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:
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.
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.
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.
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.