DC Motor Field Failure 2

[Settel1]

Can anyone offer me any insight on why a DC motors field would melt/burn.

The DC drive also had a catastrophic fuse failure requiring the drive to be replaced.

One theory I do have is that the customer did not have the cooling fan on while the motor was stopped. We were told the cooling fan was controlled externally and did not hook up the fan to the drives control. The drive constantly supplies field voltage even when stopped.

 

[waross]

Stuff happens.
I have had DC motors running on MG sets (years ago) with the field energized 24/7 and never had heating issues.
These motors had cooling fans but the fans shut down when the MG set was shut down for the night.
Most of the heat is generated in the armature windings and in the series field.
Sometimes the insulation breaks down and the field fails.
The failure may be an arcing fault to ground which may do a lot of damage before the fuses clear the fault.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Mostly, it is a failing fan that causes this. Earlier DC motors had less excitation current than newer ones (say from 1980 and on). And the acrid smell from a motor that had been energized the evening before, with fan switched off, was a nasty surprise. The fitters told me they were planning to make a nice egg and bacon on it (square frame, so you could cook directly on top of it). But decided to switch it off. I think that saved it. And me...

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Thanks for the update Gunnar. My motors were old by the 1980s.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

Any particular reason why the field has to be kept energized when the motor is not running? "Modern" motors have less thermal tolerance and a field winding with no cooling will fail thermally first before the voltage starts doing its share of damage.

Muthu

http://www.edison.co.in

 

[Settel1]

No particular reason, just how the drive is when it's in the "ready" state.

 

[edison123]

Ready state? Have no idea what that means.

Either turn off the field or turn on the cooling fan motor. I don't see any other option. Turning off the field and the cooling fan motor would both be a power saver and a winding life enhancer.

Muthu

http://www.edison.co.in

 

[Settel1]

When the DC drive has all its enables and interlocks its considered ready to run. Your ready light illuminates and you can start the drive.

After further investigation some drives do use a reduced field to stop condensation.

This motor has heaters and may not be able to handle a constant field voltage.

 

[Skogsgurra]

Sure. If left on, the excitation will overheat.

There are cases where you want to start without waiting for the field to build up. Keeping the system "ready" as you call it. For example when jogging. Then it is customary to let the excitation on. But the fan should also run.

Besides, it is good practice to have an air flow supervision (pressure monitor or simple flag+micro-switch or NTC) and also to shut excitation and fan off after, say, five minutes of inactivity. This is mandatory in all such applications that I have seen. And they are many, many.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[edison123]

Zero speed based shut down. Winding temperature based shut down. Air flow based shut down. The drive could be programmed for anyone of these? One needs a healthy motor first before it is 'ready'.

Muthu

http://www.edison.co.in

 

[electricpete]

At our plant the dc motors are relevtively small (<60hp) and relatively fixed speed (no automatic voltage adjustments). Among this dc motor population, we don't have any auxiliary fans.

So I have a question: what is the role of the auxiliary fan?

=====================================
(2B)+(2B)' ?

 

[waross]

Hi Pete;
Many times DC motors are selected for variable speed applications. Much more so before VFDs became common.
In many applications the motor will be called upon to deliver full torque (and full load current) at speeds that are much too slow for the motor's internal cooling fan to be effective. Hence an auxiliary cooling fan for slow speed cooling.
Traction motors often use auxiliary fans.
Some variable speed applications may have a load vs speed profile such that the auxiliary fan is not needed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

thanks Bill. That makes sense auxiliary fan would be required at low speed, just like for ac vfd fed motors.

=====================================
(2B)+(2B)' ?

 

[Settel1]

The fan keeps the armature and field windings cool. It's a 600hp motor.

Edison I'm not exactly sure what you're getting at here. I install VFD's for a living, but I am not a motor guru. Let's not veer off the path of the original question. Every shutdown you mentioned are interlocks which prevent the ready state. Of course a damaged field would blow fuses and open the ready state.

The installation was fully commissioned and went through a Field Acceptance Test. The motor was healthy at the time of commission. The machine was running production for three weeks.

I accept your idea of turning the field off and will write it on my list of possible fixes.

 

[waross]

Add "Fan Running" as one of the enabling conditions of "Ready State".

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Settel1]

The original post mentions that the customer supplied the control for the fans, air switch, etc. That's part of one of the theories of failure. I do not know for sure if the fan was running during operation. Their system may have not indicated no air flow.

And it is an available interlock provided already by the drive, we disabled it internally by request of the customer.

 

[edison123]

Settel - You posed the problem, "Can anyone offer me any insight on why a DC motors field would melt/burn". I offered possible causes and solutions to protect the motor (and the drive). I personally would prefer winding temperature protection for alarm and trip since it is the easiest to install and track.

Muthu

http://www.edison.co.in

 

[edison123]

Bill, Pete

I see shaft mounted fans only in pre-80's vintage DC machines. Modern DC machines (usually 100 HP and above) invariably come with an external high speed blower even when they are running at a constant speed.

During no-load, bearing temperature stabilization test runs in our facility, we have found that without these blowers switched on, the field temperature would rise rapidly and we could not complete these tests, which normally last from 3 to 6 hours.

Muthu

http://www.edison.co.in

 

[waross]

Thanks for the update Muthu.
Pre-80's vintage is a good description of My DC motor days. Motor Generator sets with Amplidynes.

Bill
--------------------
"Why not the best?"
Jimmy Carter