Magnetising current

[andym79]

Hi,
Next week I will be setting up a VFD. Given the application I would like to set it up in sensor-less vector fluxing mode.
The major problem is the nature of disengaging the motor makes highly time consuming; so running a rotating auto-tune is not really an option.

In order to auto-tune the vfd in stationary mode, I need the to know the magnetizing current. This is not listed on the nameplate, the manufacture not longer exists and I am not sure how you could calculate magnetizing current?

Any ideas?

How does magnetizing current vary from locked rotor current?

These are the details on the nameplate:

Supply Voltage: 3 ph/ 415V
Motor voltage: 300V
Locked rotor current:57A
Motor RPM: 715
Full load current: 61A
Number of poles: 8

 

[mike_kilroy]

locked rotor current LESS than rated current? seems like a typo.

assuming the 61a rated is right, just stick in 20-30 amps for isd: I would put in 25amps. if it doesnt tune well tweak up or down a tad; shouldnt make too much difference if in right area.

 

[Jraef]

I'm very curious about the motor voltage being 300V, I've never seen that. Is this a typo that supposed to be 400V? That would make more sense.

Mike kilroys suggestion seems about right, but getting that grossly wrong affects mostly the performance at the lowest speeds. Is that what you are after? Your motor is already slow to begin with, how slow do you intend on trying to get to? The reason I ask is that this appears to be an old motor ("mfr no longer exists...") and running old motors that were never designed for VFD operation is a tricky proposition at best, running them at critically low speeds is probably not a good idea at all unless you can cool them separately.

 

[Jraef]

locked rotor current LESS than rated current? seems like a typo.
...

Probably dropped a digit... 357A might be a better number on a motor with that many poles.

 

[mike_kilroy]

Just for kicks, why not tell us who this mystery mfgr is; especially since they no longer exist it can't come back to haunt you for telling.....

Someone here may have the motor specs you want....... For instance, a lot of old reliance high speed spindle motors were like 260vac @ base speed, Indramat ones even 160v @ base speed... There is a reason for this but you do not give us enough info to know if YOUR motor has good reason for this lower base speed voltage or not - like application and what the max speed the system needs to go....

And with this additional info we can maybe help you to know whether you even should put it on a vfd - some old ones will smoke rather quickly if you do as Jraef alluded.

 

[templdl]

Older motors generally have a magnetizing current of up to 13x the FLA where the newer energy efficient motors can run up to 17x the FLA.
A good rule of thumb is to look at the allowable MCP magnetic settings in NEC art 430-52.

 

[GoldDigger]

Older motors generally have a magnetizing current of up to 13x the FLA where the newer energy efficient motors can run up to 17x the FLA.
A good rule of thumb is to look at the allowable MCP magnetic settings in NEC art 430-52.

Are we using two different definitions of magnetizing current?
The definition I am used to is that the magnetizing current is the reactive portion of the total load current, and will be strongly dependent on motor speed. During starting the magnetizing current is very large compared to the resistive current. At normal operating speed, the magnetizing current will be relatively constant with respect to the load on the motor and only a fraction of the FLA. This allows for a power factor which gets closer to 1.0 as the motor is more and more heavily loaded, although it will not exceed some maximum value which is dependent on the motor design.

I could see the sort of relationship you propose between the LRA and the FLA, but not in general between the magnetizing current and the FLA.

So is the value you need to enter for magnetizing current when configuring the VFD the starting magnetizing current (i.e ~LRA) or the full speed magnetizing current?

(Quick reference: http://ww1.microchip.com/downloads/en/appnotes/00887a.pdf)

 

[Besoeker]

Older motors generally have a magnetizing current of up to 13x the FLA where the newer energy efficient motors can run up to 17x the FLA.

Maybe a few cycles of inrush when connected direct on line.
This application is for a VSD. Different starting conditions.
I think, as others have stated, what is wanted here is the current that produces the stator field. The current in the Rm and Xm branch of the equivalent circuit.

In my experience, this is generally around 30% of full load current.

 

[Besoeker]

I'm very curious about the motor voltage being 300V, I've never seen that. Is this a typo that supposed to be 400V? That would make more sense.

I wondered about that too. The name of the manufacturer might give a clue.

Your motor is already slow to begin with, how slow do you intend on trying to get to? The reason I ask is that this appears to be an old motor ("mfr no longer exists...") and running old motors that were never designed for VFD operation is a tricky proposition at best, running them at critically low speeds is probably not a good idea at all unless you can cool them separately.

Agree. Running it on a modern VFD and any speed is likely to be problematic unless it was specifically designed for VFD operation.
Excessive dv/dt and peak voltages would probably kill the insulation PDQ.

 

[templdl]

Yes, I see what you guys mean when there is a vfd involved controlling the motor. If you had an across the line starter'ot would be entirely different which I refered to which is not applicable here.
Even a soft start would be different.

 

[Jraef]

Older motors generally have a magnetizing current of up to 13x the FLA where the newer energy efficient motors can run up to 17x the FLA.
A good rule of thumb is to look at the allowable MCP magnetic settings in NEC art 430-52.

I think you are confusing magnetizing INRUSH CURRENT with regular magnetizing current when the motor us already running. The VFD vector algorithm needs to know how much current the motor needs just to make it into a motor, which is the current consumed just in keeping the iron magnetized. It needs that so that it can account for it in determining the torque producing current vector.

 

[templdl]

I think you are confusing magnetizing INRUSH CURRENT with regular magnetizing current when the motor us already running. The VFD vector algorithm needs to know how much current the motor needs just to make it into a motor, which is the current consumed just in keeping the iron magnetized. It needs that so that it can account for it in determining the torque producing current vector.

Understood.

 

[andym79]

I think you are confusing magnetizing INRUSH CURRENT with regular magnetizing current when the motor us already running. The VFD vector algorithm needs to know how much current the motor needs just to make it into a motor, which is the current consumed just in keeping the iron magnetized. It needs that so that it can account for it in determining the torque producing current vector.

That is exactly what I was asking, "how much current the motor needs just to make it into a motor"!

I had a look at this job briefly on Friday afternoon, I was so distracted by many other obvious issues, too small an earth wire to the drive; no circuit breaker between supply and VFD; 240V, 12V and 24V control circuits all squeezed into an undersized enclosure,with unnecessary relays to change voltage; no motor choke; no dynamic braking resistor; a genset highly unsuitable for the motor and a few other code infringements. So unfortunately I didn't focus on the motor as much as I should have!

Here is some background to the installation. The application is a railway traverser, so there is a very high inertia, and a lot of mass to stop. The motor is a 1946 slip ring motor, originally controlled by three resistor banks. The set up was modified in 1993, the slip rings being shorted, but not at the rings by brazing in between, rather at the terminal block, so the brushes are still in circuit! The drive was then used with a solid state starter, with a reverse and forward, as well as a shorting contactor. The solid state starter setup wasn't satisfactory in terms of speed control. All of this happened whilst the railways were run by the government, so there were log books and wiring diagrams to see the history.



In 2009, due to safety concerns the overhead supply, which was 63 years old was removed and someone installed a 40KVA genset and VSD. As I described there were many obvious defects with the installation. The under powered genset as well as no braking resistor or choke finally laid waste to the 4 year old VFD last week.

I have sent the unit off for repair, will be installing resistor and choke, mcb, an 80kva genset and bringing the setup up to code. I do have reservations about: Leaving the existing motor is place and if it is, using the ramped function for braking; would coasting be better, how will a dc current effect a motor like this?

Actual details on the nameplate were:
Rotor Current 57A (not locked rotor current)
Rotor Voltage 300V

 

[Besoeker]

Well, given that it's a slip-ring motor and its age I would be disinclined to keep it if you want variable speed.
The torque characteristics of the SR motor are usually quite different to that of a cage motor.
And then there is the insulation which will not have been designed to tolerate the output of a VFD.

We have made speed controllers for slip ring motors but these are slip-recovery drives. The stator has rated voltage and frequency applied to it. Control is on the rotor side. Normally this is by rectifying the voltage and feeding the resulting DC back to the supply, hence the "recovery" term.
The rotor voltage and frequency start at maximum (300V, 50Hz for your motor) and reduce linearly to zero at maximum speed.
So, the control method is quite different. The chief merit of this type of variable speed drive is efficiency particularly for centrifugal fans and pumps.

I'm of the opinion that it isn't the best system for your application and you would be better to change the motor for a modern cage motor if you are going to use a variable frequency drive

 

[Sahib]

The torque characteristics of the SR motor are usually quite different to that of a cage motor.

The rotor terminals are shorted here; no significant change in torque characteristic.

And then there is the insulation which will not have been designed to tolerate the output of a VFD.

Put in some 5% reactor to stop the surge from reaching the motor terminals.

We have made speed controllers for slip ring motors but these are slip-recovery drives. The stator has rated voltage and frequency applied to it. Control is on the rotor side. Normally this is by rectifying the voltage and feeding the resulting DC back to the supply, hence the "recovery" term.
The rotor voltage and frequency start at maximum (300V, 50Hz for your motor) and reduce linearly to zero at maximum speed.
So, the control method is quite different. The chief merit of this type of variable speed drive is efficiency particularly for centrifugal fans and pumps.

It is not so much efficiency as smooth speed control.

you would be better to change the motor for a modern cage motor if you are going to use a variable frequency drive

It does not appear to be an economical suggestion when it is possible to VFD the old motor with other cost effective devices such as a 5% load reactor.

 

[Besoeker]

The rotor terminals are shorted here; no significant change in torque characteristic.

Yes, I'm sorry I forgot about your in-depth technical expertise and and extensive experience relating to the application of induction motors.
But you are just plain wrong in this instance, old chap.

Let me explain some of the basics to you.
A cage *motor rotor usually has to be designed for DOL starting torque. Rotor resistance design has to take this into account. Higher resistance rotors produce more more torque, all other things considered. Some designs for high starting torque have two cages at different depths. I've heard of, but not had experience of, profiled rotor bars to give a single bar that has similar characteristics to a double bar.

By way of contrast, a slip-ring motor has a wound rotor with the windings connected to slip rings and brought out to terminals for connection to the outside world. Starting is usually by stepped dry resistances or sometimes a liquid resistance starter for some older installations. They are not designed for DOL starting - why would anyone go to the extra complication and manufacturing costs of bringing out the windings to slip rings if they are just to be shorted at the terminals. Not to mention the ongoing maintenance of brushgear.

It is not so much efficiency as smooth speed control.

Sorry.
Wrong again, maharajah.
We offer both variable frequency inverters (VFI) and slip-recovery drives (SRD).
We regularly manufacture and supply both types and, and where applicable, offer both options at the bid stage.
There are pros and cons for both. Efficiency is quite often the deciding factor.
Smooth speed control is a given for both systems.

I'm not prepared to get into an argument about these facts.
If you have nothing constructive to add, add nothing.

 

[Jraef]

I'll just add that if you DO decide to re-use the existing Wound Rotor motor with a VFD, have the rings shorted on the rotor. Brush wear and the use of a VFD in vector mode (to get the torque) can srew up the VFD performance significantly.

But I too think you should consider replacing the motor. A sorted WRIM has a very different torque/speed characteristic compared to a standard SCIM; high amps per unit of torque. A good vector drive is a great way to control that, but the age of the motor still gives me concerns. You can mitigate the effects, but by the time you remove the motor, send it out to have the rings grazed and balanced, then add the necessary mitigation devices to re-used that old motor with old insulation and no bearing grounding, you might be able to just get a new higher efficiency inverter rated motor and not have to look at this again for the rest of the decade.

PS: for this kind of application, it would HIGHLY recommend a line regerative VFD as opposed to dynamic braking resistors. More money for the VFD, less loss of production when the resistors burn themselves out.

 

[andym79]

PS: for this kind of application, it would HIGHLY recommend a line regerative VFD as opposed to dynamic braking resistors. More money for the VFD, less loss of production when the resistors burn themselves out.

True! But don't I need a mains supply rather than a generator for a line regerative VFD to be useful?

 

[Sahib]

True! But don't I need a mains supply rather than a generator for a line regerative VFD to be useful?

Then a line regerative VFD may cause problem to your generator such as over speeding.

 

[Sahib]

Yes, I'm sorry I forgot about your in-depth technical expertise and and extensive experience relating to the application of induction motors.
But you are just plain wrong in this instance, old chap.

Let me explain some of the basics to you.
A cage *motor rotor usually has to be designed for DOL starting torque. Rotor resistance design has to take this into account. Higher resistance rotors produce more more torque, all other things considered. Some designs for high starting torque have two cages at different depths. I've heard of, but not had experience of, profiled rotor bars to give a single bar that has similar characteristics to a double bar.

By way of contrast, a slip-ring motor has a wound rotor with the windings connected to slip rings and brought out to terminals for connection to the outside world. Starting is usually by stepped dry resistances or sometimes a liquid resistance starter for some older installations. They are not designed for DOL starting - why would anyone go to the extra complication and manufacturing costs of bringing out the windings to slip rings if they are just to be shorted at the terminals. Not to mention the ongoing maintenance of brushgear.

My reply to your statement colored red ( all other statements appear noise to me) is that a slip ring motor may be started with its rotor winding shorted when it has a VFD on its stator circuit or other voltage reducing starter to avoid unacceptable current peaks.

Sorry.
Wrong again, maharajah.
We offer both variable frequency inverters (VFI) and slip-recovery drives (SRD).
We regularly manufacture and supply both types and, and where applicable, offer both options at the bid stage.
There are pros and cons for both. Efficiency is quite often the deciding factor.
Smooth speed control is a given for both systems.

I'm not prepared to get into an argument about these facts.
If you have nothing constructive to add, add nothing.

The drive's purpose here is smooth speed control and for purchase option, efficiency is also a factor to consider.