Shunt DC motor question

[gar]

190113-1111 EST

This DC motor question is for those that do not know the answer.

If you have a shunt wound DC motor that has no mechanical load, not compound wound, and the shunt field current is lost, goes to zero, then what happens to motor speed? Why?

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[Besoeker]

190113-1111 EST

This DC motor question is for those that do not know the answer.

If you have a shunt wound DC motor that has no mechanical load, not compound wound, and the shunt field current is lost, goes to zero, then what happens to motor speed? Why?

.

Most DC motors that I have dealt with are compound wound. I know what happens but I'll leave for others.

 

[dpeter]

Anything to do with a Suicide circuit?

 

[gar]

190113-2511-EST

So far I have to conclude that the average electrician has no idea how a shunt wound DC motor works.

dpeter I am not really ignoring you. Just appears that way.

I would have thought that the basic concept of a DC motor would have been taught in high school physics. If one knows the basic concept, then a little thinking might at least produce a response. It does not matter whether your answer is correct or not.

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[Tony S]

Anything to do with a Suicide circuit?

In a way yes if suicide results in blue smoke and flames.

It could be worse, I tried running a series wound motor with no load. It didn’t end well.

 

[Sahib]

When field flux reduces, counter EMF tends to reduce and so armature current increases and so is speed. The speed can thus increase to dangerous level before there is no more interaction between field flux and armature current as the field flux has become zero.

 

[Besoeker3]

When field flux reduces, counter EMF tends to reduce and so armature current increases and so is speed. The speed can thus increase to dangerous level before there is no more interaction between field flux and armature current as the field flux has become zero.

Not likely to do that in a plain shunt motor.

 

[Besoeker3]

In a way yes if suicide results in blue smoke and flames.

It could be worse, I tried running a series wound motor with no load. It didn’t end well.

One of my fellow students did that. It's a lesson you don't forget.......

 

[rlundsrud]

190113-2511-EST

So far I have to conclude that the average electrician has no idea how a shunt wound DC motor works.

dpeter I am not really ignoring you. Just appears that way.

I would have thought that the basic concept of a DC motor would have been taught in high school physics. If one knows the basic concept, then a little thinking might at least produce a response. It does not matter whether your answer is correct or not.

.

Well not all of us went to a fancy high school. I didn't go, I still speech goodly and I can hook wires to the holes in the wall.

 

[__dan]

I had one semester of motor and transformer design but that was 3rd year university. Your point is a good one. The effects could be taught hands on at any age and starting early is a good bet.

The armature of a DC motor may be like a bar or short between the brushes. It needs the back emf created by rotation of the bar through the field to create voltage drop across the bar and reduced current flow.

With loss of field current in the DC motor, the armature current would be trying to go to infinity limited by the available supply and the circuit resistance. Armature current would translate to torque and without a load the motor would be trying to run over rated speed. The instruction should cover this point, I agree.

 

[Besoeker3]

I had one semester of motor and transformer design but that was 3rd year university. Your point is a good one. The effects could be taught hands on at any age and starting early is a good bet.

The armature of a DC motor may be like a bar or short between the brushes. It needs the back emf created by rotation of the bar through the field to create voltage drop across the bar and reduced current flow.

With loss of field current in the DC motor, the armature current would be trying to go to infinity limited by the available supply and the circuit resistance. Armature current would translate to torque and without a load the motor would be trying to run over rated speed. The instruction should cover this point, I agree.

Think again. The basic equation for torque on a conductor is:

T=BIL Newtons

B being the field flux
I the current
And L the length of the conductor

 

[__dan]

Think again. The basic equation for torque on a conductor is:

T=BIL Newtons

B being the field flux
I the current
And L the length of the conductor

Well it is a condition of zero field current, so with I = 0, T = 0. If there is no back emf being created on the armature (no field and no rotation) armature current is a bolted short on the supply (limited by circuit resistance).

I don't know if residual magnetism in the core will have an effect. If there is some or enough to cause rotational force and movement of the armature bar through the field, creating back emf causing voltage drop across the armature bar. If it starts to rotate, armature current will be far higher than normal.

What happens would depend on if the motor is rotating or not.

 

[Besoeker3]

Well it is a condition of zero field current, so with I = 0, T = 0. If there is no back emf being created on the armature (no field and no rotation) armature current is a bolted short on the supply (limited by circuit resistance).

Or a current limit on the supply which is usually the case for DC drives. Without that that or other overcurrent protection, the motor wouldn't survive.

 

[gar]

190114-2029 EST

Note: in my statement of the question the motor was running and field excitation was lost.

It was not a question of whether the motor would start or not.

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[Sahib]

Think again. The basic equation for torque on a conductor is:

T=BIL Newtons

B being the field flux
I the current
And L the length of the conductor

Think again. The unit for torque is wrong.

 

[Besoeker3]

Think again. The unit for torque is wrong.

Newton metres.
Well spotted, Mr Eagle Eyes.

 

[Besoeker3]

190114-2029 EST

Note: in my statement of the question the motor was running and field excitation was lost.

It was not a question of whether the motor would start or not.

.

I think your question has been answered.

 

[gar]

190115-0756 EST

Besoeker3:

There have been some answers poking around what would happen, but for the most part not indicating a basic intuitive understanding of how a shunt DC motor works.

The purpose of my posing this question was to initiate thinking.

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[Sahib]

Newton metres.
Well spotted, Mr Eagle Eyes.

Think again, Mr.Hasty. 'BIL'=Force not torque.

 

[gar]

190115-1505 EST

As an approximation what happens to the speed of a DC shunt wound motor as applied voltage is varied where the field is supplied with the same voltage as the armature?

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