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Pros & Cons of Mixers Driven by Air Motors:

There are several advantages to using an air motor versus an electric motor.

  1. 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.  

  2. Air 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.  

  3. Generally, an air motor are less expensive than an XP motors at comparable horsepower's.

  4. If 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 listed  XP 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.
  5. Air 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:

  1. Air 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 issues.  

  2. Rotary 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. 

  3. Although dry 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.

  4. The 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.  

Although air 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 proof.     

03.07.10 

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