still have induced lines of
force to interact with the rotating stator field, there's work in
changing the shaft speed, and load.... etc.

Other comments:

I don't recall anything about induction cage generators. Wound-rotor
ones, sure.

Unloaded synchronous motors are leading PF; and I recall their use
to improve power factor -- I guess before capacitors were cheap
enough/available. But nothing comes to mind re: such w/ induction
devices...

I chose escalators as I watched a crew replacing DC-drives in a
number of WMATA stations with 7.5HP AC ones. One has the longest
one in the Western world; 230 ft. and that has six of the 7.5 HP
motors.

No, they are not in pairs. There are several at a station, but it's
not unusual to have one running up and none down.



All that said; it's be decades since Prof Klingshirn's able instruction
and I could easily be out to lunch. I welcome insight.

--
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
Click to expand...
I have definitely heard of induction generators too..

Tom
 
Paul Hovnanian P.E. wrote:

No. They have lagging power factors, even when driven as generators.
They're popular because they are easy to synchronize with the line.
Click to expand...
I actually looked this up in my long- abandoned textbook, A.Draper,
"Electrical Machines", Longman 1967.

"...the induction generator is only capable of supplying power to a
system to which synchronous machines are connected...It can only supply
generated current at a leading power factor.."

The torque curve is symmetrical, so the OP should be careful not to get
too may footballers jumping on his escalator at once, or it might bounce
it over the peak to the low- torque, high slip region.

Paul Burke
 
Yzordderex wrote:
I have definitely heard of induction generators too..

Tom


Yes, induction motors will generate when overhauling. It is common
for manufacturers of induction motor speed controls to use a pair of
coupled motors to load their drives when they are burning them in.
Connect the shafts together, run the drive motor up to 60hz - 1800rpm,
throw the driven motor on the line and away you go. Just have to
adjust the drive to about 62hz and you will get full load.

regards,
Bob
Click to expand...
Thats how we did it. When you do fault-current calculations you have to
take into account the energy stored in the rotating plant - a fault on
the grid turns motors into generators, and in our case more than doubled
the prospective fault current

A fun trick with an induction motor is to bolt it down securely, attach
a large flywheel and drive it from an inverter rated at about 20x the
machine power. Spin the flywheel up to say 2x rated speed, then demand
an instantaneous reversal. If the inverter and flywheel are big enough,
the rotor will tear itself to pieces - a fun way to spend an afternoon,
but ensure you are co-linear with the axis of the machine, lest bits
come to get you :). Under fault conditions it is quite possible for
large machines to detach themselves from the floor and go walkabout -
the same is true inside power distribution transformers.

Cheers
Terry
 
Terry Given <my_name@ieee.org> writes:

Under fault conditions it is quite possible for
large machines to detach themselves from the floor and go walkabout -
the same is true inside power distribution transformers.
Click to expand...
Oh yes, I have seen some impressive pictures. A oh 12' long 5'dia
generator that someone brought on line when all three lamps
were ON, not off.

The rotor immediately tried to be 120 degrees away from where it
was, and failing that, the stator moved instead. That of course
meant the mounts failed and it blithely rolled out through a wall
or two into the parking lot where some parked car foolishly tried
to stop it.

--
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
 
Several of you seem to have my PoV reversed.

I do NOT think an oversped induction motor generates; I believe the
opposite. Why? Shaft power from an induction motor is a function of
slip, the delta between sync. speed [1800 or 3600 RPM for the most
case in 60 Hertzland] and rated speed [1725/3450 RPM]. The closer
to sync speed the motor runs, the less voltage induced into the
cage, ergo the less lines of force to interact with the rotating
stator field. In the limit, if the motor were at sync speed; it
would produce no shaft power...and draw only I^2R losses & other minor
ones.

My question is: what is different if the shaft speed is ABOVE the
sync speed? You still have slip, you still have induced lines of
force to interact with the rotating stator field, there's work in
changing the shaft speed, and load.... etc.

Other comments:

I don't recall anything about induction cage generators. Wound-rotor
ones, sure.

Unloaded synchronous motors are leading PF; and I recall their use
to improve power factor -- I guess before capacitors were cheap
enough/available. But nothing comes to mind re: such w/ induction
devices...

I chose escalators as I watched a crew replacing DC-drives in a
number of WMATA stations with 7.5HP AC ones. One has the longest
one in the Western world; 230 ft. and that has six of the 7.5 HP
motors.

No, they are not in pairs. There are several at a station, but it's
not unusual to have one running up and none down.



All that said; it's be decades since Prof Klingshirn's able instruction
and I could easily be out to lunch. I welcome insight.

--
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
 
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