VFD 6

[Reesh14]

I have an existing air handling unit with a 30 HP/460V/3 phase motor with a wall mounted motor starter. A variable frequency drive is to be installed on this air handler. I am not sure what needs to be changed do to this installation. Will the wire sizes and overcurrent protection need to be changed along with the motor starter? Not sure what design criteria is involved with the VFD installation. Please advise. Thank you!

 

[Skogsgurra]

You probably do not need to change anything. Your motor starter has protection and that may be a minor problem if your VFD is drawing non-sinusoidal current. It will heat the protection more than the good old sine did.

If you get nuisance trips, then just kill the protection. There is thermal protection built into most (all) moderns VFDs. Just make sure it has been set according to motor name plate.

At 460 V, you may get an insulation problem. VFDs have very aggressive output wave-forms that are known to kill winding insulation at voltages like 500 and 690 V. 460 V is close enough. You should talk to the motor manufacturer. If it seems to be a problem, a du/dt (or a sine) filter helps.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ozmosis]

You need to determine where the VFD will be mounted and ensure you have an enclosure classification that meets this environment. If it is inside the building and is dirty, dusty and has water sometimes splashing around, then you will need to look at a NEMA12 enclosure as a minimum. If it is clean and safe, then you could get away with something like NEMA1. If it is going outside, then its another issue altogether and NEMA3R should be looked at.
Determine what signal is going to control the speed/frequency of the VFD subject to what you are looking to control via your AHU (e.g. temperature, pressure, air quality etc). As you only had the motor on fixed speed before, then I can only assume you are fitting the VFD to replace inlet guide vane or outlet damplers. The same method of controlling these will now need to be integrated into the VFD somehow, so make sure the signal is suitable.
Determine how far away the VFD will be mounted from the motor. This will also have some effect on the nastier effects a VFD will have on the motor, as explained by gunnar. On this point, the route your motor cables take (between VFD and motor) will usually need certain planning as unshielded cable will have the potential to generate high frequency interference and if this is being routed through sensitive cables or into other control cables, you could get problems with the interference effecting other equipment.
Basic tip: Plan before you buy. Plan before you install. If not, the benefits you planned to get will be lost.

 

[Skogsgurra]

Hmm... Me forgetting about EMI! What is happening to me? I guess I was picturing a rural barn somewhere in the Mid-West. No sheriff and no FCC. Should've known better...

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[CJCPE]

The motor starter is not needed. It can be retained, but it should be used only to apply and disconnect VFD input power relatively infrequently such as if the air handler is shut down when the building in unoccupied. The VFD must be used to start and stop the motor. The existing disconnect device can probably be used as the VFD disconnect device. The existing short circuit and ground fault protection may or may not be suitable for use as VFD input short circuit and ground fault protection. You need to check the VFD manual to see what is required. Some VFDs may require semiconductor protection fuses that comply with the terms of their UL listing.

It is best to have nothing between the VFD and the motor except the dv/dt filter recommended above. The VFD input disconnect device will usually meet code requirements to serve also as the motor disconnect device. If you must have a disconnect device between the VFD and motor, you need to make sure that the VFD will tolerate opening the disconnect with the motor running. Most VFDs will tolerate that. You probably need to interlock the switch so that the switch must be closed before starting the That is required or strongly recommended for most VFDs.

If the input power system has a very high capacity compared to the VFD size, additional input line impedance may be recommended to protect the VFD. It the input power system has a relatively low capacity, additional impedance may be recommended to protect the power system from harmonic distortion.

 

[jraef]

All good advice here.

I'll only add that given you have an existing AHU that was full speed only, you undoubtedly had a damper controlled VAV system and the VFD is being retrofit as an energy savings upgrade. Very valid, but in situations such as yours I recommend leaving the existing actuator system in place, just disconnected. That also means leaving the existing starter in place, preferably disconnected or at least in parallel with the VFD using a changeover switch. This way if the VFD ever fails you still have the old system as a backup that can be readily reinstated while you get the VFD repaired or replaced.

If your VFD is a complete package with a disconnect and "HVAC bypass" option, leaving the existing starter in place would not be necessary.

The VFD input disconnect device will usually meet code requirements to serve also as the motor disconnect device.

Assuming it has one. If it is just an out-of-the-box VFD only, it will need a disconnect switch somewhere in front of it.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

The one thing not mentioned here that I see tripping Solid State Relay users up constantly is that the VFD will dissipate heat, unlike the the old motor starter. You cannot just jam one into a box it "just fits" or you will have overheating problems.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[DickDV]

Regarding motor insulation and EMI issues, at 460V and 30hp, if the motor is at least Class F insulation, you can use existing motor and leads up to about 75 foot lead lengths. Beyond that, the motor lead reactor or dv/dt filter will be needed. Even at shorter lengths, its not a bad idea.

If the motor is Insulation Class B and the leads are longer than 10 feet, figure on replacing the motor or spending the extra money on the dv/dt filter. Probably best to replace the motor as you can upgrade to premium efficiency at the same time. Buy one with MG1 Part 31 endorsement on the nameplate, as well.