Introduction
This application note provides clarification on the use of a contactor / safe-off with an ac drive, and the pros and
cons of where it is fitted.
Normally contactors may have been fitted::
a) An input contactor / isolator on the power lines to the drive
b) An output contactor / isolator on the power lines from the drive to the motor.
As the contactor / isolator placement can have an effect on the plant operation / safety, it is mandatory that a risk
assessment of the installation is undertaken. The outcome of the risk assesment should influence the decision on
the design of the control system. This application note will set out the advantages and dis-advantages of the
different contactor placements, so that an informed decision is achieved.
In addition, this application note details the use of the PowerFlex DriveGuard Safe-Off system, which provides an
easier alternative to input / output contactors.
Input Contactors
PowerFlex drives are designed so that communication adapters are normally fitted internally, and powered by the
drive. In the past, it was common practice with users to fit a mains input contactor to the drive, operated by an
enclosure entry limit switch. This means that if the operator needs to remove a blockage on a conveyor, he opens
a guard on the enclosure which opens the input contactor. Therefore drive power is lost, and the network
connection is broken.
Input contactors / isolators are often specified for reasons of cost, as a contactor / isolator can be connected to a
group of drives.
There are a number of issues that arise with drives, if an input contactor is used:
After the drive loses input power, the internal capacitor bank still contains enough power to cause motor
rotation for several seconds, thereby causing a potential Health & Safety risk.
Network connection is broken and all drive diagnostics and information across the network is lost.
Input power to the drive shouldn’t be cycled more than once per minute.
It can take from 7 to 20 s for the drive to power up and re-establish network connections, before it is
available to run.
Output Contactors
To counter the difficulties associated with an input contactor, an output contactor / isolator is a better technical
solution than an input contactor. If an output contactor / isolator is opened while the drive is operating, power will
be removed from the motor, but the drive will continue to produce a voltage at the output terminals. The motor will
then coast to rest under its own inertia. In addition reconnecting a motor to an active drive (by closing the
contactor) could produce excessive current that may the drive to fault or be damaged.
Additionally an output contactor which breaks under load will stress the output transistors and effectively reduce
the life of the drive. If this occurs only a few times a year, then maybe this is acceptable. However if an output
contactor is likely to be opened on a frequent basis, then the best technical solution for installations utilising
output contactors (frequent usage especially) is detailed below.
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The relays specified below assume a drive size of approx 4kW:
Main Contactor 100-C09DJ10 with diode gives a contact break time of 70 to 95 msecs
Early break
700DC-MB400D24 which gives a contact break of 6 to 12 msecs.
Adjustments to relay size may be necessary for smaller / larger drives.
This method of disconnection, relies on the fact that the drive enable is removed before the main contactor
opens, and then the main contactor operates a split second later. In this situation, the output to the drive is NOT
disconnected under power, and so does not stress the output devices, and the product life cycle integrity is not
compromised.
The advantages of such an installation are:
No input power cycling, so faster response of the drive from a start signal (no startup diagnostics)
As power to the drive is maintained, then the fault queue is retained. (not lost on power down).
Drive communications maintained (if on comms network), so remote interrogation is still possible.
PowerFlex40
The PowerFlex 40 drive uses the Stop input on terminal 0 as the enable “with software interpretation”.
Removing the Hardware enable jumper on the PowerFlex 40, will set terminal 0 to enable “without
software interpretation”.
PowerFlex70
The above diagram shows a PowerFlex 70 drive, with Digital Input 6 set to enable. Any input can be
assigned as enable “with software interpretation”. The PowerFlex 70EC, can set Digital Input 6 to enable
without software interpretation, by removing the Hardware enable jumper.
PowerFlex700
Similarly to the above a PowerFlex 700 drive, can have Digital Input 6 set to enable. Any input can be
assigned as enable “with software interpretation”. The PowerFlex 700VC, can set Digital Input 6 to enable
without software interpretation, by removing the Hardware enable jumper on the control cassette.
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DriveGuard Safe-Off
The best technical solution is to install a PowerFlex 70 EC or a PowerFlex700S
drive with a DriveGuard safe-off option. This solution utilises an optional card
installed into the drive, which removes the requirement for an output contactor.
This provides 2 separate inputs which switches off the output transistors of the
drive which removes power to the motor, so the user can remove the blockage
on a conveyor.
As the drive / communications adapter remains powered, so all of the
disadvantages of the input contactor are eliminated.
This solution is certified by TUV in EN954-1, and can be used to provide
Category 0 & 1 stops.
Details are provided in the DriveGuard User Manual PFLEX-UM001A-MU-P.
The DriveGuard Safe-Off option provides the best technical solution for providing
a temporary disconnection of a motor, to allow access into a hazardous area.
However it is not to be regarded as an isolation point, as leakage current could
result in stray voltage at the motor terminals.
Other solutions
If a user insists on using an input contactor, to shorten network recovery time, an alternative solution for
PowerFlex drives is to mount the communication adapter in an externally powered base. Then when a user
operates an enclosure entry limit switch and opens the input contactor, the external powerbase maintains the
power supply to the adapter. Although the drive network I/O connection is disrupted, the PLC is continually trying
to reconnect. This minimises drive recovery time and the availability to run.
This situation is discussed in an application note “ PowerFlex Network recovery with external power bases”.
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