Brake rectifier 2

[stuartfd54]

Hi

Over the last 2 weeks we have had to replace the brake rectifier 3 times on a 415v 3 phase motor with 415vac/205vdc rectifier. The complete motor and brake have also been changed along with contactor.
The rectifier is fed directly from the 415v from the motor so as the motor starts the brake releases. The operation that the motor drives requires start/stop 36 times/minute.
1st question - I presume there could be a slight delay in the motor start/ brake release but could this be causing the issue of destroying the rectifier?
2nd question - Should the rectifier be connected across U1/V1 or W1 incoming to the motor or U1 and copper bar for star connection?


Any other advise would be gratefully received.

Thank you
Stuart

 

[ScottyUK]

The rectfier really wants to be fed from a rectfied but unfiltered line-neutral supply. If you're feeding it from two lines then no wonder you're burning it out. You would be better off feeding it as an auxiliary supply switched at the starter so the brake won't see the voltage dip which occurs in the cable feeding the motor when it draws high current during start-up. The voltage dip may be enough to prevent the brake disengaging cleanly if you connect it line-neutral at the motor terminals.

That motor is seeing quite a severe duty with that number of starts per minute. How big is it, and is it a special design?

 

[waross]

The choice between connecting the brake at the motor terminals and a separate feed fro the starter is a compromise of diminishing returns.
The longer the conductors from the control to the motor, the higher the cost,
But
The longer the conductors from the control to the motor, the greater the likelyhood that you will need them.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stuartfd54]

Hi Scotty

This is designed from the manufacturer of the machine (Indosa can seamer) and has run for 20+ years, only having to replace the rectifier/motor on average every 4 years. Probably state of the art 20+ years ago and I know there's alternatives available but with a cost. Just struggling to understand why this is happening so frequently now?

Stuart

 

[ScottyUK]

Any possibility of an accidental mis-connection? Voltage-wise it would make sense to connect it line-neutral, even if you use the star point of the winding as the neutral.

 

[stuartfd54]

Hi Waross

The supply from contactor to motor are approx. 2metres.
I'm not sure how much of a delay between motor start and brake release there would be but if there was a minute delay, when the motor is trying to rotate would this increase the draw current through the rectifier and eventually result in failure of the rectifier?

Stuart

 

[stuartfd54]

Hi Scotty

You refer to neutral but this rectifier is 415v input? At current it is connected through U1 and U2 on the motor terminals. I've had a look through the internet and SEW advise U1 and star terminal in the motor for a 200vdc brake coil and 415v motor. Not sure what difference this would make?

Stuart

 

[waross]

The brake windings may be failing.
It happens even on new equipment.
The replacement may be designed for line to neutral connection.
Does the brake have surge suppression or a free wheeling diode?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stuartfd54]

Hi Bill

The coil winding measure 1kOhm which appears correct. No surge suppression, not sure about the diode?

Stuart

 

[itsmoked]

I just had to work on a '205V' brake. I can definitely tell you you should NOT be feeding that rectifier with 4xxV anything. You may have had it go bad out of lifetime issues and the replacement was possibly mis-wired back in. There is NO way to feed a typical rectifier 415v and get out the required 205Vdc.

If this motor is truly running on 415V AND the rectifier was truly being 'fed from the motor' then perhaps it was tapping somehow into the the low voltage taps of the motor. I'd have to think about that and if it's possible.

More likely it's as Scotty is trying to tell you, the motor is WYE connected and the rectumfryer is supposed to be going from one phase to the neutral.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[stuartfd54]

Hi Keith

The motor comes direct from the manufacturer with rectifier already wired up?

Stuart

 

[waross]

Is this a 6 lead motor or a 9 lead motor. Is it star connected or delta connected.
You want to check the resistance to ground with a megger at 500 Volts or 1000 Volts test voltage. You can not get a reliable insulation resistance measurement with a low voltage multi meter.
You can tap off 208 Volts from a nine lead, delta connected motor. Nothing else comes close to 205 Volts.
Is there a series resistor?
If the replacement brake is designed for AC operation then using the rectifier will quickly burn out the brake coil.
You may want to try full voltage AC to the brake. If it releses cleanly, that is your solution.
I have seen more AC brake coils than DC brake coils. Typically designed to be connected to the motor terminals and typically designed to accept the starting voltage dip.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stuartfd54]

Hi Bill

Definitely 6 lead motor and the coil is rated at 205vdc hence the need for the rectifier. The resistance of the coil was tested at 500v using fluke insulation resistance tester and came back just over 1kOhm. As said, the rectifier comes prewired direct from the manufacturer so not sure how it could be incorrectly connected, twice. The brake is releasing, it's the rectifier that is failing?

stuart

 

[itsmoked]

Bill one of the bigger brake makers uses just DC brakes they have the crazy 205Vdc coils in them.

Hi Stuart. Yes, but just because the motor comes wired to the brake doesn't mean they necessarily wired it for the correct voltage...

Got any part numbers? Like the motor? The brake make and the model you have?
A picture of the schematic associated with the brake?

Perhaps a picture into the pecker-head where the rectifier and motor leads terminate? Waross and his type would be all over that picture and could probably even read your fortune from it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[stuartfd54]

The rectifier has input 500vac max - Output just + and -
In the past I've had motors delivered that don't have the input connected if it requires an external source (240vac), you just presume that the manufacturer would wire this correctly but I will certainly review it and look at external supply for coil as the supply cable is 7 core with 3 taped up not being used, again, why waste money with a 7 core cable if you could install a 4 core to do the job? Thank you all for your help?

Stuart

 

[waross]

Insulation resistance is measured from one lead to ground. it should be hundreds of Meg-Ohms or gig-Ohms.
I did a little googling on brakes with 205 VDC coils..
If the rectifier is half wave, the connection is line to line.
If the rectifier is full wave the connection is line to neutral.
There has probably been a supply change from half wave rectifiers to full wave rectifiers.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stuartfd54]

Thank you.

 

[ScottyUK]

Good catch Bill, I didn't think anyone bothered with half-wave rectification these days.

 

[M0rt]

Hi, all,
I was mooching through this thread and thought I would throw in my 2p/2cent:
In my limited experience, motors can be supplied with "415Vac" or "230Vac" on the brake rating plate, and as mentioned, this is usually an indication of the rectifier module fitted - the actual brake coil is the same, 180Vdc - 205Vdc. the rectifier is either half-wave (415Vac) or full-wave (230Vac). One other observation I will make - I'm not convinced that the exact voltage supplied to the coil is critical, the main effect is the reaction time.
re. the OP, if the rectifier states "max. 500V", then the failure is unlikely to be caused by the supply. However, if the motor/brake is switched via a contactor, then I wonder where the back EMF from the brake coil is going when the contactor opens? It seems to me that a free-wheel diode is required on the dc side of the rectifier?
Please correct if my thoughts are wrong on this.
Thanks in advance,
Mort

 

[iop95]

I think free-wheel diode is missing or is removed from circuit if contator is at rectifier output / coil input.