VFD and Motor

[pdemapan]

I wondering if I did this calc. correctly. I'm installing a new VFD which will be used to drive 1-50 hp, 480v, 3ph cooling tower motor, and 1-15 hp, 480v, 3ph cooling tower motor. The 15 hp motor is going to be used as backup in case the 50 hp motor fails.

FLA for 50 hp = 65A
FLA for 15 hp = 21A

Wire size:
(65*1.25) = 81.25A
Choose #4 conductors (85A ampacity)

OCPD min:
65*1.75 = 113.75A

OCPD max:
65*2.25 = 146.25A

Choose 3P125A breaker feeding motors.
But in this case I have to ensure only one motor running at once. Is this correct? Anyone size the wire and OCPD for two motors running as a worst case scenerio?

Please comment away. Thanks forum

 

[Cow]

The trick to circuit sizing for motors ran on VFD's is remembering you're sizing the circuit to the VFD's, not the motors. It definitely makes a difference if you're using oversized VFD's, which we do sometimes. Your VFD's will also have min/max OCP recommendations listed.

Take a look at 430.122

 

[pdemapan]

guess its correct. thanks

 

[kwired]

The only reason for such large breakers for across the line starting is for starting current. A VFD will not have a starting surge and could be protected in most cases with 125% full load current or better yet see VFD instructions for protection - they will most likely be lower than across line starting would be.

But in this case I have to ensure only one motor running at once. Is this correct? Anyone size the wire and OCPD for two motors running as a worst case scenerio?

If you were to run both motors at the same time you would need drive large enough to handle both plus you would need separate overload device for each. You may have difficulty controlling them unless they started and stopped at same time as I don't think you normally could throw another motor on the drive while it is presently operating another one.

If the 15 hp is strictly back up I probably would not even put it on a vfd because it most likely will run at or near full speed whenever it is running, I may consider soft start if necessary or desired but not a VFD.

 

[Hameedulla-Ekhlas]

I

OCPD min:
65*1.75 = 113.75A

OCPD max:
65*2.25 = 146.25A

Thanks forum

From which reference did you get this 2.25 value. Would you please hint me the reference source.

 

[don_resqcapt19]

While the code says, in 430.122, that the supply conductors to the VFD must be sized at 125% of the VFD input current rating, the short circuit and ground fault protection is still covered by 430.52 for a single motor and 430.62 for multiple motors. In this case the maximum rating of an inverse time breaker for the 50 hp motor would be 2.5 times the FLA of 65 amps or a 162.5 amps. The code permits rounding to the next standard size which is 175 amps. If both motors are on the same feeder the maximum would still be 175 amps as there is no provision for rounding up to the next standard size in 430.62.

 

[kwired]

While the code says, in 430.122, that the supply conductors to the VFD must be sized at 125% of the VFD input current rating, the short circuit and ground fault protection is still covered by 430.52 for a single motor and 430.62 for multiple motors. In this case the maximum rating of an inverse time breaker for the 50 hp motor would be 2.5 times the FLA of 65 amps or a 162.5 amps. The code permits rounding to the next standard size which is 175 amps. If both motors are on the same feeder the maximum would still be 175 amps as there is no provision for rounding up to the next standard size in 430.62.

Note that these sections are about the maximum short circuit and ground fault protective device. If you can start your motor with smaller value device nothing prohibits that, a VFD will not have a 6-8 times FLC starting current like across the line starting can.

 

[pdemapan]

sorry yeah the maximum OCPD should be 250% times the FLA. I agree with don_res.

 

[pdemapan]

i was told this vfd also has a bypass. I guess I should use the max ocpd size? If the VFD is in bypass mode, then there would be an inrush current 6-8 times FLA correct since there is no soft start for the motor?

 

[gmohr]

VFD and Motor

Interesting that there are many threads in this forum that quote the NEC 430.122(A) requirement that circuit conductors SUPPLYING POWER CONVERSION EQUIPMENT will be sized at 125% of the VFD rated input but no one ever asked whether this also applies to the circuit conductors between the VFD and the motor (motor circuit conductors). The note in the NEC Handbook says that "Parts I through IX must be followed unless modified or supplemented in Part X". Since the motor circuit conductors are not specifically addressed in Part X, one could conclude that he should size them at 125% of the motor FLA per 430.22(A). On the other hand, the title of the section is "Branch/Feeder Circuit Conductors". This seems to include the motor circuit conductors since they are the branch circuit conductors. Any thoughts on this?

 

[mbeatty]

Interesting that there are many threads in this forum that quote the NEC 430.122(A) requirement that circuit conductors SUPPLYING POWER CONVERSION EQUIPMENT will be sized at 125% of the VFD rated input but no one ever asked whether this also applies to the circuit conductors between the VFD and the motor (motor circuit conductors). The note in the NEC Handbook says that "Parts I through IX must be followed unless modified or supplemented in Part X". Since the motor circuit conductors are not specifically addressed in Part X, one could conclude that he should size them at 125% of the motor FLA per 430.22(A). On the other hand, the title of the section is "Branch/Feeder Circuit Conductors". This seems to include the motor circuit conductors since they are the branch circuit conductors. Any thoughts on this?

It appears that 430.122 (B) would apply to this case as it covers the bypass.
Regards,
Mark