DC Brake unit.

JesperMP · Nov 17, 2006

Hi All.

I have a request for stopping a vibrating conveyor quickly.
It has 2 counter-rotating agitators.
It is an existing installation with the panel being 100 m away from the conveyor.

The problem with installing a VFD is that we would have to exchange the cables, which is a chore at 100 m length.
That is why I think that maybe a DC current injection brake unit might do the trick.

1. Are there any good suppliers of these things ?

2. Do they output pure DC current, or is it pulsating DC current ?
The latter would kill my argument about not having to exchange the cables.

Thanks.

504bloke · Nov 17, 2006

Hi Jesper

We use DOLD units, german made and very good.

I have fitted about 40 sized between a few Kw up to 75 Kw.

They inject DC into the windings (not pulsed)

JohnW · Nov 17, 2006

What about a soft start with soft-stop or braking function?

If your start/stop time is less than 3 secs you do not have to worry about EMC

DickDV · Nov 17, 2006

Most soft starter manufacturers also make stand-alone DC injection brakes for use with mag starters. These are simple, inexpensive, and work very nicely.

They send pulsating DC to the motor, not high frequency DC pulses as in an inverter, but chopped 50 or 60Hz AC. You so not have to change motor leads with these devices.

Benshaw and ABB among others make DC injection brakes.

JesperMP · Nov 20, 2006

Thanks guys. I knew I could get the answer here.

The bit about not having to change the cables was the most important to me.

Doug_Adam · Nov 20, 2006

Why do you need to change the cables?
Properly filtered, the VSD output should be OK.

JohnW · Nov 21, 2006

Doug_Adam said:
Why do you need to change the cables?
Properly filtered, the VSD output should be OK.

You have to use screened cable for VSD outputs. On large cables (over 10mmsq) a seperate earth cables is required or the use of a cable with a symetrical 3 core + 3 earth cores.

The only filtering I can think of where possibly a screened cable is not required is a sine-wave filter (is this what you are talking about, or du/dt filters?)but it would be much cheaper to replace the cable.

Manufacturers specify a copper screened cable but we find that steel wire armoured works fine in most circumstances, although it must be XLPE insulated. PVC cable lasts about 6 months due to insulation breakdown from corona effect ozone production in the air gaps.

Doug_Adam · Nov 21, 2006

JohnW said:
You have to use screened cable for VSD outputs. On large cables (over 10mmsq) a seperate earth cables is required or the use of a cable with a symetrical 3 core + 3 earth cores.

That's what I used to think, however I have found that standard inductive choke filtering works fine. I generally use SEW VSDs, of which the smaller ones come with built in filtering. For the larger VSDs, an off the shelf filter can be purchased that does the job.

JohnW said:
The only filtering I can think of where possibly a screened cable is not required is a sine-wave filter (is this what you are talking about, or du/dt filters?)but it would be much cheaper to replace the cable.

Manufacturer supplied filter is what I am talking about, generally an inductor, but possibly with capacitance.

JohnW said:
Manufacturers specify a copper screened cable but we find that steel wire armoured works fine in most circumstances, although it must be XLPE insulated. PVC cable lasts about 6 months due to insulation breakdown from corona effect ozone production in the air gaps.

What voltage are you running this at? 22kV?

Our PVC cables tend to last for a long time, the exceptions are physical damage, which can happen at any time, or UV light, which for the cables I'm thinking of have been there for more than 10 years, still work but only because they are not being moved. I have never had a PVC cable have isulation break down, even with mistakenly installed un-shielded/unfiltered cables. What I did get with the un-shielded and unfiltered cables was a lot of noise in the analogue inputs, the occasional triggering of high impedance digital inputs (again due to electrical noise), and we could not use UTP Ethernet cable in that area.
The major problem we had was noise, not cable breakdown.

JesperMP · Nov 21, 2006

Doug_Adam said:
What voltage are you running this at? 22kV?

John must be thinking of the expected reduced service life of a motor that is driven by a VFD. Didnt think that something similar applies to cables, but it sounds reasonable.

For a VFD with the motor(s) 100 meters away, I think that screened cables be used in any case, maybe with the exception of using a dU/dt filter.

Anyway. I think that the discussion about filters on VFD outputs is a material for another thread. I shall start one shortly

JohnW · Nov 22, 2006

Doug_Adam said:
What voltage are you running this at? 22kV?

Our PVC cables tend to last for a long time, the exceptions are physical damage, which can happen at any time, or UV light, which for the cables I'm thinking of have been there for more than 10 years, still work but only because they are not being moved. I have never had a PVC cable have isulation break down, even with mistakenly installed un-shielded/unfiltered cables. What I did get with the un-shielded and unfiltered cables was a lot of noise in the analogue inputs, the occasional triggering of high impedance digital inputs (again due to electrical noise), and we could not use UTP Ethernet cable in that area.
The major problem we had was noise, not cable breakdown.

Here is an extract from ABs manual "Wiring and Grounding Guide (PWM) AC drives DRIVES-IN001E-EN-P

Quote:

"Wires with dielectric constants greater than 4 cause the voltage stress to shift to the air gap between the wires that are barely touching. This electric field may be high enough to ionize the air surrounding the wire insulation and cause a partial discharge mechanism (corona) to occur. The electric field distribution between wires increases the possibility for corona and greater ozone production. This ozone attacks the PVC insulation and produces carbon tracking, leading to the possibility of insulation breakdown. Based on field and internal testing, Rockwell Automation/Allen-Bradley has determined conductors manufactured with Poly-Vinyl Chloride (PVC) wire insulation are subject to a variety of manufacturing inconsistencies which can lead to premature insulation degradation when used with IGBT drives. Flame-retardant heat-resistant thermoplastic insulation is the type of insulation listed in the NEC code for the THHN wire designation. This type of insulation is commonly referred to as PVC. In addition to manufacturing inconsistencies, the physical properties of the cable can change due to environment, installation and operation, which can also lead to premature insulation degradation.
The following is a summary of our findings:
Due to inconsistencies in manufacturing processes or wire pulling, air voids can also occur in the THHN wire between the nylon jacket and PVC insulation. Because the dielectric constant of air is much lower than the dielectric constant of the insulating material, the transient reflected wave voltage might appear across these voids. If the corona inception voltage

(CIV) for the air void is reached, ozone is produced. Ozone attacks the PVC insulation leading to a breakdown in cable insulation. Asymmetrical construction of the insulation has also been observed for some manufacturers of PVC wire.

I have experienced this where after 6 months PVC cable insulation went from 10Mohm to 100Kohm.

I replaced the cable with Belden copper screened XLPE insulated.

DC Brake unit.

JesperMP

Lifetime Supporting Member + Moderator

504bloke

Lifetime Supporting Member

JohnW

Member

DickDV

Member

JesperMP

Lifetime Supporting Member + Moderator

Doug_Adam

Member

JohnW

Member

Doug_Adam

Member

JesperMP

Lifetime Supporting Member + Moderator

JohnW

Member

Similar Topics