VFD Standard

labingtone

Member
Join Date
Aug 2002
Location
Manila, Philippines
Posts
113
Hi experts. I know this site is more on PLC, but a lot of members to this site are expert on VFD.


My customer give me project on the installation of of 2 units VFD in one panel. In his design shows that only start and stop button is present on each VFD. As contractor its my responsibility to make the system safer.
1. My question is do I really need a to incorporate emergency stop in each VFD.
2. The VFD has a built reset button. Do I realy need to put remote reset push button on each of the VFD.
 
1.YES, in a sense, the E STOP ckt should STOP the VFD. Most VFD have their own power source so you may have to use relays depending on circumstances to STOP the RUN enable and again depending on circumstances have the ability to enable the STOP ckt so it can brake if needed.
2. Explain reset
 
Safer VFD CKT

1. One may use a mushroom locking E STOP switch on the run enable input. The VFD reaction depends on the selected option (coast to rest or brake).
Most VFD's now has some capacity of braking power, but for emergency the best solution is regenerative braking: this will stop the motor in the shortest time.
A much cheaper (yet not cheap) solution is braking resistors bank, which is selected due to aticipated duty cycle.
E STOP is required any time there is no eye sight between operators console and the motor or any moving part of the machine.
The E STOP(s) has to be located near the machine at point(s) of hazard.

2. Remote reset is usually a customer's request, or derived from the required functionality, and not a safety requirement.

My expertize is limited to Reliance Electric / Rockwell Automation VFD's, but with over 100 installations.
 
1'st of all, You have to think what is most safty way for stopping?

Sometimes you can use braking resistors with VFD and use VFD's declarating properties and process will stop in best way like this. For that you need delayed EMS-relay contact for VFD's disconnecting from main voltage.

Sometimes, if we stop motor 'violently', handeled material still continue own traveling. (E=1/2*m*v*v), so free stopping is best in this case, so there can't be separate brake in the motor.

Usually you can disconnect your VFD main voltage by EMS controlled Contactor.
See attached drawings for Delayed stopping with braking resistor.
In PLC program you controlled VFD stopping with declarate ramp when EMS-circuit feedback tells it is not ok. In PLC program you need delay (< time for EMS-Relay delayed contact) for motor own Brake coil if used.
 
I do not know the 'low' in Philippines.
Sometimes I'm using common contactor in main voltage circuit for 'small' motors.
When connecting VFDs to main voltage, VFDs take charging peek from the net and sometimes I connecting VFDs one by one with small delay sequence and I can use smaller cable and circuits in MCC's supply
and Voltages quality is better in MCC.
 
My opinion is that the E-Stop for each VFD is good, inexpensive insurance. The built in stop button on most VFDs is small, and in an emergency it is nice to have that big red switch for the operator to smack. If the motors are remote (out of line of sight) from the panel then you definitely want an E-Stop adjacent to the motor, and I reccommend the maintained type with twist to release. If you have any other protective devices (motor temp switches or such) you just put the E-Stop in series with them.

On most VFDs you can program the stop function for coast to stop (power to the motor just drops out) or controlled stop - slowly decelerate with or without regenerative braking. Your application determines which is needed.

You don't usually need a separate reset button, but that presumes the drive keypad is mounted externally on the panel. If you have it inside the panel on the VFD, I reccommend a separate Reset pushbutton for each drive to allow clearing faults without having un-trained staff accessing the inside of the panel.
 
EMERGENCY STOP
Don't take this letter by letter, but everything you see must stop!
Maybe you don't need stop paper machine if you push EMS-button in warehouse, but you see the paper machine.

In attached files earlier, EMS-circuits reseting have done by plc's output and reason is what Tom told before.
DC-circuit in VFD must discharge before reseting output can activate.
Otherwise it generates Under-Voltage fault in VFD. Debendig of unused breaking resistors, discharge time can be even 25 sec.
Give VFD's stop (run off) command before VFD disconneting for declarating if necessary.
 
Here's my two cents worth on this.

First, the remote fault reset. This is a convenience item and is usually dictated by customer preference. Drives have fault logs built into them so keeping a record of faults is not a problem. You may find it undesireable to make fault reset available to machine operators since the fault may be an early warning of damage or danger somewhere in the machine. It might be wise to leave the fault reset inside the cabinet where only maintenance personnel can reach it. That way, at least someone other than the operator can evaluate the wisdom of just resetting and starting back up.

Second, E-stop buttons are a matter for careful thought. The first issue is to settle how you want the machine to behave when the E-stop button is pushed. Usually you have four choices. 1)coast to stop and Fault the drive. (This will make a record of the event in the fault log) 2)coast to stop without faulting the drive and prevent further operation until the switches are reset 3)ramp to stop at the same decel ramp rate as a normal stop without faulting 4)ramp to stop at a faster decel ramp rate than normal stop without faulting.

Number 1 is done by setting up an external fault circuit and placing the E-stop buttons in it. The record of the event is a benefit but the disadvantage is coasting to stop. Faulted drives always coast to stop and this may be too long to be safe.

Number 2 is done by setting up an external run inhibit circuit and placing the E-stop buttons in it. Disadvantages are no record and the possibility of immediate restart if the switches are reset with the run signal still on the drive. Also, some drives permit only coast to stop on run inhibit. Others have an option for coast to stop or ramp to stop. You would need to check the specific drive on this.

Number 3 is done by placing the E-stop buttons in the same circuit as the Stop buttons. You must use latching E-stop buttons or a second operation of the start circuit will restart the drive. The benefit here is that you get the same controlled stop as a routine stop. If that routine stop is fast enough you can use it. If too slow, this is not a good choice.

Number 4 is similar to Number 3 but the E-stop buttons also tie to a separate drive input that sets two different accel/decel ramps. You program the ramps so, with the E-stop buttons closed, you get the normal accel/decel ramps. With any E-stop button open, you get a much faster decel ramp (the accel ramp is not used) for a quick stop. You may need to add some braking capacity to the drive to get this fast stop.

Finally, there is mention above about pulling power off the drive on E-stop. This is usually not good practice because is assures an uncontrolled (coast) stop. The drive must have power on it to enforce any kind of ramp stop. If you want to pull the input power once stop is acheived, that is fine but a restart will take a bit longer. It might be worth it for perceived safety reasons but, at least in my experience, it is not usually done.

Now, if all that was two cents worth, imagine how long and boring a nickel's worth would be!
 
I think we may be assuming...

Actually labingtone we cant say whether your VFD's need their own E Stop buttons or connected into the E-Stop ckt somehow. THIS IS DEFINITELY A "IT DEPENDS" situation. Lets say you have an existing E-STOP ckt that kills power BEFORE the VFD, at that point the VFD cant do anything in the process.

Someone has to decide what the safety issues are in relation to the E-STOP, only you (and your associates) know the specific issues. Should everything coast to stop, should it brake to a stop, should lifts be brought down or any hanging load, the list goes on.

Conditions state what should be done, naturally you are going to have a STOP circuit involved with the VFD but you have to determine HOW you would need to include it within the E-STOP circuit.
 

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