McQuay OAC User Manual

Motor Control Options for
Vision™Indoor & Skyline

™

Outdoor Air Handlers

What fan motor control options does McQuay offer to
reduce energy costs, meet code requirements and protect
against overloads on its air handling equipment?

A wide variety of motor control options are available for
both Vision indoor and Skyline outdoor air handlers.
These include three types of disconnect switches, two
types of motor starters, and three brands of variable
frequency drives (VFDs). Complementary options such as
120V or 24V transformers, line reactors, bypass and HOA
switches are also available.

All motor control options are factory-mounted and sealed
with a professional fit and finish to maintain our low,

0.5 cfm/ft

2

cabinet leakage rate. This also reduces field
labor and installation costs. All options are ETL/c-ETL
listed for proof of safety compliance and minimized risk
when installing, maintaining and using these products.

Factory-Mounted VFDs

McQuay factory-mounted VFDs provide local or remote
speed control of a single-fan motor with minimal field
labor costs. Three brands are offered: McQuay, ABB
and Danfoss. The McQuay VFD is application-specific
and optimized for variable-torque applications (fans and
pumps). It’s also smaller in size, costs less and provides
reliable, quiet operation. See McQuay brochure A/SP
31-213 for details.

The McQuay VFD provides easy integration with
building automation systems. Available communication
options include Modbus, L

ONWORKS,

®

BACnet,®Johnson

Metasys

®

and Siemens Apogee.

®

A factory-mounted VFD with a disconnect switch and a
bypass switch offers added motor control at the unit for
mission critical air handler applications. With bypass
control, power to the VFD can be switched to allow full
operation of the fan motor in case of VFD failure.
McQuay has standardized on a simple, two-contactor
bypass in all of its Applied Air Handling products as this
provides the best value for our customers.

Note: Caution should be taken when using a bypass
switch with a VFD: When in bypass mode, the fan
motor operates constantly at full speed. This can
cause the HVAC system to shut down on a duct high­static-pressure trip or, worse yet, cause damage to the
ductwork. In addition, operating in bypass mode for
long periods of time can increase energy costs by
50% or more.

Considering the reliability of the modern VFD— with
an average service life of more than 20 years—the
need for a bypass switch may be questioned. The
switch consumes space, increases control complexity
and adds cost. For mission-critical applications, a
reasonable alternative might be to keep a replacement
VFD in inventory. In the unlikely event of failure, most
VFDs are relatively easy to remove and replace.

System Solutions From McQuay

Engineered for flexibility and performance.

TM

Non-fused disconnect switch and McQuay VFD

Vision indoor air handler with
factory-mounted VFD and disconnect switch

Factory-Mounted Disconnect Switches

McQuay unit-mounted disconnect switches provide the
means to disconnect power in sight of the fan motor as
required by the National
Electric Code (NEC). All
are housed in a NEMA 1
enclosure for Vision
indoor air handlers and a
NEMA 3R enclosure for
Skyline outdoor air
handlers. These switches
are available in
combination with any starter or VFD for a quick, easy
field connection. Three heavy-duty types are available:

• Non-fused disconnect switch. This is a simple on-off
switch with no over-current protection.

• Fused disconnect switch. This is an on-off switch
with fused over-current protection. The fuse type
varies by application. For constant-speed applications,
Class RK5 fuses are employed. For VFD applications,
Class J time-delay fuses are employed.

• Circuit breaker disconnect switch. This offers the
same over-current protection as a fused switch, but
allows the user to reset the breaker after an over-current

trip event without having to replace a blown fuse.

Factory-Mounted Starters

McQuay factory-mounted starters provide remote start­stop operation and overload protection of an individual
fan motor. Minimal field labor is required to wire the
line and control circuit
power. A Hand-Off-Auto
(HOA) switch can also
be selected for field
connection to the
building’s control system.
A 120V transformer is
also provided. Two types
are available:

• NEMA (National Electrical Manufacturers
Association) starters.

• IEC (International Electrotechnical Commission)
starters.

NEMA contactors and starters are designed in
standardized sizes from 00 to 9. Each size has a defined

current and a resultant voltage and frequency which are
dependent on the horsepower rating. No matter who
manufactures the component, the current and horsepower
rating will be identical for a given size.

IEC contactors and starters are typically smaller and less
costly than NEMA types. They are not designed to a
standardized size. Instead, the manufacturer designs them
to meet defined applications which are referred to as
utilization categories. For our application—starting a
motor for a fan—we use an AC-3.

Mounting Specifics

Vision indoor air handlers are typically furnished with
motor control components mounted on the outside of the
fan cabinet. For smaller air handlers, an optional section is
needed to provide good service access to the fan. In these
cases, the VFD is mounted in the air stream with the VFD
keypad display mounted on the unit exterior. This allows
for complete control with all access doors closed.

Skyline outdoor air handlers are always furnished with an
exterior-mounted keypad display for easy control access.
Complementary control components, including a
disconnect switch, transformer and bypass switch are also
available. All components are mounted on the exterior in a
NEMA 3R enclosure. When ordered, the VFD is mounted
inside the fan compartment. For smaller Skyline air
handlers an optional section is needed to provide good fit
and service accessibility.

Digital Ready™Controls

In addition to these cost-saving motor control options,
McQuay offers Vision air handlers equipped with factory­installed Digital Ready control components, including
limit switches, pressure transducers, actuators and sensors.
Each component is optimally located, factory-mounted
and factory-wired to junction boxes on the outside of each
shipping section of the unit. This allows you to optimize
air handler performance while minimizing the cost and
potential problems associated with field installation of
control devices. See our brochure A/SP 31-205 for more
information.

For more information on factory mounted motor control
options and Digital Ready control components, contact your
local McQuay representative. To locate your representative,
visit www.mcquay.com or call [phone].

Motor Control Options for Vision and Skyline Air Handlers

© 2008 McQuay International • [phone] • www.mcquay.com

A/SP 31-240 (04/08)