I have situation where I'm using 100A RK1 fuse for 60HP motor @480V (Chiller Pump with VFD controller). The bussman recommend to use 110A RK1 fuse and 100A RK5 fise for this size. I want to use the 100A RK1 fuse so I won't use the 200A switch. I have been told since I'm using a VFd the input current for the drive will be less than the motor, with that said, I could use the 100A fuse because the VFd will provide a soft start. Any thought on if the 100A RK1 fuse will work or I better use the 100A RK5 fuse. Thanks for your thoughts in advance...
VFD fuse sizing
- Thread starter ofares_2
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
ofares_2 said:
You were told right. The VFD makes the power factor close to unity. However, code requires you to size the fuses for whatever the drive input is rated for. The instructions for the drive will tell you this.
petersonra said:
I don't have my book with me, but I think the code only requires the feeder to be sized for the VFD. I think the fuses can be any size less than the max. allowed. (Fuses are easy to change if the motor is changed to a larger one, but feeders are usually much harder to change. I think that is the intent of the code article you are refering to.)
davidr43229
Senior Member
- Location
- Columbus, Oh
RK-5's have more time-delay than RK-1's. He wants the performance of a RK-1, whilie using the time-delay of the RK-5, which you can't expect. I understand you not wanting to jump up to a 200amp disconnect (wire, costs, etc...), since going from a 100Amp to a 110Amp involves going to a different case-size in fuses. If you set the VFD at max, Bussmann recommends a 150amp for a 60Hp 480Volts in their SPD. http://www.bussmann.com
Just my $.02
Just my $.02
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
You size the fuses, conductors, etc. for the _inverter_ input, not the motor.
Generally for motor circuits, the OCPD is used to provide short circuit protection only. The motor and its overload protection also serve to protect the conductors from overload. You might have a motor with 77A full load, supplied using conductors of 100A capacity, protected with a 150A or larger fuse. The fuse is too large to protect the conductors from overload, but must be this large in order to tolerate the inrush and locked rotor starting current of the motor.
But for an VSD supplied motor you don't have these inrush current issues. The variable speed drive adjusts its output voltage and frequency to drive the motor without the high locked rotor starting current. Depending upon the particular configuration of the drive and motor, the VSD input current may be greater or less than its output current, but the input current will always be limited by the drive itself.
-Jon
Generally for motor circuits, the OCPD is used to provide short circuit protection only. The motor and its overload protection also serve to protect the conductors from overload. You might have a motor with 77A full load, supplied using conductors of 100A capacity, protected with a 150A or larger fuse. The fuse is too large to protect the conductors from overload, but must be this large in order to tolerate the inrush and locked rotor starting current of the motor.
But for an VSD supplied motor you don't have these inrush current issues. The variable speed drive adjusts its output voltage and frequency to drive the motor without the high locked rotor starting current. Depending upon the particular configuration of the drive and motor, the VSD input current may be greater or less than its output current, but the input current will always be limited by the drive itself.
-Jon
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
It may depend on which version of the NEC your state is using. In the 2002, article 430.2 states
But in the 2005 version, that statement is now gone. The only thing it says is that the CONDUCTORS can be sized for the VFD input current.
PS, I should also mention that if your VFD has a full speed bypass, the SCPD MUST be sized for that potention use, so back to all the old rules.
In this case you could size the fuses for the input current of the VFD x 115%.
But in the 2005 version, that statement is now gone. The only thing it says is that the CONDUCTORS can be sized for the VFD input current.
You might interpret that to say that you could still size the fuses for 125% of the input current, but it is no longer as explicit.
PS, I should also mention that if your VFD has a full speed bypass, the SCPD MUST be sized for that potention use, so back to all the old rules.
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davidr43229
Senior Member
- Location
- Columbus, Oh
Additionally you may have to derate (80%)the VFD unit, depending on HZ and ambient temp. I have the A/B Power Flex 700 bulletin 140 if you need it. Have you determined the units SCCR rating for a proper Industrial installation (NEC article 409), since you seem to be the Authority of Having Juristication (NEC 100). So many things, so little time!
Jraef,
The requirement for sizing the disconnecting means to 115% of the rated input of the power conversion unit is still in the code.
430.128 Disconnecting Means. The disconnecting means shall be permitted to be in the incoming line to the conversion equipment and shall have a rating not less than 115 percent of the rated input current of the conversion unit.
The requirement for sizing the disconnecting means to 115% of the rated input of the power conversion unit is still in the code.
430.128 Disconnecting Means. The disconnecting means shall be permitted to be in the incoming line to the conversion equipment and shall have a rating not less than 115 percent of the rated input current of the conversion unit.
- Location
- Massachusetts
ECE2001 said:
The rating of the disconnecting means is not necessarily the rating of the overcurrent protection devices it contains.
- Location
- Illinois
- Occupation
- retired electrician
Jeff,
I don't think that either of the two sections that you cited have any thing to do with the overcurrent protection of the supply conductors to the VFD. Part X of 430 says that all sections of Parts I through IX apply unless modified or supplemented by Part X. I see nothing that covers the branch circuit short circuit and ground fault protection in Part X, so this protecion is still based on the rules in 430.52.
Don
I don't think that either of the two sections that you cited have any thing to do with the overcurrent protection of the supply conductors to the VFD. Part X of 430 says that all sections of Parts I through IX apply unless modified or supplemented by Part X. I see nothing that covers the branch circuit short circuit and ground fault protection in Part X, so this protecion is still based on the rules in 430.52.
Don
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