Transcript Making Motors Dance Our Tune

Making Motors Dance Our Tune
© 2012 Dr. B. C. Paul
Global Spec.com Thomasnet.com
Tricky Things We Like Motors To Do
• Go at what ever speed we need them to turn at
the moment.
• Apply torque gently and bring things up to speed
without breaking things into pieces
• Turn our load at any speed we desire even if it
does not match the motor
The Problem of Speed Matching
• Motor speeds are fixed by number of poles in the motor
and current frequency (assuming an AC motor)
• For 60 cycles per second – 3 phases – one rotor pole you will
get about 3600 rpm
• For 6 phases in the stator and for a two pole rotor you will get
about 1800 rpm
• For a 3 pole rotor you will get 1200 rpm
• Further increases in phases in the stator or poles on the rotor
usually don’t make much sense
• The Chances that what we want to drive needs one of
those 3 speeds is pretty remote.
Chain and Sprocket Speed
Reduction
Will get you about 93% efficiency
In the open.
About 95% efficiency in an oil
Tight enclosure.
V Belt and Sheave Speed Reduction
About 94% efficient.
Many Sheaves have multiple sizes so you
Can shift belts from one sheave to another for
Easier changes of speed.
Spur Gear Speed Reduction
Cut Spur Gears can get
About 90% efficiency
Cast Spur Gears only
About 85%
Gears can be noisy and
Require either multiple
Gear reduction phases or
Some pretty big gears.
Helical Gears
Single reduction
About 95% efficient
Double is about 94%
Triple is about 93%
Worm Gears
Speed reductions of about
20:1 or less 90% efficient
Speed Reduction up to 60:1
About 70%
Reductions of 100:1 down to
50%
Sizing Sheaves and Sprockets
• I would like to use an induction motor turning at
3500 rpm to run a pump using a belt drive at 850
rpm
• To avoid excessive bending of my belt I need the
smaller sheave to be 9 inches in diameter.
• How big does the other sheave need to be?
• Which Sheave is on the motor and which on the
pump shaft?
A More Friendly Coupling
• Magnetic Coupling
Take two disks – put magnets in them – when you spin one disk the other disk will try to spin.to
Keep the magnets lined up
I Can Get a Gear Ratio by adjusting
the number of magnets in each
wheel
Two Basic Designs
Radial Design
Axial Design
Now Why Would I Want to Do That?
• There is no joint contact to have to lubricate
• I can transmit power through a wall or seal
without having a hole in it
• I will not be pushing on the bearings on one side
trying to keep two sides of a gear or belt
assembly tight
• Vibration on one side does not transmit to the
other side
More Whys
• It is forgiving of misalignment up to about 2 degrees
Can Provide Torque Control
• Can design magnetic fields to slip if torque goes
above a design tolerance
• Can prevent things from breaking
• Can cause it to go about 30% slower on driven
side compared to the drive side by adjusting the
air gap.
How Efficient Can This Be
• With good alignment, small gaps and little
slippage you can get about 99 to 99.9% efficiency
• Even on bad day you should be above 97%
• Unlike other systems you don’t have frictional losses
So What is the Catch?
• Optimized magnetic coupler systems are not an off the
shelf item
• Magnetic fields do induce electrical fields and flux flow
through solids does produce heating
• Magnets don’t like heat (loose magnetism)
• Can combat with expensive magnets (get up to about 300 F)
• Can put air blown cooling systems or such on the coupling
• Speed or torque limits are generally a design
characteristic of the product and do not change on the fly
Next Chapter
How Can I Vary the Speed of a Motor on
the Fly?
How to Speed Control A Motor
• Use a DC motor
• I can control its speed with a variable resistor that controls the
voltage I apply to the electro magnetic poles on the stator.
• Use a synchronous AC motor
• The motors speed is a function of the number of poles on the
rotor and the frequency of the AC current.
• Use a solid state AC frequency controller to produce what
ever AC frequency I need.
• Use a Switched Reluctance motor and switch the DC
current to poles to make the rotor turn at the desired
rate.
Problem of DC Speed Control
• Where do I get enough DC current to run a 1000
HP motor?
• That’s a whole lot of batteries in series
• We need a source of DC current to run the motor
• Since Thomas Edison lost the fight to make the US a
DC electric system there isn’t much big DC current
around to be had
How About Our Own DC Generator
• Turns out DC Generator is not to hard to come by - It’s a
DC Motor
• If I turn the shaft on a DC Motor I force the armature to turn
• Remember that the current loops on the armature generated a
back voltage
• vg = Køη
• Now I just tap the voltage off at the ring and brush contacts
and I have my current
• Note that DC voltage is direct function of the speed at
which the armature is turned
Just One Little Problem
• As the armature coil rotates through the
magnetic field the current direction alternates
as different ends of the loop line up with the
fixed poles
• I didn’t want an AC generator
• Electricity is tapped off the loop by rings
contacted with brushes
• break the rings so that positive and negative
reverse whenever the current reverses
• broken ring system is called a commutater
The Result
Voltage Produced by One Loop on the Armature after
Commutation
Because we have several loops that peak at different times
(and capacitors) we can get an almost perfectly smooth DC
voltage
Just One Little Problem
• Need to find enough constant speed hamsters to turn a
DC generator to produce a constant voltage for a DC
motor
• How About Using an AC motor instead
• We have lots of AC voltage around
• We’ve met the Synchronous AC motor so we can get a
constant DC current from a constant speed AC motor
So Lets See How this Works
• We need speed control so we get a DC motor
• We now need big DC voltage to run the DC Motor
so we get a DC generator
• We now need something to turn the DC
generator at a constant speed regardless of load
so we get a synchronous AC Motor
Ward Leonard
• The combination of a synchronous AC motor to
turn a DC generator is called a Motor Generator
Set
• Also called a Ward Leonard System after the people
who developed it
• Through most of the 20th Century it was the only
way to get speed controlled systems large
enough for mining applications
Strengths of Ward Leonard
• It is a fully reversible system
• AC and DC motors could just as easily be generators if you
turn the motor with the shaft instead of the motor turning the
shaft
• For a lot of large machines that have inertia to change
directions you can recapture the power from the inertia
• It is very robust against fluctuations in the power supply
• It has a very large spinning reserve
• If voltage is below 70% of rated then you have a problem.
Weaknesses of Ward Leonard
• Motors are loaded with rings and brushes that mean
maintenance cost and down down.
• Everything in the system is an inductor so it will throw
your power factor off – especially at low load
• Can put extra capacitors all over the lines to try to correct the
power factor
• Inertia – those spinning rotors add the inertia of big
machines and make them respond slowly
Alternatives to Ward Leonard
• Solid State Electronic have semiconductor diodes
that only let current flow one way
• Arranged in proper circuits these diodes can be
used to reverse the oscillations of an AC Current
Simplified Thyristor Circuit
+
-
AC
Source
DC Motor Load
Simplified Thyristor Circuit
+
Limitation - Note that if the load
becomes a generator and tries to feed
power back into the line it can’t
Thyristor Controls
• Also called SCR for Silicon Controlled Rectifier
• Also called DC Static
• Thyristor Controls allow us to directly produce DC
current from an AC line
• This allows us to use DC speed controlled motors without
having to install a motor generator set
• Barrier for many years was getting thyristor
systems big enough for a competitive price
Advantages of DC Static
• Solid State Electronics replaces lots of moving
parts
• Less maintenance
• Fast change out of solid state boards
• Less inertia and faster machine motion response
Disadvantages
• It can be less forgiving of variations in line power
• It works fast to moderate the problem
• It Dislikes Dirt and Moisture – ouch – this is a mine
• Means some sort of isolation cabnet.
• It is not reversible to generate power back
• You can put in another reverse thyrister circuit but this can be
very costly compared to the recovered power (done for little
electronics – usually not big machines)
• It too lowers the power factor angle – especially at low
speeds
AC Frequency Control for Speed
• AC current is brought into a thyristor system and
rectified to DC
• The DC current is then sent to another solid state
device called an inverter
• An inverter reverses the + - value of a DC current
• It can be made to turn inversion off and on at a
regular frequency
• We have now simulated an Alternating Current
with any frequency we feel like
Controlled Frequency AC
• With our AC frequency controlled to any value we
can now speed control an AC motor using solid
state controls
• Only about the 1990s that we got to AC frequency
control large enough to run mine hoists
• Mid 1980s saw Bucyrus Erie introduce the first Freq
Controlled AC motor mining shovels
AC Freq Control
• Strengths
• 0.95 Power Factor is almost constant
• NO RINGS OR BRUSHES improves availability
• Most area under speed torque curve - great power
and response of motions
• Electronics are amenable to modularization with
board change outs (though there are more of them
than with DC static)
• Limits or eliminates brushes and high wear parts
AC Freq Continued
• Weaknesses
• Can’t regenerate power back into the line
• Need to keep those electronic isolated from a dirty
environment
• More vulnerable to dirty disrupted AC power supply
• Power utilization usually not quite up with DC static
• Can still be a little pricey (but those cheap AC motors can
save weight and offset some of electronics cost)
• AC Freq Control will probably come to dominate
increasingly in the future
Switched Reluctance
• A variation of AC frequency control
• AC current is rectified to DC
• Now instead of using an inverter to create a specific
frequency of AC current
• We use switch electronics to turn electromagnetic fields
off and on on the motor
• Often use PLC to try to control a very non-linear motor
and reduce torque ripple.
Variations on the AC Frequency
Control Theme
• AC Frequency control with a synchronous AC
motor gives strict speed control
• But AC syncronous motor is most high wear and
complex.
• AC Frequency control with an induction motor
• Simple with fewer moving parts
• But speed control is not strict
• Induction motors run a slower speed than current in order
to generate torque
Switched Reluctance
• Its really switched DC
• Computer controlled motor
• Lots of electronics to worry about
• Has good starting torque but not best for low speed
operation.
So What Am I Likely to Pick
• Cheap and simple is the first choice
• AC induction motor is cheapest and simplest
• Motor does slip relative to frequency of current but if
we have variable frequency control that may not be
important most of the time.
Now for Trying to Start a Motor
• And bring something up to speed without jerking
it to pieces
• Means both speed and torque control
• Conveyors can be a classic problem statement
Getting Starting Force for a
Conveyor
• Starting Force is the Lift required to a fully loaded
belt
• (I don’t want to shovel the thing if I have to start a
loaded belt)
• Plus double the Frictional Force
• Static friction is always greater than rolling friction
• Practice is to use a factor of 2 to be safe
A Motor Sizing Example
• The lift Force is 66667.04 lbs tension to lift
up slope
• took it out of the tension calculations on an old
example problem
• The frictional Force is
• Total Effective Tension - 73367.16 lbs effective
tension
• Minus 66667.04 lbs tension to lift up
slope
• Net Friction - 6700.119
lbs tension
Continued
• Frictional Force is 13,400 lbs
• Lift Force is 66,667 lbs
• Total Starting Force is 80,067 lbs
Converting To Starting Torque
• Need to know pully sizes and diameters
• our assumed case is 4 ft.
• Had I picked motor before belt I wouldn't have known
Force
Torque = Force * Lever arm
Lever Arm
Getting Starting Torque
• Suppose the pully is 4 ft diameter
• 80,067 lbs * 2 ft = 160,134 ft-lbs torque
• We usually need a gear reducer
• belt speed is 400 fpm
• pully radius for 4 ft diam ¶ * D = 12.56 ft per
revolution
• pully speed is 31.85 rpm
• Big motors usually 1800 or 3600 syc speed (3500
and 1750 common with slip)
Getting our Gear Reducer
• 1750 rpm / 31.85 = 54.94
• say 55 to 1 gear reduction
• Torque on Motor Shaft is
• 160,134/55 = 2912 ft-lbs
• 2912 ft-lbs/ .94 (for gear reducer efficiency)
• 3098 ft-lbs
Checking Our Full Speed Torque
Requirements
• By same calculation Full Speed Torque
• 146,734 ft lbs at pulley shaft
• 146,734/ 55 / 0.94 = 2838 ft-lbs on motor shaft
• Motor Sized on Full Speed
• HP = 2838* 1750/5250 = 946 HP
• (5250 is a conversion constant for ft-lbs to horsepower)
• Note that Starting Torque is 3098 ft-lbs
• Running Torque is 2838 ft-lbs
• Need 9.1% more torque to start (can be a lot more)
This brings us back to Motor
Ratings and Starting Heating
• Motor heating is related to the current through the
stator – big current =big heat
• We have also seen that starting torque is high
• What about starting HP?
• HP = Torque * rpm/5250
• How many rpm does the motor turn when it is
standing still?
• Starting HP must be ZERO!
The Hot Issue
• Remember that the back voltage induced in the
stator windings corresponds to the HP output of
the motor
Applied
Voltage
If the back voltage is zero,
what happens to the stator
current?
Starting Impacts
• The heat produced by resistances in the stator is directly
proportional to the stator current.
• Result #1 - Motor starting produces tremendous heat
• Implication - you can only try so many starts (usually 2 an
hour) or you will overheat
• Result #2 - Motor draws high starting current - high
current draw increases the voltage drop on the feed lines
• Can cut starting voltage back and loose starting torque
depending on mine wiring
Speed Torque Relations
.
Locked Rotor Torque
Breakdown Torque
Rated Torque
Pull-Up Torque
Torque
Speed
Torque - HP Relations
• A and B type Motors
• Little 1 HP motor may have Lock Rotor of 275% of
rated torque
• By 50 HP that surplus is only 140%
• At 200 HP there is no surplus locked rotor toque
above rated
• Really big ones above 200 HP may only give you
80% of rated torque
Checking Whether Our Motor
Will Start the Belt
• We needed about 960 HP to run the belt
• This suggests a 1000 HP standard size motor
• Check out the starting torque (Locked Rotor)
• Rated Torque = 1000 * 5250/1750 = 3000 ft-lbs
• Locked Rotor at 80% of rated 2400 ft-lbs
• Need also to consider the impact of loss of
starting voltage due to excess current draw
Voltage Torque Relations
• Remember the field strength and thus the torque
varies with the square of the applied voltage
• Thus
• Torque Actual = Torque Rated * (Applied
Voltage2)/(Rated Voltage2)
• Most mine electrical systems are designed to
allow no more than 10% voltage drop when a big
motor kicks in
Reduced Voltage Impact on
Locked Rotor Torque
• Actual Lock Rotor Torque = 2400 * (0.92)/(12)
• 1944 ft-lbs of starting torque
• To start a loaded belt we need 3098 ft-lbs
• This thing isn’t going to start
What to do about our problem
• Use a bigger motor
• 1000 HP * 3098/1944 = 1594 HP
• Oh Boy I get to tell the boss we need a 1750 HP
Motor instead of a 1000 HP - that will go over really
well
Our Little Belt Starting Problem
• Get a Class C motor
• Nema only rates these to 200 HP
• Since Nema only rates motors to 500 HP we are
already in a manufacture specific range
• Class C motor usually have 50% to 100% more
starting torque than a B
• depending on drop in applied voltage and exact rated
Locked Rotor Torque 1000 to 1250 HP would work
Pull Up torque
• For many large motors about 90% of Locked
Rotor
90% of Locked Rotor
Checking to Make Sure the Conveyor doesn’t
stall out on Start Up
Speed Torque Relationship
10000
ft-lbs
8000
6000
Motor
4000
Load
2000
0
0
50
100
% of Synchronous Speed
Line for 1750 HP B type motor
150
Plot a line
from
starting
torque to
running
torque.
Make sure
it doesn’t
dip below
the pull-up
torque.
Bringing the belt up to speed
Torque available
above the load line
can accelerate the
belt load.
Speed Torque Relationship
10000
ft-lbs
8000
6000
Motor
4000
Load
2000
0
0
50
100
150
% of Synchronous Speed
This load will make a slow start - slow down even more and
then take off.
The Breakdown Torque Problem
• As the load gets moving the stator current drops
into line and voltage comes back to normal
• This motor produced a breakdown torque of
9,450 ft-lbs
• Torque on the pulley
• 9,450*55/1 = 519,750 ft-lbs
• Translated to belt tension
• 519,750 ft-lbs/ 2 ft = 259,875 lbs tension
So that’s Why they call it Breakdown Torque
• 259,875 lbs/ 24 inches = 10,828 lbs/inch width
• We can see that this breakdown torque will force
us to drastically over-design the belt to avoid it
being torn in two.
• Generally we would like a belt that could handle
about 5 to 10% more tension than required for
start up
The Problem of Taming the
Speed Torque Curves
The Ideal Belt Motor would have a fairly flat speed torque
curve about 10% above the line for load torque
Nema class C type motors have fairly flat curves - but are
usually well above 10% extra for the load torque line
Dealing with the Jerks (Not you the motor starting)
• Use reduced voltage starting
• Torque = Lock Rotor * (voltage used/rated voltage)2
• Can make not start
• Most even uses SCR system
Why Reduced Voltage Starting?
• NEMA C type motors have the right shaped curve
- its just too much more than what is needed
• The same Torque = Rated Torque * (V
applied2)/(V rated2) lowers the Torque curve
during start-up and reduced big voltage drops
from excess current draw
How is Reduced Voltage Starting
Accomplished?
• Simple Method
• Put a line of resistors in in front of the motor and the
short them out as motor speed comes up
Solid State Starters
• Usually SCR based
• One common design limits the stator current - just
raises and lower resistance to keep a controlled
stator current
• (More difficult to reshape speed torque curve but you
can raise and lower)
• Second Design monitors and accelerates belt in a
linear fashion
• fancier but can reshape curve
A More Elegant Solution
Impeller Wheel
Motor
I can have a motor turn an impeller wheel in the air.
It will impart a small amount of momentum to the air.
Now Lets Mount a Similar Wheel
to the Drive Shaft of Something
Turbine Wheel
Conveyor
Pulley
Now Lets Put an Impeller Wheel
and a Turbine Wheel Face to Face
With an Air Gap Between them so they don’t
Touch
When the impeller wheel spins a little of the air
Momentum will transfer to the other wheel.
Of course the efficiency sucks
Maybe We Can Jack Up Efficiency
if We Slap an Enclosure Housing
Around Them
Now we can make our fluid circulate around
In a coupling loop.
Of Course Air is a Pretty Thin Fluid
– How About Something Thicker
An oil might be a good choice
Now Lets See About Starting
Something
We start and standstill. Our motor begins rotating
Our impeller. Our impeller is pretty much sitting in
Thin air so our motor is starting at as close to zero
Load as possible – well that should solve any
Problems we have with an induction motor having
Wimpy starting torque and low pullup torque
As the Motor Speeds Up Our Fluid
Starts to Circulate
You begin to see turning pressure applied to
The turbine side of the coupling. Of course if
We have a stubborn load on the turbine side
The load can hold the turbine still. As the
Motor continues to speed up our fluid circulation
And coupling become more complete
Our Start Up Torque Looks Like
This
Oh Wow – There is a God – This is just what we wanted!
The torque builds up until the load starts to turn.
Which color curve the coupling torque follows depends
On the amount of fluid in it (which gives us yet another
Idea on how to control things).
Additional Fluid Coupling
Benefits
• No direct contact between parts means low
frictional wear
• No direct contact means low vibration transfer
Do We Have A Replacement for
Magnetic Couplings?
• Not really
• The impeller and turbine wheels must have fluid
circulation between them – no transmitting power
through walls
• The impeller and turbine will spin at the same speed
(less a slight amount of slip) – you can’t set a speed
ratio by changing the number of magnets on each
wheel
• You need a steady distance between the impeller and
turbine – no using the coupling for misalignment
tolerance
Can I Use My Fluid Coupling as a
Torque Limiter?
• Yes - like the magnetic coupling increasing load
causes increasing slip – until the slip goes to
100%
• In a fluid coupling this critical slip point can be
adjusted on the fly by pumping fluid into or out of the
coupling
What Happens with Slip
• The amount by which the turbine turns slower
than the impeller is the slip – it is about 1 to 6%
• Thus when the load turns at 96% of the drive speed
the slip is 4% and the efficiency is 96%
• If the load goes to 100% slip the energy into the
coupling is converted to heat
• The coupling has a heat plug that will blow and drain
the fluid when things get hot enough
Typical Curve for Sizing a Fluid
Coupling