Efficient motor management can bring you to an optimal design of your motor and electrical installation. There is a simple approach that you can apply to every application. It will save on installation costs and avoid production losses during operation.
What are the main levers for that? I’ll cover the basics in this post.
Electrical motor installation equipment can be illustrated with the following generic figure:
Each of the four blocks contain elements that connect it to the others. What the designer needs to think about is how to select equipment in the three blocks preceding the motor to obtain correct motor operation, lower installation cost, and making sure the motor operation has the lowest possible impact on the electrical supply and parallel loads.
That doesn’t seem trivial, but Iet’s look at the main guidelines for accomplishing these goals. One of the most efficient routes is the motor management approach. It consists of five steps that collectively take every aspect into account:
- Analyze the application
The application is the load, (pump, fan, conveyor belt, gas compressor, etc.) and its operational characteristics according to the industrial process it belongs to.
The load is characterized by its relation to the speed, and inertia. The different applications have different forms of torque like quadratic, linear, constant, and decreasing. The inertia also constrains the appropriateness of specific motor controls. But everything may be changed with the requirements of the industrial process, regarding starting, stopping, speed control, and synchronization of the operation with other applications in the same process.
So you need to consider all of them to be sure that you have properly understood the application. This is the key to avoid over-performing in the ultimate solution.
- Select motor control
Once the application constraints are clarified, you can think of a suitable and cost-optimized solution for motor control. Depending on the application requirements and motor characteristics, the solutions for low voltage and medium voltage applications may be different.
Switchgear-integrated contactors or circuit-breakers, soft-starters and autotransformers or variable speed drives are often investigated. If you need speed control, synchronization to other processes, or if you estimate energy savings to be relevant, then a variable speed drive will be the most suitable. Otherwise the focus will be on motor starting. Frequent motor starts, critical motors, or weak networks will require a progressive start solution. It may be more expensive than a simple contactor or circuit breaker, but you will save in the maintenance of your motors and reduce constraints on parallel loads during start.
In the end, classical motor starters provide you with the lowest integration cost and are suitable for most cases.
- Ensure motor protection
Motor protection relates to faults and overloading. Either with protection relays or through the variable speed drive, your motor and installation require protection. Try to select the features for the real need, most motors require simple protection like overload and earth or phase fault. This setup is made with current transformers only. For more critical motors, voltage transformers allow monitoring of power, frequency, and under and overvoltage. This is a costlier solution, which increases the installation work and potentially the volume of cubicles. There are of course even more advanced protections, like differential, whose application is context-defined.
- Guarantee the power quality
Power quality impacts the motor and parallel loads. Typically, harmonics and power factor should be considered in the industrial system. They are both related to the motor control – with variable speed drives you need to check harmonics, with motor starters the power factor. Do not underestimate this topic. Power quality issues do not reveal themselves immediately, but when they do several kinds of equipment can be impacted. A variety of solutions can help you here. Active filters help you flexibly manage harmonics from multiple-speed drives in parallel. Passive filters have less efficiency but cost much less. You can also select low harmonic drives, especially for large ones, and solve the issue at the origin. For power factor improvement, capacitor banks are the best solution.
- Manage motor assets
This is a last step toward efficient motor management. Supervising the motor and load prevents failure development and avoids emergency maintenance and production losses. There are various solutions for predictive maintenance such as vibration monitoring, insulation condition monitoring, and motor and load monitoring through current and voltage measurements. The most important thing is to connect these sensors to the supervisory SCADA system, and be informed on time of developing faults so maintenance actions can be scheduled. Services-oriented drives can also perform this monitoring.
Do you want to know more? We are preparing the next post in this series now. In the meantime, please check the Schneider electric motor management web page.
Did you like this post? Please comment on which parts you would like to see in more detail.
Bernt Morten Nikolaisen
5 years ago
Part 3. Motor protection with higlights on VSD. How can you protect the motor against overharmonics that may cause damages through bearings?
5 years ago
Hello Bernt, my understanding is that this is mostly seen in MV applications. One solution would be the a multi-level technology drive which will naturally have a lower harmonic spectrum at the output. Otherwise, a more expensive sinus filter at the VSD output. It will smooth the wave and eliminate the harmonics. The choice of solution has to be coordinated with cable length. One often neglected point would be the selection of a motor with higher insulation for dv/dt. This will limit the bearing current, but depends much on the motor manufacturer, dV/dt capability can be in the range 1500-8000 V/µs. This capability is not really related with the cost of the motor, just some manufacturers make more and some less.
2 HP Variable frequency drives
5 years ago
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