DC shunt motor as a replacement of DC compound motor

[Shark96]

Hi. I am facing an issue where a dc compound motor of 220 volts, 5.5kW has failed. I have an option of replacing it with a dc shunt motor of same rating. However, the compound motor had timed external resistance in series of armature winding in the starting circuit. Can we start the dc shunt motor without the external resistance (Direct application of 220 volts dc)? The application of motor is seal oil pump for turbine.

 

[xnuke]

Since DC motor inrush current is only limited by armature resistance, you'll probably need the starting resistors. A simple Ohm's law calculation using 220 V and the new motor resistance should tell you if you need them, and a few more calculations would help you see if the resistors you have will suffice.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

There is a possibility that the original starter will start the new motor.
But the series winding of the compound motor acts to reduce the starting current surge.
You may expect a higher current surge from a similarly sized shunt motor.
The speed regulation of that size shunt motor should be quite good.
The speed regulation of that size compound motor should be better.
The starting surge of the a DC motor is too high for DOL starting.
An exception is some very small motors with enough armature resistance to limit the starting current.
The inefficiency of relatively high armature resistance makes this design unrealistic for all but the smallest motors.
The heater fan in your automobile is about the largest size of shunt motor that is started DOL.
I would consider up-rating the existing starter by adding another resistor in parallel with the first step of the starter.
Also, extend the time delay between the starter steps a little.

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

 

[Shark96]

Thank you for the replies. The current scheme with the compound motor has 2.495 ohms external resistance in series of armature winding which is bypassed in 0.8 seconds. Would the same starter scheme work for the shunt motor? The armature resistance of the shunt motor is 0.6 ohms.

 

[waross]

What was the combined resistance of the armature and series winding of the original motor?
What is your DC supply?
I may have misspoke:You may have to add some resistance in series to the starter resistance.
If you are measuring the armature resistance with an Ohmmeter, put jumpers across the brushes.
That is one end of the jumper held directly on the commutator by the brush and the other end connected to the brush holder.
The connection between the brush and the commutator is very non-linear and will introduce more metering error than you can ignore.

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

 

[Shark96]

The combined resistance of the armature and series winding of the original motor was around 0.6 ohms of armature resistance and 2.495 ohms external series resistance. Total 3.095 ohms. External resistance rated at 28 Amps.
DC supply is 240 Volts
I will get back to you with the resistance values with the method you said but kindly for now assume that the resistance of shunt motor armature is also 0.6 ohms.
Please see below drawing of the original starter of the compound motor. How much time would shunt motor need to achieve the rated speed. The compound motor series armature external resistance got bypassed at 0.8 seconds.

 

[xnuke]

The problem with replacing a compound motor with a same kW shunt motor is that the compound motor will probably have better starting torque due to its series field winding. It may take longer to start your load meaning the bypass timer setting would need to increase, or it may not be able to start it at all.

Given your values, if you replaced the compound motor with the shunt motor you assumed has the same total armature resistance, your maximum inrush current won't change. I question that assumption, though, and that's why waross said you might need to add series resistance. Without a series field winding, I'd expect a lower armature resistance for the shunt motor. After all, minimizing the armature resistance minimizes losses, and I'd expect the motor manufacturer to try to make their machine as efficient as possible.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

It's hard to say without seeing the machines.
As soon as the rotor starts to rotate, the series field will increase the back EMF, reducing the armature current.
But if it is all over in 0.8 seconds, the pump is probably easy to start.
240 Volts applied to 3.095 Ohms should give about 77.5 Amps.
I expect that the line loss and the drop across the brushes will reduce that current a little.

What would I do?
First a warning:
I have been censured at times for my "Try it and see" approach.
I would compare the two machines;
I would look for construction differences. (Is one machine built more robustly than the other?
I would compare the brush jumpers for length, cross section and general condition.
I would verify the resistances. With two brush machines I would measure the armature resistance with the Ohmmeter probes directly on the commutator bars. With a multi-brush machine I would use jumpers across all of the brushes.
Then I would decide whether to do a test start at reduced voltage or apply full voltage.
If it is possible to run the motor uncoupled, I may do some starts at 1/2 rated voltage and observe the starting current.
I expect that the starting current to be 1/2 of the full voltage starting current but of longer duration.
I would observe the current when the first stage of resistance is cut out. If I judged the current surge to be too high I would adjust time a little longer.
Similarly, if the current surge when the second stage of starting resistance is cut out and the motor goes DOL I would extend the timing.
Or,
I may decide to test at full voltage and follow the same steps to adjust the timing.
I have encountered and been responsible for many DC machines.
All were shunt machines, and all were driven by MG sets.
An anecdote concerning your DC motor starter:
I was working as an instructor teaching apprentice electricians at a vocational school.
During some free time, the head instructor asked me to help unpack some new equipment that had just arrive.
The new equipment was a DC motor starter similar to the one that you have.
The head instructor asked me what I thought of the starter.
"Well, it is a nice piece of equipment, but Pete, does anyone use these anymore?"
"Well, no. It's obsolete.
But I should say that when we ordered it it was not obsolete.
We got a call from the purchasing department asking if we still wanted it.
We thought that there was a chance that some of our students from the very old mills may still see some of these in service and it would be nice for them to see one that they could work with in our shop.
And, if we didn't take it we would not get anything, so we said 'Yes we want it'"
That was 45 years ago.
I realize that a turbine lube oil pump is a special case.
I hope that it is not too late in the purchasing cycle to consider a complete upgrade of the pump, motor and drive.
Others here are better able to advise as to what is the current practice for turbine back-up lube pumps.

Given the possible cost of damages that may result from a turbine lube pump failure, I understand if my "Cowboy" approach is not acceptable.
In your case, for such a critical piece of equipment, I may be making a case to management for an upgrade to the latest technology from a recognized turbine manufacturer.
Costs a little more but worth it.

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

 

[Shark96]

Thank you sir for the elaborate response. Just wanted your opinion on the external resistance for the field winding. Do we need that for the shunt motor?

 

[waross]

You will use the external field resistance to adjust the speed of the motor.
With no resistance the motor will run slower but with more torque.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Shark96]

There are 4 unmarked wires coming from the shunt motor. 2 for field winding and 2 for armature winding. What will be the effect of polarity of both windings if we connect it to the starter?

 

[waross]

The direction of rotation.
What is the value of the turbine that you are protecting with equipment that may be obsolete by 50 years or more?

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

 

[lukin1977]

What about replacing the resistor starting bank with a DC Drive? Then you can safely replace the compound motor with a shunt motor

 

[waross]

You may consider reducing the field resistance when starting to give greater starting torque.
Switch the field setting back to the original at the same time as the starting resistance is cut out.
I would try starting with about 2 or 3 seconds and look at the armature current.
Then set the time back to a shorter delay.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!