DC locked rotor

[ptonsparky]

Current for a DC motor is limited by winding resistance? Yes?
Locked rotor would have little or no affect on current until winding failed due to heat build up? Yes?

Two wire 48v DC motor ohms @ 1.31 via Fluke 1507. 6.10 M ohm to grnd @ 57 volt. Motor is blowing 20(original fuse), 25 & 30 amp fuses almost immediately accoridng to tech that originally tried to fix this Siren. I calculate this motor current at almost 37 amps. Am I missing something? Contactor is good and control works correctly.

2nd motor of unit works and both worked until recently.

 

[gar]

101015-1824 EDT

DC motor locked rotor current is applied voltage to armature divided by DC resistance of armature including brushes. Resistance will be slightly variable because of brush position and brush to commutator contact resistance.

Yes your calculation of 37 A is a reasonable estimate.

At any speed the current will be (applied voltage - counter EMF) / armature resistance.

Above is for a PM shunt motor. A wound field shunt motor will have the field in parallel with the armature. A series motor will have the field in series with the armature and then the story is different if looking at the motor terminals. However, in locked rotor conditions the sum of the field and armature are the resistance.

The above equation for current will be valid for a series motor if the voltage is measured across the armature rather than the motor terminals.

.

 

[ELA]

Can you compare the startup current of the unit that works to the one that blows the fuses? Using a current probe and O'scope?
Were the replacement fuses of the same time delay characteristic?

 

[ptonsparky]

Different sized motors entirely. #12 vs #2 cu. We will get a bucket up next week. The small one rotates the siren horn the other wakes the dead, it they are outside.

 

[ELA]

Here is a Torque-Speed curve for a 24Vdc motor for an example of locked rotor current vs running current. I did not see a 48Vdc motor here.

http://www.groschopp.com/Public/Performance/02001.pdf

You can see the 44 amps locked rotor current vs 7 amps running current at peak efficiency.

You said the control works correctly. Is the control a simple on/off or something with a soft start or PWM?
If simple on/off control the time delay characteristic of the fuses is more important.

 

[mxslick]

Motor is blowing 20(original fuse), 25 & 30 amp fuses almost immediately accoridng to tech that originally tried to fix this Siren.

What the heck does this "tech" think he is doing? If the fuse blows, keep replacing it with a larger one? :roll:

First off, if the fuse blows "immediately" that says one of two things to me: the motor is locked up (did this "tech" have enough brains to see if the motor turns freely?) or it is already burned out. Or both. Or even bad wiring that has shorted out..from rodents or birds chewing.

Since you measured and found this:

Two wire 48v DC motor ohms @ 1.31 via Fluke 1507. 6.10 M ohm to grnd @ 57 volt. .... I calculate this motor current at almost 37 amps.

Stop and think about that for a moment. If indeed the original, manufacturer-specified fuse is 20 amps, don't you think that a calculated motor current of nearly twice that indicates a small problem?

Sometimes the solution is the obvious one. Fried motor. Or bad wiring.

No need to overthink this.

 

[ptonsparky]

The motor is 50' in the air and with low winding resistance on motor I pretty much expect it to be fried. Tech figured that to. Lightning was invloved. Simple on/off control.

 

[gar]

101016-1945 EST

ptonsparky:

Looking back at your first post.

Current for a DC motor is limited by winding resistance? Yes?
Locked rotor would have little or no affect on current until winding failed due to heat build up? Yes?

The first sentence is correct. If you assume the motor is not driven in reverse. The critical word in the sentence is "limited".

The armature current in a conventional shunt type DC motor is as I stated earlier is Ia = (Vs - Vg)/Ra where Ia is the armature current, Vs is the voltage applied to the brushes, Vg is the counter EMF generated by the rotation of the armature, and Ra is the resistance of the armature and the brushes.

Under unloaded conditions the armature will rotate at near theoretical speed. Thus, Vs - Vg is small, maybe 1 or 2 V. If it was 2 V, then your no load current would be Ia = 2 / 1.31 = 1.5 A. 48 * 1.5 = 72 W would be the power to overcome friction, windage, and the 1.31*1.5^2 = 2.9 W dissipation in the brushes and armature.

With a load in addition to the friction, windage, and I^2*R losses there is a higher current required to supply the mechanical load. Friction and windage drop a little, and I^2*R increases, but most of the input power goes to the mechanical load. Speed will diminish as a result of the mechanical load because more current is required to power the load.

Yes, locked rotor does have an effect on motor current if you are comparing that locked rotor current with normal running current. The locked rotor condition causes a high current because there is no counter EMF. Under normal operation of the motor, up to speed, there is a large counter EMF.

Your resistance measurement with the Fluke may not tell you much because of the wire length and the inaccuracy of making low resistance measurements with this meter. However the resistance measurement does not seem out of line for this motor. Whether a series or shunt motor and its horsepower rating, or normal running current would help to identify what a realistic resistance might be for the motor armature.

These comments do not mean the motor is not the problem. The comments are a try at explaining how the motor works. You could have shorted windings on the rotor and not know this from you measurement.

.

 

[BJ Conner]

Current for a DC motor is limited by winding resistance? Yes?
Locked rotor would have little or no affect on current until winding failed due to heat build up? Yes?

Two wire 48v DC motor ohms @ 1.31 via Fluke 1507. 6.10 M ohm to grnd @ 57 volt. Motor is blowing 20(original fuse), 25 & 30 amp fuses almost immediately accoridng to tech that originally tried to fix this Siren. I calculate this motor current at almost 37 amps. Am I missing something? Contactor is good and control works correctly.

2nd motor of unit works and both worked until recently.

Starting resistance is not necessarly limited by winding resistance.
What kind of starter does it have.
I may be going way back here but I have worked on DC motors that have resistor(s) in series with the armatur. They are shorted out as the motor comes up to speed and the field coil in the motor generates a back emf.
IF you don't have series starting resistors or they shorted you get just what you describing.
Check the starting circuit and check for resistors in armature circuit.

 

[ptonsparky]

I will know as soon as I can get a ride up arranged.
Motor theory was back in '74 and at the time I knew I was never going to work on motors.:roll: (Small industrials for the last 5 years)

My total of DC motors has been three, including this one.

Thank you

 

[LARMguy]

What the heck does this "tech" think he is doing? If the fuse blows, keep replacing it with a larger one? :roll:

First off, if the fuse blows "immediately" that says one of two things to me: the motor is locked up (did this "tech" have enough brains to see if the motor turns freely?) or it is already burned out. Or both. Or even bad wiring that has shorted out..from rodents or birds chewing.

Since you measured and found this:



Stop and think about that for a moment. If indeed the original, manufacturer-specified fuse is 20 amps, don't you think that a calculated motor current of nearly twice that indicates a small problem?

Sometimes the solution is the obvious one. Fried motor. Or bad wiring.

No need to overthink this.

I agree!

 

[ptonsparky]

Armature is bad.

 

[iwire]

What the heck does this "tech" think he is doing? If the fuse blows, keep replacing it with a larger one? :roll:

First off, if the fuse blows "immediately" that says one of two things to me: the motor is locked up (did this "tech" have enough brains to see if the motor turns freely?)

Did you not read this?

Current for a DC motor is limited by winding resistance? Yes?
Locked rotor would have little or no affect on current until winding failed due to heat build up?

 

[iaov]

Take a good look at the brushes and brush cavity/holders. Worked on a motor last week that was part of a cheese salter that had a pile of salt/rust/crud in the brush cavities shorting them to ground. Brushes being worn down to little nubs can cause lots of trouble too. Also take a look at the comutator. Color, wear pattern, etc. can tell you a lot about how this motor has been operating.