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Keeping In Phase With Induction Motors continued on reverse side, column 1
Oct. 1 will be the launch of a landmark program to provide green power
to North Carolinians. NC GreenPower, a voluntary program, allows
electric customers to contribute to the development of renewable
energy sources in the state.
NC GreenPower is the nation's first statewide and multi-utility initiative
to encourage the development of renewable energy resources. While
many other states have electric utilities involved in green power, North
Carolina is the first state in which all of the utilities have come togeth-
er and agreed to offer this exciting, environmentally friendly program.
While the public launch date is scheduled for Oct. 1, with press con-
ferences in Raleigh and Charlotte, many electric customers have been
eligible to sign up for the program since July 28 on Progress Energys
website (
http://www.progress-energy.com/custservice/carbusiness/
greenpower
).
Renewable energy at work in North Carolina
Customers can participate in NC GreenPower by contributing to the
generation of "blocks" of electricity. For the residential customer, blocks
are available in increments of $4 per 100 kWh of electricity. By volun-
tarily signing up for the program with their electric utility, the customer
is agreeing to contribute an additional minimum of $4 per month to
generate power from a renewable energy source. This amount will be
added to the customer's electric utility statement each month. The
entire contribution will be sent to NC GreenPower; electric utilities will
keep none of the money.
It will be the responsibility of NC GreenPower to identify renewable
generation sources that can supply this power to the electric grid.
These new resources will include solar, wind, and methane, mostly
from landfills but also from other sources such as animal waste diges-
Keeping In Phase
With Induction Motors
Unlike residential appliances that operate on a single phase of electri-
cal supply, industrial loads normally operate using three phase power
sources. The use of three phases facilitates the efficient transmission
of greater amounts of power and in the case of induction motors this
allows for a simpler, more robust starting, and a more efficient motor.
However, the characteristics of each electrical phase are important
when considering motor operation and life.
Phase Unbalance
Because a three-phase electrical circuit has single-phase loads con-
nected to each phase, and because it is impossible to perfectly bal-
ance the single-phase loads among the three phases, the voltages of
the three phases will vary. From a technical standpoint, the amount of
this voltage unbalance, as defined by the National Electrical
Manufacturers Association (NEMA), can be determined using this
equation:
% unbalance = 100 x maximum deviation from average voltage
average voltage
Example: If the voltage (phase to phase) at the input terminals of a
three-phase motor that is rated for 460 volts is measured, voltages of
465, 465, and 474 on the three phases might be found, averaging 468.
These voltages represent a voltage unbalance at the motor.
% unbalance = 100 (474 - 468) / 468 = 1.28%
While this value may still seem small, its affect on motor operation can
be detrimental. One percent voltage unbalance in a motor has been
shown to cause a 5% current unbalance. As a result, NEMA
recommends that motors be de-rated if operating at voltage unbal-
ances above 1%. At a voltage unbalance of 5%, NEMA recommends
a de-rating factor of 0.75. That means a 10 horsepower motor operat-
Fall 2003
continued on reverse side, column 2 tion processes. NC GreenPower will, with the contributions supplied by
electrical customers, provide an incentive for renewable generators to
supply power to the North Carolina electrical grid.
In addition to the product mentioned above, a large volume product
will be offered to larger electrical users - businesses and industry - for
$2.50 per 100 kWh. Depending on the size and or needs of the par-
ticular commercial or industrial electrical customer, they may be eligi-
ble to participate in NC GreenPower by buying either of the products
offered.
This large volume product will include the resources offered in the
mass market product but will also include additional resources such as
small hydropower, and biomass.
NC GreenPower currently is in the process of identifying potential
generators for the program. Using a Request for Qualifications /
Request for Proposals (RFQ / RFP) process, NC GreenPower is defin-
ing those parties interested in supplying generation for the various
resources that qualify for the program.
Interested parties have been asked to supply information concerning
resource type, quantity and timeframe of potential generation, and
premium requested. The RFQ is available on the NC GreenPower web
site (
http://www.ncgreenpower.org
). When completed, the RFP will be
posted on that site.
Although renewable projects normally take some time to develop, NC
GreenPower will work to develop as much generation as possible dur-
ing 2004, the first full year of operation.
Green power for environmental protection
NC GreenPower is a first step in promoting renewable energy and
clean air in North Carolina. This independent, voluntary program pro-
vides North Carolina citizens and businesses with the opportunity to
increase the use of renewable energy to help protect our environment
and improve air quality.
One of the exciting aspects of this program is that it can grow.This pro-
gram offers a way for each North Carolinian to increase renewable
energy generation and help the state's environment. Energy genera-
tion is related directly to the contributions made; therefore, the pro-
gram has no growth limit.
NC GreenPower, an independent nonprofit organization, was approved by the
N.C. Utilities Commission to provide North Carolina with greater options for
their energy sources. NC GreenPower is a subsidiary of Advanced Energy, a
Raleigh-based non-profit corporation that increases efficiency and productivi-
ty in industries, businesses, and homes as they transform energy into goods,
services, and environmental conditioning.
NC GreenPower
continued
In Phase
continued
ing under these conditions should not be loaded beyond 7.5 horse-
power because motor life will be shortened due to overheating.
Correcting Phase Unbalance
Phase unbalance is not easily detected and will often increase gradu-
ally as single phase loads are added. The best way to detect phase
unbalance is through the use of a high quality true - rms voltmeter. It is
always a good idea to determine the voltage unbalance of three phase
systems. If the unbalance is unacceptably high, work should be done to
better balance the single-phase loads that are being place on each of
the phases of the three phase system.
Single Phasing
Single phasing occurs when one phase of a three-phase system is lost.
This loss can be caused by a blown fuse, downed utility line, bad con-
nection, etc. Single phasing can cause serious consequences for
induction motors. If initially running, the motor will continue to run, but it
can no longer support its rated three-phase load. As a result the motor
will draw more current on the other phases and may be damaged
because of overheating as it attempts to support its rated load. A motor
will not start using a single-phase condition, but it will draw excessive
current, which also can quickly damage a motor.
Protecting Against Single Phasing
The easiest way to protect a motor from potential damage caused by
single phasing is to install overload protection sized for a particular
application. To properly determine the appropriate size, consult the
National Electric Code (NEC), article 430.
Another means of guarding against single phasing can be accom-
plished through the use of control circuitry. Devices are available that
monitor the magnitude and phase of each line voltage. These devices
can sense both phase unbalance and loss of phase and will trip the
motor starter to take the motor offline. Typically, these devices can be
adjusted to trip at different levels of unbalance but should be set at 3%
for motors operating at or near full-load.
References for Phase Unbalance and Single Phasing
NEMA MGI. 2003. Motors and Generators.
NEC Article 430. 2002.
Single Phasing and Voltage Unbalance. 2003. Wisconsin Public
Service Corporation.
Fall 2003