What is the purpose of using a Variable Frequency Drive( VFD ) in starting a 3 phase,6600v AC motor? ?

Question

This motor is used to drive a 3 stages Nitrogen Compressor on-board LNG carrier ( Liquefied Natural Gas ) with a Reliquefaction plant.

Answer 1

A variable frequency drive is an expensive of starting a 6600V 3-phase AC motor. A soft starter or reduced voltage medium voltage starter may be a cheaper way of starting the medium voltage motor.

Answer 2

The problem with starting any motor is the high starting current. For small motors up to say 4Kw this is not considered a problem and they are often wired to a DOL direct on line starter. That is the full Voltage and Frequency is applied to the motor all the way through. Basically it is on/off.

With a VFD it is possible to limit the starting current by electronically varying the Supply parameters particuarly the frequency. This can be simply explained by comparing the motor to a light bulb.

A normal light switch turns the light on/off with nothing in between eg DOL starter

A dimmer switch allows you to vary the amount of power to the light bulb. Therefore the VFD is similar to the dimmer switch slowly increasing the speed of the motor.

As mentioned by others to use a VFD just to start a motor is an expensive method with Soft start electronic conmtrollers more economical. However if the motor requires variable speed operation as part of a process control system then the VFD is justified  

Answer 3

With a VFD, motor torque can be limited to any desired torque with motor current limited to the same level as the motor would draw during normal operation. If the current is limited to rated 100% motor current, the motor will provide rated 100% torque from standstill to the desired operating speed. For 150% of rated motor current, approximately 150% of rated torque can be obtained. Most VFDs are of the pulse width modulated (PWM) design. That means that the current at the input is proportional to input power rather than motor current. If the motor is operating at high torque but low speed, the power requirement (torque X speed) is low and therefore the input current is low, except that harmonic currents may be high unless harmonic reduction is provided. Because of on-board power system limitations, the expense of using a VFD for starting may be justified. No other starting method limits starting current as effectively.

Answer 4

If its only used for starting and not varying the speed then its seems to be a very expensive way of doing it. You don't mention the power rating and you have to consider the supply in determining if reduced voltage starting is necessary. A soft start would be much cheaper whatever.