AC vs DC Motor

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Shaneyj

Shaneyj

Senior Member
Location
Katy, Texas
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Project Engineer
Hello all, you have fielded a couple questions from me in the recent past regarding large hp motors for an oil field pump (positive displacement) application. This is an add-on to those questions.

What, if any, are the advantages of using a large hp DC motor instead of AC? Large like 800 hp and above. (800 to 2000)

My experience with DC motors runs out with my cordless 20V Dewalt drill.

Thank you for any input!
 
Shaneyj said:
Hello all, you have fielded a couple questions from me in the recent past regarding large hp motors for an oil field pump (positive displacement) application. This is an add-on to those questions.

What, if any, are the advantages of using a large hp DC motor instead of AC? Large like 800 hp and above. (800 to 2000)

My experience with DC motors runs out with my cordless 20V Dewalt drill.

Thank you for any input!
Click to expand...
A few thoughts.
I'm fairly long in the tooth and DC motors were the mainstay for my earlier life. Then came along VFD machines. For for quite a few years the DC and AC machines lived side by side but, in time, the AC machines took precedence. And that's about where we got. DC machines slipped in to their grave like Puff The Magic Dragon. (Children's tale...:)

More practically:
The motor drives, SCR, are a bit less complex and lower cost but DC motors are more complex with commutators and brushes, That usually result in significantly more maintenance costs.
The AC drives, on the other hand, are more complex but the motors are much simpler. And that's where we are at at the moment.
 
Besoeker3 said:
A few thoughts.
I'm fairly long in the tooth and DC motors were the mainstay for my earlier life. Then came along VFD machines. For for quite a few years the DC and AC machines lived side by side but, in time, the AC machines took precedence. And that's about where we got. DC machines slipped in to their grave like Puff The Magic Dragon. (Children's tale...:)

More practically:
The motor drives, SCR, are a bit less complex and lower cost but DC motors are more complex with commutators and brushes, That usually result in significantly more maintenance costs.
The AC drives, on the other hand, are more complex but the motors are much simpler. And that's where we are at at the moment.
Click to expand...
Good summation. To add to it, the ability to find service shops that can and know how to work on large DC machines is dwindling fast. I recently got involved in a project to upgrade 6 x 500HP DC drives with new DC drives, but as a separate contract, they wanted the DC motors refurbished. We negotiated the contract, ordered the drives, waited for them, built the panels, removed the old drives, installed the new, rebuilt the old FSK tone transmitter control system for them and got paid, before they even got a response on the contract to refurbish the DC motors. So we had to commission the drives into the old motors (which was difficult), then RECOMMISSION them 2 years later when they finally got the motors rebuilt.
 
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