Losing field current in a DC machine 4

[blunt80fi]

Hello!

I'm simulating a DC Motor with matlab. I have this problem with simulating the situation where the field current is lost when the machine is running in steady state. My results show that the armature current becomes very high and the rotation speed drops to zero. But what should happen with the voltages and torque? I have very little experience with these kinds of machines, and I get some results for these also, but they don't seem to be reasonable. I would be very happy for all tips and information anyone can give.

Thank you.

 

[edison123]

While you (or your program) are right about high armature current due to field loss, you have got it totally wrong about the speed. When you lose the field, theoretically, the speed is infinity and in practice, the motor will reach the speed whence it will destroy itself in the absence of protection to prevent such events.

 

[mcdm]

Be careful loosing the motor field can be dangerous
The three main types of DC motors are: Permanent Magnet, Shunt wound, and Series wound.
Permanent Magnet
These motors have a permanent magnet which generates a magnetic field. Rotating in this field is the armature through which current is passed. The armature will be wound from only a few turns of quite thick copper, giving it a low resistance. When a voltage is applied this will draw a very large current from the battery. When the motor starts to rotate the current will drop due to an effect called "back EMF". This is the voltage generated by the armature as if the motor was a dynamo. For example for a motor running at 12V with an armature resistance of 0.1 ohm, at start up the current drawn by the motor would be 120A (known as the stall current). The action of the armature wires rotating in the field may generate a back EMF voltage of 11V giving a total drive voltage across the motor of 1V. This would give a current of 10A. If load is applied to the motor the speed of rotation would drop, this would also cause the back EMF voltage to drop, and hence the drive voltage to the armature to increase. This increase in armature current would then cause the motor to try and speed up. i.e. the motor will try and maintain a constant speed.
Shunt wound
These are very similar to a permanent magnet except the magnetic field is supplied by passing a current through a fixed coil positioned around the armature. Operation is exactly the same as the permanent magnet motor, except these motors also have the possibility of field current control. This is where it gets interesting (or very boring). If the field current is reduced the motor speeds up. This can be explained by considering the example above. If the field current is reduced the magnetic field will drop causing the back EMF voltage to drop. This increase in the drive voltage to the armature will cause the motor to speed up until the back EMF is once again at its previous level. The penalty for this speed increase is loss of torque - with such a low field magnetic field the motor will be quite weak. However, by using a high field current the motor will turn quite slowly but with very high torque - the penalty here being potentially wasting current in the field when the torque requirement is low.
Series Wound
In a series wound motor the field coil is in series with the armature coil. The field coil is wound from thick wire to cope with the large armature current and has a resistance of only a few ohms. The advantage of this type of motor is its high torque output. Consider a series motor running with a given load. If the load is now increased the motor will slow causing the back EMF to drop. As with the previous type of motor the current drawn will be the difference between the voltage applied and the back EMF produced. This large increase in current will cause both the armature and the field magnetic strength to increase, resulting in a very high output torque. Hence this motor is used in high torque situation i.e. starter motors. The disadvantages are the large current drawn will soon flatten batteries, and if run for prolonged periods the motor will soon overheat. The other point that should be noted is the motor without substantial modification will only run in one direction as reversing the armature current will also reverse the field current.

 

[sreid]

The reason that a shunt wound DC motor runs away if the field is lost is that there is residual magnatism in the field iron.

 

[aolalde]

For a DC motor, in the armature circuit:

VA = E + IA*RA

VA= line voltage applied to the armature through the brushes.
IA = armature current
RA = armature resistance
E = induced voltage in the armature
E= k*Phi*n
k= winding constant
Phi = Field flux
n = rotational speed

Then; VA = k*Phi*n + IA*RA

Simplifying for n;

n = (VA-IA*RA)/ (k*Phi)

From the last equation, when Phi becomes 0 (lost of field), the denominator is zero and then:

n becomes infinity

 

[eemotor]

Despite what your simulation may suggest, it is important to note what actually occurs as a result of a DC motor loosing it's shunt field. In compound DC motors, if the shunt field is lost, there is little to restrict the armature from reaching extremely high speeds. As a result of a lost shunt field, the armature and the series field see a tremendous increase in current. I witnessed a demonstration in a lab where the shunt field was removed just momentarilly (for about 2 seconds) on a small compound DC motor (10 hp). The speed increased from about 2000 RPM to over 5000 RPM in that short time. I've also read of a study that was done on a DC motor with its shunt field removed. The people performing the experiment let the motor run away. The result was commutator bars stuck in concrete walls. Offcourse, the experiment was conducted remotely for everyone's safety but the experiment proved the destructive forces that removing a shunt field on a compound DC machine can have.

 

[starkopete]

As eemotor described, the only thing limiting a DC motor in runaway is it's structural integrity. That is why many controllers incorporate a field failure relay to open the armature circuit in the event of a field loss.

 

[cbarn24050]

hi blunt, your simulation is correct. No field means no torque, so the motor stops. No speed means no backemf so the armature current rises untill limited by the armature resistance.

 

[ScottyUK]

hi cbarn,

You are theoretically correct and possibly are correct for a brand new motor that has never been energised, but the motors we have to live with in the real world almost always maintain some small residual magnetism in the core even after the field current has decayed to zero. The result is the terrifying scenario described by eemotor where the armature current increases to a very high value and the speed increases very rapidly. An accurate model would take in to account the residual magnetism of the armature, but I think this would be a challenging thing to model accurately (for me at least!). Best of luck to blunt80fi if he chooses to extend his model to account for this factor.



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If we learn from our mistakes,
I'm getting a great education!

 

[cbarn24050]

Hi scotty, I'm afraid you are completly wrong. You could just try it, then you would see for yourself.

 

[aolalde]

I have repaired lots of (more than twenty) DC Motors large and small, with the armature rotor totally destroyed due to the centrifugal forces developed when the field was lost and the armature voltage was not removed immediately.

 

[ScottyUK]

hi cbarn,

I've seen the results of this on a medium sized motor (~200HP) and, believe me, trying it is among the last things I intend to do.

I'm often wrong, although on this occasion I don't think I am. Assuming I am wrong, please explain how you account for the real-world experiences of several people above who have all witnessed the destruction of series-wound machines caused by loss of field current? What is the real mechanism behind the motor accelerating to several times rated speed prior to the armature disintegrating?



------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[aolalde]

Thinking of the mechanism that makes the DC motor speed to increase in spite of Torque (T=B*L*i) apparently being zero.

When the machine is running, the magnetic circuits handle high flux densities leaving a residual magnetic flux (B). This residual flux is small but combined with the very high armature current ( i ) still produces a high torque (T).
Certainly the load speed-torque demand could restrain acceleration, but in some applications destructive speeds are still reached. If for some reason the motor looses the load restrain , surely the speed reached will be very high.

 

[aolalde]

Another thought is a performance similar to a switched reluctance motor. Here the forces from a magnetic field (now produced by the large armature current) react against the iron in the salient poles. The field produced in the armature continually chases the poles due to the commutator switching when each bar rotates.

 

[DickDV]

Many industrial DC motors are have compound-wound fields. By that I mean that the F1 and F2 leads energize a shunt field which is arranged physically inside the motor so that a much smaller coil laid over the top of the shunt field carries armature current. This is a series field and boosts the field strength of the shunt field in the forward direction and bucks the field strength of the shunt field.

Any motor with this configuration cannot fully loose its field since the armature current will always provide some even if the shunt field is totally de-energized.

While I have never experienced a runaway based on residual magnetism alone and certainly don't want to, it seems to me that the torque would be so small that almost any connected load would limit speed. In the case of motors with shaft fans (ODP and TEFC enclosures), I would think the fan load alone would limit speed.

Of course, with compound wound motors, torque would likely be much higher than with residual magnetism alone and destruction would occur quickly with rapid acceleration.

If I am overlooking something with regard to residual magnetism and torque, I would welcome someone explaining a more correct understanding of it.

 

[BobM2]

Blunt80fi - Does the motor speed decrease as you drop the field current? If so, there is something major wrong with your model. If the speed increases as the field current drops then your model would seem to be working correctly. How does Matlab handle the divide by 0 problem (0 current)? It would be very hard to predict the top speed as it is a function of the torque load on the shaft (windage, friction) as well as the electrical losses (IR drops through the armature winding, the brushes, the brush/commutator interface, eddy current losses and hysteresis losses). The residual magnetic field left in the field can also be difficult to predict if it is influenced by the rapidly changing magnetic field of the rotating iron. All of these factors will combine to produce a maximum speed. Depending on the construction of the motor, it may or may not be able to handle the high speed without destroying itself.

 

[cbarn24050]

Hi scotty, your right about a series motor this is because it doesn't lose all its field current. A pure shunt motor (the usual type these days) will always stop with no field. You cannot run a motor on residual magnetism the reason is because the armature field magnetises the field poles to align with it. If you try to turn a motor in this condition you can feel the magnetic forces applying a braking torque. Read the setting up instructions on a dc motor controller, note the part where you run the motor with no field in order to set the armature current limiter, if you had ever done this then you would know that the motor remains still.

 

[charlierod]

Some time ago i have the same confusion as cbarn. How can an electromagnetic torque be developed with zero field current?. Real world experience convinced me, i've seen dc motors experiencing uncontrolled acceleration and armature currents blowing fuses due to field lost. I tried to understand the phenomenon as follows:

T = K*(flux)*Ia

where

T = electromagnetic torque
K = motor constant
flux= flux produced by field current or remanent flux
Ia = armature current


Now, the decay in the term flux is compensated by an increase in armature current the overall effect being a higher torque.
Any mistakes in this reasoning?

 

[ScottyUK]

I've just realised I've had a momentary brain lapse and written series-wound when I should have written shunt-wound in my last post responding to cbarn. it changes the meaning somewhat - losing the field of a series-wound machine would break the armature circuit. Oops!

There you go cbarn, there's one of the mistakes I was talking about making!!





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If we learn from our mistakes,
I'm getting a great education!

 

[BobM2]

Charlierod - The increase in current will be less than the decrease in the flux. A high shaft speed does not mean that there is much torque available. The shaft speed will increase until the torque produced by the magnetics just balances the frictional and windage torque loads. At that point the speed will reach an equilibrium but there is no available torque to drive a load. When starting a shunt motor with zero field current the shaft may not rotate if the torque from the residual magnetism is not enough to overcome the friction.