Motors & VFD

horsegoer

Senior Member
Location
NJ
Have a motor, that has a separate disconnect and VFD. Motor requires 160 FLA @480v.

1. What STANDARD size breaker to feed that?200A? would it matter is fused switch or circuit breaker?
2. Would the same feed from panel to disconnect also motor be used to feed VFD?

Thanks
 

bwat

EE
Location
NC
Occupation
EE
See article 430....

Several parts of it show you what you need and still apply in your situation. And then Part X is the pieces that are specifically different since you have a VFD
 

kwired

Electron manager
Location
NE Nebraska
Have a motor, that has a separate disconnect and VFD. Motor requires 160 FLA @480v.

1. What STANDARD size breaker to feed that?200A? would it matter is fused switch or circuit breaker?
2. Would the same feed from panel to disconnect also motor be used to feed VFD?

Thanks
1. input to drive and overcurent protection are dependend on drive specifications, though I'd guess many the minimum sized drives that would handle this motor likely would require about 200 amp ocpd.

2. not sure exactly what you asking, but will say conductors between branch circuit OCPD and drive won't necessarily always be same size as drive to motor conductors. A drive with single phase in and three phase out (at same volts) will typically have larger input conductors because they will carry more current than the output. Once saw a 240 volt single phase input and 480 three phase output drive (20 hp I believe) and input leads were quite a bit larger than output leads.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Have a motor, that has a separate disconnect and VFD. Motor requires 160 FLA @480v.

1. What STANDARD size breaker to feed that?200A? would it matter is fused switch or circuit breaker?
2. Would the same feed from panel to disconnect also motor be used to feed VFD?

Thanks
You have a two different parts, the VFD and the motor. The conductors for the VFD must be a minimum of 125% of the input rating of the VFD (the motor rating does not matter). The OCPD for the VFD is usually a size recommended by the drive manufacturer. The motor conductors from the VFD are based on 125% of the value in T430.250.
 

horsegoer

Senior Member
Location
NJ
Don't have the code book handy buy this article confused me a bit

 

horsegoer

Senior Member
Location
NJ
You have a two different parts, the VFD and the motor. The conductors for the VFD must be a minimum of 125% of the input rating of the VFD (the motor rating does not matter). The OCPD for the VFD is usually a size recommended by the drive manufacturer. The motor conductors from the VFD are based on 125% of the value in T430.250.
Ah, got it. So need VFD info/data sheet?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Ah, got it. So need VFD info/data sheet?
Typically yes. The drive manufacturer gives you the recommended OCPD size for the VFD. If you went solely by the motor you could end up with an OCPD that is sized up to 250% which is usually much larger than the size given by the VFD manufacturer.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Typically yes. The drive manufacturer gives you the recommended OCPD size for the VFD. If you went solely by the motor you could end up with an OCPD that is sized up to 250% which is usually much larger than the size given by the VFD manufacturer.
Actually 250% is about typical for many modern VFDs. the cheaper ones usually limit it to 125%,. My understanding is this is because they are listed to an older, less robust standard, but UL allows the manufacturers to continue making them as long as they do not make any substantive design changes.

I don't think it makes much difference. A VFD is rarely going to draw even 125% input current. Most times I size the CB to the largest rating in the CB frame size that I can that is at least 125%. But I won't go to the next frame size.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Actually 250% is about typical for many modern VFDs. the cheaper ones usually limit it to 125%,. My understanding is this is because they are listed to an older, less robust standard, but UL allows the manufacturers to continue making them as long as they do not make any substantive design changes.
Lately I've been seeing them protected at about 125%. They were cheaper ones. :)
 

paulengr

Senior Member
To keep your selection simple, make sure the VFD of choice can deliver the rated 160A your motor needs at 480V. Better still, just use a small fudge factor to deal with the losses.

That won’t work well. It is application dependent.

In a centrifugal fan or pump application torque demand and thus current is quadratic in nature. Maximum torque is at full speed. So you only need a tiny amount of additional torque to reach top speed, say 110%. This is the “variable torque” rating of a VFD and typically what you see in catalogs.

Another application is say a conveyor or a compressor (mostly). In these applications we need roughly the same torque at zero speed as at full speed. And we typically need more accelerating torque, getting closer to the motor capacity. This is typically 135%+ current rating. So the VFD needs to be 2-3 “sizes” larger than the centrifugal pump/fan application.

Worst case that I’ve seen is rock crushers and their screens. 200% of rated current is where you need to be at or the motif will just repeatedly trip out.

Many people simply don’t understand this and routinely undersize VFDs.

Even if you have a pump application be aware that in process applications often the estimates on pump sizes are off by 25-50%. Within reason you can set a VFD up to run a smaller motor (up to 25-50% of name plate) but you cannot make it bigger with most drives.
 

topgone

Senior Member
That won’t work well. It is application dependent.

In a centrifugal fan or pump application torque demand and thus current is quadratic in nature. Maximum torque is at full speed. So you only need a tiny amount of additional torque to reach top speed, say 110%. This is the “variable torque” rating of a VFD and typically what you see in catalogs.

Another application is say a conveyor or a compressor (mostly). In these applications we need roughly the same torque at zero speed as at full speed. And we typically need more accelerating torque, getting closer to the motor capacity. This is typically 135%+ current rating. So the VFD needs to be 2-3 “sizes” larger than the centrifugal pump/fan application.

Worst case that I’ve seen is rock crushers and their screens. 200% of rated current is where you need to be at or the motif will just repeatedly trip out.

Many people simply don’t understand this and routinely undersize VFDs.

Even if you have a pump application be aware that in process applications often the estimates on pump sizes are off by 25-50%. Within reason you can set a VFD up to run a smaller motor (up to 25-50% of name plate) but you cannot make it bigger with most drives.
You only do that if your motor sizing is too low compared to the actual loading. During motor size selection especially for pump duty, you don't base your motor size on a Q point where the motor will get overloaded on loading changes. If you're in doubt, use API-610 in choosing the right motor drive. Note that the biggest multiplier recommended there is just 125% of the pump rating (motors smaller than 19 kW)
 

horsegoer

Senior Member
Location
NJ
Thank you all. So the sequence of the feed is usually OCPD to VFD to disconnect switch( if not mounted at motor) to motor?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Thank you all. So the sequence of the feed is usually OCPD to VFD to disconnect switch( if not mounted at motor) to motor?
That basically it. For us when the drive is adjacent to the motor the drive ON/OFF switch is the disconnecting means.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Thank you all. So the sequence of the feed is usually OCPD to VFD to disconnect switch( if not mounted at motor) to motor?
OCPD to VFD, minimum conductor size is 125% of VFD Input Current Rating (NEC 430.122).

OCPD is sized per VFD instructions (NEC 110.3(B)), but are usually listed as MAXIMUMS, so anything smaller is dealer’s choice. But also pay attention to the instructions. MANY smaller VFDs are ONLY listed by UL with fuses or SPECIFIC circuit breakers, as in brand and model number. This is becoming more and more prevalent because UL listing rules have recently changed. So don’t be caught off guard.

VFD to motor conductors, standard NEC 430 rules for motor circuit sizing applies, nothing special (125%of motor FC from NEC charts).

If there is a disconnect ahead of the motor, it does not change the conductor sizing, but NEVER put a fused disconnect downstream of a VFD, and use an aux contact in the disconnect to send a “disable” signal back to the VFD, most instruction manuals will describe this.

Side note regarding sizing of the VFD; to ensure success, learn the difference between “Variable Torque / Norml Duty” vs “Constant Torque /Heavy Duty” ratings. Basically if it is a centrifugal pump or fan, it is Normal Duty, EVERYTHING else should be sized as Heavy Duty.

Also, ALWAYS size a VFD per the motor nameplate FLA, NOT the HP, because on some motors, the FLA is much higher than normal. And although as electricians we rarely get to be involved in this part, some people like to use motors into their Service Factor, even though the motor mfrs tell you that you cannot do that if running from a VFD. If you see something, say something.
 

horsegoer

Senior Member
Location
NJ
OCPD to VFD, minimum conductor size is 125% of VFD Input Current Rating (NEC 430.122).

OCPD is sized per VFD instructions (NEC 110.3(B)), but are usually listed as MAXIMUMS, so anything smaller is dealer’s choice. But also pay attention to the instructions. MANY smaller VFDs are ONLY listed by UL with fuses or SPECIFIC circuit breakers, as in brand and model number. This is becoming more and more prevalent because UL listing rules have recently changed. So don’t be caught off guard.

VFD to motor conductors, standard NEC 430 rules for motor circuit sizing applies, nothing special (125%of motor FC from NEC charts).

If there is a disconnect ahead of the motor, it does not change the conductor sizing, but NEVER put a fused disconnect downstream of a VFD, and use an aux contact in the disconnect to send a “disable” signal back to the VFD, most instruction manuals will describe this.

Side note regarding sizing of the VFD; to ensure success, learn the difference between “Variable Torque / Norml Duty” vs “Constant Torque /Heavy Duty” ratings. Basically if it is a centrifugal pump or fan, it is Normal Duty, EVERYTHING else should be sized as Heavy Duty.

Also, ALWAYS size a VFD per the motor nameplate FLA, NOT the HP, because on some motors, the FLA is much higher than normal. And although as electricians we rarely get to be involved in this part, some people like to use motors into their Service Factor, even though the motor mfrs tell you that you cannot do that if running from a VFD. If you see something, say something.
So basically I can size the VFD from the motor nameplate info?
 

kwired

Electron manager
Location
NE Nebraska
If the motor is accelerated, decelerated or even reversed frequently that can impact things also. Of course can also impact what size motor is needed to start with as well, but driven load characteristics play into this as well.
 

paulengr

Senior Member
You only do that if your motor sizing is too low compared to the actual loading. During motor size selection especially for pump duty, you don't base your motor size on a Q point where the motor will get overloaded on loading changes. If you're in doubt, use API-610 in choosing the right motor drive. Note that the biggest multiplier recommended there is just 125% of the pump rating (motors smaller than 19 kW)

Don’t care about petro industry standard because the fluid properties are known. Try mineral slurries that are non-Newtonian or plastics. And this is just the start.

Say the process engineer calculates 100 HP. So it makes all the sense in the world to put a 100 HP VT motor there with say water. With a given pump the turndown ratio is limited. No matter what you do there is a limit to turn down ratios and it’s pure fluid dynamics that is the limit. Note that most comical engineers do NOT get this. All problems, even process ones, are electrical until proven otherwise.

So say they really need 150 HP. And in reality most systems need a certain amount of kilowatts per pound of material moved but we mess this up by getting lost in units like CFM or HP or Amps. So at first they crank it up to 60 Hz. Then they ask you to unlock it, get to say 65 Hz and it trips. It doesn’t matter what point this happens at. At some point they are going to hit the motor thermal limit. The thing is that with pumps and fans the affinity laws say power draw is proportional to the cube of speed. So they are going to play this game and find out it makes very, very little difference before we even get into issues with cavitation.

So if you had the budget to do it and went up 50% on the VFD or at least just size everything “HD”, no problem. They buy their bigger pump and motor and you just change the motor parameters in the drive. But if you sized to name plate now it’s changing the VFD, cables, breaker, and maybe transformer.

Now saying this there is a big difference between say buying a 20 HP VFD for a10-15 HP pump and buying a 750 HP VFD for a 500 HP pump.

And you should have a feel for it. The gas & oil and power generation industries tend to look at things in a more pure cost/benefit point of view and won’t hesitate to rip out a million dollar drive system and put in another million dollar drive system 6 months later. That’s far different from wood or food plants that will cry over an engineering mistake but live with it for years. And mining notoriously undersized things then may or may not fix it but will blame you for selling a “cheap/weak” VFD, especially quarries.

The best way to answer this is make sure you size for the application as a bare minimum. Offer a size upgrade too. Leave it on the customer. Often I will look at frame vs. model size and just offer the lframe size as the upgrade since it doesn’t change any dimensions drastically and often has only a small price increase.
 
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