Think About Energy Saving For Fix Speed Application

[marke]

 

 

 

For fixed speed applications, provided that the fixed speed is equal to the line speed, there will not be energy savings when a VFD is used compared to a soft starter.

[marke]

Adding a VFD (Variable Frequency Drive) to a motor running at a fixed speed and continuing to run the motor at that speed, does not make the motor efficiency improve except where:

The motor is very underloaded and a reduction in voltage will reduce the flux in the iron and thereby reduce the iron losses in the motor, BUT

1. The reduction in voltage applied to the motor will :
   1. reduce the magnetising current
   2. reduce the flux in the iron and therebye reduce the inductive current flowing in the motor
   3. Increase the slip in the motor, thereby increasing the slip losses
   4. increase the work component of the current flowing in the motor
2. Reducing the flux in the stator can result in an element of energy saving BUT
   1. Reduced Flux ==> reduced Iron loss
   2. Reduced Flux ==> increase slip loss
   3. Reduced Flux with an open shaft motor ==> reduced line current and reduced copper loss
   4. Reduced Flux in the motor with load current equal to or greater than the magnetising current of the motor ==> increased motor current and increased copper loss.
3. At line speed, the potential to save energy by using a VFD is limited by the true losses due to the use of an induction motor. With many modern machines, the efficiency is is typically quite high and until the load is very low and the actual iron loss is commonly less than a few percent of the full load of the induction motor.
4. The iron loss is independent of the load, but there will only be a net gain when the load is very low, BUT the addition of a VFD to control the motor can reduce the iron loss by a small amount, but there is an immediate loss of two to three percent due to the the additional losses inherent in the additional losses in the from the use of a VFD.

[Andre]

Hi Mark,

great topic and helps customers to understand the pros and contras from a VFD. There is some get input as well from the CAPIEL which is the European Coordinating Committee of Manufacturers of Electrical Switchgear and Controlgear.

see the link, https://www.capiel.eu/img/6685-Capiel-Tryptique-EN2-Version-Web.pdf

 

[marke]

Hi Andre

A good paper, but it needs to be read by a lot more people. There is a perception that is prominent in the industry, that put a VFD on a motor, and it will save energy, even when the work output does not change.

The reality is that there is little energy wasted by the motor that can be reduced by adding the drive, and you have to offset that by the losses in the drive and harmonic mitigation.

In some applications where the motor is driving a circulation fan that is freely circulating at an unnecessarily high rate, then energy will be saved by slowing it down. The energy saved, is not by reducing losses in the motor, it is by reducing the work being done to what is required.

The energy saving fraternity have for years implied that the energy consumed by the motor is it's full load rating, and an energy saving device (such as a VFD) will reduce this down to what is required. There are also many claims that an oversized motor wastes energy, once again based on the premise that the energy consumed is related to the motor size.

An oversized motor is often more efficient than the correctly sized (smaller) motor, plus an induction motor is often at it's maximum efficiency at around 75% load. In many cases, I have found that an oversized motor that runs continuously at around 75% of it's rating is significantly more efficient than a smaller motor running at 100% of it's rating.

Too often, the VFD is used as a band aid to mask poor design. Don't do a full design, just guess and apply a drive and if you get it wrong, no one will know because the drive will make it work.

Best regards,

Mark.

[selinawilliam148]

Very informative article and i really appreciate your whole research! I can conclude your whole discussion by saying that when it comes to fixed speed applications for your electrical supplies, consider ways to save energy. Making small modifications and using energy-saving features can help reduce your company's energy usage, leading to lower costs and a smaller carbon footprint. You can see more details on the topic via the following link: https://agmelectrical.com.au/