APPENDIX D (Continued)
f) Motor Protection. Motors require overload
protection. The most common practice is the use of a motor
overcurrent relay system that will protect all three phases and
protect against single-phasing. This type of protection will
respond to motor overcurrent conditions of an overloaded motor,
but will not detect overtemperature conditions.
A motor operating at reduced speeds will have
reduced cooling; as a result, it may fail due to thermal
breakdown of the motor windings insulation. Thus, the optimum
protection for a motor is thermal sensing of the motor windings.
This sensing is then interlocked with the VFD's control circuit.
This is highly recommended for any motor that is to be operated
for extended periods of time at low speeds.
(1) Humidity and Moisture. As is the case with all
electrical and electronic equipment, high humidity and corrosive
atmospheres are a concern. Drive units should be installed in a
noncorrosive location whenever possible, with ambient humidity
ranging between 0 to 95 percent noncondensing. Avoid locations
subject to rain, dust, corrosive fumes, or vapors, and salt
water. In some cases, appropriate NEMA enclosures may be
specified where some of these locations cannot be avoided.
Consult VFD manufacturers about the location and application
before doing so.
(2) Vibration. Do not locate VFD's near vibrating
equipment unless appropriate vibration isolation methods are
(3) Line Transmitted Transients. The VFD is a
solid-state electronic device, therefore, surge and transient
protection (from lightning strikes, circuit switching, large
motor starting, etc.) should be specified, either integral to the
VFD or external, as appropriate.
a) Successful installation of VFD's, as with nearly
all electrical equipment, is derived from an orderly, well
planned start-up procedure. After reading the entire VFD manual
and before energizing the VFD, make a physical inspection of the
VFD and look for the following: