Regenerative Elevator Drives What_ How and Why by dfgh4bnmu

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									 Continuing Education: Technology



Regenerative Elevator Drives: What, How and Why
by Donald Vollrath, Magnetek, Inc.




                                         cities. Despite many improvements
 Learning Objectives
                                         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
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were converted and built to be pow-      ergy is the act of doing work, in                    CERTIFIED
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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
                                                                            Continued

                                                                                                      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
                                                                                                                      Continued

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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-
                                                                  HVAC EQUIPMENT
                                                                                                              tem and cogeneration. A cogenera-
                                                                  WALL OUTLET                                 tion (wind turbine, fuel cell,
                                                                  DISTRIBUTION       WALL
                           UTILITY                                   PANEL          OUTLETS
                           POWER
                                                                                                              photovoltaic or waste gas recovery
                          METERING
                                                                    LIGHTING
                                                                                                              generator) system is intended to op-
                                                                                      ROOM
                                                     DOMESTIC       CONTROL         LIGHTING                  erate at near capacity almost contin-
                            LOCAL                                     PANEL
 UTILITY                   BUILDING                 STEP-DOWN
 MAINS
              UTILITY
                           METERING                                                   ADJ
                                                                                                              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
                                                                    SAFETY
                                                                                                              ergy required for the local premises
                           DISTRIBUTION BUS          POWER
                                                                 DISCONNECTS
                                                      RISER                                                   or even sell electricity to the utility
                                                                                      ADJ
                                                                                    SPEED          ELEVATOR   company for distribution to other
                                                                                   ELEVATOR         MOTOR
                                                                                                       2
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
                                             METERING                PANEL
                                                                                                              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
UTILITY    NEIGHBORHOOD
MAINS        STEP-DOWN                                                               COMMON LIGHTING          will absorb the energy before it reaches
                                                                                       ADJ                    the kilowatt-hour meter. If local loads
                                         BLDG SERVICE                                SPEED         ELEVATOR
                      BUILDING                                                      ELEVATOR
                    DISCONNECT
                                           METERING
                                                                                      DRIVE
                                                                                                    MOTOR     are not large enough to absorb all the
                                                         COMMON SERVICES PANEL
                   & DISTRIBUTION                        ELEVATOR SERVICE RISER
                                                            WITH DISCONNECT
                                                                                                              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
                                          Instructions:
                                        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-
   ergy go?
   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.




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