VVVF bus volts

You need to determine if the problem is over-voltage or low voltage. If its low voltage, find out why it is low. Is it the utility's problem? If its over voltage you should also check with the utility. You can install a dry type transformer and use it to reduce the voltage. What is the nameplate voltage on the unit?

I am assuming that you are talking about a Variable Frequency Drive. From what I gather from your description a resistor will not help. Resistors are used to dissipate energy from a regenerative load. Most any drive I have worked on removing the input power while "running" will cause a under voltage fault. There are a number of things that can be done depending on how the control of the drive is accomplished.

I am not sure why it is necessary to remove power from drive in your situation for a e-stop (there may be very good reason). There are many things that can be done depending on your specific situation here's just a couple....

• Most drives have a programmable input for e-stop
• if you have to remove power from drive in e-stop you can make a additional input reset the drive after the e-stop is cleared

Hope this helps.......

The way your discription reads is the way a device normally should function after an emergency stop. I would recommend that you strongly consider leaving the function the way it is. This is a safety and Code compliance issue.

NFPA-79 is invoked by the NEC (NFPA-70) for industrial machinery and control equipment design and is a Code requirement. Refer to the following requirement from NFPA-79:

NFPA-79 Electrical Standard for Industrial Machinery

Section 9.2.5.4* Emergency Operations (Emergency Stop, Emergency Switching Off).

"(2) Once active operation of an emergency stop (see Section 10.7) or emergency switching off (see Section 10.8) actuator has ceased following a command, the effect of this command shall be sustained until it is reset. This reset shall be possible only at that location where the command has been initiated. The reset of the command shall not restart the machinery but only permit restarting."

The rationale is that you generally don't want equipment to be starting without warning and potentially catching someone unawares in moving machinery. E-stops are there for emergency use. For normal starting and stopping, use run/stop or auto/reset switches.

Which make VVFD you are useing,What type of controls you are useing,Is it DCS system control or Conventional Control.

It is clear from your statement that when you are removing Power Supply from VFD,Naturally the VFD senses it has affected by undervoltage.Hence it is tripping.As long as you are not resetting it will not start.

This option can be removed.Contact your VVFD Supplier and ask him for necessary correction.

Brains trust

Thank you all for tour comments so far. I will now try to answer some of the issues raised.

1: It is a policy where I work that when an emergency stop is activated on a circuit containing a VFD that the supply to the unit is withdrawn. My concern is that the sudden drop in supply voltage is triggering a bus voltage disturbance or that the unit can not cope in dissipating the motor voltage.

2: Normally any VFD unit in service is continnually powered and is started via an enable input. I am at a loss as to why this process does not work after an emergency stop. The only way to start the unit is in manual from the unit keypad.

3: There is no unit fault output when in this state, ie the operator interface software(CITEC) does not regester the unit as being in fault.