Contactors used with VFDs

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lawtons

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Are contactors required when using VFDs? I have seen VFDs with, and without contactors. I know if a contactor is installed between the drive and the motor the output of the drive must be turned off before the contactor opens. This also brings up the question on disconnects between the VFD and motor????

I have seen machines where the main drive motor that makes the machine mechanically operate has a contactor ahead of the drive. This contactor drops out with the MCR relay. This makes since for personnel safety. The only problem is that there is a delay in restarting the machine.

I don?t think the NEC specifically calls for a contactor. It does seem to allow for the upstream lockable disconnect to act as the motor disconnect switch.

I currently have an application where I have a pump room with 9 pumps from 5 to 30 HP controlled by VFDs in an adjacent room. There is also one 20HP fan motor outside.

Any help would be appreciated
 
My experience has been that the derive manuf's recommend against a contactor ahead of the VFD. The only place I have seen them is on Japanese stuff and I think it was some safety S/O after the drive is shut down or some such. Personally, never used them. Good way to fry a drive.

RC
 
With today's VFD's a contactor is not required if the VFD is rated and programmed for the motor overload rating.

Contactors are typically used in HVAC and pump applications when down time of the motor in question is unacceptable. A 2 contactor bypass will remove power from the drive input and allow the motor to operate across the line. A 3 contactor bypass also removes the line voltage from the output of the drive so maintenance/repair can be done with no voltage on the unit. Both bypass arrangements will use standard overloads between the motor conactor and the load.

Good practice is to not remove the line voltage from a drive since you may lose programming parameters and the startup time to initialize the drive can take a while. Use a start/stop relay or PLC contact to start and stop the drive.

Good luck and don't let out the smoke!!!!
 
lawtons,
This exact question was asked in an earlier thread that may be of interest to you. Use the search function in this forum. (Type - VFD + Contactors)

No, Contactors are not required for the line or load side of a "Variable Frequency Drive". Bypass contactors are the exception. Contactors are sometimes used for E-Stops on line side but they are not needed. The control circuit in the VFD should be disabled for E-stop. Opening a line contactor will not allow electronic braking or possibly not allow a motor brake to energize. Pumps and fans usually do not have any braking involved.

Contactors should never be used on the load side of the VFD. The current spike associated with closing and opening of contactors is not conducive to proper VFD operation. They will trip the drive out.
Contactors on the line side were widely used on older and some current DC drives. The manufacturers of these DC drives encourage and sell these contactors.
That is why you see contactors on the line side of VFD's. People either leave the contactor in place when they convert to AC, or they install one thinking they need one.
You will need to protect the input section of the VFD using the input current and voltage listed on the VFD's nameplate. Use the VFD input rating, not the VFD output rating. They will be different.

Some will use a contactor on the line side to start the motor. This is not advisable. The VFD needs some time to power up. It must check itself before it will send power to the motor. Just like your PC, except much faster.

I never install a disco on the load side unless it is specified for lock out. Disconnecting the motor from the VFD while in operation will also trip out the drive. This could be an issue if the operator cannot reset the VFD. Makes for easy service call.....LOL.
Note: Some constant torque VFD's are sending current to the motor even when the drive is stopped. ( to lock motor shaft).
That is why I only provide disconnecting means on the line side of the VFD.

The output section of the VFD is considered seperate from the line section, as it may use more current and or voltage to operate the motor. The VFD is the protection on the load side.

The 20hp fan motor is a variable torque application as are the pumps. Variable torque applications are not treated as constant torque and use smaller VFD's.
Example: 20hp motor uses 15hp drive.

PM me if you want to discuss further......John
 
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The 20hp fan motor is a variable torque application as are the pumps. Variable torque applications are not treated as constant torque and use smaller VFD's.
Example: 20hp motor uses 15hp drive.
Click to expand...

John,

I agree and disagree. I agree with everything up to the example. I disagree with the example. A 20hp motor in a variable torque application will use a 20hp VFD. A 20hp motor in a constant torque application will also use a 20hp VFD. However, the VFD used in the constant torque application will have a higher current rating and most likely be a 25hp or 30hp VFD derated to 20hp. I think that is what you are pointing out.

tony
 
NFPA 79 states that a Category 0 or Category 1 stop must be accomplished by means of electromechanical components which is why I always place a contactor on the line side of VFD?s. The contactor is part of the emergency stop circuit and shuts down the drive immediately when the circuit is broken. Of course each application must be evaluated to make sure that an unsafe condition is not created by this procedure. If you are using a current source drive then you must take precautions when installing a disconnect between the motor and drive. If a current source drive is run into an open load there is a good chance it will be damaged. I usually add an auxiliary contact to the disconnect which is part of the drive enable circuit so that the drive cannot be energized if the disconnect is off. Most drives on the market today are current source, Toshiba is the only manufacturer that I am aware of that produces a voltage source drive.
 
Tony,
Please look at any Motor/VFD catalog or their online specs for Baldor, Weg, AB, Teco, ABB, Reliance, ect, and see the current demands for variable torque drives. Baldor has even designed a drive that is for variable torgue applications. Guess what? Same drive just one size smaller than the constant torque drive.
I also have represented all the companies above. I am very familiar with AC and DC drives. I also am a factory trained warranty/repair technician for Baldor, Weg, Teco Westinhouse and ABB drives.
Have you ever looked at a variable torque vs constant torque curve drawing? Current, Voltage, RPM, Torque in real time?
What you will see is a very different picture for each. Variable torque loads may reach full load or more at times. But not all the time. That variable is built into every drive on the market. You see, it is not how much current a drive will produce, it is how much current over a preset time.
Most drives allow 150% current for a predetermined amount of time. One example would be start-up

Example:
1) Drive rated at 10 amps nameplate reaches 11 amps for 20 seconds.
2) Same drive (10 amps) reaches only 9 amps for 1 hour.
3) This scenario happens 24 -7 With different values.
4) The control is constantly reviewing these values and is ready to take action when needed. (Trip Out)

If you use a 10 hp drive on a 10 hp water pump motor you would be wasting money on the cost of the drive.

In summary: Variable torqe conditions are (variable). That is why my example is correct. And an excepted practice with all VFD manufacturers.

GWPowell.....I do not understand your terminology "current source drive". Can you be more specific. Are you refering to a "Flux Vector Drive/Control"?
 
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gwpowell said:
NFPA 79 states that a Category 0 or Category 1 stop must be accomplished by means of electromechanical components which is why I always place a contactor on the line side of VFD?s. The contactor is part of the emergency stop circuit and shuts down the drive immediately when the circuit is broken. Of course each application must be evaluated to make sure that an unsafe condition is not created by this procedure. If you are using a current source drive then you must take precautions when installing a disconnect between the motor and drive. If a current source drive is run into an open load there is a good chance it will be damaged. I usually add an auxiliary contact to the disconnect which is part of the drive enable circuit so that the drive cannot be energized if the disconnect is off. Most drives on the market today are current source, Toshiba is the only manufacturer that I am aware of that produces a voltage source drive.
Click to expand...

A disconnect switch is a category 0 stop. You have to have that anyway, Why add a contactor?
 
John,

I think we are in the same ballpark, just sitting in different sections. From my experience, manufacture's derate VFD's for use in constant torque applications. Is that your experience too? A 30hp variable torque becomes a 20hp constant torque. Hence the variable torque VFD being a smaller size than an equally labeled hp constant torque VFD. Having said that, we, I, whoever, typically select single motor VFD's based on motor HP and application. So, if I call ABB and tell them I need a 10hp VFD for a fan, they are going to give me the part number for a 10hp VFD. If I tell them I need a 10hp VFD for a conveyor they are going to give me the part number for a 10hp VFD. Those part numbers and VFD sizes and cost will not be the same because one application is a variable torque and the other is contant torque. Is that also true from your experience representing the various VFD companies?

I agree that applying a 10hp constant torque VFD to a 10hp variable torque load is a waste of money. Isn't that why you would always ask the customer the application to determine what they need, constant or variable torque?

tony
 
Tony,
Yes, we are definately in the same stadium. However, if you call ABB and ask them for a 10 HP drive for a fan they are going to send you a 10 hp drive. Unless you get an experienced voice at ABB that will tell you that you don't need 10 hp. (you will be lucky to talk to anyone). It's up to you to ask for the 7.5 Hp.
You have to make the decision on the application. I never let a customer make that decision. I make it, then I present it to them.

If you are installing or working with VFD's, I would suggest that you look at the specs for each one, and try to find a qualified rep to answer any questions. I still talk with the guys on a regular basis for information. We will never know it all.......LOL
All the drive manufacturers have authorized distributors (easier access) with drive guys on staff. They love to talk drives.......as you can see!
 
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