E-STOPPING motors with drives

CT782

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Join Date
Jan 2004
Location
NC
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Does anyone know what OSHA, NFPA, or NEC codes mandate as far as E-Stopping motors driven by Inverters or DC motors run by drives? Is there a national standard or is it a 'local regulations' issue?
I've dealt primarily with imported Eurpean machinery for the past 10 years, and I've seen contactors between ac drives and the motor, contactors before the drive killing power to the drive on e-stop, and most recently circuits letting the drive 'fast-stop' the motor and then opening a contactor between the motor and drive. They LOVE electro-mechanical brakes so coasting is usually not an issue with these motors.
I'm not a big fan of killing the power to the drive for an e-stop myself, but haven't really heard or read anything one way or the other.
I'm not doing a project or designing anything, I was just relaxing on my day off here and thinking about motors and drives (WHAT HAS BECOME OF ME!!!) and thought about the many methods I've seen and wondered what some of your opinions are.
Any of you drive reps ""DickDV"" know how this is supposed to be done? Also if you buy a European machine and it's not done to the American standard, does it have to be changed? or is it ok since it wasn't built here?

Hope you all are having a wonderful weekend!!

Chris
 
Chris.

My thoughts from a non US regulatory perspective are:

1. A contactor upstream of the drive is worse that useless. It doesn't deal with the energy stored in the DC bus and if an operator toggles the E-Stop repeatedly there is a good chance the input section of the drive will exceed it's starts/per hour rating.

Many VSD's have some form of inrush current limiting on the input side, and it has a maximum number of "starts/hr" before it typically overheats and goes bang.

2. Contactors downstream of the VSD are ok so long as the VSD is rated to handle it. All the AB 1336 Plus and Powerflex drives I know do this with ease.

3. Better still include in the contactor an Auxiliary that indepedently drives the VSD "Enable" input. Again on the AB drives I am familiar with this input is a direct hardware interupt to the processor/s and absolutely prevents the drive from running.

In fact in some ways the VSD "Enable" technique could be argued to be a better E-Stop than an electromechanical contactor. It is externally wired NC, it is internally hardwired to the processor, there is no programming involved and it absolutely forces all the output transistors OFF instantaneously.

Now I would not argue that it makes an adequate ISOLATOR..(for the purposes of someone working on the motor an isolator should be a seperate lockable switch) but using the VSD "Enable" input certainly makes sense as an E-Stop.

Whether in practise you would use a downstream contactor AND the VSD Enable input, or rely on the Enable input alone is an open question. Any thoughts??
 
Also if you buy a European machine and it's not done to the American standard, does it have to be changed? or is it ok since it wasn't built here?

Once you work on a machine's controls (even if you just troubleshoot & replace a bad relay coil) there are lawyers a-plenty that will take on a lawsuit against you anytime after that if anyone is injured or worse on that machine.

If I see any safety problems on a machine I am working on I immediately stop work & notify the customer about it.

If they decide to take it on themselves to continue operating it that way, I note on my workorder the specific unsafe condition(s), along with the reccommendation to take the machine out of use until modifications are made to meet all OSHA, MIOSHA and general safety requirements.

Greg
 
E-stop braking is a really big subject and, in my experience, there is little in the way of standards to go by. I do know that OSHA dictates the stopping time from max speed depending on the type of machine. A roll press, winder, or slitter often has an OSHA requirement of half-turn on the roll. I have seen some requirements for no more than 6 inches travel on the roll surface. On the other hand, many machines have no stopping requirements at all. Fans inside air handlers, centrifugal pumps, or the main flywheel drive on a stamping press are examples.

If there is a requirement for E-stopping, the first issue to settle is whether normal stopping and E-stop are at the same ramp time. If so, then you will have to have enough snubber braking or regen braking in the drive system to handle both since they are the same. This is not often the case unfortunately. Usually normal stop is a more gradual long decel ramp where E-stop is basically a "slam" stop.

Any notion of cutting input power to the drive needs to be rejected as bad practice for the simple reason that the drive cannot manage the stop intelligently when it is without power. Further, this guarantees the LONGEST possible stop since, after the input power is cut, the DC bus capacitors have a large amount of energy stored which will all run out to the motor keeping it running until exhausted. At that point, the motor and machine begin coasting to a stop. Simply unacceptable!

In some applications, the use of a mechanical brake can be justified because it can, if sized right, stop the machine very suddenly. This can bring its own problems such as stripped gears, broken motor feet, and even a broken motor shaft if the brake is mounted backside and the brake torque is 2 or 3 times the motor torque. If E-stopping is very infrequent, a mechanical brake carefully sized can be a good solution.

When using a mechanical brake, you have to pilot the brake from the drive in such a way that the drive applies the brake only when it is telling the motor to coast to stop. If the drive is ramping to stop and the brake engages, the drive will generally fault on overcurrent and now the drive is incapable of managing the stop any further.

Another good way to stop is to use DC injection braking from the drive to the motor. Keep in mind that, if your braking requirement is 30ft-lbs and the motor is only 10ft-lbs motoring, you cannot get the needed stop rate. Some (few) applications actually need to have the motor and drive sized for the braking requirement, not the motoring requirement. Also, some brands of drives do not give you DC braking from full speed, only engaging the DC injection at around 5hz or so. Clearly, this won't get your machine stopped. You can use DC braking at the same time as applying a mechanical brake since there is no ramp being enforced by the DC from the drive.

Finally, use of a contactor between the drive and motor is great for isolation safety but is no good for E-stop braking since it also prevents the drive from managing the stop. The best you can hope for with a simple contactor in the motor leads is coast to stop.

The best system I have found (it also makes a lot of safety people happy) is to use enough snubber or regen braking in the drive to stop the machine to spec on E-stop and use a contactor in the motor leads that is piloted by the drive RUN relay. That way, you can take full advantage of a fast stop thru the drive system and, as soon as the stop ramp is completed, the contactor opens. You can even wire the E-stop circuit so it selects a second decel ramp (faster than normal stop) which permits easy accel/decel ramps for normal operation and quick stops for E-stop.

The only disadvantage with the system above is that it loses stopping control on power failure. This is minimized in most applications by the fact that the machine likely has also stopped due to lack of power. If not, in some critical applications, this must also be considered in the stopping scheme. Spring-applied, power-released safety brakes are about the only answer.
 
DickDV said:
You can even wire the E-stop circuit so it selects a second decel ramp (faster than normal stop) which permits easy accel/decel ramps for normal operation and quick stops for E-stop.

Excellent idea, Dick!... (y)

beerchug

-Eric
 
E Stopping of Drives

Not aware of you codes but in NZ our company has used a lot of VSD drives. We have connected them both ways
IE Isolator, Fuses, contactor, VSD, Motor.
or Isolator, Fuses, VSD contactor, Motor.
In both cases the E Stop is hard wired into its required function so any cable, control failure, manual E stop stops the motor. preferred one is the first set up.
1) The E Stop tells the drive to stop
2) Disconnecting the drive from the VSD can upset the drive as it knows something is wrong. You may then need to turn the main isolator off, turn the contactor between the VSD and motor back on then turn the main isolator back on. Not very pratical
3)With some drives it can force the drive to ramp up even if disconected from the motor.

Care is need to ensure the drive does not stay ramped up as when you turn the contactor back on say via the E Stop it may start as a DOL motor.

Running two motors of the one VSD drive requires planning as to how protection to both motors can be achieved to comply with wiring regulations.
Our E Stops must
1)disconnect the motor instantly from the power supply. Ramping down first is not allowed.
2) Starting cannot be actioned by simply pulling the E stop back out.
 
NFPA 79

Time for another plug for using NFPA 79 Electrical Standard for Industrial Machinery. Some day the US will have a "standard way" of designing machine control.

Excerpts from NFPA 79 2002 edition:
9.2.2
Category 0 is an uncontrolled stop by immediately removing power to the machine actuators. [i.e. electric, pnuematic, and hydraulic]
Category 1 is a controlled stop with power to the machine actuators available to achieve the stop then remove power when stop is achieved. [i.e. this is what DickDV is suggesting]
Category 2 is a controlled stop with power left available to the machine actuators. [non-emergency stop]

9.2.5.3.1
Each machine shall be equipped with a category 0 stop. [part of lock out-tag out rules]

9.2.5.4.1.3
The emergency stop shall function as either a Category 0 or a Category 1 stop.
 
The majority of drive manufacturers state that you should be dependant on electronic e-stopping of a machine. What if the fuses were taken out during the initial phase of an e-stop?

Recommend that a fail safe pneumatic bake is fitted !!! which would become your principle e-stop source, the drive braking would be an extra.
 
Notwithstanding all the good information posted so far, my own experience has been:
1) OSHA and NEC require that there is a separation of the motor from power during an E-Stop, so the output contactor idea works...but the alternative ramp does not (even if it would theoretically work better). When using a second ramp, it is more correctly called a "quick stop". If bidding into a system which calls for an E-stop, or for the drive to interact with an existing E-stop, I would always include an output contactor on the BOM to CMA...
2) If the E-stop requires very fast stopping, then disconnect the drive using the contactor and apply a manual brake of some sort...these are usually held open when energized so that loss of power won't defeat the protection.
3) Drive or PLC logic never replaces real world mechanical safeties.
 
What I have done is the inclusion of a safety timer. Disconnect the contactor after sufficient time for the drive to stop the motor. If the drive doesn't stop it in time, I wouldn't have stopped anyway if you just pulled the mains.
 
We have a lot of Concrete Products Machines implementing Freq Drives and have recently replaced our old equipment with New. In the old control panels we had the contactor between the voltage source and our freq drives, however in the new panels they're is no contactor and the drives power is always on. When I asked the manufacturer about it they said that Canadian Standards required the use of the extra contactor and since their once booming Canadian market has "dropped off" they now follow the American Standard -
(No Contactor)

So I'd say you probably don't need it...my source is VERY reliable.
 
If you are in the US market Jim Dungar's response in post 7 is the way to evaluate it. In any e-stop situation you must ultimately remove supply power from the machine. If you choose to use a Cat 1 e-stop you can keep power applied through the e-stop action. But at the end of the e-stop (there is a time limit in this that escapes me; I always use 3 seconds) power needs to be removed.
Note I didn't say ENERGY. To the best of my knowledge there is no requirement that an e-stop take you to a zero energy state. In fact you may not want to go to a zero energy state as that may exacerbate an already bad situation.

Keith
 

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