VFD's and motor sizing by NEC 430

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megloff11x

Senior Member
Motor disconnects and overload protection is sized under sec. 430. What if your controller is a VFD instead of an on/off switch? VFD's are typically rated a few amps more than the ampacity rating of the corresponding motor HP being controlled. The tables 430.148-430.151 don't take this into account. Or do they?

If I'm using a VFD, do I go by amperage from motor HP ratings, and the method of 430.110 for multiple motors, or do I go by what the VFD says it will draw and multiply that by what "fudge factor?"
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: VFD's and motor sizing by NEC 430

This is a question I have blown before so I remember it well. :D

Take a look at 430.2 Adjustable-Speed Drive Systems.

The conductors are matched to the input rating of the drive and the disconnect is not less than 115% of that.
 

pwhite

Senior Member
Re: VFD's and motor sizing by NEC 430

the vfd unit has an output current range. i would check to see what the range is and compare it with the motor requirements as a check.

also, the motor you are placing on the vfd unit must be an inverter duty motor or else it will overheat and burn up in most cases.
hope this helps
 

tony_psuee

Senior Member
Location
PA/MD
Re: VFD's and motor sizing by NEC 430

Article 430.2 is the reference point for the VFD. You made reference to running multiple motors with a single VFD? If so, each motor will require protection in accordance with Article 430. If it is single motor the integral overload protection of the drive is sufficient.

IMO an inverter duty motor is highly recommended however is not a requirement. Most motor manufacturers have guidelines for motor lead lengths regardless of duty type. A non-inverter duty motor within the allowable lead lengths will have a shorter service life because of the additional heat than an inverter duty rated motor. How much shorter, who knows. Run it longer than the recommended lead lengths and the service life could end at start up.

Tony
 
Re: VFD's and motor sizing by NEC 430

if you use multiple motors on (1) vfd, you will need to derate the vfd unit. see the manufacturer for specific requirements.
 

John Valdes

Senior Member
Location
SC.
Occupation
Retired Electrician
Multiple motors on a VFD do not need to be derated. Just add up all the motors nameplate current value. Use this value to choose the size inverter needed. Example: If your total motor load is 35 amp you will need a 40 hp drive. It's not complicated. Now, you will need to protect each motor individualy. One overload relay per motor. The VFD overload parameter value will be the total of all the motors. Whatever you do, do not put any contactors on the load side (between VFD and Motor). This is a common mistake. When a contactor closes it will spike the line and trip the drive. I hope your speed tolerance is not too close. Not one of the motors will run exactly the same speed. It's called (SLIP). Even if they are all the same brand and RPM.
 

John Valdes

Senior Member
Location
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Occupation
Retired Electrician
Yes Frank, You cannot install contactors between a VFD and the motor. The current spike associated with the contactor energizing will trip the drive. VFD's are temperamental and will not allow switching of loads while in operation. Example: 4 motors 1 VFD. Now lets just pick a current, 10 amps per motor. 40 amps is what you program into the drive. When the contacter closes on one or more motors while the others are running the VFD senses this increased starting current and trips on overload or other protection feature on the drive. The VFD does not know if you have 1 or 100 motors on its output. It only knows the current you program in it's parameters. That is why you will need a seperate OLR for each motor (rated for that motor). The programed current in the drive is for the total current for ALL the motors. You cannot protect each motor individually with one VFD. Thus OLR for each.
 

tony_psuee

Senior Member
Location
PA/MD
You cannot install contactors between a VFD and the motor

John,

That is incorrect. It is common to have a contactor on the output of a VFD. You can have the contactors, you don't want to cycle them as the means motor control for the reasons you stated. Their purpose is to isolate the VFD output components in the event of a VFD failure or if the motor is being run in bypass mode, contactor in parallel with VFD.

Tony
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
benaround said:
John,

That's a tuff order. "Put one overload relay per motor" and " Do not put any

cont actors between the VFD and the motor"

this is a common application for standalone overload blocks.

normally one would put a separate overload relay on each motor and wire the contacts in series to the VFD enable.
 
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John Valdes

Senior Member
Location
SC.
Occupation
Retired Electrician
Tony.....Why put a contactor or contactors on the load side. If you are trying to protect the VFD and motor/motors you put it on the line side. But if you use the correct size breaker or fuses (semiconductor fuses) you don't need a contactor period. You stated "It is common to have a contactor on the output of a VFD". Where did you get that information. VFD bypass contactors should be located to bypass the VFD. If you put a bypass contactor on the load side you still have power to the VFD in event of VFD failure.
Bob, you are correct to series the overload circuit so all the motors stop. If you want all the motors to stop.....LOL
 
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tony_psuee

Senior Member
Location
PA/MD
John,

The line contactors are not for VFD protection, it is for isolation. A fuse/circuit breaker will protect the line side of the VFD, however if the input bridge fails with a short they will trip/blow and you will not be able to run the motor in bypass. The load side contactor does protect the VFD by isolating the output power components from your source when in bypass mode. Without a load isolation contactor, anytime the motor is in bypass you backfeed the VFD. So, the first time you run the bypass to check the motor rotation may be the last time you VFD worked.

Tony
 

John Valdes

Senior Member
Location
SC.
Occupation
Retired Electrician
Tony,
Your saying the same thing I am saying. You are just looking at it from another angle. Bypass contactors should isolate the motor and VFD. That is why the bypass should be comprized of two contactors, one to bypass the VFD and one to open the load side of the VFD to prevent backfeed. If you have a breaker feeding the VFD that can be turned off,and still have line power to the bypass contactor then only one contactor (load side) would be needed. This will not work if you need auto restart in bypass mode.
Your point is well taken, as I did not address the backfeed issue. Of course we do not want to apply line power to the output of the VFD. However, we do not want line power to the VFD input either in bypass mode.
My original statement about contactors on the load side of VFD is still true, as they cannot be operated while the VFD is operating. Bypass is a totally different situation.
 
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tom baker

First Chief Moderator
Staff member
tony_psuee said:
IMO an inverter duty motor is highly recommended however is not a requirement.
Tony
A invertor duty motor is a motor that has a high turndown, IE it can run a lower speed without overheating, perhaps 5 hz. A standard motor can go to 30 hz, as the motor cooling is via a fan on the motor shaft, slow speed mean less cooling. A centrifugal pump will not have to run loaded below 42 hz as it does move any fluid below that speed
But what is also needed is a inventor spike resistant wiring motor. the VFD puts out a 1500 volt p-p wave and this will damage the windings of a non spike resistant motor.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Tom,
A invertor duty motor is a motor that has a high turndown, IE it can run a lower speed without overheating, perhaps 5 hz.
Are you sure? I thought that only difference was the winding insulation is designed to withstand the spikes generated by the VFD.
A centrifugal pump will not have to run loaded below 42 hz as it does move any fluid below that speed
That changes with the pump impeller design. I have installed a number designed to work at 1/2 speed or 30 hZ.

Don
 

John Valdes

Senior Member
Location
SC.
Occupation
Retired Electrician
Turn down ratio's are a product of motor manufacturers advertizing campaigns. ABB markets their metric ac motors at 1000:1. Baldor at 10:1 and so on. Turn down ratios are a tool the manufacturer uses for warranty also.
Look at any motor catalog. The truth to this matter is of motor construction and minimum speed of the motor. There are two types of inverter duty motors.
1) Fan Cooled (TEFC) & (TENV) speced for Inv. Duty
2) Blower Forced (TEBC) speced for Inv. duty.

Another way to look at this is from the issue of Torque. If a motor is running slower than base speed (top speed). The amount of torque is reduced. If a motor is running faster than base speed the torque is also reduced.
If you do the math you can see the affect that speed has on motors.

T = HP X 5250 divided by RPM = ft lb's

Some inverter duty motors can run at 1 rpm, full torque, all day for years. But they have forced blowers and encoder feedback.

Note: Most pumps are variable torque loads. Allowing slower speeds and reduction in VFD size.
 
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tony_psuee

Senior Member
Location
PA/MD
Invertor duty motors

Invertor duty motors

Tom,

"A standard motor can go to 30 hz, as the motor cooling is via a fan on the motor shaft, slow speed mean less cooling."

If it is a TEFC motor then lower RPM can be an issue for the additional heat associated with being operated by a VFD if it is not invertor duty. The invertor duty motors we use are 10:1 turn down and typically TEAO. However, they are in a cooling application and moving a fan at 6Hz provides little if any cooling, so most of the time it is 15-20Hz.

To my knowledge the main differences between inverter duty and standard motors are:
-higher insulation, "spike resistant wire", 1600-1700V insulation vs. 1000V
-larger rotor, more wire
-more phase paper between the windings
-insulation tubes on the end turns

The NEMA motor standard does have what the actual requirements are for NEMA rated motor to be invertor duty.

Tony
 

John Valdes

Senior Member
Location
SC.
Occupation
Retired Electrician
Tony,
I left the (TEAO) motor out. I didn't know you could get inverter duty in a TEAO frame motor.
 
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