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A series DC motor has a high starting torque thanks to its field wiring consisting of thicker wires and fewer turns resulting in high starting torque. And a shunt DC motor do not have a high starting torque because of a field winding consisting of thinner wires and more turns.

But why couldn't wiring be made a choice depending on the use so that it does not matter which motor is being used ?

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What you propose is commonly done: motor drive systems which allow the motor to be switched from shunt to series, while the motor is in use. You use the series wiring to start the machine and then above a certain speed you switch it to shunt. This is done in electric locomotives.

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A series DC motor has a high starting torque thanks to its field wiring consisting of thicker wires and fewer turns resulting in high starting torque. And a shunt DC motor do not have a high starting torque because of a field winding consisting of thinner wires and more turns. But why couldn't wiring be made a choice depending on the use so that it does not matter which motor is being used ?


The answer to this is extremely easy. The name of this type of motor is a compound motor.

They have existed for over a hundred years. Compound motors usually use both series and shut wound excitation fields. The series winding can be removed from circuit by an external contactor arrangement's when the motor has started, if required. As with Locomotives as posted by "niels nielsen".

However true Compound motors use both winding's simultaneously and how the effects are inter-coupled has some very different effects on the motor and it's usage. Compound Motors

https://electrical-engineering-portal.com/4-types-of-dc-motors-and-their-characteristics

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Thank you for making me think about this.

In a real electromagnet, the ultimate strength of the magnetic field developed is a strong function of the amount of space filled by the wiring, the geometry of the armature, and the power dissipated in $I^2R$ losses in the wiring. If you take a motor whose field coil is packed full and rewind it with wire that has four times the area, then as long as you pack it full you'll use 1/4 the length of wire. The resistance will go down by a factor of 16 ($4^2$).

To get the same magnetic field, you'll need to drive four times the current -- which means that your "new" $I^2$ is 16 times as much as your "old" $I^2$. Since your new resistance is 1/16 as much, the overall $I^2R$ losses will be the same.

Which means -- on a basis of equal heat generated in the field winding, as long as you have a source of power for the field coils, it doesn't matter if it's series or shunt.

I think the reason that shunt windings are good for starter applications is because you get more heat in the windings than the motor can stand for long periods, but the amount of heat in the field winding is self-regulating -- it's only super-high in that (hopefully short) period of time when the motor isn't moving; once things get going, the field winding current just naturally goes down, without needing the intervention of humans or even automatic circuitry.

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  • $\begingroup$ The initial paragraphs clears things quite a bit. Please tell me: What do you mean by 'more heat in the windings than the motor can stand for longer periods'. Isn't winding a part of the motor ? And also how is heat helpful for starter applications ? $\endgroup$
    – Zam
    Aug 29 '19 at 9:06
  • $\begingroup$ Yes, the winding is part of the motor. It's an inside part of the motor. And if you dump too much heat into it, then the winding can get too hot and either burn or melt. Heat (or high temperature) is not helpful for starter applications -- it's a consequence of high current, which is what you need for high torque. $\endgroup$
    – TimWescott
    Aug 29 '19 at 14:45
  • $\begingroup$ @TimWescott sorry you are very incorrect. First The use of a low resistance shut winding is a big No. You would need high currents to produce a back EMF. This is extremely undesirable. Also expensive. Back EMF's need to be produced to regulate the speed of the motor so you would continually need high currents flowing through the field winding. $\endgroup$
    – Brad
    Sep 1 '19 at 2:51
  • $\begingroup$ @TimWescott Secondly, Shunt winding's are not good for starting, series winding's are good for starting as they are low torque. electrical-engineering-portal.com/… $\endgroup$
    – Brad
    Sep 1 '19 at 2:55

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