Strange phenomenon with a 3PH motor of a suction blower

[dor123]

Look at my video:
https://www.youtube.com/watch?v=h4GNQ5iRPxs
This is the suction blower of a clinic in Carmel hospital that I don't remember its name. It have a 3PH motor
When the emergency generator connects the hospital back to the mains voltage, during the power interruption, it begins to slowly slow down its speed.
Than after the power gets back, it continues to slowly slowing, but after few sec, just braked (Immediately) to 0 RPM and than ramping up to its operating speed.
Why the motor breaks immediateky to 0 RPM before ramping up to its operating speed, and don't just ramping up from the speed it was, when the power restored???

[FrontSideBus]

Might be because it's on a variable speed drive and the rapid braking is the drive ramping up the frequency from a "stopped" state which causes the motor to change speed suddenly to match.

[dor123]

It have 50hz humming, and its ramp up sound, as well as the time it take to reach its operating speed, implying a standard 380V 3PH motor without VFD control.

[Ash]

Most motors are optimized to run at 50/60 Hz, so even with VFD control installed, many will have the frequency set for 50 Hz anyway (with the VFD allowing smoother starting)

In the video there is the typical chirp sound of a VFD, so i would think it is indeed connected to a VFD



If it is not on VFD, let's think of other possible explanation. The motor install may have a few features typical for motor installations :

- The motor is controlled by a contactor

- It may be on an ON-delay timer (to prevent it from drawing starting current in the first instant when power is first applied to the board it is powered from, so reducing inrush current arcing on any change over switches, starting torque on a generator powering the board, and so on)

- It may be on a Wye/Delta starter

When running on generator, the frequency may be a bit below 50Hz, and the voltage a bit below nominal (resulting in higher slip) which make it spin a little slower than when powered from the grid

After the changeover back to grid, the ON delay timer is still in delay, so the motor is spinning by inertia with no power applied. There may be remanent magnetization in its armature, resulting in it outputting voltage back towards the board. As the contactor is open, this voltage does not go anywhere

When the contactor closes, the starter switches initially to Wye, so the backfed voltage from the motor might (?) be higher than the voltage supplied from the board, in which case it may briefly cause braking of the motor. I actually doubt this one here, as - 1. once power is applied it will overpower any remanence in the armature quite immediately and return to accelerating even in Wye, and 2. the motor appears to lose speed quite significantly during its open phase, so it probably does not backfeed that high voltage anymore. (And if it does not backfeed higher voltage than the supply, then it will not brake)

So let's think of another explanation for the last one instead. I can think of two, but both involve some issues with how the board is made to actually happen :

Maybe there are 2 tiers of contactors in series in the board, one powering an entire subcircuit from the supply, and the second powering this motor from the subcircuit. Maybe the contactor connecting the motor to the subcuircuit closes before the first one (due to times of delay timers being configured incorrectly), and then the motor brakes once it gets connected to an unpowered subcircuit with other loads

Maybe the phase order of the generator is reversed (due to improper wiring), so when finally grid power is applied, the motor screeches to a halt to change direction. Being a centrifugal blower, it will appear to "work" while spinning backwards, but fail to actually move much air

[dor123]

The motor always spinning in one direction. It is only that it breaks when it getting the power again, in both power outages.
The motor don't have VFD, as it never changes speed (And don't needs to changed speed).

[BC5-80]

I agree with Ash, you can hear the inverter squeak. Could it just be the soft starter that uses the VFD facility, and when power is restored, it starts the soft-start sequence from zero speed?

[dor123]

This blower working at a fixed speed. Probably it should work faster in the case of fire in the clinic.