AC vs. DC drives

Should I specify an AC drive or a DC drive in my application?

In short, consider the total cost and performance benefits of each. AC is perceived as cheaper because AC motors are cheaper than DC motors. However, you must consider the total installed cost and performance needs:

Consider AC DC
System Purchase Price More attractive in some power ranges. Best at high power or very low power.
Drive Purchase Price and Maintenance Usually more expensive using the latest technology. Uses well proven technology that is cost effective.
Motor Purchase Price and Maintenance Less expensive and more reliable. More costly due to Commutator and brush technology. Accessory faces readily available.
Installation Cost Several subtle issues may drive cost up. Usually less expensive and straightforward.
Overhauling Load (Braking) RED ALERT Easily accommodated
Efficiency Two power conversion stages. One stage conversion
Power Factor Constant and better. OK at high speed
Harmonics 6, 12, 18, 24 pulse converters available at a cost. 6 or 12 pulse converter
Performance Servo like Lower bandwidth

System Purchase Price

It is usually the case that the combined cost of a DC drive and motor is lower for very small HP applications: AC wins from 5HP to about 100HP, then DC wins again as power increases. If you already have a DC motor, then DC is probably going to cost less.

Drive Purchase and Maintenance Costs

The "brains" of both AC and DC drives cost about the same amount and use similar processors. The power and firing electronics dictate the total drive cost.

DC drives use SCRs to handle the switching power. AC drives use IGBTs as switching elements. Fewer IGBTs are required in a power bridge, but they are much more costly than SCRs, especially at higher power levels.

AC drives also include an AC to DC conversion section. This uses diodes and bus capacitors not necessary in a DC drive. These capacitors are expensive and become a maintenance issue over time.

Motor Purchase and Maintenance Costs

AC motors are usually less expensive than DC motors because they are simpler. AC motors don't have brushes like DC motors. Brush maintenance cost is usually sited in any cost comparison. The only good thing about DC brush maintenance is that it can be scheduled and usually doesn't result in unexpected down time.

Installation Cost

For multiple drives, common bus AC drives can save money on system installation since only one mains connection is required vs. individual DC drive mains connections. AC motors require only three power conductors; DC motors require four. However, most AC drive suppliers recommend special shielded power cables to minimize electrical interference from the AC drive. A motor power lead filter may be required if the cables to AC motor are over 100 ft. long. Also, specially designed EMI filters may be required to be installed on the input side of the AC drive when it is installed in close proximity to EMI sensitive instrumentation devices. Add up all the extras when comparing installation costs.


Does the application require the drive to provide braking torque? If so, how much and how often? Even a little braking torque can be an issue if it is continuous. It adds up.

DC drives can easily pump braking energy back to the AC mains. In fact the cost for full regeneration is so reasonable that Nidec-Avtron offers regenerative braking bridges for free up to about 300HP. Above that, non-regenerative DC bridges are available and cost less than their AC drive counterparts. Alternatively, dynamic braking can be provided by adding an external DB contactor and DB resistors. The advantage goes to DC.

AC drives are more complex. Braking energy goes back to the DC link (bus) inside the drive. This makes the DC voltage rise. If it rises too much, the drive trips off line. There must be a way to absorb this energy before that occurs. This can be by DB resistor and DB chopper circuit or by the more complex means described next.

AC drives may be stand-alone or common bus. Most are stand-alone drives that are powered from the AC mains. (Each drive is powered separately.) This type of drive usually relies on DB resistors to absorb braking energy. Care must be exercised to specify the maximum braking and duty cycle to avoid overheating the resistor or the DB chopper in the drive.

Common bus AC drives are available, and can use braking energy more efficiently. They are powered by a common DC bus with one AC power supply. Several common bus AC drives are tied to the same DC bus and can dissipate braking power among them. This works so long as the combined sum of all the drives is always drawing energy from the AC mains. Emergency stop may require DB resistors. A more complex and expensive power supply can be purchased that regenerates back to the AC mains line. Alternatively, a regenerative DC drive, or for even more money, an inverter can be used to regenerate to the line for improved harmonics.

In short, AC drive applications that require braking cost more than DC applications, and require more complex components that fail more often.


DC drives are more energy-efficient. AC drives have two stages of power conversion (AC to DC then DC to AC). DC drives have one stage (AC to DC). Each stage has energy losses, in the form of heat generated in the drive. More heat is generated during switching (switching losses). The higher the switching frequency, the more the losses. If braking is required, the regeneration available in most DC drives will increase efficiency further beyond the DB resistors used on most AC drives. Due to their additional stages and higher switching frequencies, AC drives generate more heat than DC, and are less efficient.

Power Factor

AC drives have a better power factor in most applications. DC drives approach the same power factor as AC drives only when the DC drive is operated at or near maximum speed.


AC and DC drives often produce similar harmonic problems on the AC mains. In either case, harmonics can be reduced by going to a 12 pulse (six phase) rectification scheme. This requires a phase-shifting transformer. The cost added for a 12 pulse AC drive is less than it is for the 12 pulse DC drive. AC drives can also be purchased in 18 or 24 pulse configurations. An active front end AC drive which utilizes an inverter for the DC bus supply, though costly, has the lowest harmonic content.


Higher switching frequency in the AC drive generally results in higher transient response capability than possible in a DC drive. AC motors permit to higher speeds. The AC motor may also be lower inertia. If the application requires "servo-like" performance or operation at high speed, then AC is usually a better choice.

AC and DC can both be operated without an encoder. Performance suffers in either case. Low and zero speed performance are most severely affected.

For more information, Contact Nidec-Avtron using the Contact Us box on this page.

DC Digital Retrofits v. Entire New DC Drive Replacement

Digital AFM retrofits offer the benefits of a drive replacement, at a fraction of the price!


  • Spare Parts Available
  • Higher Machine Performance
  • Increased MTBF (Mean Time Between Failures)
  • Reduced MTTR (Mean Time to Repair)
  • Modern Diagnostics

SCRs and power bridges are highly reliable units with few design changes over the last 20 years and spare parts are widely available if needed. The big improvements that have been made in drives are in the drive "smarts," and these are the parts on old drives that are hard to troubleshoot, repair, and replace.

Nidec-Avtron can replace the "brains" of your drive, using our AFM, often during normal maintenance outages, and give you the benefits immediately!

Existing hardware which generally remains in the case of an AFM DC drive retrofit:

  • SCR Bridge including SCRs, heat sinks, and bus bars
  • SCR fuses
  • SCR snubbers (resistor/capacitor network)
  • SCR bridge cooling fans
  • Drive cabinet and associated cooling fans
  • Armature current sensing transformers or shunt
  • AC circuit breaker
  • DC main contactor
  • AC power wiring into the cabinet
  • DC power wiring from the cabinet to the motor (armature and field connections)
  • Tach feedback wiring from the motor to the cabinet
  • Motor thermostat wiring
  • Motor blower starter and wiring to the AC blower on the motor

Components which are generally replaced in a retrofit:

  • Drive regulator modules. Entirely replace the electronics in the drive regulator.
  • Motor field controller and field power bridge. Function built into the Nidec-Avtron unit.
  • SCR gate pulse transformers. Replaced with modern picket fence firing type.

Struggling with this decision? Use the Contact us page on this page to request more information.

Open Loop v. Closed Loop - Is Encoderless good enough for my application?

It's all about performance and safety. Most AC drives, and some DC drives, are applied without encoders. Most of the AC drives sold today are for stand-alone applications which don't require great speed regulator performance or position control such as common pumps, fans, and compressors. System applications may frequently benefit from the improved speed and torque regulation performance available with an encoder.

An encoder is generally indicated for the following application situations:

  • Best available speed regulation performance at low speed (less than 10% of base speed)
  • Precise positioning requirements (metal cutting, forming)
  • Holding loads stationary under force, load float (hooks, cranes)
  • Highly non-linear load behavior (injection screws, presses, mixers, gas handlers)
  • Operation above base/nameplate motor speed
  • Coordination of several drives on a process line (steel, paper, films, etc.) where precise speed control and/or draw control are needed.
  • Cranes, hoists, mining shovels, people movers, and other applications which require verification of position and/or speed for safety.

Struggling with this decision? Use the Contact Us box on this page to request more information.

Contact Avtron

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