Inrush current and motor power

[Besoeker]

power is power
V x I, so V can be constant and I varied to vary power/torque OR I can be ~constant and V varied to achieve the same result

A couple of very basic things that misses.
Motor starting isn't a constant power application. Your power relationship is thus irrelevant.
You can't vary the voltage and current independently.

 

[Besoeker]

Generally speaking and in basic terms and elementary motor principles: The motor's rotor's rotating magnetic field induces a counter-voltage in the stator.

For the majority of 3-phase motors the rotating field is in the stator. The rotor is usually a cage. Hence the term "squirrel cage induction motor".

 

[Meterman Eng]

Induction Design Classes

Induction Design Classes

Understandablly, the stator magnetic field is rotating (because AC voltage is applied), but the rotor's magnetic field is spinning. My post before was more towards synchronous motors, but it is still relevant. Sorry to get technical, but I am a EE. Here is a good technical paper on induction motor theory: http://www.pdhonline.com/courses/e176/e176content.pdf

Looking into induction motors more, NEMA (in the US) and IEC (in Europe) classify induction motors by their characteriscs:
Design Class A: standard design, normal starting current, low slip... known to be problematic for their high inrush currents (at about 500% to 800% FLC).
Design Class B: normal starting torque, lower starting current than class A, low slip. Mostly have replaced Class B in new installations.
Design Class C: high starting torque, low starting current, low slip. Made from double cage rotors.
Design Class D: very high starting torque (275% or more of rated torque), low starting current, high slip at full load. Designed by increasing the resistance of the rotor bars.

Reference: Electric Machinery Fundamentals, Stephen J Chapman (ISBN : 978-0-07-246523-5)

 

[Jraef]

by limiting current
I suggest YOU read it, and understand it

some drives can put out full voltage and ramp current
I thought a self-proclaimed expert like yourself would know that

power is power

OK. What mechanism do you use to ramp the current whilst applying full voltage?
Be precise and succinct if you can.

solid state devices
convertors can be voltage sourced, or current sourced
we have applications where the drive holds a load/tension and modulates at +/- 1/2 Hz, the voltage is close to line, the current modulated/limited, otherwise you would essentially be at locked rotor
power is power
V x I, so V can be constant and I varied to vary power/torque OR I can be ~constant and V varied to achieve the same result
precise? like you? lol
like parsing posts and picking out one line to be corrective/confrontational and passive aggressive about? :lol: you need to lighten up skippy

now to get back to something actually useful
Laplace transform application to the study of power system transients

have fun running in circles

A couple of very basic things that misses.
Motor starting isn't a constant power application. Your power relationship is thus irrelevant.
You can't vary the voltage and current independently.

You guys are fun to watch sometimes...

Reduced Voltage Solid State Starting is ALWAYS "reduced voltage", even if you are using Current Ramp as the control methodology, so Besoeker is correct. Within the RVSS world there are many ways to skin the cat; Voltage Ramp, Voltage Ramp into a Current Limit, Current Ramp, Constant Current (Pedastal) starting, Torque Ramp, Speed (Tach Feedback) Ramp, etc. but ultimately, the BASIS for ALL of it is the phase angle firing of SCRs, which reduces the RMS voltage going to the motor. ALL of the rest of those are different only in whether or not there is a feedback loop, and what the feedback loop is attempting to accomplish. No matter what though, it remains "reduced voltage starting".

"Current Sourced" and "Voltage Sourced" would apply to DRIVES, not Starters. Drives changes the entire discussion, so lets just not go there in this thread.

 

[Besoeker]

You guys are fun to watch sometimes...

Reduced Voltage Solid State Starting is ALWAYS "reduced voltage", even if you are using Current Ramp as the control methodology, so Besoeker is correct. Within the RVSS world there are many ways to skin the cat;

Which he would/should have known from the article he cited.
But my bad. I shouldn't give the time of day when he comes up with such contradictory rubbish.

 

[GoldDigger]

Which he would/should have known from the article he cited.
But my bad. I shouldn't give the time of day when he comes up with such contradictory rubbish.

Playing the Devil's Advocate, one could contort the language to make the assertion that full line voltage is applied to the output, but it is a pulsed rather than a constant application of that line voltage.
You and I would look at that and say that the average voltage is reduced even if the instantaneous max voltage is not changed. As would any person who was using the language the way a professional in the field like jraef normally would.

By that same argument, all VFDs always output full DC bus voltage to the motor windings but control the current by means of the pulse timing. Not a useful way of looking at it for most purposes, but definitely valuable when looking at insulation breakdown.
I.E. if you never run the VFD output at more than 1/2 nominal motor voltage you still cannot use a motor with 1/2 the insulation breakdown voltage rating.

 

[Besoeker]

Playing the Devil's Advocate, one could contort the language to make the assertion that full line voltage is applied to the output, but it is a pulsed rather than a constant application of that line voltage.
You and I would look at that and say that the average voltage is reduced even if the instantaneous max voltage is not changed. As would any person who was using the language the way a professional in the field like jraef normally would.

Average, RMS or however one describes it, it's not line voltage. Line voltage isn't pulsed.

By that same argument, all VFDs always output full DC bus voltage to the motor windings but control the current by means of the pulse timing.

It actually controls the voltage and frequency/speed. The current will be whatever the current needs to be for the load at that speed.

 

[GoldDigger]

Average, RMS or however one describes it, it's not line voltage. Line voltage isn't pulsed.


It actually controls the voltage and frequency/speed. The current will be whatever the current needs to be for the load at that speed.

So a typical VFD can apply a voltage which is pulsed DC whose peak value is lower than the internal DC bus voltage? How does it do that? Making use of an internal series reactor?

I understand that by controlling only pulse duration and the interval between pulses you can produce an average voltage at any AC frequency you want to approximate (below the switching speed that is.)

 

[Jraef]

So a typical VFD can apply a voltage which is pulsed DC whose peak value is lower than the internal DC bus voltage? How does it do that? Making use of an internal series reactor?

No, I think (if I may) that what he was referring to was in the assertion on your part that a soft starter is "pulsing" line voltage. It is not. Phase angle firing of the SCRs delays the turn-on time of each half-cycle until AFTER the voltage sine wave has already begun, so the peak never rises as high in the foreshortened half-cycle as it would have, ergo the RMS is lower.

This is totally different than how a VFD works, hence my reluctance to go down that path. Conflating them can be very confusing if you lose track.

 

[Besoeker]

So a typical VFD can apply a voltage which is pulsed DC whose peak value is lower than the internal DC bus voltage? How does it do that? Making use of an internal series reactor?

The peak is the same. What the motor sees is RMS.

I understand that by controlling only pulse duration and the interval between pulses you can produce an average voltage at any AC frequency you want to approximate (below the switching speed that is.)

That's about the sum and substance of it. Switching frequency is usually in the kHz range. The motor inductance makes the current a fairly good sinewave.

 

[Besoeker]

This is totally different than how a VFD works, hence my reluctance to go down that path. Conflating them can be very confusing if you lose track.

Quite right. It is not relevant to the topic.

 

[GoldDigger]

For the majority of 3-phase motors the rotating field is in the stator. The rotor is usually a cage. Hence the term "squirrel cage induction motor".

And while the rotor is rotating, the field of the rotor is also rotating. Both the rotation of the rotor and the "dragging" of that magnetic field through the rotor material combine to produce the actual rotation speed of the rotor field.
The stator field always rotates at a speed fixed by the applied frequency and the number of poles. The rotor field does not.

 

[GoldDigger]

No, I think (if I may) that what he was referring to was in the assertion on your part that a soft starter is "pulsing" line voltage. It is not. Phase angle firing of the SCRs delays the turn-on time of each half-cycle until AFTER the voltage sine wave has already begun, so the peak never rises as high in the foreshortened half-cycle as it would have, ergo the RMS is lower.

This is totally different than how a VFD works, hence my reluctance to go down that path. Conflating them can be very confusing if you lose track.

I was using a more general application of the word "pulsed". "Gated" would probably have been a better word.
At any given moment the applied voltage to the motor is either zero or the instantaneous value of the input waveform at that time, which for some gating phase angles will include the voltage peak itself.

I think that all of us, except perhaps Ingenieur, seem to be in agreement that it is not possible (when applying a fixed frequency voltage waveform) to reduce the RMS current without reducing the applied RMS voltage, regardless of what kind of sophisticated electronics you use to achieve the result of reduced current.

 

[Jraef]

I was using a more general application of the word "pulsed". "Gated" would probably have been a better word.
At any given moment the applied voltage to the motor is either zero or the instantaneous value of the input waveform at that time, which for some gating phase angles will include the voltage peak itself.

I think that all of us, except perhaps Ingenieur, seem to be in agreement that it is not possible (when applying a fixed frequency voltage waveform) to reduce the RMS current without reducing the applied RMS voltage, regardless of what kind of sophisticated electronics you use to achieve the result of reduced current.

Ok, we agree then. However it comes about, it is voltage control even if for the purpose of affecting current.

 

[iwire]

Sahib,

You are not going to screw this thread up. I moved your questions and thoughts into their own thread here

http://forums.mikeholt.com/showthread.php?t=179311

Any further posts of yours in this thread that are not directly on the OPs topic will be removed.