Rubber Mill Slip Ring Motor Experience ?

[DickDV]

Leadfoot, I usually think of dynamic braking as being a DC motor issue. I hear it mentioned in connection with AC motors a lot and, while that's ok with me, I find the term "snubber braking" a little more descriptive of what a brake chopper and brake resistors actually do in an AC drive.

With the brake chopper being essentially a voltage sensitive switch for transfering excess DC bus energy to the brake resistors, it effectively snubs the DC bus voltage. Or, said another way, the braking system only operates when the DC bus voltage rises above a specified voltage level transfering some of the bus energy to the brake resistors to be wasted as heat. Either term works for me.

There is, of course, regenerative braking as well. This involves either a drive specially built with a bi-directional front end so it can pass braking energy backward to the AC power supply thus saving it, or a separate regen brake unit like Bonitron builds which does the same thing externally when the drive has a conventional rectifier front end.

swhite65, sparkz has it just right. As the motor poles go up beyond four poles, the FLA for the same hp goes up also. As a result, if the drive were selected based on hp, you very well may not be able to fully excite a 12 pole motor.

Isn't this drive/motor business just a barrel of fun! Now I know why I avoided becoming knowledgeable about sensors, PLC's, servos, etc. etc. My brain hurts just keeping up with the drive world!

 

[JohnW]

I have a complete design for one of these mills including PLC based starter control and a Pilz multi safety relay for safety. also my safety system design has been validated by Pilz. The control system HAS to meet the requirements of RUBIAC and you should stick with the slip ring starting as this provides the easiest means of providing the reverse plugging.

 

[504bloke]

I have a complete design for one of these mills including PLC based starter control and a Pilz multi safety relay for safety. also my safety system design has been validated by Pilz. The control system HAS to meet the requirements of RUBIAC and you should stick with the slip ring starting as this provides the easiest means of providing the reverse plugging.

Hi John

Could you let me know which Pilz system was used and how it was integrated onto the mill. At present there is a bar apparently in front of the roller that reverses the motor if hit. (This is what i have been told as i havent been to site yet). The old system controls and motor are Laurence Scott 1930's era. I was told today the old panel has asbestos within/seperating the rotor resistors...

 

[Leadfoot]

Leadfoot, I usually think of dynamic braking as being a DC motor issue. I hear it mentioned in connection with AC motors a lot and, while that's ok with me, I find the term "snubber braking" a little more descriptive of what a brake chopper and brake resistors actually do in an AC drive.


Isn't this drive/motor business just a barrel of fun! Now I know why I avoided becoming knowledgeable about sensors, PLC's, servos, etc. etc. My brain hurts just keeping up with the drive world!

I vote we name the means of reducing the DC voltage in VFD's "BUS VOLTAGE REDUCING CIRCUIT" "BVRC" for short and leave it at that. Too many names and methods of describing things.

You said a double mouthfull on keeping up with things. Just about the time I get comfortable with things, I get to learn something new. Unfortunately my hard drive is an OLD small one. Everytime I learn something new these days I have to forget something. Then my company wants me to become the PLC, sensor guy too.

 

[JohnW]

Dave,

Get a copy of the BSEN 1417 and the RUBIAC guide on two roll mills. The safety device is a Pilz Multi http://www.pilzsupport.co.uk/products/pnozmulti/index.htm

programmable safety relay.
These milss are really dangerous and if you do not have experience of them them I would advise that you walk away.

Attached is my Functional Specification for a mill safety system, I hope it helps if you decide not to walk away.

Talk to your local Pilz supplier (probably Routeco) and ask to be put in touch with a Pilz Product Engineer

One further note, the plugging may not work with electrolytic starting as you need a constant rotor resistance.

 

[504bloke]

Thanks John

I doubt we will walk away, just make sure the price we quote covers everything from a safety point of view.

In your word doc, can you explain how and what the VSR does exactly when sensing rotor voltage ?

With respect to the Vapormatic Starter if this is sized correctly for Plugging duty on the mill then there should be no problems. The size needed for this 145 Amp motor is a 285 Amp unit with 4 electrodes instead of 2.

edit: btw i have a love hate relationship with routeco: I hate them and will never deal with them - ever

 

[JohnW]

In your word doc, can you explain how and what the VSR does exactly when sensing rotor voltage ?

The VSR detects the rotor stalled when it is plugged as the the rotor voltage is high when the rotor is stationary and power is applied to the windings in reverse during the plugging. This is used to detect standstill, a timer then allows the motor to reverse for a specific length of time to give the quarter turn reversing.

I will PM you with my contact at Pilz.

I couldn't use a safety rotation sensor as these were only rated as cat 3.

 

[504bloke]

Saw the twin roller today and the slip ring motor is a direct drive to a right angle radicon gearbox driving the rollers.

We will be quoting for 2 systems, one using a standard reversing slip ring starter (electrolytic type) and the other using an SSD Invertor, both will be Cat 4 using Siemens ASI Safe Bus system.

 

[Riche007]

504Bloke, how did your project work out? I just saw this thread.

I'm also at a Rubber man plant. We carried out similar project last year and we seem to have some relatively good success so far.

 

[504bloke]

Very well, customer went for an invertor.

Rotor was shorted out at the windings when the motor was rewound (stator and rotor)

The drive monitors the stopping distance through sensors to make sure after every stop that the stopping distance is within allowed tolerences or the system locksout.

The ASi monitors the four corners of the mill that has a two safety switches on each corner of the LunBar, the four corners are then divided (By software zoning in siemens ASi safety software) into 2 zones across diagonals so if one zone fails the other being still active still recognises both lunbars.

We also had a mechanical brake added to the main drive shaft that brakes to instant standstill if the drive isnt braking enough and/or if any estop condition occurs or a power failure.

After comissioning i have never had to go back, system runs 24/7.