Shunt DC motor question

[dpeter]

GAR, not feeling ignored at all but thank you. I deal with some older DC traction elevators and that suicide circuit is in there for good reason. Learned some more from all the posts too.

 

[Tony S]

190117-1040 EST


I think a number of responses indicated a lack of a basic understanding of a DC shunt motor. Should an ordinary electrician know how a motor works? I don't know. If one never works with a motor, then probably no need. Otherwise I would say yes.

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DC motor knowledge is dying out. When I started as an apprentice we had numerous DC motors and MG sets on site, they were part of our life. Ward Leonard systems galore!

One time I did screw up was changing a fan drive, the commutation point was way out. I’d got the panel door open, normal starting current would be around 900A. The drive tripped at around 2500A and I had had singed legs from the arc flash.

 

[kwired]

190113-1111 EST

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

I'm still stuck on this, you want answers from those that don't know the answer? If someone accidentally gets it right - are they wrong for answering?

 

[gar]

190119-1340 EST

kwired:

Yes.

The purpose of the question was to get a number of readers to think about the question that they may have never been exposed to. Thus, an answer from persons that known exactly what happens does not help others think about how to study the problem.

An accidental answer that is correct is not wrong. This was not like a multiple choice type of question where there is a certain probability that you guess the correct answer by chance.

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[Besoeker3]

190119-1340 EST

kwired:

Yes.

The purpose of the question was to get a number of readers to think about the question that they may have never been exposed to. Thus, an answer from persons that known exactly what happens does not help others think about how to study the problem.

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Do they really need to given that DC motors are no longer mainstream the movers and shakers in industry?

 

[kwired]

190119-1340 EST

kwired:

Yes.

The purpose of the question was to get a number of readers to think about the question that they may have never been exposed to. Thus, an answer from persons that known exactly what happens does not help others think about how to study the problem.

An accidental answer that is correct is not wrong. This was not like a multiple choice type of question where there is a certain probability that you guess the correct answer by chance.

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I understand what you are trying to do - make people think. Something that doesn't happen anymore and problem solving is taking a beating in more recent years because people don't know or have forgotten some fundamentals. Doesn't help that a lot of things are cheaper nowadays to just replace than they are to repair when they have a problem.

Wording in describing an issue or asking a question is also important to get accurate results. Got a question on FB asking how many years will you be celebrating 2019 - my response was I would probably celebrate 2019 for about one year - intent of the one asking was how many years will you have been married, together, etc. in 2019:blink:

 

[gar]

190119-1529 EST

Another question.

What happens to a hair pin loop of wire if I pass a high current pulse thru the wire?

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[Sahib]

190119-1529 EST

Another question.

What happens to a hair pin loop of wire if I pass a high current pulse thru the wire?

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Tends to straighten out?

 

[gar]

190129-1354 EST

Can anyone provide a reason why Sahib's answer is correct or incorrect? And have you ever tried the experiment?

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[Sahib]

190129-1354 EST

Can anyone provide a reason why Sahib's answer is correct or incorrect?

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For each small segment carrying current in the loop there is a corresponding diametrically opposite segment carrying current in opposite direction and they may be considered parallel. Applying right hand rule, the force is directed out on either segment. Extending the argument to other similar segments in the loop, it may be seen that the entire loop is subject to outward force tending to straighten it out.

 

[Skokian]

Run-Away Motor.

Run-Away Motor.

I agree that if the field of a shunt wound D.C. motor with no load is lost, the motor speed ramps up without limit until the commutator segments start flying away.

This is a very dangerous situation for any persons or equipment in the line of the commutator.

I saw this happened while I was still in school at I.I.T. in Chicago when a student accidentally opened the field of a shunt would motor of around 10 or 15 Hp. The motor started accelerating VERY rapidly. Fortunately the lab instructor was able to dive toward the distribution panel and pull one of the patch coards feeding the motor.

I took a stand behind a steel I-beam as soon as I heard the motor speeding up.

BTW, transit motors (busses, locomotives, etc.) are always series wound since this gives very good accelerating torque. In the case of busses and rapid transit trains, series resistors are used to accelerate the motor(s). In the case of locomotives, the field current to the generator is used to lower the voltage to the motor for starting from zero speed. As the motor accelerates, it's back EMF increases the effective impedance of the motor so less series resistance can be used until the motor is in "shunt" (across the line).

 

[Reggie Lavoie]

Ward Leonard System

Ward Leonard System

There is an older DC motor control system called Ward Leonard. On a shunt DC motor, the speed of the motor is controlled by varying the armature voltage while applying full field voltage, up to base (nameplate) speed. This is a constant torque mode. Once reaching base speed, the controller will then start to reduce the field voltage to increase the motor speed above base speed. This is now a constant HP mode (speed increases while torque capability reduces). If the field is reduced too much and the load is light enough, the motor could easily overspeed and destroy itself. This arrangement could be found on elevator and printing press controls. Elevator controls could also to the opposite, overexcite the field (applying higher than nameplate voltage) for short periods of time when running the motor slowly, such as when leveling the car to the floor. As a number of you have stated, reducing the field voltage reduces back emf (essentially the motor acting partially as a generator). Back emf opposes the applied voltage (similar to placing batteries in series the wrong way). With less back emf, the armature current increases, causing the motor to speed up.

 

[Tony S]

A use of Ward Leonard drives was with mine winders.

Using a slipring motor with partial resistance in the rotor circuit allowed a degree of slip. The main generator set had a flywheel between the motor and generator (one I know of the flywheel was 14 tons).
Accelerating the cage the flywheel gave up kinetic energy, braking the cage it absorbed energy. Not very efficient but it worked as it prevented sudden loads on the pit head supply.

They aren’t new “efficient” drives but they served their purpose at the time.