Tech Tip: Adjustable-Speed Drives

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Why consider the environment when it comes time to select and apply an AC variable speed drive?

When it comes time to select and apply an AC variable speed drive, don’t forget about the physical environment you are putting the drive into.  In many cases, people will install their drives in a Motor Control Center room with a moderate ambient temperature.  However, if you are mounting the drive out by the machine or on a wall where the climate is less controlled, you will want to consider the environment.  The ambient temperature is key.  Many drives are rated for 50 to 55 degrees C and operating slightly lower than these temperatures is a good thing!  However, if the drive will be in an abnormally cold environment (Below 5 to 10 degrees C), be sure to review the temperatures with your drive supplier.  Also, you will want to consider if the drive will be in a dusty, corrosive or wet area.  If this is the case, then you will want to consider a more robust NEMA Enclosure such as a NEMA 12 or a NEMA 4 drive rather than lesser NEMA Enclosure such as a NEMA 1.   

Why Utilize Adjustable Speed Drives?

Adjustable speed drives are more convenient than changing belts, offer slower start-up speeds and are utilized when multiple speeds are required.  They are the industrial solution to maintaining continuous shaft speed, overcoming an inactive disposition on start-up, offering constant torque despite changing shaft speeds, and applied when two shafts must be synchronized to run at the same speed.  They also reduce operating and maintenance costs and improve product quality!

Reduce Energy Costs in a Fan or Pump Application

Centrifugal fan and pump power consumption is proportional to the cube (third power) of the flow rate or motor speed. Therefore, any reduction in speed, by using a variable frequency drive, results in a big drop off in energy demand. For example, at 80 percent speed, the energy consumption is 50 percent of rated.

Why Can't I Use Any Three-Phase Motor with a Variable Frequency Drive?

Modern variable frequency drives produce a Pulse Width Modulated (PWM) waveform output that not all motors can tolerate. Also, some motors are not adequately cooled at low speeds. For new installations use inverter or vector duty motors or check with the motor manufacturer regarding the potential application of premium efficiency motors.

Advantages of a Flux Vector Control (Closed Loop) variable speed drive 

Drives are run in one of three modes of operation; Volts/Hertz, Sensorless Vector (Open Loop) or Flux Vector (Closed Loop) control.  The big advantage with selecting and operating a drive in Flux Vector Control mode is that it can deliver a wide controlled speed range.  Of the three modes listed above, the torque control is best in this mode.  A Sensorless Vector drive typically can provide you torque control down to 1Hz, whereas the Flux Vector Control drive can provide you full torque down to zero speed.  So if your application requires excellent torque control, consider going with the Flux Vector Control offering!

 

The information provided in Tech Tips is not meant to be all-encompassing, but rather to draw attention to and provide information about the particular subjects covered.  All suggestions and recommendations contained in Tech Tips are based upon information that is believed to be accurate to the best of the experience and knowledge of PTDA's contributing members, but are made without guarantee or representation as to results.  PTDA and Tech Tip contributors expressly disclaim any warranties or guaranties, express or implied, as to the accuracy or completeness of any information published in Tech Tips, and disclaims and makes no warranty that the information in Tech Tips will fulfill any of your particular purposes or needs.  PTDA and Tech Tip contributors disclaim liability for any personal injury, property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on Tech Tips.