DC Motors replacement

[Lc85]

Hi,
This is my first DC Motor contract and I'm a little confused.
Our client has given us the data sheet for a lube oil skid DC motor. We are replacing this lube oil skid and so replacing the motor, However our lube oil supplier has offered a permanent magnet DC motor. My question is would it be possible to use this permanent magnet DC motor if the original was not?

Here is the data we received.

Manufacture : Evar Bardello-Varse Italy
Type : DC F 100L
KW : 0.75
Field current : 0.2A
Voltage : 125 V DC, 10A
RPM : 1450 SI
Serial # : 242570

Thanks very much

 

[Skogsgurra]

It would help tremendously if you could tell us the specs for the existing motor.

Gunnar Englund

http://www.gke.org

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

 

[iceworm]

Lc -
I'm confused too.

Our client has given us the data sheet for a lube oil skid DC motor.

That sounds like someone has given you a data sheet for a motor they want to buy

We are replacing this lube oil skid and so replacing the motor

Now this sounds like you are buying (or specifying) a lube oil skid to replace one on a piece of your machinery or perhaps you cleints machinery

However our lube oil supplier has offered a permanent magnet DC motor

And this says the company that sells you lube oil also sells DC motors? Or maybe they also sell lube oil skid packages and their package uses a PM DC motor?

My question is would it be possible to use this permanent magnet DC motor if the original was not?

This does not appear to be an engineering question. It's about meeting customer specs.

So, who is the client and what did they ask for? I can't tell from your post.
Did the client ask for a lube oil skid with specified pressure and volume and no other specs?
Or did the specifications also include a specific motor type?

For the first, you get to decide if the substitute will meet spec. For the second, I'd give the customer what they asked for.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[Lc85]

There is an existing lube oil skid with the DC motor data shown above. We are replacing the skid, we have a quote from a supplier who wants to offer a permanent magnet DC motor in place of that shown above.

I was wondering if the original motor is not a permanent magnet DC motor can we replace it with a permanent magnetic DC motor if the client is using the same starting equipment?

Thanks

 

[iceworm]

I was wondering if the original motor is not a permanent magnet DC motor can we replace it with a permanent magnetic DC motor if the client is using the same starting equipment

Probably - likely with some mods. Get out the prints for the existing equipment that will still be in use. Get the new motor mfg book. See what the new motor requires for controls. See what the existing can provide.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[ScottyUK]

If you use a PM motor how will you adjust the speed? Emergency oil pumps are usually much smaller in capacity than the AC-driven auxiliary pumps, and it is important that they achieve the correct speed to ensure that the minimum oil flow is maintained to the bearings. Most EOP starters are designed for simplicity and robustness - typically they use a resistor timestarter to limit the armature current during starting and trim the speed in using a field weakening resistor. I have not seen one which uses an electronic controller. A PM motor will run at whatever speed is determined by the battery voltage and the load.

 

[Lc85]

On this contract we are also replacing the drive unit dc motor which is used as a back up to the ac motor. The client wishes to use their existing starter and have also given us details of the existing dc motor.

5.5 kw
750 rpm
Excitation voltage 125vdc
Armature voltage 100vdc
Nominal current 64 A
Inrush current 255 A
Resistance is 0.39 ohms

I think the in rush current is wrong should this not be 125/0.39=320A

I agree with the nominal current (125-100)=25/0.39=64A

But should the power not be the armature voltage times the nominal current 100x64=6.4kw am I doing something wrong here?

Thanks

 

[Skogsgurra]

5.5 kw
750 rpm
Excitation voltage 125vdc
Armature voltage 100vdc
Nominal current 64 A
Inrush current 255 A
Resistance is 0.39 ohms

"I think the in rush current is wrong should this not be 125/0.39=320A"

Inrush is not a common specification for DC motors. And is definitely not equal to excitation voltage minus armature voltage divided by armature current.

If this motor has been used DOL or with a starting resistor, the 255 A is tha maximum instantaneous current in the armature. And it is somewhat consistent with 100 V divided into armature resistance (100/.39=256 A).



"I agree with the nominal current (125-100)=25/0.39=64A"

You should not agree. Nominal current is delivered Power/armature voltage and then corrected for efficiency. Or I=5500/100=55 A. Divide by typical efficiency to get nominal armature current: 55/0.9=61 A. Which is Close enough.

You have confused a lot of things here. And you have tried different numbers until some of them happened to deliver an answer that seemed right.

Shame on you! That is not at all engineering.


Gunnar Englund

http://www.gke.org

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

 

[Lc85]

Thanks Skogsgurra.
I followed example 5.1 of the following link to get my values:

http://www.pssurvival.com/ps/Motors/DC_Motor_Calculations_2004.pdf

Are these equations not correct?

Thanks

 

[Skogsgurra]

The equations are correct. But you interpret them like the Devil reads the Holy Bible.

Nowhere do the equation say that you shall subtract the Excitation voltage from Armature voltage Before Dividing by Armature resistance.

Nowhere do the equations say that you get Nominal current the way you tried to calculate it.

I was rather harsh on you, and you reacted very well to it. That's a good sign and I think that you deserve some guidance.

Gunnar Englund

http://www.gke.org

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

 

[rcw retired EE]

In my experience with many gas and steam turbine generator lube oil skids, setting the motor starter resistance steps for the DC lube oil motor is very critical to proper operation of the lube oil. If it fails, you just wiped out a multi-million dollar turbine. (Been there. Got the T shirt).

The goal is to get oil pressure back as quickly as possible without damaging the motor, wiring or battery system with excessive accelerating current. GE had a field instruction on how to set the first step resistor and timing. The maximum allowable inrush current determined the the first stage starting resistance. The resulting acceleration time determined the step time delay. If the initial current was set too low, the second or final step would have too large a current or the first speed jump was too small.

Imagine the speed versus time curves as stair steps with a rounded front edge and the current curve as a peak decaying to a steady state. If the first step is too small the second step has to be larger and that second current peak is higher. Allowances were also made for reduced DC voltage or increased circuti resistance.

I may be wrong, but I recall that the peak current was limited by some characteristic of the series winding brushes.

A system was only accepted if the recorded peak current was below the maximum and oil pressure came up in time.

The DC starters had unusual safety features. Many were de-energize to start so any control wiring problem dropped the start/run contactors closed and started the pump. The field (which a PM motor won't have) was always energized and monitored by a current relay that alarmed if it dropped. The energized field also worked as a motor space heater and prevented the motor speed from varying due to increased field resistance as the motor warmed up.

Maybe your lube oil system is not that critical. But you should probably check the actual application and criteria. The PM motor may be fine, but you need more investigation to make sure the advantages of the PM motor, such as no brushes to fail, are not negated by poor starting torque or starter control issues.

 

[Skogsgurra]

Curt

We do not know if the PM motor is a three-phase motor or if it is a DC motor. Siemens used to have brushed PM motors before they switched to Three-phase servo motors.

I am worried about Lc85 and his complete lack of understanding. It doesn't help much to pick up fragments of what he thinks he observes and try to come to some sort of answer in public. So, I invited him to contact me for a more in-depth discussion. He has not done so and I think he is a lost case.

Lc85 doesn't seem to understand that he needs to put an experienced drive guy on this and that also worries me.

I have been involved in risk analysis of such lube pump systems. The system under study was built with standard three-phase induction motors and standard VFDs supplied from a bank of batteries. The main concern here was that the DC link overvoltage protection would trip when the VFD was hit by transients on the grid.



Gunnar Englund

http://www.gke.org

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