DC Motor

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
DC Siren Motor blows fuse about once per year. Usually we replace the fuse and leave. Today I had my help amp clamp the DC side when it cycled for the 'noon whistle'. No more than 5 seconds run. Meter was set on 'Inrush'. Flukes terminology for the setting used. Capture was 468 amps DC. Rectifier output, two diodes, was the 48VDC listed output. He did not have time to switch meter to collect run amperage. Nominal operating current is 100 amps, via specs.

Does a DC motor have an inrush? I thought not, but have never looked for it before. Across the line start on load side of power supply.

468 amps would explain the occasional fuse blowing as the time current curve for the fuse used shows 10 seconds for 500 amps.
This is the only siren of its type that we have this problem with.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Absolutely.
It’s almost like a short when the motor first comes on. AKA locked rotor
DC inrush current limiter AKA starter..
 
Last edited:

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Absolutely.
It’s almost like a short when the motor first comes on. AKA locked rotor
DC inrush current limiter AKA starter..
Ok.
I knew that was true for an AC motor, but assumed (yes, bit more than once) that was not valid for DC.
We have been given the ok to put test equipment on for a more detailed testing. Residents need to be notified it will be sounding during the day.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Where in the five seconds does the fuse blow?

Is a greater-delay version of the fuse available?
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
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. It's only AFTER the magnetic fields are created that you have back-emf to act on the coil as a current limiting device. So for that first instant as the magnetic fields come into being, the high current that flows is the "inrush current" and can be up to 10x the FLA or more. Doesn't matter if the motor is AC or DC, a coil is a coil. Magnetic inrush current is VERY VERY short in duration so MOST of the time, fuses survive it, especially TD fuses.

With AC, there is ALSO the high current (around 6x FLA) that takes place to START the motor because the Power Factor is very very low until the motor accelerates to around 80% speed. That is called "Starting Current", but people often lump it in with the Inrush Current as the same thing, they are not. In a DC motor, you don't have that.
 

myspark

Senior Member
Location
SCV Ca, USA
Occupation
Retired EE
Ok.
I knew that was true for an AC motor, but assumed (yes, bit more than once) that was not valid for DC.
We have been given the ok to put test equipment on for a more detailed testing. Residents need to be notified it will be sounding during the day.
Inductive loads ie coils, motors are reactive loads. It doesn't matter whether it runs on AC or DC.
Motors require extra power to get started as JR explained.

And to add to this extra demand--it undergoes thru a sequence where kVAR (reactive power) gets in the act.
kVAR is an "invisible" power that is both useful and useless in an electrical circuit.
Useless because it doesn't do anything to do any (specific) work but it is useful for engineers to determine the proper sizing for designing a system that can handle incidents that might occur during the normal operation of a system.

Think of it as a safety factor that is used by structural engineers to handle an unexpected or a dead load of a bridge that can be subjected to.
During startup where no back emf is apparent. . .an inrush (elevated ) power demand is needed but diminishes as the motor continues to spin.
Newton's Law of Energy Conservation takes over the wheel.

Improper incidences of kVAR can also become a burden if not occuring correctly.
It can cause unproductive power losses.

It requires knowledge of calculus to determine reactive power that's being generated by an inductive load.

It can be presented in a horse-and-buggy analogy. . . but, that would be for another day.

OK, what's the possible remedy to the problem?:

Nowhere in your post mentions capacitor. I assume there's no reactive power-compensating capacitor in your application. You also mentioned this is the only one of this type among the rest.

Does any of the other motors have capacitors?
Capacitors are used to overcome the elevated power demand during the 5-second crank-up stage.

In inductive loads, voltage leads the current. . .while current leads the voltage in a
capacitive circuit.

ELI the ICEMAN.

To achieve the ideal situation presented above. . .a reactive -power- compensating-capacitor will overcome this issue.
Time to recruit Mr Google.
 

gar

Senior Member
200730-2424 EDT

Jraef:

You have not had a college electrical engineering course on DC Machine theory.

First, assume the magnetic field intensity in which the rotor is immersed is constant. A reasonable assumption in many cases. This means the induced voltage produced in the rotor is proportional to rotor RPM. True for either a motor or generator. In a motor this is called the Counter EMF.

If you lock the rotor, then there is no Counter EMF. This is the normal start up condition. The current vs time is defined by the series L and R circuit of the rotor, and the applied armature voltage.

Unlock the rotor and let the motor come up to its steady state running RPM. Now the circuit looks like two opposing voltage sources connected by the rotor L and R. Ignore L because you have no changing current. Also you may ignore L at start up.

At no load running speed you actually have load of friction and windage. Thus, you will never get theoretical maximum output RPM. Armature current at running speed is determined by the (Source Voltage minus Counter EMF) / Rotor R.

How motor current and RPM will vary with time will depend upon the motor and load.

There is more to what can be calculated, but the above is the essence to an answer to the original post.

.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Ok.
I knew that was true for an AC motor, but assumed (yes, bit more than once) that was not valid for DC.
We have been given the ok to put test equipment on for a more detailed testing. Residents need to be notified it will be sounding during the day.
Does this motor have a starter?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
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. It's only AFTER the magnetic fields are created that you have back-emf to act on the coil as a current limiting device. So for that first instant as the magnetic fields come into being, the high current that flows is the "inrush current" and can be up to 10x the FLA or more. Doesn't matter if the motor is AC or DC, a coil is a coil. Magnetic inrush current is VERY VERY short in duration so MOST of the time, fuses survive it, especially TD fuses.
I am going to disagree on the above point with respect to DC devices.

Whenever you have changing current in a coil, you have changing flux cutting that coil, and you get 'back EMF'. This is the nature of inductance, and it applies immediately upon the application of voltage.

For _AC_ devices, the flux alternates in the core, and typical design places the _peak_ flux level just below saturation.

When an AC inductor is initially energized, the flux can drive into saturation leading to 'inrush' current, which tapers off as the alternating flux becomes symmetric positive to negative.

This sort of inrush doesn't happen when DC is applied to an inductor.

-Jon
 

Besoeker3

Senior Member
Location
UK
The usually part is what I’m curious about.
Im wondering if this particular install has one.
Typically, the VSD drives were around about 100kW. Many of those were in paper mills. Each section had a variable speed section to get up speed then normal drive.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Typically, the VSD drives were around about 100kW. Many of those were in paper mills. Each section had a variable speed section to get up speed then normal drive.
And it doesn’t... see post #13...
 

Besoeker3

Senior Member
Location
UK
The motor in question in post #1. It doesn’t have a starter...
all it has is a contactor.

maybe we’re on two different paths...😅
Pedantic.............
But starters I have had came with proper starters. Old ones had face plate designs.
 
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