Starting torque of series vs shunt DC motors

Question

Recently, I read that starting torque of series motors are much higher than shunt motors.

I try to find out what is reason behind it and I get to know that this is because in DC series motor torque is proportional to square of armature current (and hence we get maximum torque at starting because armature current is maximum at that instant) because field current is same as armature current.

While in DC shunt motor torque is proportional to field and armature current but in this case value of field current is very less (because we take resistance of field winding very high) hence we can say that starting torque is proportional to armature current only.

But there are few points which I still don't understand. Hope someone can help me!

1. Why we keep high resistance of field winding in shunt DC motor, isn't low resistance of field winding in shunt DC motor can provide same benefits as of series like high starting torque?

2. Why we don't consider inductances of field winding for starting torque calculation (while I think inductance value must be high as it is wrapped on a ferromagnetic material) for starting torque calculation?

And isn't starting torque should be zero​ in case of series and shunt motor as Inductances of field winding doesn't allow current and hence starting torque is zero?

Answer 1

while in DC shunt motor torque is proportional to field and armature current but in this case value of field current is very less ( because we take resistance of field winding very high )

Not quite -- a typical shunt-wound motor has more turns of wire in the field to go with that higher resistance. Since the field is proportional to turns * current, and smaller wire is both higher resistance and smaller, it's easy to wind the coil such that for the same amount of power dissipated, the generated field is the same.

1.Why we keep high resistance of field winding in shunt DC motor , isn't low resistance of field winding in shunt DC motor can provide same benefits as of series like high starting torque ?

Power dissipation and convenience. In a series-wound motor you want most of the voltage available to the armature -- so you wind the field with a short length of thick wire.

For series-wound motors that are starting up, the start-up event is -- hopefully -- not going to be happening for long. So you can dissipate more -- potentially lots more -- power in the field coil during start than you could continually.

2.Why we don't consider Inductances of field winding for starting torque calculation (While I think Inductance value must be high as it is wrapped on a ferromagnetic material) for starting torque calculation ?

Because the electrical time constant of the coils is very much shorter than the amount of time it'll take the motor to spin up. So it can generally be ignored.

And isn't starting torque should be zero​ in case of series and shunt motor as Inductances of field winding doesn't allow current and hence starting torque is zero?

Technically yes, but again, the time L/R time constant of the coils is very much lower than any mechanical time constants, so you can almost always ignore the electrical dynamics.

Answer 2

ad 1) You can have a very high current and only few turns, or very tiny current an lots of turns to produce the same amount of excitation field. For the sake of simplicity, the excitation is made of winding with lot of turns, that has a resistance that limits the field current. So you do only need to rectify the mains voltage and you have a fairy stable excitation current without any current controller.

ad 2) The inductance of the excitation winding has no meaning, as it has to provide only the excitation. In series motor this winding is somehow different as it has low nr. of turns and high current VS. shunt motor that has large nr. of turns and small current.

The armature current ramps up with L/R constant, which is low, therefore when switching on the supply voltage the torque is practically immediate - looking more precisely the torque starts from zero and it reaches its value very fast.

Answer 3

A low resistance shunt is simply a waste of power. So for a shunt wound motor, a high impedance field winding makes perfect sense.

If you need high starting torque in a shunt motor, add a series winding : during start, the very high starting current adds to the field strength and the starting torque. This was known as a compound wound motor.