AC vs DC Motors

[CuriousElectron]

Greetings,

What are the advantages of using DC motors for automatic slide gates and other type of gates in water conveyance applications? Apart from providing a battery as a backup for powering the motor when the utility power is lost. Is better gate position accuracy achieved when employing DC type motor? I think pulsed controls are used for positioning the gates, so perhaps better setpoint control is achieved with the DC motors.

Thanks for your thoughts,
EE

 

[jraef]

If you were using DC Servo Motors for positioning, I would say yes, maybe slightly more accurate but I can't imagine the +-.01% improvement would be meaningful in something like a slide gate for water flow control. If the power need is low (i.e. under 3HP/2.2kW or so) and battery backup is important, I would go with Permanent Magnet DC motors (permanent magnet to eliminate the maintenance headaches of having a brushed DC motor exposed to the elements).


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Doesn't your control scheme have position monitoring separate from the motor?

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

 

[che12345]

Dear Mr. CuriousElectron

1. I presumed:
a) you are not using DC Servo Motors [for positioning],
b) providing a battery as a backup for powering the motor when the utility power is lost, [is essential].
2. I am of the opinion:
a) that an [AC three-phase (NOT single phase) squirrel] motor would be better in terms of cost, availability from stork, reliability and requires little or minimal maintenance. A DC motor with brush or w/o brush (PM) would not work for more than two? years without [careful regular] maintenance.
b) To remain operational when utility fails may be taken care of by installation of a (UPS+battery bank). The extra item is the UPS, while the battery bank would be about the same (Ah) capacity irrespective of whether using DC or AC motor. BTW the motor operation is likely to be up(open)-down(close) for say <10min, twice a day. It is NOT going to be 7/24 operation!
Che Kuan Yau (Singapore)

 

[vamc23]

While both A.C. and D.C. motors serve the same function of converting electrical energy into mechanical energy, they are powered, constructed and controlled differently.The most basic difference is the power source. A.C. motors are powered from alternating current (A.C.) while D.C. motors are powered from direct current (D.C.), such as batteries, D.C. power supplies or an AC-to-DC power converter. D.C wound field motors are constructed with brushes and a commutator, which add to the maintenance, limit the speed and usually reduce the life expectancy of brushed D.C. motors. A.C. induction motors do not use brushes; they are very rugged and have long life expectancies. The final basic difference is speed control. The speed of a D.C. motor is controlled by varying the armature winding’s current while the speed of an A.C. motor is controlled by varying the frequency, which is commonly done with an adjustable frequency drive control.
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