VFD Blew up

[JST]

The motor is ONLY a generator when the drive "excites" it, meaning supplies a signal which would cause it to turn slower than it is currently turning. That is what the slowdown (decel) does.

Without that, it will not generate except under very special conditions which won't be present with a VFD.

The motor is a "load" when it is turning slower than the synchronous speed. It is a "generator" when turning faster than synchronous speed, which is what happens as the drive reduces the frequency going to the motor. The excess energy is removed from the motor by the generator action, gradually as the VFD slows it's output frequency according to the "decel" curve.

Older large wind turbines were simply large induction motors connected to the blades by the shaft and the mains by wires. When the blades tried to turn the motor faster than synchronous speed, the energy was fed back to the line.

The self-protection occurs when the drive ceases to provide a signal to the motor stator. At that point, there is no longer any magnetic field in the rotor, and generator action stops.* Everything is essentially identical to a "coast to stop" action from then on.

As for the rotary converter, it is not externally "driven" so it itself is always a "load". It only "generates" out to the "generated leg". Theoretically, it could receive and pass energy back to the line, if a load motor were driven faster than synchronous speed.

A motor connected to it can generate if something drives it faster than synchronous speed. That rarely happens, although a CNC may have means to dump excess DC bus energy into the line as AC by means of something akin to an inverter.

* An induction motor can be made to generate on its own. I have done that. However, it requires the connection of capacitors which will draw a current which is correctly phased to do the excitation. The action must be "started" by residual magnetism in the iron core, which does the initial generating to get everything started. The capacitor value is reasonably critical, and generally motors on a VFD will not have capacitors at all.

So the chances of generation after the VFD stops providing excitation to the motor coils is small.

 

[EmGo]

What's the attraction of these vfd things ? A rotary is so easy to make, and cheap, and none of these problems that are all over here "my vfd blew up !"

And oh yeah, the wave form is much nicer (even if one leg is higher).

I can see it for a manufacturer who can avoid making gears and shafts and shift forks and all that stuff, but for running stuff at home ? Why ?

(Please don't say "so it can have variable speed !" because that's close to pointless 95% of the time).

 

[JST]

Actually, one leg of the RPC is LOWER.*

You can boost it at no load with capacitors, but the effect drops with load. You can do better boosting with a transformer, but that leg will always be weaker than the "pass-through" legs (the other two).

That's not really a big negative for most motor applications. It is less good for "rectified loads" such as most CNC machines.

* it has to be, since it is the "back EMF" of the motor, and that is always lower than the mains voltage. If it were not, the motor would not draw power.

Since an RPC puts out "stinger" 240V, the voltage to ground from that wire is about 210VAC, higher than from the other two wires. But the voltage from the generated leg to either of the "pass through" wires is lower than that between the pass-through wires.

I like RPCs. They are as simple as a stone hammer by comparison to a VFD, and the parts for a simple RPC are widely available, needing very little in terms of special equipment even to manufacture from scratch. The technology is well over 100 years old.

VFDs have their uses. When you need what they do, then you use one. But, very often, they end up getting used for reasons that come down almost to "because I can".

 

[SomeoneSomewhere]

Yup, attempting to do hard braking on a drive without a brake resistor or active front end (which can push energy back into the mains) will typically result in a "DC BUS OVERVOLTAGE" or similar, and the drive tripping and allowing the motor to coast to a stop. Not an explosion. Most drives have a feature that attempts to make the ramp less aggressive if the voltage rises too high, but this often doesn't react fast enough if the jerk (rate of change of acceleration) is too high.

Seen it happen in fan/pump setups where a PID loop is too aggressive, and shutting a door, wind gusts, or underdamped oscillations caused the PID loop to try to brake the fan quickly. Solved it by sticking in a negative torque limit.

It's not impossible for a drive without a brake resistor to brake a motor. It depends on inertia, how fast you want to brake, and how much capacitance the drive has. The drive itself consumes a non-negligible amount of power from the DC bus to power fans, electronics etc. Oversized drives with lots of capacitance can absorb quite a bit, especially if they're running at 380VAC but the drive is rated up to >480VAC.

The only case where a motor can *force* energy into a drive even where the IGBTs are off is if the motor is outputting higher peak voltages than the DC bus, at which point the freewheeling diodes start conducting and the electronics can't do anything about it. But that almost never happens - as noted, it's basically impossible on a simple induction motor. It's plausible on permanent magnet motors, but a) they're uncommon, and b)the motor would have to have been spinning higher than the maximum that the VFD could deliver in the first place, as voltage on those is proportional to RPM.

 

[johansen]

So a rotary converter just feeds back into the line ?

As you probably know induction motors consume reactive energy. but It Is theoretically possible for a drive to regen and then switch to free wheeling due to over voltage limit, and the dc bus would continue to climb very briefly, but not by much. I would guess a few percent.

 

[candeservices]

So a rotary converter just feeds back into the line ?

Yes, but a rotary converter has contacts that open when you push stop thus not being able to feed power back to service.
While it is spinning down you can measure voltage across the motor leads .