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Motor control

Motors are controlled in many ways.

Next to fuel, electricity cost is the next expense in an industrial plant. Most of this electricity is consumed by motors. These motors are moving material in the form of conveying systems or air in the form of fans. For this reason motors are a critical part of every plant operation and maintenance. Only 2% of the lifetime cost of a motor is related to its purchase price. The remaining 98% is the cost of electricity to run it. A standard industrial motor can consume 40 times its original cost in its first year of operation. The small premium paid for a high-efficiency motor can often be paid back in less than one year.

Standards for high-efficiency motors start at approximately 80% ratings for smaller motors and increase to approximately 95% for larger motors (more than 500 horsepower). As a rule of thumb, purchase motors with efficiency ratings of 90% or higher.

However, efficiency ratings are mostly irrelevant if a motor is operating at less than approximately 75% of its rated capacity. The tendency to oversize motors contradicts attempts to become more energy efficient. Therefore, when replacing motors, ensure that the specifications meet the application requirements (including a reasonable safety factor) rather than just using the old motor data.

In addition to the efficiency there is different “duty”. This can define the application or environment in which the motor has to perform. This duty can specify the insulation rating, bearing type, bearing seals, drive shaft characteristics, and cooling. In most plant application a TEFC or Totally Enclosed Fan Cooled will work just fine; however, special attention should be required if this motor is under unusually high starting load or if it is being used with a speed controller (ACVF).

As motor horsepower increases the required current increases. For this reason, it requires that the motor voltage be increased. This cost cutoff is normally close to 600 HP. Wire length, control type, transformation cost, and motor cost must be looked at for this decision. So as the motors become larger the voltage is also jumped.

MCC (Motor Control Center)
There are many motors in a plant setting. For this reason a special area or room is allocated to house all the components for motor control. In this room is a MCC. A Motor Control Center is a modular cabinet system for powering and controlling motors. Several MCCs in a factory may be powered from main switchgear which in turn gets its power from a transformer attached to the incoming line from the power company.

A typical MCC cabinet is an enclosure with a number of small doors arranged in rows and columns along the front and flat, mostly featureless back and sides. Behind each door is a unit called a bucket which connects the electrical devices inside to the electrical bus bars behind it. In the back of a MCC are large busses that carry the current. The buckets are varying sizes based off the size of the motor they are controlling. The bucket is intended to be easily removable so they are easily replaced or serviced. Today MCCs can be purchased with regular starters, reversing starters, soft start, and small (>150 HP) variable frequency drives. Sections can be added as required for expansion.

ACVF (Variable Frequency Drive)
A variable frequency drive (VFD) changes the speed of a motor by changing the voltage and frequency of the electricity supplied to it, based on the system or load requirements. Properly engineered and controlled, a VFD will provide a wide variation in motor shaft speeds while offering substantial energy efficiency during lower speed or partial load operations. On a variable application a VFD is a simpler and a less costly method than using a pulley, gear reducers, and hydraulic clutches. Using a VFD over a damper will yield better control and better energy use; however, if you are running the motor at near or at maximum capacity, a VFD offers no improvement. In fact because VFDs have 1 – 3 % losses it may actually reduce efficient.
Drives have also been blamed for causing disturbances or harmonics on power lines. This can cause the power quality and overall transformer efficiency to be reduced. Sometime isolation transformers are used to reduce or eliminate.

For a complete definition

Manufactures Reference Manual