High cycle DC injection braking

[Barry1961]

A customer has a small high cycle indexing conveyor with a 1.3 second cycle. Almost all the braking with be done with decel but they are thinking of using DC injection braking to insure the motor comes to a complete stop. This is a 1/50th hp, 3 ph motor on a 300:1 high efficiency gearbox.

The load is very small and the only inertia is the rotor and input pinion.

I am trying to get an idea of how often I can lightly DC injection brake without burning the winding and how I can tell if the winding is about to burn. Will the motor case get hot before the winding burns?

Is this something doomed to fail no matter how lightly we DC injection brake?

Any ballpark guesses on what we can get away with?

Barry1961

 

[Skogsgurra]

Could work.

I have done "fine feed" with rapid cycling small induction motors on-off. Didn't need to brake them, though. Those motors lasted for many years and were removed when the plant eventually was rebuilt.

If you do not use more than rated current and switch the DC off at stand-still, I think that you may get overheated motors - but mostly because the fan doesn't run all the time. Rotor heat is usually the weak point when fast cycling, but I don't think it would be with motors as small as yours.

I would try adding some friction to the motor output shaft instead of using DC braking. Or, I would use a small 12 or 24 V DC motor (have a look at Bosch) and short the armature (possibly with some small resistor) to get a fast stop.

Gunnar Englund

http://www.gke.org

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[Barry1961]

I really hate to say this but I am not sure how to accurately measure the DC current. I would guess the VFD is still using the same carrier frequency and just turning off most of the IGBTs to get the DC so I need a RMS reading. Will the winding act as a good enough filter to allow a accurate current reading with a standard DMM?

The parameters in the VFD deal with percent torque but I have no idea what the efficiency of DC braking is.

Also the Fluke web site is down now.

Barry1961

 

[itsmoked]

Why not use a vector VFD and just let it control the motor position directly? You will automatically have braking.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

OK, so there's a VFD involved? You didn't say so in the OP. Then do what Keith says. Not much rotor heat.

BTW: Where did you find a 0.02 HP three-phase motor? And a VFD that suits it?

A DMM will probably not work beacause the cycle time is too short for a stable reading. Use a DC clamp and a scope or recorder if you really need to verify what's going on.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Barry1961]

It is a vector drive, they are just worried about drift. I don't think they will need it but they like the idea of the rotor being locked in place after decel. There is no feedback now and they don't want to add any.

Barry1961

 

[Skogsgurra]

You will be fine then. But no need for DC injection. Just add a braking resistor to the DC link, but doubt if even that is needed.

The chopper is usually alredy included.

What make are drive and motor?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

I'm amazed you need locking on a 300:1 gear box for your described conveyor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[CJCPE]

DC braking will not hold an induction motor at zero speed. Zero torque is produced at zero speed. I don't remember where I got the attached torque vs speed curve, but I believe it is valid for some range of motor sizes.

 

[Barry1961]

Sorry about not putting the VFD in the OP.

It is an AB PowerFlex and a Brother gear motor. Brother and Sumitomo both make very strong high reduction, high efficiency gearboxes down to 1/50th hp in 3 ph. They are just single voltage at 1/50th, 230 V or 460 V. I have used these on several brands of inverters and they seem to perform well.

You do have to have an output filter when running on 460 V even with short motor leads or there is a very good chance the windings will go in a month. You also need to have a OL parameter that is scaled for 1/10th amps.

Sumitomo and Brother both get the motor from the same manufacturer in Japan but I can't remember the name.

I have an application where sometimes I back drive a 750:1 right angle brake gear motor. Brother has an option for a light spring set brake for this to protect the gears. The back driving torque of this gear motor is 130 in-lb which is near it's 148 in-lb continuous rating.

FYI on the 1/50th, sort of "slippy" for a 3 ph.
208 V 1610 rpm .13 amp
230 V 1650 rpm .13 amp
460 V 1700 rpm .12 amp

Barry1961

 

[Skogsgurra]

"Brother has an option for a light spring set brake"

I think that is all you need.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Barry1961]

Brother lists the max brake cycles at 10 per minute. From what I understand this is due to the brake coil getting hot from the high inrush. I don't know how much a light spring would improve the rating. Maybe a light spring and a resistor?

I have done my best to convince the customer not to do the DC injection braking and am hoping they will not need it.

Thanks for all the help!!!

Barry1961

 

[Skogsgurra]

The spring is just a stand-still brake. The stopping is done via the VFD ramping down. That doesn't produce much heat. Problem may be that you cannot set the ramp fast enough for that 1.3 s cycle.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ozmosis]

If your AB Powerflex is a Powerflex 70 (the smallest 'vector' VFD that AB do) then you may have some challenges setting the vector performance up. The smallest rated drive is 0.5Hp and at best, the effectiveness of setting up a vector control within the VFD on a motor with a rating <VFD is about 1:6.
I think the motor will be too small to really use the vector performance of the drive.
If this doesn't work too well, try setting the VFD to standard V/F (scalar)control. The actual size of motor may work in your favour due to the inefficiency of a motor at this rating. Braking torque will be dissipated in the motor first with a low eff.
The PF 70 below 30Hp also has a built in brake resistor for light duty braking, so this would also serve to provide additional dumping rather than dc braking.

 

[Barry1961]

ozmosis

In past applications I did not have much luck generating torque quickly in V/F mode. In a recent application with a Mitsubishi E700 it took about .5 second to reach 100% FLT in V/F mode. I tried raising the voltage boost with not much effect. The way I got torque quickly was in Vector mode and adjusting the anti-slip parameter. I am guessing that a major part of the anti-slip is adding hertz in proportion to the amount of slip measured. Using a two-step acceleration or minimum starting hertz might be able to mimic this but I hope the AB can do the work in Vector.

Barry1961