In this time some power factor corrections are included active filters, so in some cases we do not need to have harmonic filter for each equipment. Active filters analyze the harmonics drawn by the load and then inject the same harmonic current to the load with the appropriate phase. As a result, the harmonic currents are totally neutralized at the point considered. This means they no longer flow upstream and are no longer supplied by the source. A main advantage of active conditioners is that they continue to guarantee efficient harmonic compensation even when changes are made to the installation.

In design stage can calculate how much harmonic can reduce by this device, and which one needs independent harmonic filter, especially in high number of variable speed drive in service.

Let us consider simple network of RLC and power supply. The current with the voltage will generate power in the resistance. The current and the voltage in the inductor will generate the magnetic field in the inductor, this field increase and collapse 50 times per second (due to the frequency) so every collapse the source will compensate the requirement (imaginary power). The inductor will build the field which will induce back EMF that will act against direction so it will cause the current to lag and can't follow and be in phase with the voltage.

The current and voltage in the capacitor will generate the electrical field between the capacitor sides. this field increase and collapse 50 times per second (due to the frequency) so every collapse the source will compensate the requirement (imaginary power). The capacitor will allow the current to pass through it initially and voltage builds up and this voltage gradually resist the current causing it to decrease exponentially. So the high initial current before voltage build up means the current leads the voltage and become max before the voltage.

There are twice as many power components in an AC drive as in a DC drive. In the past, the regulator has been approximately twice as complex as that of a DC drive. Present with digital technology or state-of-the-art electronics allows this factor of complexity to come much closer to 1:1 than 2:1. Circuits that were difficult to produce with discrete parts can now be accomplished with large scale chips and microprocessors. This has greatly reduced the complexity differences between AC drives and DC drives.

In summary, conversion of DC to AC is possible but hard to achieve if you can not have your manufacturer or drive supplier provide you already with some similar or identical application. You need to get the answer from him but the project has to be a viable option (cost) for you also! The conversion will be very expensive compare to staying with DC (existing or brushless). However, if you can improve your production, process etc... in the long run, it will be a good choice.

Variable speed drive solution will solve in most cases the torque problem when using standard asynchronic motor. Soft starter that reduce the toque might not deliver the required torque in effort to get to full speed. In case no need for speed regulation variable speed drive with synchronize bypass system can solve the problem This way you can have high torque during start, low current and after getting into full speed you bypass the variable speed drive and save all the energy loses and noises that variable speed drive can cause.

Energy saving with variable speed drive is achieved due to better control of magnetizing and rotor currents. It optimizes total current to motor thus reduces motor (and network) losses caused by excess current. If you use speed control, it is good to set a torque limit to control the maximum torque produced. If in torque control, then speed limit is needed.

Variable speed drives like any medicine, have potential side effects. But with careful consideration of these side effects such as harmonics, EMI / RFI, Common-Mode Currents, Reflected Waves, etc. etc. variable speed drive can be successfully applied with minimal impact on other systems.

From an energy savings perspective the only applications that really qualify for potential significant savings are centrifugal fans, pumps and compressors where the application bears out that most of the time they will be operated below the base speed of the motor. The decision to use of variable speed drives on all other applications should be made on the basis of process and mechanical requirements, not energy savings.

There are also different types of variable speed drives for different types of motors, that is synchronous or induction for example. Therefore, evaluating case by case makes sense and other considerations along with the above mentioned also needs to be carried out. Here are some examples.

• There are multiple motors that must start with the variable speed drive to reduce the starting current. In this case, one should consider the number of motors that can start in succession, using the same variable speed drive.
• Check with the variable speed drive manufacturer whether they offer a simplified variable speed drive for starting only. For starting only, the losses aren't of greater importance. Preferably you may want the variable speed drive to come with bypass switch already incorporated to it.
• As it comes to low cost soft starters, those vary only voltage, never forget that these soft starters reduce the torque of the motor to the ratio of voltage to the power of 2.2 (yes 2.2, not 2), so you will want to evaluate if the motor will accelerate the load with the desired voltage, and if so, without overheating. Also remember that some loads are more demanding than others. Check for torque requirements and load inertia. If the motor can accelerate, then a soft start can be used. If not, a variable speed drive can be one solution.
• When the process requires, or the output of production varies, the application of a variable speed drive makes sense.

If energy savings (reduced speed) is the issue a variable speed drive will almost always be the first choice, but again the driven load, (VT) and application require review and sizing considerations. If speed control is the consideration then a constant torque load will require a variable speed drive. The motor basics we have had to live with, NEMA design A, B, C & D are there for a reason as one design doesn't fit all applications. The controller is just another tool to use within it's limitations to provide the desired result for an application.

Soft starter - considerably cheaper than variable speed drives, much more efficient, does not require harmonic mitigation, but draws 3 - 4 times start current and does not offer variable speed feature. Understanding the application and exactly what you are trying to achieve would be the most important factor.

The most critical selection factor: working current. Actually, choose the VSD according to the working current is the final step in the entire selection process. Here is to emphasize. Sizing the variable speed drive according to the motor actual operating current (not the nameplate current) rather than the nameplate power.
Basic principle, while working in a long time: variable speed drive output current > Motor current

Other factors for selecting variable speed drive

• Altitude.
• Ambient temperature, transport and storage temperature, enclosure.
• Input line voltage level, frequency, variable speed drive output frequency range.
• The variable speed drive efficiency, overload capacity, cooling methods.
• Size, structure, installation way.
• etc...

Selecting a variable speed drive (VSD) for a centrifugal fan application is a complex decision that should take into account a number of factors:

Motor type: Is it a new or retrofit installation? Some older motors don't have the proper insulation to withstand the pulse-width-modulation output of a variable speed drive. Select an inverter-duty motor if possible. Determine if an output line reactor and filter is required to protect the motor.
Distance limitations between the variable speed drive and motor.
Motor data: voltage, HP or kW, full load amps (always make certain that rated variable speed drive amps exceed motor full load amps), and base speed data.

The variable speed drive (VSD) is one part of a total system that includes a motor and a load. The motor acts as a power transducer, converting electrical power to rotational mechanical power. AC induction motors, 600V and below, often are paired with a variable speed drive. These AC motors fall into classes with different torque speed curves. Most variable speed drive manufacturers assume use of a Class A motor and that the torque speed curve will be almost linear at the operating point. The variable speed drive will shift the whole curve left or right to change the operating point. Note the "slip" in the figure. Every motor suffers from some slip or difference between rotor and stator fields. This is quite different than stiction, the term used with control valves for the needed stem force to overcome static friction. The load - mechanical conveyor, pump, fan, compressor or the like - has inertia, rotational friction and stiction. The process also has dynamic characteristics that may change when using a variable speed drive instead of a control valve. It takes time to accelerate the load to operating speed and this is proportional to the inertia and the motor torque.