From: Essential Information <ei@igc.apc.org>
Newsgroups: sci.energy
Date: 31 Dec 92 13:54 PST
Subject: Re: Energy Ideas - HVAC-2
Nf-ID: #R:cdp:1466300134:cdp:1466300142:000:4976
Nf-From: cdp.UUCP!ei    Dec 31 13:54:00 1992



VARIABLE SPEED DRIVES PROVIDE FLEXIBLE POWER

One of the most energy-intensive activities of heating,
ventilating and air conditioning (HVAC) systems is the operation
of pumps and fans. Frequently, when a thermostat or other energy
management control device signals the HVAC system to increase or
decrease the temperature in a building, the HVAC system operates
at full power. This is seldom needed. Since frequent operation of
a pump or fan at a low rate consumes less energy than infrequent
operation of a pump or fan at a high flow rate, the installation
of a motor that varies its speed saves energy. These motors are
called adjustable speed drives (ASDs).
How They Work

The speed of a motor can be controlled by varying the amount and
frequency of the voltage applied to a motor, and by varying the
current passing through a motor. ASDs alter the speed of both
direct-current (dc - the type of current produced by a battery)
and alternating current (ac - the type of current produced by a
generator) motors by varying a fixed input voltage and current.

When used to drive dc motors, ASDs typically control both the
voltage and the current. At lower speeds, the current is
maintained at the value rated for the motor while the voltage is
increased to produce increased speed and torque (the force of the
twist provided by the motor). Above the rated speed, the voltage
is maintained at its rated value while the field current is
decreased to produce higher operation speeds.

ASDs control ac motors by controlling the magnitude of their
voltage as well as the frequency of the supply. An ac ASD
consists of three components: the rectifier, which converts an ac
signal to a fixed dc voltage or to an adjustable ac voltage; the
filter/dc link, which smooths the output of the rectifier; and
the inverter, which converts the dc output into an ac output of
adjustable frequency and adjustable voltage. 

Improved Efficiency and Other Benefits

According to an article in the September, 1992 Energy User News
("Graham Adds 300-Hp Unit to Its Series of Adjustable Speed
Motors," p. 18), installing an ASD can reduce energy consumption
by up to 60 percent. These motors can be used in chilled water
circulating systems, cooling towers, heat exchangers, exhaust and
make-up air systems, fume hoods, dust collecting systems,
domestic hot water and waste treatment systems. As shown in the
above diagram, the ASD compares the signals received from a
sensor and the setpoint and adjusts the motor speed to match the
two signals. 
Electronic speed control offers much more than just energy
savings. Less maintenance is one benefit, since electronic speed
controls often replace a mechanical transmission system. The
mechanical stresses on the machines, bearings and shafts are
lower, which prolongs the service life of the equipment. Because
of the low starting current, the thermal stress on the machine
itself and the electrical stress on the power supply are
substantially reduced.

A motor controlled by an ASD should not be operated at its
maximum output. The ASD
components use electricity to function and increase the electric
load. A 150 horsepower (hp) ASD motor operating at full power
consumes more energy than a constant 150 hp motor. The advantage
to the system is its variability; if you need a motor to operate
at a constant power level, do not purchase an ASD.

CASE STUDIES:

Installation at Police Station
The City of Phoenix installed a single ASD on a 20 hp tower fan
motor at the Police Training Academy. The ASD has saved about
30,000 kWh per year, resulting in annual savings of $2,172. The
installation of the ASD is also expected to reduce tower fan
maintenance. The cooling tower has a fan motor running at 100
percent speed intermittently to maintain a certain condenser
water temperature. The ASD varies the motor speed from 30 to 40
to 60 percent of full speed to maintain a constant cooling water
temperature. Since the energy consumption of the motor varies
with the cube of the speed, these speeds represent energy
reductions of 97 percent, 93 percent and 78 percent,
respectively. The expected payback period of the motor is 2.5 to
3.0 years. (Contact: Dimitrios Laloudakis, Energy Conservation
Manager, 2631 South 22nd Avenue, Phoenix, AZ 85009, (602)
261-8813.)


ASD Installation at School
Between 1988 and 1991, the Phoenix, Arizona, Union High School
District #210 added ASDs to the chilled/hot water systems on all
campuses. Each ASD cost about $180,000 and reduced the energy
consumption of each motor by approximately 60 percent. The pumps
were resized and some were replaced with higher efficiency pumps.
The district has not experienced any operating difficulties and
has an ongoing maintenance agreement with the manufacturer of the
ASDs. (Contact: Kenneth E. Wissinger, Assistant to the
Superintendent for Business and Operations, Phoenix Union High
School District #210, 4502 North Central Avenue, Phoenix, AZ
85012, (602) 271-3301.)