It does not need to be at a standstill, but it should NOT be rotating in the opposite direction as it will when you energize it! A very slight rotation (1 or 2 RPM?) may be tolerable, but any more can significantly add to the mechanical stress on the shaft and/or increase the acceleration time to full speed so as to cause unnecessary heating in the motor, particularly the rotor.
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As Jeff says. As long as it rotates the "right" way and if you do not have a VFD on it, it os only good for the motor and everything else if it rotates. But has to be same direction as it is supposed to work.
A VFD (probably an MV type with motors as big as this) it all depends. If there is a flying start function in the VFD, it should take care of it. If not, you better stop the fan before starting it.
My understanding is that reverse rotation presents primarily mechanical issues.
As I understand motors, the primary issue with forward rotation is induction generation or back EMF. If the motor is not generating a significant voltage then it is safe to energise it while it is spinning forward.
Capacitors permanently connected to the motor leads may increase the magnitude and probability of back EMF.
Energising a motor when significant levels of back EMF are present is similar to connecting a generator to the line without regard to synchronization. The odds are heavily against you.
Problems with energising a spinning motor with a VFD are a result of VFD capabilities as well as back EMF issues.
Energisation of forward rotating motors without permanently connected capacitors or motors rotating at low speeds should be safe. You can use a voltmeter on the motor leads to check for back EMF. Be aware that the frequency of any back EMF will be a function of motor speed. The voltmeter should be frequency agile.
Back EMF is a serious issue when power is interupted to a running motor for only a few seconds and then re-applied.
For a motor of this size it may be worth the effort to actually check for back EMF with a meter. It depends on the speed of rotation.
respectfully
Driving an ID fan I assume you have an ISCM. You should allow 4 to 7 cicles (around 0.11 sec on 60 HZ line)after the power supply is disconnected to avoid an out of step reconnection.
After that, your limitation is the motor thermal capacity, you have to consult the OEM for verification of the intended motor cycle.
