VFD
VFD
(OP)
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!





RE: VFD
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
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD
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.
RE: VFD
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD
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.
RE: VFD
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.
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
RE: VFD
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: VFD
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.