multiple motors controlled by vfd's

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sgroff

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Having a facility with 42 motors started by vfd's and the vfd is controlled by a plc via dry contact for fwd or rev on the vfd. normally on a motor starter situation the control voltage for the coils could all be the same and run through the emergency stop circuit via a master control relay or contactor, meeting osha requirements as well. proposed way to meet requirements for emergency stop situation to shut off the run command for the vfd's? can't rely on the plc to shutoff those run commands.
 
sgroff said:
Having a facility with 42 motors started by vfd's and the vfd is controlled by a plc via dry contact for fwd or rev on the vfd. normally on a motor starter situation the control voltage for the coils could all be the same and run through the emergency stop circuit via a master control relay or contactor, meeting osha requirements as well. proposed way to meet requirements for emergency stop situation to shut off the run command for the vfd's? can't rely on the plc to shutoff those run commands.
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Most VFD's I have been around usually have a control terminal that is essentially an "enable" input. This input must have a signal or the drive won't run regardless of any other input signals. You may need relay(s) or multi-pole switch to connect multiple drives to a single master control though.
 
Shutting off the run command alone would not satisfy the requirements for a E-stop based on NFPA 79.

Some VFDs (for example some Allen-Bradley drives) offer a Safe-Torque-Off feature which is an approved method of isolating the output transistors from the load. Otherwise, one would need to have a contactor or perhaps two contactors in the circuit to satisfy the requirements for control reliability. The level of protection required is based on the determined risk. One must also consider whether the contactors are located before the drive or after the drive. Most drives would not take kindly to opening a load-side contactor while under load.

An emergency stop could also be accomplished by opening the main disconnect on the panel. but if an emergency stop push button is required, that would not do.
 
richwaskowitz said:
Shutting off the run command alone would not satisfy the requirements for a E-stop based on NFPA 79.

Some VFDs (for example some Allen-Bradley drives) offer a Safe-Torque-Off feature which is an approved method of isolating the output transistors from the load. Otherwise, one would need to have a contactor or perhaps two contactors in the circuit to satisfy the requirements for control reliability. The level of protection required is based on the determined risk. One must also consider whether the contactors are located before the drive or after the drive. Most drives would not take kindly to opening a load-side contactor while under load.

An emergency stop could also be accomplished by opening the main disconnect on the panel. but if an emergency stop push button is required, that would not do.
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In general I think most drives have no problem with opening the load while it is operating, what they don't like is suddenly connecting a load to an active drive output. So if you open the output lines you also should stop the drive output, and close the output lines before starting the drive again.
 
I jusy rebuilt a tromp dough sheeter. A 45' long machine with 8 emergency stops and 3 light curtains. The emergency stop circuit opens the line side of each vfd through individual contactors. The plc stays on as well as the hmi, but cancels the enable output to each drive.When the e stop is released, the machine goes back into standby mode and starts with the push of a button.
Using Allen Bradley powerflex 4 vsd's, a slc500 plc using automation direct hardware.

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