A primer for matching the right motor to application and operational requirements
With the multitude of motor types available — and competitive pressures — it is imperative to make the best selection the first time. Therefore, to meet specific packagingmachinery requirements, first consider the major motor selection criteria:
• Operating environment
• Duty cycle
• Speed and motion requirements
• Mounting considerations
Important motor-selection criteria include where it is to be mounted and its exposure to the environment. In clean applications, an open drip-proof (ODP) motor may be used. It has openings in its case to circulate outside air through the motor. If the area is dirty or dusty, a totally enclosed fan-cooled (TEFC) or a totally enclosed nonvented (TENV) motor should be used. These types have no vents through which dirt can enter.
A TEFC enclosure — the most commonly used enclosed motor — has an external fan mounted on the motor shaft that blows air over the outside of the motor. However, for areas with powder or dust that would clog the external fan or the fins on a TEFC motor, a TENV motor should be used. For a given power rating, a TENV motor will be in a larger frame so it can dissipate the heat by convection rather than by forced air. Thus, a TENV motor will cost more than a TEFC motor of the same rating.
Wet locations. Washdown motors, designed for high-pressure cleaning, are best for applications where water is present. Washdown motors are made with:
• Lip seals with slingers on each end of the shaft to help prevent water and contaminants from entering the motor.
• Neoprene gaskets to seal the conduit box.
• Sealing compounds between the motor housing and endplates to also aid in keeping water from entering the motor.
• Rust-preventive epoxy paint on the exterior surfaces, except the shaft.
• Stainless steel shaft and hardware.
• Drain holes to allow internal condensation to drain out of the motor. This condensation is drawn into the motor as it cools after running.
• FDA-approved, rust-preventive, epoxy paint may also coat the inside of the motor housing including the end plates and the rotor’s steel laminations.
For applications using strong caustic solution or high pressure during cleaning, the exterior of the motor may also be stainless steel.
For packaging in many food industries, a BISSC (Baking Industry Sanitation Standards Committee) standard details requirements for motors used in bakery operations and other food installations. These special washdowntype motors require a smooth radius at all joints and seams so food won’t be trapped in any pockets. BISSC motors also include many of the special construction features included on washdown motors. A special BISSC logo is affixed to the motor noting its certification.
Hazardous duty. For hazardous locations, explosion-proof ac and dc motors are available. These motors, with special housing and design features, have been tested and approved by either Underwriters’ Laboratories or Canadian Standards Association. Explosion-proof motors are listed for use in specific environments based on the hazardous material in the environment — either a powder or a gas — and its combustion temperature. The motors are not tightly sealed from the hazard, but offer a flame path at the joints and openings to allow an internal ignition to be sufficiently cooled by the time it exits so the external environment is not ignited.
If the motor control is exposed to this hazardous environment, it too must be mounted in an approved enclosure.
Ambient temperature. Most motors and controls are designed for a maximum ambient temperature of 40 C (105 F). Derating is required above this level.
Low temperatures, such as in a freezer, often present some surprising problems. One example is when one end of a packaging machine is in a typical plant environment with temperatures in the 70s, and machine runs through a hole in a wall and into a freezer at subzero temperatures. Here, motors can deliver poor performance. The most common is lack of lubrication, because the grease is too viscous to lubricate the bearings. Plus, some permanent magnet materials (such as those in PM-dc and brushless dc motors) reduce their resistance to being demagnetized by inrush currents. Also for these low temperatures, dc-motor brushes must be selected for low temperature and resulting low humidity.
Thus, for operation in unusually high or low temperatures, work closely with your motor supplier to assure you get the motor you need.
Whether continuous or intermittent, duty cycle is another important consideration for both technical and economic reasons. In many cases, a continuousduty motor is not required. If an intermittent duty motor is sized correctly to a load that occurs infrequently, significant savings may be realized over the price of a continuous-duty motor.
However, a 30-minute duty-cycle rating does not mean that the motor will be 30 minutes on and 30 minutes off. Rather a 30-minute duty-cycle rating means that it will take 30 minutes of operation at rated load for the motor temperature to rise from ambient to its rated maximum temperature. Then, there must be sufficient cool-down time so the next operating cycle doesn’t exceed the motor maximum temperature. Small motors may cool down in 30 minutes and a larger motor may take 2 hours or more. Determining the motor for a specific duty cycle requires either calculating the root-mean-square of the load or giving the operating cycle information to a motor manufacturer that will suggest the proper motor.
Speed and motion requirements
With the increasing needs for faster production, machine versatility, and complex packaging, manufacturers are turning to servo systems to power all or parts of packaging systems. Such servos provide fast and controlled acceleration and deceleration to quickly move driven loads. These loads vary from small labels to large containers. Programmable motion controllers typically command drive amplifiers, which activate the servo motors to provide specific application-oriented movement profiles.
Servo motors typically used in packaging machines include brushless dc, permanentmagnet brush-type dc, ac servo, and typical ac induction motors often used with vector controls if high performance is required.
Once the motor rating and enclosure type are selected, the mounting style needs to be determined. Many motors are simply base mounted and close-coupled or belted to the load. If belted, the pulley sizes and number of belts need to be reviewed to ensure that the motor bearings are not overloaded.
Coupling a motor to a gearbox is common in the packaging industry. A NEMA C-face provides a mounting rabbet which engages the gearbox, and the motor shaft fits into the gearbox. The two components are joined by bolts through the gearbox threading into the motor face. A D-flange is similar except the bolts go through holes in the motor flange and into threaded holes in the gearbox.
Machinery built overseas is likely to have motors and gearboxes conforming to metric IEC standards. Domestic motor manufacturers offer motors with metric mounting dimensions enabling replacement with U.S. built motors.
John Malinowski is drives specialist at Baldor Electric Co., Fort Smith, Arkansas.