The only reason for the possible power saving will be to let the fan run at reduced speed. As a main rule of thumb the power required for a fan is proportional to the speed to the power of three. So in order to reduce the power to a third, the speed shall be reduced by around 30% to near 70% of the original speed.

That can just as easily (and cheaply) be done by installing a slower (and smaller) motor, or by installing some VFD. Remember, the standard induction motors have long been preferred because of their simplicity, and minimal maintenance requirements. No brushes and the like, that wears out over time as in DC motors.

Installing DC motors in any industrial application is (were) only done if there are very special requirements to starting torque or speed control. It is definitely not quite as simple as just changing some MCC drawer. There is no why a DC motor will draw less power than your standard induction motor for the same application.

Typical squirrel cage asynchronous motors are capable of 200 to 225% locked rotor torque generation at start, and without the slip in this type of motor, the starting torque generation isn't there. Synchronous motors are not used in any hoist and crane applications I'm aware of because of this, for the 36 years I've been in the industry. Once in a while we run into a crane repaired by people not as experienced as they should have been, one recently in Texas where they had installed a synchronous motor as a replacement for the original center drive bridge motor. (Both 5 HP Rated) They called here for advice as the motor wouldn't operate properly with the existing ac drive. I ran it and identified the motor. The end result was they opted to repair the original, asynchronous motor and the problem was solved.

Your mechanical engineer is going to be very concerned about matching shaft diameter and shaft height. Shaft diameter can be adapted by changing one half of the coupling to suit. Remember to purchase the coupling. Your civil guy is going to want to know if the new machine will fit on the same foundations as the old one. You may need to build a sub-frame to adapt the old foundation to the new machine.

If your machine is big and you are on a weak supply then look at 12, 18 or 24 pulse converters. Be careful how you tune the new drive because AC drives can respond way quicker than DC drives and this can lead to mismatched response times with existing equipment or other instabilities. To get true DC performance you may need to install an encoder (a must on hoist applications). Cabling needs to be proper variable frequency drive cable or your new installation may not play nicely with the existing equipment - especially instrumentation.

Surge testing is not the only test that can detect weakened insulation between phases or coils in a connected winding. IEEE 43-2000 is the standard that applies to insulation testing in the field at industrial plants. The guideline it provides is for DC voltages at or below nameplate voltage (ref Table 1 in the standard). Surge testing is not considered an optimal technology for regular testing to be applied in the field on in service motors for several reasons: Danger to personnel; potential damage to the motor; and conditions required for surge testing are rarely valid in the field. Anyone involved in the condition assessment of insulation on motors in the field, is encouraged to carefully read IEEE 43-2000. It is frequently mis-quoted.

IEEE 43 covers only the recommended practice for insulation resistance testing. IR is an excellent test for detecting certain issues with aging, moisture and surface contamination. But, it is not the only recommended field test for insulation. IR can not detect weak ground, phase and turn insulation in every case. The more rigorous hipot, surge and PD tests are also necessary to properly test an insulation system. These tests are routinely used to test insulation systems in motors, transformers, cables, etc. in the factory and in the field.

Having a higher rating AC drive will not damage a low rating motor as long as it is within the control range of the motor. The AC drive is rated in amperes and nor motor horsepower.

In the old days we used to oversize the AC drive by a factor of two to be sure we didn't end up with egg on our faces. It is also possible that to have enough starting torque for your process you may have to install a larger motor. At least with the larger motor the excess power factor penalty will be reduced by the AC drive.

AC drives are one of the most effective tools managers can use to control a facility's energy use. As organizations struggle to curtail costs in every possible way, AC drive applications offer some of the best available returns on investment. Simple payback for an AC drive typically falls between six months and three years.

AC drives offer an alternative means of matching system output to load requirements by slowing the motors that drive HVAC system components. Unlike throttling, where a motor's energy use decreases only slightly with the decreasing system load, slowing a motor to match a decreased load results in a rapid drop-off in a motor's energy requirements.

AC drives control motor start-up by initially applying a very low frequency and voltage to the motor. The voltage and frequency ramp up at a controlled rate, greatly reducing the in-rush current, as well as heat and stress on motor windings, thereby extending motor life.

In simplified terms, the control circuit, Figure 1, receives inputs for the commanded values for speed or torque and from the motor model. In a few microseconds, the circuit calculates the proper switch positions (on and off) for the six power switches. Unlike most ac drives that have specific modulation (also called chopping and switching) frequencies, the DTC technology determines the on state for each switch needed at that instant to produce the needed flux and its various components to develop the required torque. Generally, the on times produce a modulation frequency between 1.5 kHz and 3.5 kHz.


Granted, these frequencies are in the audible range, but the ABB engineers point out that there is no one prominent frequency as is often noticed in conventional drives. Instead, the DTC drive produces a white noise much like a fan or other "background" noise device.

In China domestic, the total estimate market capacity of AC drives should be 110 to 170 billion Yuan, with a scale of low voltage drives for about 70%, middle voltage and high voltage drives share 30 % market. Due to the high-voltage power electronic devices limitation, high voltage ac drives have not been applied widely, it needs the manufacturers make further marketing promotion.

Technology is always the basic of a brand to survive in the market. China ac drive manufacturers should establish their own research and development efforts to improve the drive's technology level, creating new ac drives to adapt to market demand. For a long time, China ac drive suppliers are in the low-end technology level among the global markets, but in recent years, the manufacturer realizes it's better to improve their own R & D abilities and production lines capabilities, rather than to emulate someone's technology. China ac drive manufacturers establish their own R&D teams base on domestic market demand, build their own brand and increase market share, thus, compete with other drives brands in near future.

The utilization of AC drives comprehends currently the most efficient method to control the speed of induction motors. AC drives transform a constant frequency constant amplitude voltage into a variable (controllable) frequency-variable (controllable) amplitude voltage. The variation of the power frequency supplied to the motor leads to the variation of the rotating field speed, which modifies the mechanical speed of the machine.

AC drives suit both variable and constant torque loads. With variable torque loads (low torque demand at low speeds) the motor voltage is decreased to compensate for the efficiency reduction normally resultant from load reduction. With constant torque (or constant power) loads the system efficiency improvement comes from the feasibility of continuous adjustment of speed, with no need to use multiple motors or mechanical variable speed systems (such as pulleys and gears), which introduce additional losses.