Field Weakening Theory On DC Motors

After looking up on the web and here I found no good info on Field weakening on DC motors. Has any one a link or an article with a good explanation that say more than the lower the field voltage the higher the speed.

I don't have any specific links but is there something specific you want to know? You already seem to know the reason why you want to do it. Just remember you don't get anything for free.

Field weakening (decreasing the field current in a shunt wound DC motor) decreases the field flux, which decreases the back EMF generated as the armature spins through the field. Since a DC motor can't run any faster if the back-EMF equals the applied motor voltage, decreasing the field allows the motor to run faster by decreasing the back-EMF at any given speed. To a large degree the achievable speed is inversely proportional to field current. So if the motor can run up to 2000 RPM with full rated field current it can run up to 4000 RPM with 1/2 the rated field current applied. There are other loss mechanisms that may prevent getting all the way there, however.

However, a DC motor genrates torque due to the interaction of the shunt field and the armature field. If you decrease the shunt field current the armature current must increase to maintain the same amount of torque. So for a given load there is a practical limit to the amount you can decrease the field current.

Keith

Keith:
Tks for your prompt answering.
My motor data is:
Armature 163 Amps FLA, 500 VDC
Field 300 VDC @ 5.63 Amps
Base speed 650 RPM, max speed 1950.
My customer wants to work almost all the time between 650 and 1950 rpm.

My questions are as follows:

1) What is the minimun armature voltage (threshold) that field weakening should start working. My guess is that if my armature voltage is 500 VDC field field weakening should start about 400 or 450 volts.
If that is true I only have 100 or 50 VDC to control 60% of speed range and I do not like that.

2) How stable is to control speed when field weakening is operating.

Oddly enough, I was going through some old notes I had from a D.C. Drive class I had taken a couple of years ago last night. I had scribbled on my notepad... "Do not lower the field without full armature voltage, strange things will happen."
Maybe someone can elaborate.

What kind of drive is it? Any digital drive made in the past 15 or 20 years should automatically be able to set itself up for field weakening.

When using field weakening, you must have a feedback device connected on the motor. (Well, it is not absolutely necessary, and can be done unsafely without, I really advise against this).

Basically though, you will get right to 480 to 500VDC on the armature before you start weakening the field.

From 0 to base speed, with full field, the motor runs in constant torque mode. From base speed and up, the motor runs in constant horsepower mode.

As rdrast said, I would be surprised if your drive won't handle this for you on it's own if you tell it to. While the upper level concept is pretty straightforward (decrease the stator field intensity to get more speed) the details can get a bit involved. The drive ends up controlling multiple variables concurrently to get this to happen.

Make sure your load is a candidate for field weakening. You are atempting to run three times base speed. This will leave you with 1/3 of your rated torque at full speed (remember the nothing comes for free axiom). Center driven winder manufacturers can use field weakening very successfully as that is effectively a constant horsepower endeavor (for a given line speed, motor torque requirements decrease as motor speed requirements increase). For something like a conveyor, where the load is usually the same at all speeds, field weakening doesn't work as well unless the motor is already significantly oversized in torque for the application.

Keith

rdrast said:

What kind of drive is it? Any digital drive made in the past 15 or 20 years should automatically be able to set itself up for field weakening.

When using field weakening, you must have a feedback device connected on the motor. (Well, it is not absolutely necessary, and can be done unsafely without, I really advise against this).

Basically though, you will get right to 480 to 500VDC on the armature before you start weakening the field..

Click to expand...

- It´s a powerflex DC drive from AB, 100 HP.
- Yes, I´m using a encoder as a speed feedback.
- My guess was to start field weakening at 475VDC.
- I´m supposed to set it up this afternoon, i´ll let you know. My application is a slitter (steel coils) and field weakening is going to be applied on the recoiler drive.

Hi folks:
Finally I convert my drive to field weakening.
What I learned:
- Field weakening must start very close to maximum armature voltaje (500 VDC) in my case 475 VDC.
- You can not use armature feedback when using field weakening, I use an optical encoder as a feedback.
- It works great with no problem and even under field weakening range speed is stable.

Thanks for all who helped me.

Actually, widelto, when you set up field weakening in an active field regulator drive, you have speed control thru the armature voltage up to the field weakening point (in your case about 475VDC) and then speed control shifts to the field regulator with higher speeds holding the armature voltage nearly constant and acheiving overspeed by reducing the field current.

widelto said:

Hi folks:
Finally I convert my drive to field weakening.
What I learned:
- You can not use armature feedback when using field weakening, I use an optical encoder as a feedback.
- It works great with no problem and even under field weakening range speed is stable.

Click to expand...

I'm glad you got it set up. Modern drives do make the process painless, but I thought I'd comment some on your feedback point, and clarify what I said earlier.

Namely, you want a separate feedback element (tach/encoder), simply because, as you've noted, once you hit the crossover point, the armature voltage (or, back EMF) stays essentially constant. What can you regulate off of?

Way back when, the company I was with used to make purely analog 'Tachless Crossover Controls', which worked, but IMHO were much more costly and less reliable then just having a tach. Anyway, the tachless controller worked by actually using two regulators, (duh), one for Armature Voltage, and one for Field Current. The controller itself would be adjusted to hit the crossover point (ex: 480VDC on a 500 VDC motor) and then start working off of an analog calculation of motor flux as the feedback while controlling the field. 15 or 20 pots to adjust 'just so', no real good way to do a safety system... Much hate for that system.

My best reason for leaving Field Control to newer drives though, is pure safety. Seriously, you do not want to see what happens, if you are running a 650 RPM base motor, at 2500 RPM or so with field weakening, and then have an ESTOP apply FULL FIELD to the motor. Nope.. You do NOT want to see that.

On your second point, Speed control, especially nowadays, is fairly exceptional, even through full field weakening. Then again, even in the dawning of DC Machines, extended speed through field weakening was still accurate and reliable, as a DC machine is really a nice, simple design.

rdrast, I'm not sure I follow what happens under E-stop conditions as you describe. I suppose it matters just what E-stop does to the circuit but, it seems to me that, if a brake resistor is connected across the armature and full field is applied, you would get something over full torque braking. That, of course, would be adjustable with resistor sizing. What am I missing?

Another thing, there is a way to do field weakening without the complexities of the "crossover point". Simply permanently fix the field at its weakened (lower) current point. That way, the speed range of the armature voltage shifts to cover the range up to full field weakened speed. The drawback with this is that the available torque from the motor is lowered over the whole speed range. The advantage is simplicity and no speed feedback required.

The comment about a DC machine being nice and simple is lost on this oldtimer. I've always regarded working on a DC motor as courting a caged tiger! Two reasons: speed runaway (and armature explosion) with field loss and huge stored energy in the field that can produce literally a lightning bolt if the field circuit accidently opens. (I have experienced the later) Give me a nice simple AC induction motor. I know ahead of time what speed it will run (close to the frequency) and, if the motor leads are opened, a small arc is all that occurs.

DickDV said:

rdrast, I'm not sure I follow what happens under E-stop conditions as you describe.

Click to expand...

I think what is meant is that if you have a 500V motor with a base speed of 650 rpm field weakened to say... 1300rpm; the motor is running at 2 * base speed. If you then apply full field current at 1300rpm the back EMF will rise to 2* base speed voltage, i.e. 1000V What do you think might happen if the motor were running at 2600rpm? and what do you think might happen to the control electronics?

Fire extinguisher anyone?

Nick

Manglemender said:

I think what is meant is that if you have a 500V motor with a base speed of 650 rpm field weakened to say... 1300rpm; the motor is running at 2 * base speed. If you then apply full field current at 1300rpm the back EMF will rise to 2* base speed voltage, i.e. 1000V What do you think might happen if the motor were running at 2600rpm? and what do you think might happen to the control electronics?

Nick

Click to expand...

That is exactly the problem. It's just that it is a case that is often times not fully thought out by a lot of people. I've seen too many designs where the first thought was "I want to fast stop on ESTOP, so we'll DB it with full field for 5 seconds"... Except then things blow up, commutators need replacement, and in general bad things happen.

At least with feedback, you can (and should, and modern drives absolutely do) control the rate of field rise within safe limits.

OK, I see the basic problem with the CEMF rising higher than the rated armature voltage.

But, if we think about this for a minute, that condition does not occur nearly as quickly as mentioned above. The reason is that the CEMF at full base speed is not 500V. It will be 500V - (arm amps x arm resistance) which in many cases equals about 300VDC. So, with a 500V armature, you could go 1.67 (300 x 1.67 = 500) times the base speed before the CEMF exceeds the armature voltage rating.

So, yes, at double and triple base speed it could be a problem. And, no, I hadn't thought about it until you mentioned it. Thanks.

Hi folks:
Thanks to all who answered this thread.
In my application (steel coil slitter), it was almost impossible to use separates power sources for armature and field voltage, main reason is that I need to change speed very acurately on the recoiler as its diameter is increasing, this part is working great, I used a formula that I got from a thread here. For some reason my customer bought the wrong motor so I have to change my setup from base spped to field weakening, I would have preferred to use base speed and a bigger drive and motor but due to money I had to do it based on field weakening.
Rdrast, i´m going to pay close attention to your recommendation, thanks.