With the advent of the micro-processor, it became reasonable and economical to provide AC motor drives which could vary the frequency of power (Hz) delivered to three phase motors, thus making them variable speed motors.  This opened up tremendous opportunities for process engineers and machine designers, but also new challenges for the motor designers.

Inverters present the motor with power that is not clean, sinusoidal, three phase. In fact, it is rather dirty three phase, laden with high voltage spikes which can destroy the insulation of the motor winding. Modern inverters are far better than earlier generations, but there are significant differences between brands and models. The length of wire between the motor and inverter can amplify the problem, and most manufacturers specify a maximum permissible wire length unless special corrective measures such as reactors are employed. Designers must make significant and costly improvements to the motor winding design to ensure long life on inverter power. Better wire and phase/phase insulation along with improved lamination and winding design all contribute to improved motor life on inverter power.  

Although the motors winding may be suitable for inverter power, motor life may still be cut short due to motor heating. As the motor speed is reduced, cooling is diminished (TEFC motors) and the motor will run hotter unless the load (torque) is substantially reduced. Designers provide inverter duty motors with extra thermal capacity for this reason.  Longer core lengths and other proprietary measures are utilized to provide a higher thermal reserve.  Our Pharmaceutical Series motors are rated for constant torque turndown to 4:1. In addition, thermostats are provided as standard so that overheating and subsequent motor failure can be avoided.

For the best possible constant torque performance at turndown ratios to 10:1, alternative cooling means such as TEBC (Totally Enclosed Blower Cooled) are employed. As its name implies, a TEBC motor has a constant speed blower to maintain a constant flow of cooling air over the motor at all speeds. This is accomplished by fitting a separate small fan motor to the larger, inverter driven motor. Separate wiring is required for the fan motor, as it must be fed line voltage whenever the main motor is being operated.  Our TEBC motors feature junction boxes with both the main motor and blower motor leads, so that only one conduit is required, thus enhancing the cleanability, or GMP, of the installation.

Other cooling options exist as well: TECAC (Totally Enclosed Compressed Air Cooled) in which compressed air is passed through the motor at low pressure; TEFCAC (Totally Enclosed Fan Cooled Compressed Air Cooled), which is a cross between TEFC (Totally Enclosed Fan Cooled) and TECAC, a standard cooling fan is used and the compressed air is utilized only when necessary; TELC (Totally Enclosed Liquid Cooled) in which cooling liquid (usually water) is circulated in a special cooling jacket built into the motor; TEOT (Totally Enclosed Oil Through) where a high dielectric oil is circulated directly through the motor winding.

Stainless Motors, Inc. can supply any of the above cooling options to satisfy the most demanding or unique applications.