DC MOTOR Series Vs Shunt.

[tim_callinan]

Hello Folks,
I hope you are keeping well. Im a mechatronics instructor at a community college and you lads have helped me bigtime in the past. I'm teaching a little module on DC motors in a electric motors course and Im planning on using these ancient DC motor trainers I was given. I made a quick video of me playing around with it and I had some questions. If you have 5 mins spare - could you have a look and see if Im on the right direction please?
https://www.youtube.com/watch?v=x_sp5EXj4ng
Thank You,
Tim Callinan

[rdrast]

Your shunt configuration is wrong, actually. A shunt field is powered continuously, from a separate source from the armature.


Series was fine.



It's all about how the magnetic field for the armature is created.



For larger motors, a series field and a shunt field are wound very much differently.



In Series, the field winding has to be able to handle full armature current. A Shunt Field is typically much smaller wire, with many more turns. Series Field motors were most often used as traction motors, as the field coil would develop a very strong field at zero speed, with a huge amount of current, but would eventually moderate the field strength as the motor speed (and back EMF) increased. A series field motor can generate a LOT of torque at standstill. The ability to weaken the field in a controlled manner really doesn't exist, but could be done in a limited (and expensive) fashion with different field taps.


The Shunt Field motor has a relatively low field current, but again, with so many windings, it doesn't need to be very high current wise, and the field strength remains constant (more in a minute) no matter what the armature load is. They don't have the standstill pullout torque of a series field machine, so many larger shunt wound motors had an internal, smaller (magnetically) series field, to provide boost torque at low speeds. These worked fine, for many applications, but not all, as reversing the motor is an issue. Swapping the armature polarity would result in the series field partially cancelling out the shunt field. So did reversing the shunt field partially cancel out the series field.



In most cases, for modern (newer than 30 to 50 year old) DC Machines, it is generally just an armature and a shunt field, which can still deliver full rated torque down at standstill.


A shunt field, also allows for an easy method of running above the base full speed of the motor by reducing the field current. Base speed is the point where the motor essentially switches over from a constant torque machine to a constant (horse)power machine.


Two things to never do with a DC Motor: Never run unloaded with zero field. Residual magnetism can make it accelerate destructively, and Never just disconnect the wires on an energized shunt field. The resulting arc is Very Impressive, due to the stored energy in the field.

[Mickey]

https://www.electrical4u.com/series-...-series-motor/

https://www.electrical4u.com/shunt-w...c-shunt-motor/

[tim_callinan]

Thank you both for those answers! My feeling is - i could be totally wrong here. Is series vs shunt types of motors - are not that common anymore. and its only with very large DC motors. Like I couldnt find a single DC series/shunt motor in McMaster Carr. 2 HP motors were still using permanent magnets. I suppose my instinct is not to get too deep into series vs shunt and concentrate more on permanent vs field windings. I can buy cheap scooter DC motors and I think these are all using permanent magnets - having them wire these up to a controller, etc would be a good lab.

[gas]

Depends....
If in your local most industrial plants are recent then yes you are correct.

I have been in areas where that old stuff is still running and making money for the owners. They are not about to interrupt that revenue stream until they have to.


That is a good question for your curriculum advisory committee.

[magdoleme]

rdrast answer is correct except for the function of the small series field usually found in larger (say bigger than 10 HP) dc motors. A shunt field dc motor is capable of full nominal torque in standstill even at rest, once its shunt field is powered at nominal current. The reason a series field is used is to compensate for the armature reaction on the magnetic flux of the motor poles (magnetic steel structure surrounded by the field coil). The current of armature wiring creates a magnetic field that can partially saturate the field magnetic circuit, decreasing the overall magnetic flux of the shunt field. The extra flux generated by the series field is to compensate this flux loss (and then, the torque loss).
If we design the motor using more magnetic steel we may reduce this loss, but that will also increase the motor size and the costs.
Playing with dc motors is very dangerous if you don´t know how to (as you demonstrated in your video). As mentioned by rdrast, it may reach very high speeds if the shunt field is not powered or have a current much lower than the nominal. At these speeds it nearly blows away.
Using permanent magnetic motors is a good idea since you do not have this safety problem.

[hagos]

Rdrast

My understanding is that the shunt winding is excited from the same source of the armature.

The seperately excited winding is excited from an other source

[hagos]

Seperately Excited D.C Motor wiring diagram

[widelto]

Same power source for field and armature was used in the 1930's aprox., when there was no speed variation , I remember having seen the still in the 1980's. Normally 240 VDC.
When speed variation was used , field and armature had different power sources, speed could be changed by changing armature voltage. If field voltage was changed, that was called field weakening, this also changes speed.

[magdoleme]

The first shunt field wiring show in Hagos drawing is not possible with industrial motors, since they have a very low armature (rotor) resistance, and the starting current would be around 15 to 20 times the nominal current, well above the commutator capacity. Maybe with very small motors (battery powered) that works, I don´t know. In very old times, motors with this kind of connection would have several series resistors connected with the armature circuit for starting, which would be short-circuited in steps to limit the current as the motor speeds up and generates the back electromotive force (“voltage”) to oppose the applied voltage.
The armature motor current “I” is generated by the difference between the applied voltage “V” and de generated voltage “EMF” divided by de armature resistance “R”:
I = (V – EMF)/ R
The EMF exists only when we have field flux “F” (created mostly by the field current) AND rotation (RPM), and depends on the motor design (represented by “k”):
EMF = k . F . RPM
The second Hagos´ drawing shows the typical wiring of a dc generator connect to a load. The dc generator is a dc motor whose shaft is driven by some external rotating device (typically, at constant speed - by an ac induction motor, combustion motor, etc.), and produces output voltage based on the EMF equation above. When rotating, its voltage is produced by the flux of the shunt field. Controlling the field current controls the armature output voltage.
These explanations are simplified for understanding, and do not consider the voltage drop in the armature circuit, for instance.
Finally, when d.c. motors are used in industry, it always uses a separate shunt field power supply.
Today, and in the last decades, the armatures of these motors are fed by thyristor static converters capable of bidirectional power flow.

[magdoleme]

DC drive and motor

[hagos]

Thanx Magdoleme for the valuable informations,

I'm an old folk,my drawing goes back to the era of Edward Huges where we have graduted in the year 1982.

Once again many thanks
Hagos

[magdoleme]

You are welcome, Hagos.
I´m not a young guy as well. Graduate as an Electrical Engineer in 1973 ...
There was still many electronic tubes around ...

[tim_callinan]

Thank you for all the replies. This is a great forum and its made my life as a teacher a lot easier. I needed to think about the replies in this thread. It seems like series vs shunt is not that common in fairness. I feel if I was going to spend time - brushed vs brushless DC motors would be more practical and realistic for them? Anyway thank you for your advice and time.
Tim