VFD Retro with existing mag starter

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njhvac

Member
When adding a VFD to an existing combination starter and motor, should the VFD be between the starter and motor or before the starter?
 

sc57ford

Member
Location
South Carolina
Check out Rockwell Automation for some good advice. Your Allen Bradley dealer can help you. Depending on your particular circumstances, you may need to change out the cable to the motor. Wome applications need shielded cable to prevent nuisance tripping, etc. They can advise you.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I would put the VFD between the existing motor and starter.

It can be bad karma to open the load side connections of a VFD while it is running, so I wouldn't want a separate starter on the load side of the VFD.

Steve
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
You also need to make sure that the motor can operate with a VFD. There are issues related to the materials of which the bearings are made, and related to grounding the shaft. I don't know more than that, but I think you need to check with the motor manufacturer.
 

njhvac

Member
Weve been wiring the vfds after the motor starters to provide a soft start to the motors.
An apprentice posed a question as to why didnt wire them ahead of the existing starters.
My contention is that the drive would fault if the starter disconnected the motor while the drive was controlling it.
 

genesparky

Member
Location
Kingman Az.
You loose many of the functions of the VFD if you open the circuit after the VFD and before the motor, for example controlled decel, and dynamic breaking, I have NEVER seen a starter after the VFD.
 

tom baker

First Chief Moderator
Staff member
You don't need a starter.
Be carefull when sizing the VFD, it can be constant torque or variable torque.
Also I recommend a line reactor. I lost two expensive ($6,000) VFDs before I installed a line reactor and a $2,000 TVSS.

And I agree call a tech rep. There are many mfgs of VFDs some are OK, a couple, Danfoss and AB lead the rest.
 

iwire

Moderator
Staff member
Location
Massachusetts
How about leaving the starter in place but wire it in parallel with the VFD and it can be used as a bypass contactor with the addition of come interlocking relays.

If your application happens to be critical process having a bypass contactor can be handy.
 

racerdave3

Senior Member
I agree, remove the old starter as it is just another, unnecessary link in the system. All you need is your disconnecting means, the new VFD, then your motor. As someone mentioned earlier, pay close attention to the motor. Your newer motors suitable for the VFD application have designations on the nameplate indicating inverter duty.
 

John Valdes

Senior Member
Location
SC.
Occupation
Retired Electrician
Iwire, good thought. You never know when the drive is going to fail. If he dosen't use the starter as a bypass as you suggested it should not be used at all, except to kill VFD line power such as E Stop button. NEVER put contactors on load side of VFD!!!!!!!
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
John Valdes said:
NEVER put contactors on load side of VFD!!!!!!!

I would not say never. But, it would be an extreme rarity. If you do, you would want to wire a NC interlock from the motor starter to the VFD enable to shut off the VFD when the load side starter opens.
 

John Valdes

Senior Member
Location
SC.
Occupation
Retired Electrician
Not a good idea to have your VFD "programmed to run on power up". Electronics like to be told discreetly when to start and stop. You don't want your VFD powering up at the same time you are starting the motor.
 
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