& Cons of Mixers Driven by Air Motors:
There are several advantages to using an
air motor versus an electric motor.
Air motors are intrinsically safe, which
means that they can be used in just about every explosion proof environment,
whereas electric motors can only be used in Class I, Group C & D, Division I
explosion proof vapor environments. Group D explosion proof motors are
more common. Explosion proof motors
are also classified by dust environments, such as the common Class II, Group
F& G designs.
Motors have variable speed capability, in that the airflow can be throttled
back using either a needle value or a ball valve to reduce its speed.
Needle valves constrict the maximum available flow, whereas ball valves
enable full flow capability.
an air motor are less expensive than an XP motors at comparable horsepower's.
the application calls for both variable speed and an explosion proof
environment, a special inverter duty explosion proof motor must be utilized,
which explicitly states a hertz range beyond 50/60 hertz on the UL
nameplate. There are also
additional costs to consider for wiring, in that the variable speed inverter
must be located in a non-XP environment, where the power source is brought
to the inverter and then the motors power leads come from the inverter to
the XP motor environment.
motors stall when they experience an overload condition, whereas electric
motors, during excessive over-current situations burn out to failure. This
stalling feature is an inherent safety feature unique to air motor designs.
Air motors also have numerous
disadvantages versus electric designs:
Motors are notoriously inefficient. It
takes a 10 horsepower compressor to operate a 1 horsepower rotary vane
motor. It is quite common for plants to develop overall air capacity
vane air motor designs utilize wearing elements that steal motor efficiency
over time. In other words, air
motors require regular maintenance and parts servicing, which is not
inherent compared to electric motor designs.
running air motors are available, they are still considered specials in
that they are much more expensive as compared to their lubricated cousins and
they generally require a longer lead time to procure.
more common lubricated rotary vane air motor requires a filter, regulator
and lubricator for its operation. The
continuous lubrication requirement is paramount in providing a lubricated surface for the wearing
rotor vanes. What this also means is that lubricant steadily discharges
from the outlet port (air muffler), which can be messy. Generally this type design is shunned in both clean
environments and for food applications.
motors are unique in that they are both intrinsically safe and have variable
speed (0 – 3,000 RPM’s) capability, when they are applied to mixers, they do
not have a safety speed lockout feature, which is now a common feature available
on many variable frequency inverters (VFD's), which are commonly used with Class
F insulated electric motors. Most portable mixers have two common
elements: they are single speed, and they operate continuously above their first
natural frequency. Once the mixer changes to variable speed design, most
have an indicated unsafe range of operation to avoid this natural frequency
range of operation. With an air motor design, this range must be manually
controlled by an experienced operator, or in other words, the design is not fool
proof. Although a tachometer can be applied to indicate speed, generally
this unsafe range must be negotiated either blind or by feel (indicated visual
vibrations). VFD's with lockouts avoid this range of operation, and instantaneously
pass through this unsafe range toward full speed capability. Since
continuous operation during the unsafe range is avoided an electric motor design
is inherently fool