Since the main purpose of variable frequency drives is precise speed control, the motor speed can be increased and decreased, and the connected load can be maintained at the desired speed, using only the required energy.
When an AC motor is started "over the line," starting the motor and load may require up to 7 to 8 times the motor's full load current. This current bends the motor windings and generates heat, which shortens the life of the motor over time. The variable frequency drive starts the motor with zero frequency and voltage. As the frequency and voltage "build up", it "magnetizes" the motor windings, which typically takes 50 to 70% of the motor's full load current. Additional current above this level also depends on the connected load, acceleration rate and acceleration. This variable frequency drive advantage extends motor life.
Starting an AC motor across the line, and the subsequent demand for 300 to 600% of the motor's full load current, places a huge drain on the power distribution system connected to the motor. When the power supply voltage dips, depending on the size of the motor and the capacity of the distribution system, the voltage dip can cause sensitive equipment connected to the same distribution system to go offline due to the low voltage. Items such as computers, sensors, proximity switches, and contactors are voltage-sensitive and may drop out of line when affected by a large AC motor line activated nearby. One of the advantages of variable frequency drives is that this voltage dip is eliminated because the motor starts and accelerates at zero voltage.
If the power is proportional to the current multiplied by the voltage, the power required to start an AC motor across the line is significantly higher than using a variable frequency drive. This is only true at startup. The main problem is that some power distribution systems may be at their limit at certain times of the day. When industrial customers start their motors during these peak hours, customers are often stabbed by power surges during peak hours. The advantage of variable frequency drives is that these demand factors will not be a problem.
The VFD advantage is also that the VFD starts at zero speed and accelerates smoothly on a customer-adjustable ramp. Conversely, an AC motor started "over the wire" triggers a higher mechanical shock load for the motor and mechanically connected loads. Over time, this shock increases wear not only on the connected load, but also on the AC motor. Applications that include products that are prone to dumping greatly benefit from a slow power ramp-up, which allows the conveyor belt to accelerate smoothly rather than suddenly jerking to full power.