DC Shunt Motor : Starting

Have a look at http://www.lmphotonics.com/vsd/vsd_02.htm

I'll take one piece:

DC motor is started by connecting to DC power supply. The armature conductor will carry the current. Base on Lorentz force "A current carrying conductor when placed in magnetic field experiences a force that tends to move it". So, all the conductors place in DC motor are subjected to these forces. These forces causes the armature rotates.

Starting DC motor is not easy. Because at the time of starting, the armature is not rotating so the back emf in the motor is zero plus the small value or armature resistance which cause an extremely high starting current if the full voltage is applied across the terminals.

In order to start a DC motor, a series of external resistor must be connect in series with mature circuit to reduce the applied voltage. When the armature begin to speed up, these external resistors are disconnected from the armature circuit.

Never apply the full voltage to armature to start a DC motor.

Can anybody explain me why is DC shunt motor not used as universal motor?

For me, it is because of its characteristic. Universal motors can be used with DC or AC power source but the DC shunt motor cannot. That is what people will not say DC universal motor.

A series field machine with commutator brush-gear on rotor, which works on a, say, 120V DC supply can have a reactance low enough to work on a 120VAC with little reduction of the stationary rotor current [hence starting torque]. But a shunt field with resistance to work continuously on a 120VDC supply will have an inductance so high that with AC you will get negligible field current, hence negligible field strength and rotor torque. The other "crunch" with the shunt field is that the inductance makes the field current lag the voltage up to 90 degrees. Instead of field current being maximum when the rotor needs maximum back e.m.f. it can be near zero. With field and rotor in series, they get the same current and cannot have fields out of phase.

@ TonyS

Thanks A lot for the link. It enchanced my knowledge.

@ Hien

Can you please explain How does characteristics of DC shunt motor not let it be a

Universal Motor?

@ 67

You rock buddy. I got it.

Well, I just can give a plain explaination is that when you reverse the voltage applied across the terminals of series motor, the field current also reverse. This keeps the direction of the torque produced unchanged.

Base on this fact, people tried it with AC power. Note that the polarity of AC varies in every half circle). Series motor still work fine even there are times during the current is reversed when not torque is really made. The average torque still can cover it and the armature will still rotate. Series motor then to be called as Universal motors.

For shunt motor, the rotation will reverse when the voltage applied is reversed. So, it cannot by use with AC power.

Hope it helps,

Mmmm..hien.nquyenquoc,. Rotation of DC shunt motor is reversed if you swap polarity of field winding relative to armature. But if you just swap supply polarity (with same field - armature interconnection), the motor still goes same way! Compare the shunt terminal connection and series terminal connection drawn below. A1,A2 are two terminals of armature and F1, F2 are field terminals. N.B. Positive supply to both "1" terminals would give standard rotation direction (clockwise looking on drive end). For shunt A1 is "bolted" to F1 and A2 is "bolted" to F2 ; for series A2 is bolted to F1.Sorry I cannot make the diagram better, CR4 keeps taking out spaces and double-spacing lines!!

Shunt:

A1 F1

A2 F2

Series:

A1

A2F1

F2

Imagine positive battery current flowing top to bottom through motor, it flows A1-A2 and F1-F2 in both shunt and series cases. If positive current flows bottom to top it reverses in both windings i.e. flow is A2-A1 and F2-F1, similarly for both shunt and series connection.

Your point about the fluctuating torque with AC [it falls to zero every half cycle at current zero]; but direction is always same and "average" does the job of turning the shaft is true.

@ 67model

Thanks a lot.

Is there any rotational magnetic field in DC Machines like we have in Induction Machines?

Does AC current flows in armature windings of Dc series motor ?

But By the best of my knowledge , Dc armature when supplied with Dc causes current in armature winding to be AC. As commutator in Armature circuit act as rectifier so Alternating current flow in the armature circuit inspite of giving DC. Does it not be the reverse case if A dc motor is given AC supply there also Commutator must act as a rectifier.

Please CR4 Help me out from this doubt.

Hi again, Classy Lass,

First question :- there MUST be a rotating field in a simple DC machine (one rotor winding, with 2 segment, each almost 180 degrees, on commutator). Because the rotor carries a current (which makes a magnetic field passing through it) and is rotating. Typical DC machines have many segments and windings, so I guess the field wobbles back and forth as the commutator steps from one winding to another, rather than rotating all round.

No time to reply further, got to read a text at church today.

Second Question:-

Have a look at http://en.wikipedia.org/wiki/Electric_motor#DC_motors for a [mostly good] diagram of a basic DC motor. The associated explanation is not very complete or helpful.

It is best to consider that the DC current from the battery cycles from minimum to maximum in a regular cycle as the rotor turns at a steady speed.

1. The field (provided by a permanent magnet or a current in a coil of wire {electro-magnet, which would, on the diagram, consist of a coil with its axis horizontal}) links through the coil on the rotor.
2. According to a basic rule of electromagnetism, the field (of the permanent magnet) linking the rotor coil induces a voltage in the rotor which is proportional to the rate of change of the magnetic flux linking it {and to the number of turns}.
3. The field magnetic flux does not change (ideally) BUT the linkage to the rotor coil changes as the rotor rotates.
4. It is easiest to imagine the linkage with the field as an electromagnet with north pole at the left of the coil and south pole at the right [so it matches the permanent magnet drawn on the diagram]. When the field and rotor coil axes are coincident, plenty of the field flux passes through the rotor coil. When the two coil axes are perpendicular, very little of the field flux goes through the rotor. And, of course, the alignment of the rotor compared to the stationary [stator] field reverses as the rotor turns. So a steady turning speed makes an alternating voltage in the rotor.
5. The trick in the motor is to swap the rotor connections over every half revolution, so that the rotor coil (coil and winding mean the same) induced voltage always opposes the battery voltage.
   
6. Because the voltage available to drive current through the rotor resistance [ the battery voltage less the induced voltage] cycles up and down in magnitude, the rotor current cycles in sympathy. But note it always goes in the same direction, thanks to the commutator.
7. One can view this as as an average battery current, always flowing in the same direction, upon which an alternating current is superimposed.

So there is an AC current in the rotor of a DC motor [shunt or series] BUT it is in combination with the DC current, not magically in place of it. Good DC motors have a low AC current - it just does not contribute to a steady torque. Series motors are more difficult to analyze, because the field current fluctuates with the rotor current (in a series motor, rotor and field currents are the same, because the same current passes through both).

I think you will now see that the idea in your "But by..... " sentence is incorrect, the armature current has both AC and DC.

Your sentence "As commutator in Armature circuit act as rectifier..." is incorrect use of "rectifier". Rectifier is reserved for when a DC voltage is got from an AC source. Also, as explained, the armature current is not just AC [in fact, it is only the DC part which contributes to turning the motor in a steady direction, the AC would just make the rotor vibrate to and fro].

Your final (question mark-less) question.... A DC motor with brushes and commutator given an AC supply works because it has an electromagnet series field which carries the rotor current. A commutator is something that switches or turns-off something at the right point in a cycle. It does not usually "rectify".

The reason what is fundamentally a DC series motor can work on AC is that, as a DC motor, it would not matter what polarity with which you connected it to the battery - it would still rotate the same direction. So if you keep swapping the supply polarity [AC] it still works (noisily, with a torque which fluctuates during each revolution, but effective). Why AC does not work well with a shunt DC motor was given in my preceding post.

Thanks for reply, it is encouraging to know a message was received and helpful. For the avoidance of doubt, please note that 1. to 7. of post #10 are about a Shunt motor, in which the field strength is fixed by the supply voltage and shunt winding resistance [but since the resistance of a copper winding increases about 39% between 0 and 100 Celsius (about "hot" temperature of a motor winding), the field strength ampere-turns is not constant in practice].

Some students get a "bad name" on CR4, because they ask for a "worked" answer to a homework problem, without any effort at explaining what they think is the answer (suggesting they do not want to put any effort of their own into getting a real understanding of the subject).

And, if hien.nguyenquoc took note of post #7, his time would not be wasted, since he would understand how to describe "how to reverse a DC motor" explicitly!

Hey, you finished me.

Oh my Godness, I have wasted my time for a student.