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I want to power and control the RPM of three 115v .47amp shaded pole axial fans each powered by a separate leg of the 3-phase coming from a VFD. Can it be done? I'd like them to run at full speed (60hz) down to possibly half of that or less if that's an option.

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  • \$\begingroup\$ Two legs, not one \$\endgroup\$ Jan 15 '18 at 19:42
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Most VFDs have a mode of control that is not senseless vector. That mode is often called the "V/Hz" mode. If you select that mode, it should be possible to power three shaded pole motors driving fan loads. Some VFDs may have a "load loss" or "load imbalance" protection feature, but I don't think that either of those features is common. VFDs usually have sufficient configuration adjustment ranges to allow setting up for 115V output at 60 Hz.

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Given any reasonable VFD has a current regulation, no.

  • Reactive currents in the individual legs will differ depending on speed.
  • Active currents in the individual legs will differ depending on the load situation.

This will make the VFD go into failure mode, expecting the single motor it was designed for to be broken.

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I have been working on a related question, so although I do not know for sure (please check on the bench before deploying) I believe the answer is maybe, depending on how the vfd is designed and other factors.

Let's look at some factors that can push the answer in one direction or another.

  1. Are your motors and loads identical? the more unbalanced your current loads on the vfd are unbalanced the more likely the control loops in the vfd will be confused.

  2. How coupled are your loads? The good news is that unless all your fans share a shaft, your load can tolerate unbalance (synchronization of air flow is probably not necessary). The bad news is that depending on layout the fan layout air from one fan could affect load on another (either reducing or increasing), and other airflows may affect different fans disproportionately. this may significant affect the vfd current enough to cause problems.

  3. Where is your neutral? I have never seen a vfd with a four wire output (not counting ground), which means that either you need to use line neutral (which probably will start a fire), find a neutral inside the vfd (which means you need to modify the vfd, which I do not recommend), or you need to wire the fans in either a delta (which does not need a neutral) or a wye (which can float it's neutral). remember that the output voltage of your vfd is a phase to phase voltage so if you are wiring a delta you use the name plate voltage, but if you are using a wye you need to multiply the nameplate voltage by the square root of three. Also not that wye configuration is probably more vulnerable to a lost phase and neutral imbalance.

If you got this to work, I would love to hear about it. ( I would also be interested in any problems you have, including HCF events. )

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