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- Jun 12, 2014
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I believe you mean external braking resistor. There is no down side to adding a braking resistor as long as the VFD has the provision to support it. You do not need to buy the factory braking resistor, VFD manufacturers list the recommend ohms (minimum) and the wattage, you can connect it to any power resistor that meets the minimum specifications. So a 1 Hp Hitachi WJ200-007SF lists 75 ohms 300W, so a resistor in the 75-125 ohm range 300W is around $40-50. It is not uncommon for factory VFD lathes to connect the brake to a switch which activates the VFD braking, I also have seen people use a foot switch that if released will stop a machine. Please be aware that you typically need some form of interlock or relay system to prevent the machine from restarting once a fault mode is removed or with power up. How this is implemented depends on the VFD, so it is not a simple matter of wiring the VFD to your motor and wiring the run inputs to a switch. You can use 3 wire control which uses momentary buttons for run and stop and a sustained for reverse (this varies by manufacturer, some have a momentary for for-stop-rev). Two wire control which is the default, requires a sustained switch for the direction for-rev, this can be dangerous if the machine is in a run mode after a fault is cleared or power up. I use some relay designs with 2 wire control which prevents this, but it all takes some figuring out. Mill I usually recommend 3 wire control. How 3-wire control is implemented varies by VFD manufacturer, so the way it is implemented say with the Fuji is a bit atypical from what I have seen from other manufactures.
You are also limited as to braking based on the chuck mounting (i.e. threading) and the drive connection to the motor, older flat belts are limited as to power delivery and braking.
Since I do not know what kind of switch gear or how you want to implement the run/direction commands, it is a bit more difficult to give you specific directions. Attached are two designs for use with a lathe using momentary buttons for for-stop-rev, the first is specific to the WJ200 which has an internal 24VDC power supply that can supply up to 100 mA (most are like 20 mA so not enough power to close a relay), the second design can be used on 2 wire control VFD (the jog circuit is specific to the WJ200), it requires a 24VDC power supply. Each VFD model has strengths and weaknesses, typically the more expensive ones have more features/controls, but it comes down to what you need/want it to do.
You are also limited as to braking based on the chuck mounting (i.e. threading) and the drive connection to the motor, older flat belts are limited as to power delivery and braking.
Since I do not know what kind of switch gear or how you want to implement the run/direction commands, it is a bit more difficult to give you specific directions. Attached are two designs for use with a lathe using momentary buttons for for-stop-rev, the first is specific to the WJ200 which has an internal 24VDC power supply that can supply up to 100 mA (most are like 20 mA so not enough power to close a relay), the second design can be used on 2 wire control VFD (the jog circuit is specific to the WJ200), it requires a 24VDC power supply. Each VFD model has strengths and weaknesses, typically the more expensive ones have more features/controls, but it comes down to what you need/want it to do.
