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AC VS. DC DRIVE COMPARISON
AC and DC drives both continue to offer unique benefits and features that may make one type or other better suited for certain applications.
AC DRIVES MAY BE BETTER BECAUSE. . .
They use conventional, low cost, 3-phase AC induction motors for most applications.
AC motors require virtually no maintenance and are preferred for applications where the motor is mounted in an area not easily reached for servicing or replacement.
AC motors are smaller, lighter, more commonly available, and less expensive than DC motors.
AC motors are better suited for high speed operation (over 2500 rpm) since there are no brushes, and commutation is not a problem.
Whenever the operating environment is wet, corrosive or explosive and special motor enclosures are required. Special AC motor enclosure types are more readily available at lower prices.
Multiple motors in a system must operate simultaneously at a common frequency/speed.
It is desirable to use an existing constant speed AC motor already mounted and wired on a machine.
When the application load varies greatly and light loads may be encountered for prolonged periods. DC motor commutators and brushes may wear rapidly under this condition.
Low cost electronic motor reversing is required.
It is important to have a back up (constant speed) if the controller should fail.
AC DRIVES - PRINCIPLES OF OPERATION
Adjustable frequency AC motor drive controllers frequently termed inverters are typically more complex than DC controllers since they must perform two power section functions, that of conversion of the AC line power source to DC and finally an inverter change from the DC to a coordinated adjustable frequency and voltage output to the AC motor. The appeal of the adjustable frequency drive is based upon the simplicity and reliability of the AC drive motor, which has no brushes, commutator or other parts that require routine maintenance, which more than compensates for the complexity of the AC controller. The robust construction, and low cost of the AC motor makes it very desirable for a wide range of uses. Also, the ability to make an existing standard constant speed AC motor an adjustable speed device simply by the addition of an adjustable frequency controller creates a very strong incentive for this type of drive.
DC DRIVES MAY BE BETTER BECAUSE. . .
DC drives are less complex with a single power conversion from AC to DC.
DC drives are normally less expensive for most horsepower ratings.
DC motors have a long tradition of use as adjustable speed machines and a wide range of options have evolved for this purpose:
Cooling blowers and inlet air flanges provide cooling air for a wide speed range at constant torque.
Accessory mounting flanges and kits for mounting feedback tachometers and encoders.
DC regenerative drives are available for applications requiring continuous regeneration for overhauling loads. AC drives with this capability would be more complex and expensive.
Properly applied brush and commutator maintenance is minimal.
DC motors are capable of providing starting and accelerating torques in excess of 400% of rated.
Some AC drives may produce audible motor noise which is undesirable in some applications.
DC DRIVES - PRINCIPLES OF OPERATION
DC drives, because of their simplicity, ease of application, reliability and favorable cost have long been a backbone of industrial applications. A typical adjustable speed drive using a silicon controller rectifier (SCR) power conversion' section, common for this type unit, is shown in Figure 2. The SCR, (also termed a thyristor) converts the fixed voltage alternating current (AC) of the power source to an adjustable voltage, controlled direct current (DC) output which is applied to the armature of a DC motor.
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