VFD disconnect clarification needed

petersonra said:
That's just not true. You can have motors with the same HP and sf rating that have different fla ratings.
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Because they have differences in power factor and/or efficiency.

1 hp motor as mentioned has 1 hp available at the output shaft - when applied rated volts and frequency and will draw rated current if 1 HP of load is being demanded.

Now give that motor a 1.15 SF, that mostly means it can put out 1.15 HP without causing any significant damage to the motor windings. Probably still a good idea to design a situation that you expect to see 1.15 of continuous load the majority of the time with a larger motor though. A motor that you expect to occasionally be loaded up to 1.15 or maybe something that runs nearly continuously but on occasion will be drawing up to 1.15 for short durations is a little bit of what this service factor is all about. They intentionally designed motor winding to be able to take that more so than a motor designed for 1.0 SF.
 
kwired said:
Because they have differences in power factor and/or efficiency.

1 hp motor as mentioned has 1 hp available at the output shaft - when applied rated volts and frequency and will draw rated current if 1 HP of load is being demanded.

Now give that motor a 1.15 SF, that mostly means it can put out 1.15 HP without causing any significant damage to the motor windings. Probably still a good idea to design a situation that you expect to see 1.15 of continuous load the majority of the time with a larger motor though. A motor that you expect to occasionally be loaded up to 1.15 or maybe something that runs nearly continuously but on occasion will be drawing up to 1.15 for short durations is a little bit of what this service factor is all about. They intentionally designed motor winding to be able to take that more so than a motor designed for 1.0 SF.
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The point was not why but that SF is not allowed to be considered in setting the vfd current overload setpoint.

Sort of related trivia maybe. I seem to vaguely recall that square d had a catalog number for heaters that had a 1.15 SF built in and another that did not for nema starter heaters. But perhaps I am remembering wrong. I think that was because virtually all motors sold in the USA at one time were 1.15 sf.

I suspect most people selected the overloads based on the HP charts in the code rather than the nameplate so they were usually oversized anyway.
 
petersonra said:
Sort of related trivia maybe. I seem to vaguely recall that square d had a catalog number for heaters that had a 1.15 SF built in and another that did not for nema starter heaters. But perhaps I am remembering wrong. I think that was because virtually all motors sold in the USA at one time were 1.15 sf.
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Yes, Square D had different selection tables based on the SF motors. I don't know if it was because of what SF was common, as much as it was because the SF was built into their tables so there was no math required to select a T.U..
 
Jraef said:
You are required, by code, to follow the manufacturer's installation instructions, even if you think there is a conflict with some other code article. Instructions trump all.

Besides, the 250 or 175% rules are MAXIMUM, there is nothing wrong with less.

Worth noting here that in addition, the CONDUCTORS feeding the VFD must be rated for 125% of the VFD Maximum Input Current, not the motor current. The MOTOR leads (VFD to motor) must be sized per the motor FLC table. It generally works out that the inpout conductors could be smaller, but you have to double check to make sure.
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If I am not mistaken the conductors between a motor starter or a drive and the motor only have to be sized for the setting of the motor overload protection x 1.25. At least I thought that was the way it was or used to be.
 
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petersonra said:
The point was not why but that SF is not allowed to be considered in setting the vfd current overload setpoint.

Sort of related trivia maybe. I seem to vaguely recall that square d had a catalog number for heaters that had a 1.15 SF built in and another that did not for nema starter heaters. But perhaps I am remembering wrong. I think that was because virtually all motors sold in the USA at one time were 1.15 sf.

I suspect most people selected the overloads based on the HP charts in the code rather than the nameplate so they were usually oversized anyway.
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Still deal with many Square D thermal units, in one particular place that still has a lot of them. Instructions say the values in the selection charts are for motors with 1.15 SF, for motors with 1.0 SF you need to multiply motor rated current by 87, or something close to that.
 
Eddie702 said:
If I am not mistaken the conductors between a motor starter or a drive and the motor only have to be sized for the setting of the motor overload protection x 1.25. At least I thought that was the way it was or used to be.
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Not quite. Common misconception though.

You must use 125% of the FLC table values in Article 430 for the motor HP, unless the motor nameplate value is higher than the table values (it rarely is). Has been this way for as long as I’ve been in the industry (coming up on 50 years now).

430.6(A)(1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate.
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@Jraef
Could have sworn it was code to size the load wires off a starter or VFD based on the overload setting. Was that changed some time ago?

I never did that as I just always used the same conductor size that fed the VFD/starter. Just wondering if my memory is shot.
 
jim dungar said:
It has not been this way in the 50yrs I have been doing this.
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I have only been at it 44 years and I don't recall a time it was done to code in some other way. But there is a lot of things done in the electrical industry as a whole that are based on myth and legend and generally don't come back to bite you because of how conservative the numbers in the code actually are.
 
I find this issue is not uncommon, I attribute it to people assuming without checking. You likely learned it that way as an apprentice from someone who learned it as an apprentice from someone who learned it as an apprentice, etc., and all along nobody actually checked. I was lucky enough to work with a very anally retentive J-man who insisted that I tell him where I found a bit of info in the Code. I made the mistake of using the tables to select heater elements, which was wrong too. I seem to recall having my knuckles whacked for that.
 
Eddie702 said:
@Jraef
Could have sworn it was code to size the load wires off a starter or VFD based on the overload setting. Was that changed some time ago?

I never did that as I just always used the same conductor size that fed the VFD/starter. Just wondering if my memory is shot.
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As mentioned has always been you must run conductors sized for the NEC motor currents (other than for some motor that isn't listed in the tables then you can go with the motor nameplate.

I think the idea is should you replace that motor with one of same rating you should be covered, as those NEC tables list the current for worst case efficiency/power factor you would likely find in the common standard sized motors that are listed in those tables.
 
Master Nater said:
yes, and where do you think they come up with the FLA rating? it's usually HP x 746 x SF.

SF is a factor in the usage of VFD, because it's what they generally use to come up with their nameplate ratings. therefore, the ratings the VFD must be set to for overload protection because of the motor's service factor ratings.
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Every time that I contacted our talented Danfoss salesman to order a replacement drive would say " we need a drive for a 100 HP 480 volt motor " and he said they are not rated for HP but rather ampere. So several 100 HP motors that had a motor nameplate 124'amps on a drive rated for 125 amps guessing should only attempt to run short periods @ 1.15 service factor. Always set the ampere on drive relays to nameplate ampere. Cracked me up every one of theses replacement drives had a HP rating on information label. Many years ago we were taught that an average AC motor draws approximately 1,000 VA per HP due to the four losses.
 
Jraef said:
I find this issue is not uncommon, I attribute it to people assuming without checking. You likely learned it that way as an apprentice from someone who learned it as an apprentice from someone who learned it as an apprentice, etc., and all along nobody actually checked. I was lucky enough to work with a very anally retentive J-man who insisted that I tell him where I found a bit of info in the Code. I made the mistake of using the tables to select heater elements, which was wrong too. I seem to recall having my knuckles whacked for that.
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Seems like this is a perfect opportunity for everyone to review Charlie's Rule for Reading the NEC
 
garbo said:
guessing should only attempt to run short periods @ 1.15 service factor.
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In case you (or anyone reading this) is unaware… when you run a motor from a VFD, the Service Factor becomes 1.0. Good motor mfrs put this info right on the nameplate of inverter duty motors, but not all do, they bury it in their technical documentation somewhere and force you to look for it.
 
Jraef said:
In case you (or anyone reading this) is unaware… when you run a motor from a VFD, the Service Factor becomes 1.0. Good motor mfrs put this info right on the nameplate of inverter duty motors, but not all do, they bury it in their technical documentation somewhere and force you to look for it.
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I had a customer that routinely put larger vfds on motors than the motors they were rated for and routinely ran them at 10-30% over current. Never seemed to harm the motors any. Did not understand that practice.
 
Jraef said:
It comes down to luck at that point…
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Oddly, at least to me, when they did this they usually ended up running the pump that was attached to the motor at about half speed but way over current. Maybe they just learned over time this worked.

Every time I saw it I sort of expected the motor to get really hot but not so much. I looked at the current in the scada logs and sometimes these pumps had run well over the FLA on the np for weeks. Still did not overheat.
 
petersonra said:
Oddly, at least to me, when they did this they usually ended up running the pump that was attached to the motor at about half speed but way over current. Maybe they just learned over time this worked.

Every time I saw it I sort of expected the motor to get really hot but not so much. I looked at the current in the scada logs and sometimes these pumps had run well over the FLA on the np for weeks. Still did not overheat.
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Submersible or hollow shaft pumps? If so, some of the heat gets carried away with the water.
 
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