Drive Braking Resistors

[elevmike]

CT782,

In my experience, if positioning is important your dont stop on the brake. The motor & drive stops the machine, and the brake holds t he position. In the "old days" you can "plug" and ac motor with dc to stop it, but better accuracy was achived with a DC Gen field control. You may get buy by installing a 3 HP motor (if the size is right) and use your current drive with regen resistors. You must consult with Fuji, and/or review the manual.

This thread makes for an interesting dialog, but you really need to approach somebody like Baldor who can assist in helping you figure out the whole drive/motor/regen thing, and provide you with all the components. Be sure to give them ALL of the details of your requirements for speed load and stopping accuracy etc..

Good luck with it,

Mike.

 

[kamenges]

CT782-
At this point I think everyone will agree that you need a braking resistor. The remaining question is what size.
As stated in a previous post, the resistance value will be pretty well fixed by the drive design. That leaves you with power rating.
Mike's regen wattage calculation should get you pretty close to soemthing you can use. My question to you is how we look at duty cycle.
You state that the total up/down cycle takes about 1 second. How long between up/down cycles? This will define your duty cycle and should help you get a ballpark number.

Keith

 

[DickDV]

There is a nifty way to size brake resistors if you already have the drive. In the instruction manual, they will tell you what the minimum brake resistance (ohms) can be. This is set by the current limit of the brake chopper resistor.

Using this resistor value and (for 460V applications) a voltage of 750V, figure the brake wattage using the formula watts=volts x volts/ohms. This will give you a resistor that has enough wattage to handle the full drive horsepower continuously braking. This is almost always way more wattage than you need.

A handy rule of thumb with resistors is that a cold resistor can handle ten times its normal wattage for about 15 seconds. This will allow you to use a much smaller and less expensive resistor if you only have to use the brake infrequently (note the same ohms but lower wattage).

If your application needs braking somewhere between these two extremes, it becomes a matter of judgement on just how much wattage is really needed. Since resistors are very inexpensive especially at the 5hp level, err on the high side with extra wattage and you'll be ok.

If you are starting from scratch with drive sizing and braking, you will need some of the good formulas presented above or a lot of "seat of the pants experience".