vfd retrofit

[ultramag]

We are getting ready to replace two across the line starters for two 200 hp. supply air fans with vfd's. The starters are in a motor control center, the vfd's will be on a wall about 20 feet away from the mcc. Is there any reason why we can't use the existing breaker in the starter bucket as our ocp, run the feed over to the vfd and bring the motor leads back to the mcc and splice them to the existing motor leads in the wireway of the mcc? We would be doing this for two motors.

 

[mcclary's electrical]

We are getting ready to replace two across the line starters for two 200 hp. supply air fans with vfd's. The starters are in a motor control center, the vfd's will be on a wall about 20 feet away from the mcc. Is there any reason why we can't use the existing breaker in the starter bucket as our ocp, run the feed over to the vfd and bring the motor leads back to the mcc and splice them to the existing motor leads in the wireway of the mcc? We would be doing this for two motors.

It sounds compliant to me

 

[weressl]

We are getting ready to replace two across the line starters for two 200 hp. supply air fans with vfd's. The starters are in a motor control center, the vfd's will be on a wall about 20 feet away from the mcc. Is there any reason why we can't use the existing breaker in the starter bucket as our ocp, run the feed over to the vfd and bring the motor leads back to the mcc and splice them to the existing motor leads in the wireway of the mcc? We would be doing this for two motors.

If they are fuly sized thermal-magnetic circuit breakers you can use them, except you would be violating the UL lsiting of the starter. If they are magnetic only MCP's then you can not use them as they are not listed for individual use. You can use the entire starter with properly sized OL elements as feeder OCPD's.

But there is a heck of a lot more you ned to look at. Are the motors inverter rated? Are they far enough that you are going to have overvoltage problems or even unable to run the motors because of the distance? Are the existing cables comply with the manufacturer recommendation? Are you going to have shaft current problems? Are you going to have noise and harmonic problems that affect other systems in the area, or even on-line? Are you prepared to handle mechancial resonance problems with the lines that the fans are feeding, varied flow can change things?

ASD's are not just slam-bam-thank-you-ma'am drop in replacements.

 

[mcclary's electrical]

If they are fuly sized thermal-magnetic circuit breakers you can use them, except you would be violating the UL lsiting of the starter. If they are magnetic only MCP's then you can not use them as they are not listed for individual use. You can use the entire starter with properly sized OL elements as feeder OCPD's.

But there is a heck of a lot more you ned to look at. Are the motors inverter rated? Are they far enough that you are going to have overvoltage problems or even unable to run the motors because of the distance? Are the existing cables comply with the manufacturer recommendation? Are you going to have shaft current problems? Are you going to have noise and harmonic problems that affect other systems in the area, or even on-line? Are you prepared to handle mechancial resonance problems with the lines that the fans are feeding, varied flow can change things?

ASD's are not just slam-bam-thank-you-ma'am drop in replacements.

I'm assuming his engineering has been done, and he is asking about the code compliance of the install, but I might have misunderstood what he's asking

 

[ultramag]

Thanks for the replies. Yes all the engineering has been done and they are full size thermal magnetic breakers. There is a new hvac control system going on this air handling unit. I just want to make sure of code compliance.