Retrofits of DOL motors: Low-hanging fruit?
Despite clear benefits of VFDs in motor drives, most industrial motors are still of fix-speed type. This is true for newly manufactured motors and even more for the installed base. Hence there is a huge potential for retrofits of fix-speed direct on-line (DOL) motors. How does such a retrofit look like? What needs to be considered?
Motor operation at variable-speed utilizing power electronic based VFDs provides many benefits, such as substantial energy savings, smooth start, improved process control, better dynamic response etc. Yet, the higher investment cost may still lead to a decision to go for a fix-speed drive. Fortunately, the advantages of variable speed are so significant that it becomes also commercially very attractive.
One area is a new (greenfield) installation where the use of a VFD fed motor is quite straight forward. However, there is also large installed base of fixed-speed industrial motors. Can they be retrofitted and adjusted for variable speed operation?
Benefits of DOL motor retrofits
Of course they can be retrofitted (at least most of them) and it is actually quite common practice. It may also be seen as a low-hanging fruit. Why? Just consider following:
— Low initial investment (only VFD plus eventually an input transformer)
— Short payback time (in most cases)
— No or minimum changes on the mechanical side
— Keeping most of existing interfaces
— Smooth starting and extended lifetime of mechanical components
— Minimum scheduled downtime for upgrade
Affordable investment cost
Low investment cost is linked to the fact that only a VFD and eventually an input transformer needs to be purchased. The motor is kept original. An assumption is that the motor health condition gives a perspective for reasonable long operation. For “standard” designs the cost of a new motor may be comparable to the cost of a VFD. Let’s assume following cost distribution:
Input transformer: 15%, VFD: 45%, Motor: 40%
In such case retrofitting an existing motor with a VFD including a dedicated input transformer will be approx. 60% of the cost of a new drive system. The cost benefit becomes even much more significant if existing motor is of a special design that was tailor made for the project.
Of course, there will also be cases where purchasing a new motor is a better alternative. However, in most situations a motor retrofit is an attractive solution.
Payback time, energy savings and improved power factor
Short payback time is linked to the relatively low investment cost and potential energy savings. The VFD helps to save active power, especially in square torque type of applications. The energy and cost savings along with a payback time can be calculated with dedicated energy saving tools (e.g. MVD FanSave, MVD PumpSave). As a reminder VFD does not only reduce the active power consumption (kilowatts). It may also improve the input power factor and reactive power consumption. Older asynchronous motors (and particularly those with higher pole number) tend to have lower power factor when operating direct on-line.
Let’s illustrate it on a small example of 5 MW/6-pole asynchronous motor with rated speed of 994 rpm:
DOL operation: pf = 0.87 at 100% load → 3’179 kVAr reactive power consumption
VFD operation: pf = 0.95 at 100% load → 2’312 kVAr reactive power consumption
DOL operation: pf = 0.86 at 50% load → 1’653 kVAr reactive power consumption
VFD operation: pf = 0.97 at 50% load → 1’013 kVAr reactive power consumption
Keeping the interfaces
A retrofit project usually allows to keep most existing interfaces, particularly between the motor and the driven load as well as the load and the process control. In other cases small adaptations may be required. The control system of a VFD is very flexible and regarding the setpoints the VFD accepts wide range of reference signals.
Keeping the same interfaces means less effort on site and lower risks.
Smooth starting
As reported in previous articles, smooth motor start is beneficial for the grid (elimination of high starting current and voltage drops/sags). Moreover, the load side will be thankful as well. Smooth VFD motor start removes large torque oscillations that are present during DOL start. Consequently, the mechanical components are much less stressed. Their lifetime can be extended and eventually also maintenance intervals can be adjusted.
Minimized impact on production
Downtime required for the retrofit can usually be minimized which is another significant advantage. The VFD installation works go on while the motor still operates as direct on-line. Some basic VFD cold commissioning can be made without the motor as well. Just when everything is really ready the process needs to stop for a while and the hot commissioning is performed. Moreover, this short downtime can be well planned and eventually combined with some motor maintenance activities that would have to be performed anyway.
Use of historical data
In our view there is one more advantage when retrofitting a DOL motor with VFD. The user shall have all the historical data and knows very well what the real load demand is. Therefore, the system can be dimensioned based on exact needs and overdimensioning is avoided. It helps to further reduce the investment cost and boost the efficiency.
Summary
Retrofitting an existing DOL motor to variable speed brings many advantages. In most cases it really is the ‘low hanging fruit’. For reasonable initial investment the user can achieve attractive energy savings (both active and reactive power) and get a smooth motor starting capability. One of the goodies is the short downtime during hot commissioning.
Retrofits of DOL motors may be an excellent starting point for companies to gather their first experience with VFDs and modern variable speed control.
P.S. Are you interested how to select a suitable VFD for existing DOL (fix speed) motor? Watch out for our upcoming articles. For more detailed information consider purchasing our premium plan with many advantages.
References
[1] Energy efficiency series (blog posts), https://mb-drive-services.com/category/energyefficiency/
[2] Motor starting methods: Direct on-line start, https://mb-drive-services.com/direct-on-line-start/
[3] Motor start with VFD, https://mb-drive-services.com/vfd-motor-start/
1 Comment
Right strategy to retrofit fix-speed motor into VFD operation - MB Drive Services · August 30, 2021 at 9:16 pm
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