VFD Breaker Tripping

[Besoeker]

Here is one sceanrio I can think of.

If for some reason, the power is failing at 6 AM (or earlier and coming back on at 6 AM), there is an initial inrush of current that occurrs when the capacitors are charging. This is a very fast rise because it is charging the capacitors almost instrantaneously. Many VFDs have various methods of mitigating that, i.e. charging resistors, DC bus chokes or using SCRs on the front-end instead of diodes, but some do not.

I don't think that is a very likely scenario.
TBH, I have not come across any VFD that did not have a precharge circuit.
Inrush is one problem. But not the only one. Without the precharge, the step change in input voltage would result in capacitor overvoltage - you have simple LC circuit with a potential transient at twice steady state operating voltage.
Input line reactors or a DC choke are likely to exacerbate this, so won't mitigate that fundamental fundamental issue.
I don't think capacitor inrush is the problem.

 

[steve66]

TM,

Check the time setting on ramping up the VFD for starting.

- JWC

I'll second that, along with increasing the breaker size to the right size.

For some reason, VFD's always seem to default to a very fast ramp-up time. And the HVAC startup people never turn it down. I've seen breakers trip, belts fly off, and all the lights in an entire building flash because of very fast starting ramps. All fixed just by changing the ramp up time.

I'm not a VFD expert, but I'm starting to think a VFD can accelerate a motor even faster than "across the line" starting.

 

[Besoeker]

I'm not a VFD expert, but I'm starting to think a VFD can accelerate a motor even faster than "across the line" starting.

It can.
Without taking excessive supply current.
For a standard induction motor, direct on line starting can take five or six times motor rated current without a propotionate starting torque.
Best torque per Amp is developed at low slip, the difference between synchronous speed and rotational speed.
A VSD can maintain that low slip.

 

[steve66]

It can.
Without taking excessive supply current.
For a standard induction motor, direct on line starting can take five or six times motor rated current without a propotionate starting torque.
Best torque per Amp is developed at low slip, the difference between synchronous speed and rotational speed.
A VSD can maintain that low slip.

So a VFD can start a motor by applying a low freq. (maybe 5 or 10 hz), which keeps the slip low, and creates more torque.

More torque means faster acceleration, and the motor gets up to speed faster.

That seems to make sense, but I was assuming that the faster acceleration also resulted in a higher starting current. It seems like faster acceleration should require a higher input current. (Power in = Power out) Unless across the line starting is just that inefficient??

 

[Jraef]

When a motor is started Across-the-Line (DOL), starting current is high because the slip is high and although the current is high, the power factor is very low so not much of the total power is doing useful work. A VFD can use slip compensation methods to keep slip in check throughout the acceleration process, so more (most) of the current is used in creating motor torque.



Besoeker,
I have run across 2 brands / models of VFDs with virtually no pre-charge circuit. Allen Bradley's original 1336 drives over 100HP used a phase-angle controlled SCR converter section to ramp voltage into the caps and no pre-charge resistor. But their early versions had a glitch in that if the supply was dirty (noisy), sometimes the SCRs would self commutate when first powered on, leaving you with no ramp. The result was usually the breaker tripping on instantaneous current.

The other was a cheap Chinese drive I think from Rich Electronics (looked like a reverse engineered knock-off of a Yaskawa G3 design). No pre-charge circuit that I could detect, high field failure rate.

It's also an issue on old drives where they used a relay for shorting out the the precharge resistor after initial power up, because sometimes the contacts weld and the resistor is no longer in the circuit.

But now that you mention it you're probably right, a choke would not likely solve those problems.

 

[Besoeker]

So a VFD can start a motor by applying a low freq. (maybe 5 or 10 hz), which keeps the slip low, and creates more torque.

More torque means faster acceleration, and the motor gets up to speed faster.

That seems to make sense, but I was assuming that the faster acceleration also resulted in a higher starting current. It seems like faster acceleration should require a higher input current. (Power in = Power out) Unless across the line starting is just that inefficient??

The VFD can ramp up the frequency and voltage from zero. Keeping the two in the same ratio (often called constant V/f) keeps the motor flux essentially constant and can allow it to produce rated torque at any speed.

This is the manufacturer's speed torque curve for a fairly standard induction motor.

It shows that, at rated load, the slip is very low at about 0.2 Hz for this particular machine*. As long as the slip doesn't exceed this value, the motor won't take more than rated current.
By contrast, at anything above about 3% slip in this case, you get increasing motor current and decreasing torque.

On your point about faster acceleration requiring higher input current. Well, not generally. Power is speed times torque so the lower speed during run-up means lower power. Below are the characteristics for one of our high speed machines showing the variation of input power with speed - it's part of a spreadsheet I use for estimating performance.

Ignoring waveform distortion, input current is approximately proportional to input power.

*This was a fairly large machine but I haven't come across many with a slip of more than about 1 Hz.