The pre-charge circuitís main purpose is to prevent the drives line fuses from blowing when power is applied. The dc bus has a significant amount of capacitance which momentarily appears as a short circuit when power is applied until the caps are fully charged. To prevent the large in-rush of current a pre-charge circuit such as the one below is employed.
This type of circuit is used in many small to medium sized drives. The relay is initially open when power is applied. Current charges the bus bank capacitors through R1 until the bus is up to specified voltage which usually takes under 1 second. The low voltage power supply is fed from the bus. Once the bus is charged the low voltage power supply starts up the logic which immediately closes the relay and the drive is ready to run.
There are many other types of pre-charge circuits depending on the type and size of drive. Some smaller drives replace the relay and resistor with an NTC thermistor which has a resistance when cold but almost zero resistance when hot. Initially the NTC is cold until the pre-charge is over at which point the resistance is low. The downside of this technique is that the NTC stays hot while the drive is on which contributes to inefficiency.
Another technique used on large drives is replacing the 3 phase bridge rectifier with a half bridge(shown below) which can be slowly turned on by phase angle firing the SCR's which comprise half the bridge during the pre-charge time. This added complexity means it is only used if it is really required, such as on high horsepower drives.
Note: AC/Servo motor drives always employ a pre-charge system which slowly charges the DC bus capacitors to prevent the incoming line fuses from blowing when power is applied. Often this system is designed for limited duty, meaning they canít keep repeatedly charging the bus on start up without the resistor failing. Therefore, when troubleshooting a drive problem, donít keep turning the power on and off. This will lead to failure of the pre-charge circuitry which means the drive will not power up. Turning the drive on and off a couple of times shouldnít cause the pre-charge circuitry to fail but it is prudent to wait a couple minutes between applying power to give the pre-charge circuitry a chance to cool.