D700 VFD Braking help

[bdahlka]

I am performing a durability test on some vehicle components. I have a Teeter-Totter table that is cycled using a 408 motor. For controls I am using a Mitsubishi D700 VFD with a 24 volt 3 pole control relay interrupting voltage between the VFD and the motor. This cycles twice a day for 70 minutes then sits for 8 then 16 hours. Also this is at -15°C so I dont want to use any additional air cylinders or anything.

Problem when the relay opens the motor coast down to a stop. I need it to stop in the horizontal position. I have a Prox sensor to monitor table location and would like to apply a break at that time. The motor is not moving very fast (48hz).

-The table could slow down before it stops (it doesn't have to stop on a dime just needs to stop horizontal.

Please let me know if you have any additional questions. Thanks in advance

 

[Skogsgurra]

Breaking the VFD output is the worst way of stopping the motor, some say no-no. Issue a stop command instead and let the motor ramp down. It may be necessary to add a braking resistor - or it may work without one. It depends on inertia and friction and how fast you ramp down.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

bdahlka; If you look at section 1.4.3 of the manual you will see:


You also say for "70 minutes". Are you reversing the motor back and forth continuously for that entire time? If so you have to be VERY careful you don't fry it. That is abusive service if you don't limit the current correctly. Your low temp will help a little but if you "test" the system without the low temp you will need to be doubly careful.

If you've done the design correctly with a bellcrank so you aren't thrashing the motor you can disregard the above current limit precaution.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[bdahlka]

The motor only spins in one direction. It is geared down. With a breaking resistor my understanding is this will just help the table stop faster. Is this accurate? I need the table to stop and then stay in the horizontal position. I am looking into DC injection breaking now but I am unfamiliar with this. Concerns I have read so far is that the DC Injection should not be provided over long periods of time. Any input on this?

 

[LionelHutz]

Disconnecting the motor from the VFD with a contactor while it's running is not a good thing to be doing.

Just stopping at the horizontal position vs holding the table in the horizontal position are much different requirements.

DC injection is incapable of holding an induction motor at zero speed.

It's possible to use the VFD to hold the motor stopped - running at 0Hz with certain VFD's can do this.

It's possible to use a brake on the motor shaft. Certain VFD's can co-ordinate the application of the brake.

You should thing about changing the drive and motor. I expect a servo motor is much more suitable for this application. No point trying to put lipstick on a pig by forcing what you have to do things it's not very capable of doing.

 

[jraef]

Or put another way, VFD braking, in any form, is not intended to be used as a continuous HOLDING brake, especially for safety reasons, because if you lose power, you lose the holding cability. Yes there are VFD options that allow holding torque at zero speed, but the INTENT of that capability is to allow for safe removal of a mechanical holding brake without having the load move before the drive can attain control of it, for example a hoisting application.

So bottom line, it sounds like you need a MECHALICAL brake, either built in to the motor or mounted externally. You can add dynamic or DC injection braking from the VFD to do MOST of the braking work with the motor and thereby all but eliminate wear and tear on the mechanical brake, but set the mechanical brake for the long term hold. Think of it as the Parking Brake on your car.

"Will work for (the memory of) salami"

 

[bdahlka]

*update I now have it configured to stop using a relay to remove the jumper from STF to SD. Is it possible to then apply the DC injector break after the prox switch is on? As the motor coast down it passes the prox switch. At that time I would like the injector break to come on and hold the table in position. I have been reading the 450page manual and haven't come across a way to do this yet. Please let me know.

Thanks in advance!

 

[bdahlka]

What about a brake kit to add to the existing motor like this one?

http://www.grainger.com/product/USEM-Motor-Brake-Kit-23J947?searchQuery=23J947

 

[LionelHutz]

OK, I will repeat this again. DC injection braking is incapable of holding an induction motor at zero speed. You'll be able to turn the motor by hand with the DC applied.

Still, there should be a way to use a digital input for turning-on DC injection. Not really needed though because you can just decelerate the motor to a stop.

And yes, add a mechanical brake to the motor.

 

[bdahlka]

Thanks, yes I realize now that I need a mechanical break. Problem with the one I put in the link is the table utilizes both shafts of the motor so this break would not work. Is there any other options?

 

[itsmoked]

bdahlka; Erase ALL your existing braking ideas with respect to a VFD. You've got the commonly made concept errors noobs usually have when seeing all the 'brake' words associated with VFDs. I made them too.

You need to realize all the "brakes" in any VFD are DYNAMIC brakes. The DC injection only works at speed. The "Brake Resistor" should really NOT be called a brake resistor it should be called a "braking load resistor" as it only allows the VFD to dissipate overrunning energy that is supplied by the spinning load's inertia.

There is no 'parking brake' what-so-ever available.

You need to acquire a proper spring brake. There are zillions of them out there. They're a spring applied - electrically released brake. You will need to apply power to the brake to release it, then run your VFD. Use your VFD in vector mode so it can bring the motor to a controlled stop and hold it there MOMENTARILY, and then, remove power from the spring brake to permanently lock things in place. Next you "STOP" the motor via the VFD. Now all power can leave your facility and the shaft will still not move nor will your teeter-totter.

You need to look for a "Spring applied electrically released fail-safe brake". They are everywhere though you need to dig a little because "brake" has so many connotations on search engines. Use lots of '-terms' while searching. Especially '-trailer'

Check ebay. This is on the first page:
[link]http://www.ebay.com/itm/DODGE-140-DBSS-031377-ELECTRIC-MOTOR-BRAKE-208-230-460-VOLT-C-FRAME-NEW-/121105328974?pt=LH_DefaultDomain_0&hash=item1c3270ab4e[/url]

[link]http://www.jrcweb.com/Spring_Actuated_Electric_Fail_safe_Brake_p/sab.htm[/url]

There's a guy at Applied Technologies Matt Pandolfi (831)722-0243 that helped me with one of these.

These are merely examples. There are hundreds of different styles and sizes.

There are two sub-styles 1) can stop your load if power fails and 2) one can only hold your load once it's stopped. You'd probably be fine with the latter which are much smaller and cheaper since they don't need to dissipate a bunch of kinetic energy.

Keith Cress
kcress -

http://www.flaminsystems.com