When applying adjustable speed drives for the control of induction motors, several operating impacts should be considered in order to ensure proper operation and equipment life. What follows is a brief overview of some common considerations:
Motor torque, speed, and temperature: Many modern adjustable speed drives (ASD’s) are capable of controlling torque by directly manipulating motor flux, such that torque is maintained constant across the full zero-to-base speed range. That said, when operating a self-cooled motor at reduced speeds, temperature rise must be factored in. This means that in many cases it is advisable to de-rate a self-cooled motor to ensure temperature rise is maintained within the range dictated by insulation classification. It is generally stated that each increase of 10 degrees Celsius in winding temperature above rated levels reduces winding insulation life by 50%, so proper cooling is essential. In addition to de-rating, there are several other ways to address this issue, including auxiliary cooling (such as a motor-mounted blower) and increasing motor frame mass and/or stator configuration to permit better heat dissipation. All of these methods also rely on the motor’s being operated within its rated ambient temperature range, however.
Operation above base speed: Where the application calls for the motor to operate above base speed, it should be noted that from base speed to maximum speed horsepower will remain constant and torque will be reduced. This is effectively because the volts-to-hertz ratio, which determines torque output, begins to drop once base speed is exceeded. Ultimately, motor break-down torque will determine the upper limit for constant horsepower operation. NEMA Design A and B general purpose motors are rated for constant horsepower operation up to 90 Hz, that upper limit being derived based on the approximate peak torque capability for these motors of 175%.
Running current: ASD’s should be sized to accommodate the continuous rms value of motor current to allow for harmonic loading, which may increase total demand by 5 – 10% above the sinusoidal value. Drives should also be capable of periodic overload to allow for response to changing load conditions. This is typically done when specifying the drive; overload ranges are usually either 110% or 150% for 60 seconds, although this may vary somewhat for larger capacity drives. Also, if the motor/driven load mechanical design is such that motor speed cannot be changed quickly enough and slip increases beyond rated value, higher current draw may need to be factored in.
Starting current: an ASD is typically set up to start a motor at very low frequency and ramp up from there to achieve the desired speed. This low frequency start requires low current draw sufficient to overcome rotor and stator resistance. Depending on starting torque required, a voltage boost may be needed at start-up; this is usually programmed at drive set-up to be a percentage of rated voltage. If rated torque is required at start-up, then at least rated current will be required.
Motor efficiency: Efficiency of the motor drops when controlled by an ASD, due to harmonics losses and the heat and skin effect losses induced by circulating currents. Note, however, that depending on the application and on the presence of output filters on the ASD, the overall system (drive, cables, motor, and load) efficiency may improve because of the capability of operating the system at optimum levels.
Sound levels: experience has shown that operating motors with pulse-width modulated (PWM) ASD’s can result in an increase in A-weighted noise of 5 dB to 15 dB, due primarily to the high-frequency switching of the drive’s electronics. This can be affected positively or negatively depending on the drive’s selected carrier frequency, the motor’s mechanical construction, and other environmental factors, so the impacts must be considered on a case-by-case basis. Torque ripple is a less significant problem with modern ASD’s than in the past, but resonant frequencies in the motor can increase noise and/or vibration levels and should be avoided. ASD’s usually can be programmed to allow specific frequencies to be “jumped” (skipped) to assist with addressing resonance issues.
Voltage stress: due to the rapid rise time and frequency of drive switching transients, and depending on cable length and physical characteristics, voltage seen at the motor terminals when supplied by an ASD can be upwards of 2 – 4x rated supply voltage. NEMA MG1 Part 31 requires motor insulation systems for 460V rated motors to be capable of withstanding 1,600 volts peak, at a rise time of 0.1 microseconds. Although numerous manufacturers label their motors as “inverter duty”, the only mandated standard remains NEMA MG1 Part 31. So it is wise to confirm with the manufacturer that the selected motor’s insulation system complies with this standard.
While the benefits of using an ASD can be significant, the associated requirements for effective specification and use should be kept foremost in mind. Considering the details above, as well as the wealth of information obtainable from numerous manufacturers and web references, including contacting us at JP Motors and Drives, you can be assured of achieving an efficient, properly operating drive and motor system.
Please note: the following information is derived from the NEMA MG 1-2007 condensed standard. More information, as well as the full NEMA MG 1 standard, can be found at www.NEMA.org.