Istilldontknow (Customer) asked a question.

July 23, 2023 at 7:55 PM
Using Symax Permanent Magnet motor as a holding brake motor

I am using a Symax 1/2Hp or 1Hp (low amps) PMAC motor with with a GS13 or 23 drive. They are a permanent magnet design so I am putting +15Vdc to 2 of the lines (L1 & L2) and 0Vdc from the same power supply to the other line (L3) when the drive isn't running. This creates an excellent holding brake that will never mechanically wear out and provides more than enough holding torque for my application. (NOTE: I tried a 24V PS and the motor got hotter than I liked so I scaled back to 15V and it never gets over 120degrees F.)

 

I originally used a motor contactor with auxillary contacts to wire it through: The contactor is controlled by the drive, whenever the drive runs, it turns on the contactor sending power to the motor. When the drive is off, the 15VDC power supply sends power thru NC contacts to the motor acting as the brake.

 

Since I couldn't find a contactor with 3NO and 3NC contacts I used a 2NC auxillary contact block on a 3NO, 1NC contactor to get the 3NO 3NC contacts I needed to swtich between the drive and the Power Supply. I think that they did not switch mechanically fast enough on one of the 5 installations and that drive would trip intermittently on a ground fault and had to be manually reset. (NOTE: Only one out of five did this and I replaced the drive, replaced the contactor, etc. It still faulted intermittently. maybe the problem was elsewhere in the wiring but I couldn't track it down)

 

So, I'm looking for a cleaner solution without auxillary contact blocks and I see no reason not to use a standard 3PDT ice cube relay to switch between the drive and the power supply other than that they are only rated for 230V max. The drive is ramping the voltage up to 460V, the braking power supply is only 15VDC. Since I'm not really switching the motor with the relay at full voltage and the motors are pulling no more than 3 amps (I'm using 15A rated relay contacts) is there any REAL reason to be concerned with this arrangement?

 

I've already tested the ice cube relay solution with no problems whatsoever so if anyone can tell me why this would be a horrible idea I would really appreciate it. This arrangement makes a very nice brake motor replacement with nothing mechanical ever to fail or wear out. I have five units running (almost) flawlessly for over a year now.

  • 8 answers
  • 382 views

  • Adisharr (Customer)

    What are you holding and what happens if the contact were to fail losing all your hold torque?

    • Istilldontknow (Customer)

      Just some minor slippage of a registration position. I'm not really concerned about the slipping. The "accepted" design uses a regular fail-safe brake motor and these brakes are almost always worn out and don't do any holding at all.

       

      I think the inherent magnetic clogging in the PMAC motor might be enough to prevent the slippage of the load but with the extra 15VDC holding torque I know for sure that it works. Theoretically, DC Injection braking would work perfectly but the drives can't supply that for an indefinite time period. If they could, I would just always inject some DC into the motor through the drive.

       

      HEY AUTOMATION DIRECT PRODUCT TEAM, MAYBE YOU COULD GET DELTA TO INCORPORATE FULL TIME DC INJECTION INTO THE GS DRIVES WHEN USING WITH PMAC MOTORS TO USE AS A HOLDING BRAKE?

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  • PouchesInc (Customer)

    I wouldn't use the ice cube relay if you have to pass 460v through it and the relay isnt rated for that. Perhaps you could use the contactor to pass the 460v to the drive, and use the single built-in aux contact to drive the coil of the ice cube relay, and then the DC voltage passes through the relay? It is not as clean as you would like with only a single device, but would keep the voltages nicely separated and requires no additional aux contact blocks to make work.

  • Cap (Customer)

    The Ice Cube cant handle it.. You are switching from A/C to D/C then Back to A/C.. on the same contacts.. I think the Inductive Spike will kill the Contacts.. and GENERALLY they do not have enough separation between the Contact Surfaces to keep from welding.. And if they weld, you might be blowing the VFD back into the PowerSupply..

     

    Generally you want a Reversing Contractor with a Mechanical Interlock.. So it is not Possible for both contractors to be energized at the same time.. One Contractor will have the VFD Feeding it.. One Contractor will have the 15Volt PS on it..

     

     

     

    Contactor.. Need Two of them.. I'm using 24VAC for my Coil Switching

    https://www.automationdirect.com/adc/shopping/catalog/motor_controls/iec_magnetic_contactors/iec_contactors/sc-e02p-24vac

     

    Interlock, that Mechanically prevents both contractors from engaging at the same time..

    https://www.automationdirect.com/adc/shopping/catalog/motor_controls/iec_magnetic_contactors/magnetic_contactor_accessories/sz-rm

     

     

    And then 2 of the Aux Contact Tops for the Contractors.. So the Logic knows what is actually happening..

    https://www.automationdirect.com/adc/shopping/catalog/motor_controls/iec_magnetic_contactors/magnetic_contactor_accessories/sz-a22t

     

    Overbuild it rather than making it small and cute.. it will last forever..

     

    Cap

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    • Istilldontknow (Customer)

      Thanks for the input Cap!

       

      Here is what I am going to do because I want to test this.

       

      1st: This motor is a positioning motor so the greatest amount of time the drive is ever run continuously is 60 seconds. the typical run time is less than 15 seconds. So, first and foremost, this is more of a holding brake arrangement rather than a running motor arrangement.

       

      2nd: The relay isn't really "switching" the AC Voltage from the drive. it basically works like this. A) Drive starts operation and turns on its internal relay output, B) The relay output switches the ice cube relay (power selector). 3) the power selector relay switches from the DC supply to the Drive AC output. (FYI: I put a RC network on the DC power side to snub any arcing). IN THEORY, we are only switching very minimal volts and amps when the drive starts and the drive turns off its output when it stops and releases the relays. The drive is set to coast to stop, so as soon as the drive stops, the power selector relay switches back and motor immediately stops via applied 15VDC (just like DC injection braking that the drive itself would use).

       

      So, I have 4 running systems with contactors and auxillary blocks and I know this way works for over a year (with the one anamoly)..

      I like your 2 interlocked contactors idea. I will do it this way on the next two units.

      As a TEST, I will build one unit with the ice cube relay idea because it is the most simple arrangement.

       

       

       

       

       

       

       

       

       

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  • OkiePC (Customer)

    Have you looked into using the VFD DC Braking option? If suitable for you application, it could eliminate the contactors altogether, but the the VFD will usually limit how long you can apply DC Injection braking, and it might not allow enough time for what you're application demands.

    • Istilldontknow (Customer)

      Yes Okie, It needs to act like a fail safe brake that is always on or I would use DC injection.

 

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