VFDs and starting current

[Besoeker]

Not only was he wrong about how VFDs function, he was wrong about soft starters as well!
What the heck is solid state series impedance? :dunce:

I assume he means SCRs which are the usual method of controlling the starting of reduced voltage starters.
But they operate as switches - either on or off with little voltage drop in the on state. Calling them an impedance is misleading to say the least.
Bad article, I agree.

 

[RumRunner]

180706-2037 EDT



Alternately:

In the quote of the first post "VFDs deliver much higher frequency source power ---- " we conclude the question is about motor input current, or in the case of the VFD it is also the VFD output current.

If motor starting is primarily controlled by ramping up of a low frequency apparent approximate sine wave current frequency to the motor by short modulated current pulses of a much higher frequency, then the motor wants to follow that low frequency. Per unit time there is not much energy required to slowly accelerate the rotor and load. Thus, not a lot of input current to either the motor or the VFD input is required compared to across the line starting.

When properly modulated the high frequency current pulses to the motor are averaged or smoothed by the circuit inductance to produced an approximate low frequency sine wave. A scope plot of motor current will show a relatively smooth low frequency current waveform, not short pulses, but with some of the high frequency as ripple.

I believe the quote of the first post is wrong because a high chopping frequency of current to a motor is present whether starting or not, just different modulation, and no mention is made of the synthesized low frequency sine wave current as the primary way that starting peak current is reduced.

What I have just said may be clear as mud. So if not clear question me.

.

I will try not to excoriate the person that wrote the article because I think that he's received enough flak to make him look like he's based his comments on thin air.--and will even cause to him to burn and be reduced to ashes.

Easy to say that he's had some good grounding in Newton's Law regarding energy preservation that is analogous to Lenz's Law with help from Faraday.

Lenz's law named after the physicist Heinrich Friedrich Emil Lenz who formulated it in 1834, states that the direction of current induced in a conductor by a changing magnetic field due to induction is such that it creates a magnetic field that opposes the change that produced it.

Based upon this law in physics-- the article is correct. Not so much about the OP which is only acting as the messenger.

Here's why:

“Impedance is the total contribution of both -- resistance and reactance. This is important for AC analysis and design. At DC, reactive elements can be replaced with their steady-state model (capacitor->open,inductor->short) and resistance can be considered. (this isn't true for transient analysis)

This may sound a little abstract. Impedance really is an abstraction of things that are far more complicated (things like time constants and rise times) that electrical engineers have to constantly consider. The idea of 'impedance' allows for many of these things to be wrapped up into one subject so that they are easier to communicate.

The short answer is -- impedance includes reactance, and reactance includes effects which vary with frequency due to inductance and capacitance.

Reactance, however, is a measure of the type of opposition to AC electricity due to capacitance or inductance. This opposition varies with frequency. For example, a capacitor only allows DC current to flow for a short while until it is charged; at that point, current will stop flowing and it will look like an open. However, if a very high frequency is put across that capacitor (a signal that has a voltage which is changing very quickly back and forth), the capacitor will look like a short circuit. The capacitor has a reactance which is inversely proportional to frequency. An inductor has a reactance which is directly proportional to frequency.”

I'm not singling out your comment that prompted me to post this but is also directed to other posters who seem to impose the “slash and burn” tactics to discredit-- albeit lacking discernible explanation as to why the article writer was prompted to make a statement which seem controversial at a quick glance.

To be honest, this borders on ethics protocol in the practice of the profession.

If an engineer has a “beef” on a statement of another engineer—he should take it up with him first before making statement that disagrees with him (the article writer).

That viewpoint, I think, should be upheld if we want to project ourselves as a portent harbinger of knowledge and professionalism.