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VFD Braking Resistors - another question.

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  • #16
    I see the graphs but have no idea what they mean. What is a unit? And who runs a mill motor at 10-20% That's like 6-12hz for a 60hz motor.

    When I'm on low pulley and back gear, the spindle is turning ~80 rpm but the motor is still turning at 1755 rpm at 60hz. How are all the guys power tapping on YT getting a quick stop? That's what I'd like to know. The graphs don't help me.

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    • #17
      The graph actually does make sense...... I had to remember the basics here.....because it seems a bit counter-intuitive.

      When the RPM is high, the "slip frequency" of the DC braking is high, and the rotor current is less due to inductance of the cage conductors being a higher impedance at that frequency. That reduces braking.

      At the same time, the voltage goes up, so the curve is not linear.

      As the motor slows, the generated voltage is less, but the impedance is also dropping. At some point there is a maximum current point. Below that, the voltage is low, and the current again drops off.

      However, don't let it fool you, the effect of DC braking can be pretty violent from high speed even so. And it can be very noticeable at low RPM also.
      CNC machines only go through the motions

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      • #18
        On a semi related note, my hybrid car uses regenerative braking. It uses a similar concept to brake by using the electric motor as a generator. The generated DC goes back to the battery pack. The deceleration is very predictable right down to the last few MPH. That's when it switches over to mechanical disc brakes. From what I read, they did not go with DC injection for that last part because of overheating fears.

        It would seem that using the same combination on a lathe would work well there too.

        Dan
        At the end of the project, there is a profound difference between spare parts and left over parts.

        Location: SF East Bay.

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        • #19
          Originally posted by danlb View Post
          On a semi related note, my hybrid car uses regenerative braking. It uses a similar concept to brake by using the electric motor as a generator. The generated DC goes back to the battery pack. The deceleration is very predictable right down to the last few MPH. That's when it switches over to mechanical disc brakes. From what I read, they did not go with DC injection for that last part because of overheating fears.

          It would seem that using the same combination on a lathe would work well there too.

          Dan
          With the VFD that is in the car, they can program a very good braking curve. Standard VFD braking is not a heat issue because it is basically just running the motor in a normal way.

          DC injection is really "eddy current braking", that induces currents in the rotor conductors. That energy goes into I^2 * R losses, and ends up as heat in the rotor. If you beat on a motor hard enough, you could actually melt the aluminum conductors. It would take a bit of doing, but is possible.
          CNC machines only go through the motions

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          • #20
            I found it interesting that the Toshiba link I provided mentioned using DC injection it to drive moisture from the motor.
            Last edited by lakeside53; 04-17-2018, 10:36 PM.

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            • #21
              Originally posted by lakeside53 View Post
              I found it interesting that the Toshiba link I provided mentioned using DC injection it to drive moisture from the motor.
              It clearly would work to do that. you need to use a fair bit of DC, maybe 10% or up to 25% of FLA, but it will warm things up for sure.
              CNC machines only go through the motions

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              • #22
                Originally posted by CCWKen View Post
                I see the graphs but have no idea what they mean. What is a unit? And who runs a mill motor at 10-20% That's like 6-12hz for a 60hz motor.

                When I'm on low pulley and back gear, the spindle is turning ~80 rpm but the motor is still turning at 1755 rpm at 60hz. How are all the guys power tapping on YT getting a quick stop? That's what I'd like to know. The graphs don't help me.
                I think the units in the grahps are nominal speed ie 60hz = 1.0 and nominal current.

                You should start by telling (again) what VFD and brake resistor you have and what are your settings. Or start separate thread specific to your hardware.
                My bench drill would reverse "pretty fast" if would trust the pulley axle nuts to hold... oversized VFD that is capable of dynamic braking at around 400% nominal torque from full speed to zero speed and accelerate again at 400% torque to full reverse.
                Location: Helsinki, Finland, Europe

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                • #23
                  With it set as you say in back gear and with 80 rpm spindle you can stop pretty quick with just dynamic braking (resistor); I could probably get a fraction of a second. BUT it's compromise. It will only stop as fast as your deceleration setting - and you will have to reprogram the deceleration/acceleration setting to be more practical for other speeds or you will trip out. If I add DC braking, my VFD ignores the deceleration times and "slams on the brake". That's one way to get a "quick stop" but...

                  This is why I want both mechanical and vfd braking (both dynamic and DC injection) on a lathe. The inertial mass varies massively on my lathe (collets, 6 inch to 16 inch chuck just for a start, never mind the work from grams to 100's of lbs); it's hard to find any single sweet spot for deceleration with just dynamic braking.

                  I need to play with the PLD on my vfds - in theory I can read a selector switch and auto-set parameters - would be nice to have multiple changes at the flick of a switch. I can already set 2 "different" motors, so I guess I could have two discrete sets of parameters entered.
                  Last edited by lakeside53; 04-18-2018, 11:50 AM.

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                  • #24
                    This effect ( DC injection) was a problem back in the days of the early induction motor vfds based on scrs
                    with forced commutation by capacitors.

                    If an scr failed to commutate, there was uncontrolled rise of stator current, basically into a circuit similar to Matti's #15.

                    On time as a young skinny guy I was testing one of those early vf drives into a motor about 35 kW
                    sitting unbolted on a pallet in the test bay.

                    On a decomm at full speed the motor lifted up into the air, started rolling over and crashed down onto the floor.

                    The 6 pulse dc drives of about that era had add on pack of dynamic braking resistor and DC bar contactor.
                    usually bolted on top of the cabinets in mesh housing. That was braking by resistance across spinning armature, not DC injection.

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                    • #25
                      Originally posted by lakeside53 View Post
                      With it set as you say in back gear and with 80 rpm spindle you can stop pretty quick with just dynamic braking (resistor); I could probably get a fraction of a second. BUT it's compromise. It will only stop as fast as your deceleration setting - and you will have to reprogram the deceleration/acceleration setting to be more practical for other speeds or you will trip out. If I add DC braking, my VFD ignores the deceleration times and "slams on the brake". That's one way to get a "quick stop" but...

                      This is why I want both mechanical and vfd braking (both dynamic and DC injection) on a lathe. The inertial mass varies massively on my lathe (collets, 6 inch to 16 inch chuck just for a start, never mind the work from grams to 100's of lbs); it's hard to find any single sweet spot for deceleration with just dynamic braking.

                      I need to play with the PLD on my vfds - in theory I can read a selector switch and auto-set parameters - would be nice to have multiple changes at the flick of a switch. I can already set 2 "different" motors, so I guess I could have two discrete sets of parameters entered.
                      You can KNOW what is possible and what is not.... if you care that much.

                      And, dynamic braking (meaning the regular deceleration via programmed stop) can simply be programmed for the desired speed.... It will NOT matter what the mass is, so long as it results in less than the maximum braking energy per unit time.

                      If you need to always have the optimum for whatever you have in the machine, then you can calculate it.

                      The braking resistor pulls a certain current, dependent on the duty ratio. And there is a certain current draw from the power supply, etc, plus any load current. That is the drains.

                      The energy content of the capacitors is 1/2 C*V^2, and you can figure the difference in energy content between normal voltage, and the trip-off voltage. The VFD will accept up to that total energy without tripping off.

                      The drains (brake resistor, etc) subtract energy at a given total rate, so the net energy per unit time that the unit will accept with a given resistor and duty cycle is knowable. That will give you the stopping rate that is possible, if you know the spinning mass (and effective radius) of the drive train and load.

                      In general you will not care very much for home shop applications, and it will just work if you add a resistor. Or maybe even if you do NOT add any "braking resistor". But in some industrial operations, with a given "process cycle", it is actually required to calculate that all out and verify that the cycle will actually work.
                      Last edited by J Tiers; 04-18-2018, 12:13 PM.
                      CNC machines only go through the motions

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                      • #26
                        Some VFDs can be set to limit decelleration based on the DC bus voltage.

                        That's how the Huanyang on my Harrison L5 used to be set up; With the collet chuck fitted, it would stop withing the programmed decceleration time. With the three jaw fitted, you could hear the motor pulse a couple of times as it hit the DC bus limit. With the big 4 jaw fitted, it would still occasionally trip if stopped from high speed.

                        Now the VFD has the braking components and an external resistor fitted and stops fine all the time.
                        Paul Compton
                        www.morini-mania.co.uk
                        http://www.youtube.com/user/EVguru

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                        • #27
                          Originally posted by EVguru View Post
                          Some VFDs can be set to limit decelleration based on the DC bus voltage.

                          That's how the Huanyang on my Harrison L5 used to be set up; With the collet chuck fitted, it would stop withing the programmed decceleration time. With the three jaw fitted, you could hear the motor pulse a couple of times as it hit the DC bus limit. With the big 4 jaw fitted, it would still occasionally trip if stopped from high speed.

                          Now the VFD has the braking components and an external resistor fitted and stops fine all the time.
                          And on some fancier VFD's you can set the maximum desired torque used for deceleration and/or maximum motor power limit for deceleration.
                          Location: Helsinki, Finland, Europe

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                          • #28
                            Another thing to consider - If you demand rapid starts and stops you are operating in the range that most VFD manufactures would call Heavy Duty operation and they would suggest a larger unit, 1-1/2 to 2 times larger. This would also allow for faster stops as there are more capacitors to charge.

                            While you may be using the VFD for the convenience of running a 3 phase motor off single phase they are more designed for cost savings when operating motors. By running a motor on a pump or fan at less than full speed the cost savings can be enormous. Also - if you are starting and stopping often you may be in a Heavy Duty classification of operation and actually need a bigger VFD.

                            If you are using a braking resistor and it is not heating up it is either not installed correctly, the VFD is not configured correctly, or it really is not needed in your application.

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                            • #29
                              If you ever get a chance to look over a Hass TL 1. They just use an old fashion spiral Electric stove heating element as the breaking resistor.

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                              • #30
                                Originally posted by H380 View Post
                                If you ever get a chance to look over a Hass TL 1. They just use an old fashion spiral Electric stove heating element as the breaking resistor.
                                While that sounds cheap and effective, you need to be careful if you try that.

                                There is a minimum resistance that is allowed, because the discharge IGBT ("Braking resistor IGBT") is almost always smaller, and has limited current capability. If you pop that part, you are out of luck, because they cannot be repaired except by replacing a module that is usually as expensive as the whole VFD, if it is even available.

                                OK, maybe I and a few other folks could do something else, but in general it is just not practical....
                                CNC machines only go through the motions

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