From what I gather, the primary factor is the motor voltage and drive hp. A 2.2KW motor with a 2.2KW drive on 208v is 10.6 amps. Well, the phase currents are a different calc, and we're also talking about the main bus cap in the VFD which is rectified off the 208v source. So these numbers have a lot wrong with them. But if the drive's transistors are rated for 10.6 amps, then a 20ohm resistor would be needed to create enough sink current to stop from full RPM with full braking torque.
If you had a 5kw 208v motor but only put a 2.2kw VFD on it, you'd need that 20 ohm resistor because the transistors can't produce more stopping current. If you have a 4.4KW drive and a 4.4KW 208v motor, it would have 21.2 amp transistors and thus need a 10 ohm resistor. Now note this- say this is a 2.2KW motor and a 2.2kw VFD, but it's a 480v motor and VFD, that runs on higher voltages and lower currents. The same 20 ohm resistor would sink 2.3x more current off the higher voltage bus cap- yet the transistors are sized for 1/2.3x the current. The resistor is far too small, it needs to be 107 ohms to do the same thing. The VFD manual describes how this works. As I said before, the higher wattage brakes seem unnecessary for stopping a spindle at such low duty. But search "braking resistor" on ebay, high wattages aren't expensive. Getting excess wattage is not a problem. However, getting too high a resistance means the bus cap can still get an overvolt error resulting in reverting to freewheeling and you'll have to increase the stop time. Conversely if the resistance is too low, the resistor will actually be sucking too much current off the bus and depress to bus cap. The VFD will probably switch it on and off but it's a bad idea. The transistor the VFD uses as the resistor switch is only rated for a certain amperage and you're exceeding it by using too small of a resistor. Again, DO NOT use my resistor ohms examples here. They are not calculated correctly for 3-ph systems. Danny On 3/13/2016 8:30 AM, John Thornton wrote: > I just used the the one for by the drive. I "think" not all drives would > use the same resistor. Just looking at the GS2 and there are 12 > different braking resistors. > > http://www.automationdirect.com/adc/Shopping/Catalog/Drives/GS2_%28115_-z-_230_-z-_460_-z-_575_VAC_V-z-Hz_Control%29/GS2_Drive_Units_%28115_-z-_230_-z-_460_-z-_575_VAC%29/GS2-23P0 > > http://www.automationdirect.com/adc/Shopping/Catalog/Drives/AC_Drive_(VFD)_Spare_Parts_-a-_Accessories/GS_Braking_Options_(Braking_Units_-a-_Resistors)/Braking_Options_(All_GS_Drives)/GS-23P0-BR > > JT > > On 3/13/2016 8:03 AM, Gene Heskett wrote: >> On Sunday 13 March 2016 07:40:25 John Thornton wrote: >> >>> On my GS-2 with the braking resistor I go from 1800 fwd to 1800 rev in >>> 1 second. I have both acceleration and deceleration set to 1/2 second. >>> >>> JT >> That would be considerably more usefull than the controllers default of >> 30 seconds. Do you recall the specs of the resistor you used? >> >> Thanks John. >> >>> On 3/12/2016 10:12 PM, Gene Heskett wrote: >>>> Greetings everybody; >>>> >>>> Given that I note the vfd's seem to have an adjustable accelleration >>>> control, and that my couple of hours playing with it from its own >>>> front panel, I am led at ask the question of just how fast can one >>>> of these things be reversed from say 1000 rpm at the motor spindle? >>>> >>>> I ask because my present setup, using the OEM PM DC motor which is >>>> all gear drive to the spindle, one of Jons pwm servo amplifiers and >>>> a 125 volt dc supply, reversals at the turn around depth of this >>>> cycle of a peck tapping operation, with the spindle doing 300 rpms, >>>> is a very small fraction of a second, far less than 30 degrees of >>>> rotation. Based on its reversal performance at 2500 rpms being a >>>> slim second, I am concerned that the vfd may not be able to reverse >>>> so quickly, and will impact my use of G33.1 because of the >>>> turnaround over shoots. On my lathe, with its fragile drive train >>>> and a 5" chuck, this overshoot is between 2 and 3 turns at 250 >>>> running rpms. Usable, but if I don't stop it soon enough, the tap >>>> may bottom out and break. >>>> >>>> What has been the experience of others using smaller vfd's and >>>> motors? >>>> >>>> Can you still do rigid tapping safely? >>>> >>>> Thanks all. >>>> >>>> Cheers, Gene Heskett >>> ---------------------------------------------------------------------- >>> -------- Transform Data into Opportunity. >>> Accelerate data analysis in your applications with >>> Intel Data Analytics Acceleration Library. >>> Click to learn more. >>> http://pubads.g.doubleclick.net/gampad/clk?id=278785111&iu=/4140 >>> _______________________________________________ >>> Emc-users mailing list >>> Emc-users@lists.sourceforge.net >>> https://lists.sourceforge.net/lists/listinfo/emc-users >> Cheers, Gene Heskett > > ------------------------------------------------------------------------------ > Transform Data into Opportunity. > Accelerate data analysis in your applications with > Intel Data Analytics Acceleration Library. > Click to learn more. > http://pubads.g.doubleclick.net/gampad/clk?id=278785111&iu=/4140 > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > ------------------------------------------------------------------------------ Transform Data into Opportunity. 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