Continuing Education: Technology
Regenerative Elevator Drives: What, How and Why
by Donald Vollrath, Magnetek, Inc.
cities. Despite many improvements
toward safety and ease of use, the el-
After reading this article, you
evators of today still use the same
should have learned about:
counterweighted lifting mechanism
N How mechanical and electrical
and a traction sheave on the suspen-
power are related
sion ropes for the simple reason that
N How the cycling of an elevator
counterbalancing the weight (mass)
affects the flow of energy
of the car against gravity reduces the
N How various motors and drives
amount of force required to move
react to bidirectional energy flow
the car up and down the hoistway.
N How regeneration can lower Also, when additional counterweight
energy costs is added to balance the system with
N Why safety concerns of regen- an average payload, the least
eration need to be dispelled amount of effort is required for the
expected range of typical loads.
Various forms of elevators have Less effort to move an average
been with us for several hundred load also means less average energy
years. At first, they were crane-like consumption. In today’s world, en-
mechanical lifts with drum-and- ergy conservation is one of the major
winch mechanisms powered by hu- goals for improvement. We typically
mans, draft animals, water wheels or pay for energy as measured by the
steam. Block-and-tackle roping allowed kilowatt-hour utility meter. What-
even the heaviest loads to be lifted. ever we can do to conserve and min-
For repeated use, inventers imize energy use will certainly help Value: 1 contact hour
quickly learned to counterbalance save money and conserve energy (0.1 CEU)
the weight of the lifting platform or supplies.
car cage in order to reduce the effort While it is recognized that many
This article is part of ELEVATOR WORLD’s
required to move expected payloads short-rise buildings use hydraulic lifting
Continuing Education program. Elevator-industry
up and down the hoistway. Many im- mechanisms to operate an elevator, personnel required to obtain continuing-education
provements toward elevator safety the illustrative discussion within this credits can receive one hour of credit by reading
and practical electric motors were article pertains only to traction ele- the article and completing the assessment exami-
invented near the end of the 19th vators. nation questions found on page 127.
century. Despite scientific disputes Energy For this article and more continuing-education
about AC versus DC electric power What is this thing we call energy? opportunities, visit www.elevatorbooks.com
distribution, almost all elevators From a scientific point of view, en- NATIONAL ASSOCIATION
OF ELEVATOR CONTRACTORS
were converted and built to be pow- ergy is the act of doing work, in CERTIFIED
Approved by NAEC
ered by electric motors by the 1920s. which the basic definition is a force EDUCATION PROGRAM
Drum-and-winch operation gave multiplied by the distance an object
Approved by NAESA
way to counterweighted systems is moved. There are two major forms International
with direct traction sheaves on steel of mechanical energy involved when
suspension ropes. operating elevators: potential and ki-
Approved by QEI Services, Inc.
The electric-powered elevator be- netic energy.
came a catalyst, allowing a boom of Potential energy is gained or lost
tall-building construction in large by an object (mass) as it is raised or
June 2010 | ELEVATOR WORLD | 95
Continuing Education: Technology Continued
lowered against the force of gravity. occurs at the same time. The flow of justing the voltage (and frequency of
An elevator must put energy into the power is also bidirectional. When the an AC drive) is the primary mecha-
mass of the payload when it is lifted demand for electric power is meas- nism by which to control how much
(weight X distance). That same ured and accumulated over a period power flows to or from the motor.
amount of potential energy will of time, the result is the sum of all When the power converter pro-
come out of the payload when it is work accomplished over that period. vides voltage in excess of the
lowered by gravity. Electric meters at the utility entrance counter voltage (counterelectromo-
Kinetic energy is the energy of to buildings do exactly that. They tive force [CEMF] generated inter-
motion stored within a mass as it is total the amount of energy flow at nally by the rpm of the motor), elec-
moving. We often refer to this as in- the end of the month. This is a true trical power flows into the motor to
ertia or momentum. The magnitude measurement of electrical energy create the accelerating or lifting
of kinetic energy in a moving object supplied by the utility company to torque on the shaft and elevator
is equal to one-half the mass X the the measured premises. sheave. When the power converter
square of velocity. It must be added The motors that move elevators output voltage is less than motor
to any object in order to make it ac- are power-converter devices. Elec- CEMF, the motor acts as a generator
celerate or move. Likewise, that tric power goes in (volts X amps or to transform mechanical power
same amount of kinetic energy must watts) and gets converted to me- (torque X speed) into electricity that
be removed from the mass to make chanical power (torque X rpm or flows backward into the power con-
it stop. Kinetic energy becomes im- horsepower). Motors and generators verter. The design and type of elec-
portant when we consider how ele- are constructed alike and operate in trical power converter used to adjust
vators are constructed and how fast much the same way. All electric mo- motor speed affect what happens to
they are moving. tors can act as electric generators. regenerated energy provided by the
All of the major parts of an eleva- Each time the shaft of a motor is me- lift motor.
tor must move in order to move the chanically forced to turn faster than DC Elevator Motors with Ward-
payload. The need for mechanical the electric power source, it causes Leonard (MG Set) controls
strength dictates that the car frame mechanical power (torque X speed The Ward-Leonard MG set is an
and counterweight have significantly of the turning shaft) to be converted AC induction motor connected to
more mass than the payload rating to electric power (volts X amps). utility lines that turns the shaft of a
of the elevator. As a result, the total Therefore, power can flow in either DC generator at a relatively constant
kinetic energy added to accelerate direction through an electric motor. speed. The DC elevator motor is con-
and then removed to decelerate the This is true of DC motors, with or nected to the output of the genera-
elevator apparatus is four to five without brushes; asynchronous AC tor. Adjusting the field strength of
times what it would be if only the induction motors; and permanent- the generator directly varies the out-
payload were moved. Note that en- magnet (PM) synchronous motors. put voltage applied to the DC eleva-
ergy flow being discussed is me- This principle allows energy tem- tor motor. The value of generator
chanical and bidirectional. That is, porarily stored in the mass of a mov- voltage in relation to elevator motor
we must put energy into the me- ing elevator to be recovered. CEMF determines the direction and
chanical parts and payload to make Adjustable speed drives (ASDs) magnitude of actual power flow.
it accelerate and rise. Energy comes are a form of electrical power con- When the elevator motor is provid-
out again when we stop the motion verters typically used to convert the ing braking force on the load, power
or lower the load. With an elevator, fixed voltage electricity of utility-line flow is from motor to generator. The
this action repeatedly occurs many mains to adjustable voltage that con- generator then acts like a motor and
times during the day. The cyclic na- trols the speed of elevator motors. attempts to speed up the shaft con-
ture of energy flow yields an ideal ASDs are often thought of as being nected to the AC induction motor. As
opportunity to minimize the overall only AC drives and motors, but the the induction motor is pushed slightly
consumption. speed of DC motors can be easily ad- faster than the utility frequency and
The rate at which energy flows to justed by voltage control and have voltage allow, it essentially becomes
accomplish work is called power, or been doing so to operate elevators an induction generator, creating and
force times speed in mechanical for many years. Ward-Leonard-style pushing AC power back into the util-
units. The electrical units are watts motor-generator (MG) sets are a ity lines. Equipment efficiencies
(or kilowatts), which is voltage (the form of DC drive, as are silicon-con- come into play, but the fact that an
force) multiplied by amperes (the trolled rectifier (SCR)-DC adjustable MG set regenerates is often over-
rate or speed of electron flow) that speed controls. In either case, ad- looked. Even though the AC motor
96 | WWW.ELEVATOR-WORLD.COM | June 2010
on the MG set draws a significant and motor to the mechanical load. Regenerative Add-on Modules
amount of current at a low power When the applied voltage and fre- A power-regeneration module
factor when idle, the utility-line cur- quency are less than the generated can be added to most conventional
rent is always free of harmonics. CEMF, the motor will act like a gen- motor-control inverters to regulate
DC Elevator Motors with erator. Mechanical power pushing power flow from a DC source back
SCR-DC Drive Controls on the shaft is converted to electrical into utility power lines. Operation is
An SCR-DC drive converts utility AC power by the motor, and the in- similar to a second inverter operat-
AC voltage to a variable DC voltage verter directs that power back into ing in synchronization with the
by a selective rectification process the DC bus. power-line frequency. When eleva-
known as phase control. The SCR Non-Regenerative Inverters tor motor and inverter action pumps
devices act like switches to connect Many inverters for AC motor con- power back into the DC bus, causing
specific portions of the AC utility trol have a one-way power rectifier the voltage to rise higher than the
voltage sine waves to the DC motor. on the front end to convert AC volt- utility line peaks (empty car up or full
Adjustment of the phase timing of age from utility lines to DC voltage car down as explained above), the
each SCR switch in relation to the in- that is temporarily stored on an in- regeneration module pushes excess
stantaneous AC voltage varies the ternal capacitor filtered power bus. power back into utility lines. This ac-
average DC voltage applied to the This is the ideal low-cost way to pro- tion keeps the rise in DC bus voltage
motor. A dual SCR bridge circuit al- vide DC voltage for operating an in- under control such that the DC brak-
lows motor current to flow in either verter controlling an AC motor. ing resistor is not used. The energy
direction to and from utility lines. However, the rectifier can pass that would have been wasted as heat
Adjusting the average DC output power in only one direction. When- in a braking resistor is returned to
voltage of the power converter in re- ever elevator operation is such that the utility distribution system. This
lation to motor CEMF controls the di- the motor and inverter are regener- makes a significant difference in
rection and magnitude of power. The
ating power to control the speed of a overall energy consumption, particu-
SCR power conversion process is
gravity-fed overhauling load (empty larly with a low-friction PM gearless
particularly efficient when motoring
car up or full car down) or simply ex- elevator system.
or regenerating. However, the vari-
tracting out kinetic energy to slow Where Does Regenerated
able-voltage phase control mecha-
down the moving mass, energy will Energy Go?
nism does create significant current
be pumped into the DC bus, resulting Electric power distribution within
harmonics on utility lines in either
in an increase of voltage on DC bus a building facility often takes one of
operating mode with a widely vary-
storage capacitors. the forms illustrated in Figures 1 or
ing power factor.
The conventional way to resolve 2. A single-use building, such as a
AC Elevator Motors with Inverters
excessive voltage buildup is to add a corporate office or hotel, may have a
Speed and torque of an AC motor,
circuit that temporarily turns on a single utility meter located where the
be it synchronous or asynchronous,
is controlled by an electronic power switch to dump excess energy into a utility power enters the facility.
inverter adjusting the frequency and bank of resistors. Resistive braking is Mixed-use commercial buildings
voltage applied to motor terminals. one way to handle energy returned have a combination of common-use
There are several types of inverters, from the elevator equipment, but it electrical equipment for hall lighting;
and all use rapidly controlled elec- represents pure losses to the meas- heating, ventilation and air-condi-
tronic switch pulsing (pulse-width ured electrical consumption. There tioning (HVAC); and elevators, as
modulation) to convert constant are other losses related to efficiency well as individual metering for multi-
voltage from a temporary DC voltage of the elevator equipment system, ple tenants, such as restaurants,
source, or bus, to three-phase, vari- but 25-40% of the total elevator en- business offices or apartment
able-voltage AC at an adjustable fre- ergy consumption is typically lost in dwellings. The utility distribution
quency. Although various technical the resistor bank. Heat released by voltage step-down transformer may
schemes are used for precise con- the resistors must be removed from be outside in a neighborhood utility
trol, motoring occurs when the volt- the control room. One must pay an vault or on a local utility pole for
age and frequency applied to the AC additional amount for any power smaller buildings. Large buildings
motor is greater than the CEMF gen- used to operate ventilating fans or often have a substation step-down
erated inside the motor in proportion air-conditioning units that may be transformer built into the basement
to rotating speed. Power flows from required to remove heat away from or other convenient location. In ei-
the DC source, through the inverter the resistor bank. ther case, there is a metered power
June 2010 | ELEVATOR WORLD | 97
Continuing Education: Technology Continued
distribution center (breaker panel) ment regenerates, power can flow di- using the same energy twice results
where multiple wiring feeders rectly back to the distribution panel in reduced utility costs.
spread out to other areas within the and out to some other equipment al- Be aware that the nameplate rat-
building. Elevators usually have their ready in use within the building with- ing on an elevator motor (in volts,
own power riser dedicated to that out passing through the utility meter. amps, horsepower or kilowatts) is
service, but elevator feeders are al- During the few seconds in which based on the power it takes to raise
most never separately metered. elevator regeneration occurs, the the maximum rated payload of the
Even though the combined horse- meter doesn’t need to run backward; elevator at rated speed. However,
power rating of all the elevators may it simply slows down while the en- the elevator is not always loaded to
be large, elevator operation is spo- ergy recovered from the elevator rated capacity, and it only operates
radic, and the actual electrical load is momentarily helps to power a differ- for short periods of time. Also, effi-
relatively small compared to the sum ent elevator, operate lights in the ciency losses dictate that the total of
of continuous loads of other feeder hallway or run the HVAC fans. There regenerated power will always be
branches attached to the same distri- are no other noticeable effects, such less than that required for full-load-
bution panel. The total of all power as voltage surges or distortion, up service, and it can regenerate power
drawn by electrical loads in the build- which can cause lights to blink or less than half of the operating time.
ing is measured by the utility meter, other electronic equipment disrup- The end result is that only a small
and the rate that the utility meter tion. Some of the energy that was portion of the full load electrical rat-
spins depends on night or day occu- once metered and used to start the ing of the elevator is regenerated
pancy conditions, outside temperature, elevator moving or lift the load is re- during a typical day, and it is an even
other machinery use and whether or covered and used again to perform a smaller portion of the total energy
not the elevator lift is in operation. different task within the building. A consumption of the entire building.
Note that when the elevator equip- smart building owner will notice that Regeneration Versus Cogeneration
Although it may sound the same,
OTHER MACHINERY there is a significant difference be-
tween a regenerating elevator sys-
tem and cogeneration. A cogenera-
WALL OUTLET tion (wind turbine, fuel cell,
UTILITY PANEL OUTLETS
photovoltaic or waste gas recovery
generator) system is intended to op-
DOMESTIC CONTROL LIGHTING erate at near capacity almost contin-
UTILITY BUILDING STEP-DOWN
uously. The goal of cogeneration is
SUB-STATION SPEED ELEVATOR
STEP-DOWN ELEVATOR MOTOR to supply a major portion of the en-
TRANSFORMER DRIVE 1
INTERNAL POWER ELEVATOR
ergy required for the local premises
DISTRIBUTION BUS POWER
RISER or even sell electricity to the utility
SPEED ELEVATOR company for distribution to other
Figure 1: Single-point utility metering
DRIVE paying customers. Power regener-
ated by elevator equipment is spo-
NEXT BUILDING radic and relatively small. It is in-
tended to save energy by recycling it
NEXT BUILDING to be consumed by other loads
within the building. Using reclaimed
TENANT TENANT 1
METERING PANEL energy for a useful purpose rather
than wasting it as heat is an effective
TENANT TENANT 2
form of overall energy conservation.
When an elevator drive regener-
TENANT TENANT 3
METERING PANEL ates, the recovered power actually
flows backward into the utility system.
COMMON MISC EQUIPMENT
In most cases, local electrical loads
MAINS STEP-DOWN COMMON LIGHTING will absorb the energy before it reaches
ADJ the kilowatt-hour meter. If local loads
BLDG SERVICE SPEED ELEVATOR
MOTOR are not large enough to absorb all the
COMMON SERVICES PANEL
& DISTRIBUTION ELEVATOR SERVICE RISER
regenerated power, it will still flow
Figure 2: Multiple-tenant building with individual metering backward into the utility system.
98 | WWW.ELEVATOR-WORLD.COM | June 2010
However, many utility meters are Other safety concerns related to generation captures a large portion
ratcheted so that they will not run regenerating elevator drives are un- of the mechanical energy operating
backward, but power will still pass founded. Motor control drives, those elevators and puts it back into the
through them toward the distribution with and without regeneration abil- electrical system, where it can be
grid. Energy will be conserved, but the ity, are designed with features that reused by another elevator or to
building owner will not necessarily re- sense the condition of utility power. power other electrical loads. Capturing
ceive the full monetary benefit. This is a design necessity in order to the regenerative energy can help to
In the event that emergency be a reliable product in the elevator conserve and minimize energy use,
power generators are used to oper- industry. If utility power is weak or which will certainly help save money
ate elevators when utility power is fails unexpectedly while the drive is and conserve energy supplies.
not available, energy-absorbing in operation, it must sense the poor
loads must also be attached to the power condition and shut down in a
controlled manner in order to sur- Donald Vollrath is a principal engineer of the
same generator. An emergency-
vive. If utility power is interrupted Elevator Drives division of Magnetek, Inc., head-
power consumption analysis should
quartered in Menomonee Falls, Wisconsin. He has
be performed in order to determine while an elevator is in flight, the
more than 40 years of experience developing AC
the minimum size of generator to be drive will quickly sense that condi-
and DC motor drives and controls for industrial
used for this purpose. Very often, tion and shut down. According to el-
applications. During the past 20-plus years, Vollrath’s
lighting and HVAC loads powered by evator safety code rules, interlocked work has shifted almost exclusively toward de-
the same generator will absorb all control circuits will also engage the signing and perfecting the application of several
regenerated energy from elevators. mechanical brake to bring the eleva- generations of general-purpose and custom motor
However, operating only a single el- tor to a safe stop when the drive control equipment for elevators. He holds a BS in
evator from an equally sized emer- ceases to operate. Elevator code also Electrical Engineering from the University of Illinois.
gency power generator will not requires overspeed protection in the
work, due to the sporadic nature of form of a governor with an electrical
switch that opens control circuits Learning-Reinforcement Questions
elevator operation and the inability
that turn motor controls off and en- Use the learning-reinforcement
of the generator to absorb any signif-
gage the mechanical brake. On high- questions below to study for the
icant amount of regenerated energy.
speed lifts, there may be an addi- Continuing Education Assessment
One can reduce the magnitude of
tional motor dynamic braking circuit Exam available online at www.
regenerated energy by reducing ele-
to further assist in the braking effort. elevatorbooks.com or on page 127
vator speed during emergency power
The combination of these features of this issue.
situations. The maximum amount of
means that an elevator will be N Why are elevators counterweighted?
regenerated power and energy will
quickly and safely brought to a halt, N What is the difference between
be reduced in direct proportion to any
even if it happens to be regenerating power and energy?
reduction in operating speed. The in-
at the time of power disconnection. N Which three main factors deter-
ternal losses in most elevator systems
The faulty-power-sensing mecha- mine how much power is required
will allow operation up to 10-15% of
nism of regenerative drives mentioned to operate an elevator?
rated speed without regenerating
above will cause the drive to shut N What is meant by bidirectional
any significant power back into util-
down within fractions of a second in power or energy flow?
ity or generator supply lines.
the event of a utility power disconnec- N How does weight of the elevator
In the case of elevator drives with
tion. Energy of the moving elevator equipment figure into the energy
an add-on regeneration unit, it may
will be transferred to the mechanical requirement?
be possible to use control logic to
braking system as the elevator is N What controls the flow of power
disable regeneration and engage
brought to a halt. It will not continue through an elevator motor?
conventional dissipative resistor
to be injected into the utility line. N What provides the energy to lower
braking under emergency power
Regeneration occurs naturally in an elevator load?
conditions. It is more difficult (and
thousands of buildings all over the N Which type of motor controls can
costly) to provide for backup dissipa-
world, as long as the utility system and cannot regenerate?
tive braking on elevator drives de-
N Where does the energy go if the
signed to automatically regenerate remains healthy and strong. The
drive cannot regenerate?
while controlling utility-line har- massive capacity of the utility grid al-
N How does regeneration actually
monics and power factor. Most ele- lows the momentary energy required
save on the utility bill?
vator equipment furnished today is to operate an elevator to flow in ei-
N What happens to the energy if
not designed to have its regenera- ther direction and be relatively un-
there is a utility power failure?
tion feature turned on and off. noticed in large buildings. True re-
June 2010 | ELEVATOR WORLD | 99
Continuing Education: Technology Continued
ELEVATOR WORLD Continuing Education
Assessment Examination Questions
N Read the article “Regenerative Elevator Drives: What, How and Why”
(page 95) and study the learning-reinforcement questions.
N To receive one hour (0.1 CEU) of continuing-education credit, answer the
assessment examination questions found below online at www.elevatorbooks
.com or fill out the ELEVATOR WORLD Continuing Education Reporting Form
found overleaf and submit by mail with payment.
N Approved for Continuing Education by NAEC for CET® and NAESAI and QEI
Services, Inc. for QEI
1. What does the utility meter measure? 7. What happens if there is a utility power failure while an
a. Peak kilowatt demand. elevator drive is regenerating?
b. Average kilowatt-hour demand. a. The drive will blow up.
c. Kilowatt-hour consumption. b. The elevator uses the brake to stop.
d. Kilovolt-ampere consumption. c. People in the neighborhood may get electrocuted.
d. The elevator overspeeds.
2. What most affects the peak power demand of an ele-
vator? 8. If an elevator drive is not designed to regenerate,
a. Whether the drive can regenerate. where does the energy go?
b. Operating speed X capacity. a. It is wasted as heat into the brake.
c. Acceleration rate X equipment mass. b. It is wasted as heat into the motor.
d. The type of elevator drive. c. It is wasted as heat into a resistor bank.
d. It is wasted as heat into the drive.
3. What most affects the overall energy consumption of
an elevator? 9. Does regeneration disturb other utility customers?
a. Operating speed X capacity. a. No, voltage distortion that other customers see will
b. Acceleration rate X equipment mass. be no different than with a non-regenerative motor
c. The type of motor. drive.
d. Whether the drive can regenerate. b. Yes, they must be disconnected.
c. Yes, they will be disturbed by current harmonics.
4. How do electric motors regenerate? d. Yes, regeneration causes the line voltage to sag and
a. The motor runs faster. surge.
b. The motor acts like a generator.
c. They have PMs. 10. To be compatible with a regenerative elevator drive,
d. Regeneration has nothing to do with the motor. an emergency power generator needs to:
a. Be equal in kW size with the elevator motor.
5. Which drives can regenerate? b. Be greater than two times the kW size of the eleva-
a. MG sets. tor motor.
b. AC inverters with diode rectifiers. c. Have other loads attached to absorb all regenerated
c. SCR-DC drives. power.
d. Both a and c. d. Have switchable loads to accept regenerated power.
6. If an elevator drive can regenerate, where does the en-
a. It heats up braking resistors.
b. It temporarily powers other loads within the building.
c. It heats up the motor.
d. It gets stored in the adjustable-speed motor drive.
June 2010 | ELEVATOR WORLD | 127
Continuing Education: Technology Continued
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