DC Motor

GoldDigger

Moderator
Staff member
To some extent the turn on surge behavior, as well as the current versus speed curve, will depend on how the motor stator is wired (series, shunt or compound). There will be no counter-EMF in the field winding.
 

paulengr

Senior Member
Sorry - what doesn't?
It’s a contactor and maybe a rectifier, that’s it. Look I realize you and the educated idiot that flaunts his college ego or I mean degree live in a world of power electronics. The transistor wasn’t even invented until the early 1950s (ignoring double cat whiskers on crystal sets). The SCR came even later in that decade, originally with a whopping 16 A output. A GE MD624, 1200 HP DC motor has a stall current of 6000 A, used on large mining equipment of the day. See the problem? Simply put there can’t be a drive because those didn’t exist for over a decade later! Even if it did, it was a toy suitable for engineers in school not the real world.

When I went to school to have my common sense removed (I hold two bachelors and a masters), in power class the professor first covered salient pole motors then told us he was skipping the DC chapter because we will never use or need it, and jumped right into synchronous machines, followed by induction machines. See? No common sense to be found when the Pentium CPU just came out. Computer technology was just on the cusp of field oriented AC control and IGBT based low voltage AC drives were just coming out albeit V/Hz only. Anything over about 100 HP was either a GTO (big explosions every few months) AC or DC. So as I said, the professor was just an educated idiot with a PhD, as in doctorate in dumb.

That was the early 1990s. Little did I know then that I would be working for one of the largest regional motor shops in the country and that not only would DC be a regular part of my job but that it’s making a comeback in high efficiency and very compact applications. Even the older wound rotor motors are prominent in car shredders and modern wind mills. How wrong that professor was.

And yes I am VERY familiar with Ward Leonard loops and MG sets. Those would not be used for this purpose. Military tech in this era is simple and reliable unlike say an M-16.

There are three kinds of DC motors. If you look at the wiring in a “modern” one you will find four wires and if they are labeled, A1, A2, F1, and F2. It may have more F wires and maybe S1 and S2 if those aren’t internally connected but that’s it. You drive the field and armature separate. Often rectified DC goes on the field. On the armature in years past you would run an AC motor mechanically coupled to a DC generator. The excitation.on the generator rotor comes from a rheostat fed with again rectified DC or in large generators an exciter which is just another stage of DC generators. This is Ward Leonard technology. Came out in the 1930s and revolutionized the older wound rotor AC tech that worked but was much more difficult to control. Later the MG set was replaced with solid state SCR drives in “smaller” DC applications but I’ve still personally worked on larger DC systems that still use MG set technology even in 2020. It’s still around.

That’s all shunt wound DC motors. Compound wound usually has S1 and S2 on the series or commutating coils or those are just internally connected and A2 is really S2 but I digress. That’s not the case here.

In this case what you have is a large “universal” motor. Today we get into brushless designs and even old WWII vintage toys used them. On small motors like the kind prominent now in battery powered tools the rotor is a permanent magnet (PMDC) but this has size and cost limitations. On larger DC motors of the time both armature and field are wound but on a siren they will be connected in series so just two wires.

To the OP your brushes could be arcing or by now it’s so dirty with carbon dust it’s probably flashing over occasionally and blowing fuses. DC motors all require periodic cleaning. Take it to any competent motor shop that still does DC. It’s expensive but your motor should hold up another decade or more. If you want to test first get an electrician with a digital Megger to disconnect the leads and test it for one minute at 500 V. If it’s under 2-3 megaohms time to wash it out and bake it, then retest and change the bearings. If you haven’t even been cleaning it, it will fail and what you are seeing is flash overs. Redip while you are at it for cheap life extension. Then every year take the brush cover off and clean all the dust out with a shop vac and check brush wear from now on. That’s it. Once a year.

If you went to school in the 1980s or earlier they would teach this. DC tech has all but been forgotten.
 

Besoeker3

Senior Member
Location
UK
A contactor, to me, has no overloads. A starter would.
The motor is On, or Off. No ramp up or down.
The systems I have used had no ramp. These were mainly paper mills, typically 100kW upwards, but also other applications.
And yes, I did go to school earlier than 1980s.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
DC motors tend to have lots of iron in them and are harder to kill then ac motors in larger sizes. Anything under about 75 HP though, most applications an AC motor with an appropriate drive seems to be a better choice.

I can remember my first job out of school using pm DC motors to run smallish chemical pumps because the drives were cheap and it was simpler to do it with DC motors than ac. These days I can't even remember the last time I saw a DC motor on one of my projects.

There was a DC motor that popped up a couple months ago though. One of our other engineers asked me what it was because it came in on a machine for rebuild and he couldn't figure out how to get it to run. Said it had four wires coming out of it and never seen anything like that. Sure enough it was a fairly small DC motor. Someone had taken the drive out of the cabinet before shipping the machine to us.
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
If you went to school in the 1980s or earlier they would teach this. DC tech has all but been forgotten.
I received my BSEE in 1972. All I remember from my ONE power course was the instructor verifying the field connection and that it wasn't series. There were stories of motors exploding in the EE lab every year or so.
 

paulengr

Senior Member
Is your contactor in the incoming AC line or in the outgoing DC line?

Maybe that one time a year something is abnormal about the supply volts?
DC contactors are incredibly expensive and best avoided. In AC the current goes to zero 120 times per second. With DC the contactor has to physically rip the arc apart. This is what makes DC contactors much larger and more expensive.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
DC contactors are incredibly expensive and best avoided. In AC the current goes to zero 120 times per second. With DC the contactor has to physically rip the arc apart. This is what makes DC contactors much larger and more expensive.
Depends on the rating. Under 200 volts @ 50 amps or so they are not too awful. In fact you can often just upside an iec style contactor.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
....


To the OP your brushes could be arcing or by now it’s so dirty with carbon dust it’s probably flashing over occasionally and blowing fuses. DC motors all require periodic cleaning. Take it to any competent motor shop that still does DC. It’s expensive but your motor should hold up another decade or more. If you want to test first get an electrician with a digital Megger to disconnect the leads and test it for one minute at 500 V. If it’s under 2-3 megaohms time to wash it out and bake it, then retest and change the bearings. If you haven’t even been cleaning it, it will fail and what you are seeing is flash overs. Redip while you are at it for cheap life extension. Then every year take the brush cover off and clean all the dust out with a shop vac and check brush wear from now on. That’s it. Once a year.

If you went to school in the 1980s or earlier they would teach this. DC tech has all but been forgotten.
These are 60 feet in the air so no maintenance is ever done. Only at failure. I know we started working on this siren in about 2011 but have never touched the motor itself.

We just replaced the brushes in one of these units for another village and I would guess that this is similar in age.

We can Meg and will recommend we open it up at the time.

Thanks to all, again.
 

Russs57

Senior Member
I have a lot of old elevators with MG's (Ward Leonard technology). A little maintenance, such as what Paul proposes, goes a long ways. It sounds like these motors have never had any. Might even need to polish/resurface commutator. Caught in time most things are cheap and easy to fix.
 

paulengr

Senior Member
I have a lot of old elevators with MG's (Ward Leonard technology). A little maintenance, such as what Paul proposes, goes a long ways. It sounds like these motors have never had any. Might even need to polish/resurface commutator. Caught in time most things are cheap and easy to fix.
Elevators is what Ward Leonard was invented for. Wound rotors and mechanical gearing was jerky or difficult to smoothly control compared with DC systems which smoothly accelerate and decelerate. There are still a few around North Carolina including one in the state capital building itself.
 

myspark

Senior Member
Location
SCV Ca, USA
Occupation
Retired EE
DC Motor
Texts in italic bold are mine.

. . . . When I went to school to have my common sense removed (I hold two bachelors and a masters), in power class the professor first covered salient pole motors then told us he was skipping the DC chapter because we will never use or need it, and jumped right into synchronous machines, followed by induction machines. See? No common sense to be found when the Pentium CPU just came out. Computer technology was just on the cusp of field oriented AC control and IGBT based low voltage AC drives were just coming out albeit V/Hz only. Anything over about 100 HP was either a GTO (big explosions every few months) AC or DC. So as I said, the professor was just an educated idiot with a PhD, as in doctorate in dumb.

That was the early 1990s. Little did I know then that I would be working for one of the largest regional motor shops in the country and that not only would DC be a regular part of my job but that it’s making a comeback in high efficiency and very compact applications. Even the older wound rotor motors are prominent in car shredders and modern wind mills. How wrong that professor was.

And yes I am VERY familiar with Ward Leonard loops and MG sets. Those would not be used for this purpose. Military tech in this era is simple and reliable unlike say an M-16.

There are three kinds of DC motors. If you look at the wiring in a “modern” one you will find four wires and if they are labeled, A1, A2, F1, and F2. It may have more F wires and maybe S1 and S2 if those aren’t internally connected but that’s it.

A compound series DC motor can have more than four wires. The field and stator can be connected through a drum switch where the armature can be connected either series or parallel.
Connection scheme as described above gives it more controllability as the load varies.


You drive the field and armature separate. Often rectified DC goes on the field. On the armature in years past you would run an AC motor mechanically coupled to a DC generator. The excitation.on the generator rotor comes from a rheostat fed with again rectified DC or in large generators an exciter which is just another stage of DC generators. This is Ward Leonard technology. Came out in the 1930s and revolutionized the older wound rotor AC tech that worked but was much more difficult to control. Later the MG set was replaced with solid state SCR drives in “smaller” DC applications but I’ve still personally worked on larger DC systems that still use MG set technology even in 2020. It’s still around.

That’s all shunt wound DC motors. Compound wound usually has S1 and S2 on the series or commutating coils or those are just internally connected and A2 is really S2 but I digress. That’s not the case here.

DC motors are a common sight in older paper making machines --that date back to the late 30s and still operating to this day. Although modern plants use VFDs.
DC motors are used in conjunction with Ward Leonard MG Sets. Paper-making requires very subtle (soft) motor control because--when the slurry-- with the mixed paper pulp still suspended in water it has no strength to be able to be rolled in huge cylinders.
As the soft sheet travels through the production machine--it slowly gain strength as moisture is removed.
As tensile strength increases-- torque and tension have to be adjusted to prevent breakage (a big job for the operators) in the event of breakage.

Paper-making may sound easy -especially when you re seating on a throne and then grabbing a roll to clean yourself.

Yeah, it requires more than "shop talk" that is often seen as alluring to read --by some waiting to read something attractive--like looking at a photogenic picture of Britney Spears.
A big part of your exhortation doesn’t speak well when you consider yourself an engineer with multiple degrees.
I’m not going to dig into your credentials-whether or not you have taken an oath as a practicing professional in the engineering field.

It is an unwritten “code of conduct” that is adhered to by all members of educated “elites”
(nulli secundus) .

To engage in such abhorrent name-calling behavior . . . diminishes the credibility of the otherwise honorable calling.
What I can tell you though is; professionals don’t denigrate a colleague. . . whether he belongs to the same discipline --or apart from what he’s been trained for.
If you have a grudge with someone’s claim that you disagree with. . . you should take it up with him before heralding to the entire world that he is an idiot, stupid or whatever adjective you can come up with.
That’s why there is such a thing called EMAIL.
You may have a reason to say whatever you have in mind. . . but you also have the moral responsibility to act like someone who spent years in formal education.
How would you feel if someone called you a doofus, stupid?

I can’t imagine being in a symposium with different groups of professionals--exchanging ideas--and one attendee telling someone he is an idiot, foolish, baboonish, half-cracked, fat-headed doltish bovine.
 

gar

Senior Member
200802-2133 EDT

Jraef:

In your post numbered #6 you said
"Any time you have a coil, like in a motor winding, for the first instant after being energized there is nothing slowing down the rise of current except the resistance of the wire itself. ----"

This is an incorrect statement that I seem to see quite often on this forum.

The differential equation for an equivalent series RL circuit fed from a battery and switch is
L di/dt + Ri - V = 0
a solution for this equation is
i = V/R ( 1 - e exp - t*L/R )
for initial current of 0 at t=0.
After t = 0+an extremely small increment of time i is still 0.
Note: e exp 0 is equal to 1. After about t = 6 time constants e to its exponent becomes near 0

You can also look at https://www.electronics-tutorials.ws/inductor/lr-circuits.html

The above is valid for either a shunt or series motor.

What happens after t = 0 is very dependent on the motor and load.

After reaching steady state speed, then ordinary algebra works.

To see what is possibly happening a digital scope should be used to measure and compare current vs time of a working motor ( no problems ) vs the motor of post #1.

.
 
Last edited by a moderator:

gar

Senior Member
200810-1619 EDT

Jraef:

I ran a simple test on a convenient PM DC motor with a 30 to 1 spur gear box and no external mechanical load.

The motor is a Bodine 32D3-BEPM-3.
Some of its specs are:
130 V DC 0.71 A 1/12 HP
83 RPM 30/1 ratio
I measured loop resistance at 25 to 26 ohms (DC), and
L = 48.6 mH, and a Q = 4 @ 1kHz.

The calculated L/R time constant is 48.6/26 = 1.87 mS.

My experimental circuit consisted a 12 V DC battery, a switch, 2 ohm current sensing resistor, and the motor.input leads.

I would like to have been able to lock the rotor, but I did not want to spend the time so my 63% time point is somewhat of a guess. This guess is about 2 mS. Good correlation with the calculated value.

As the rotor is starting to rev up I get peak current5 at about 8 mS. Motor reaches full speed about 100 ms.

My comments here so far possibly have no bearing on the original post. I have continued on this because I have seen so many incorrect statements in this area.
 
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