A lot has been written and said about AC drives in extrusion machine systems. And about DC drives in extrusion applications. Let's get to the basics.
- Even though the DC drive is more efficient than the AC drive, the AC motor is more efficient than the DC motor by a much greater amount. So overall, the AC system is more efficient than the DC system.
2. The AC system has a Power Factor of about 98% whereas the DC system varies proportional to speed from about 1% to 82%. Thus, the overall power distribution system is more efficient with AC than with DC.
Now, for some more details.
- DC drives switch line voltage to the motor using a Phase Control method; the AC drive switches the line using a diode to a DC bus.
- SCR switching will cause line notching (due to the relatively slow turn off time) which can be devastating to adjacent equipment. With the AC drive, diode switching does not cause line notching due to its relatively fast turn off time.
Extrusion machine’s control system’s response
- The plastics industry for years considered DC king because of its speed regulation with tach feedback. AC did not rate because it had poor speed regulation.
- Today, through Vector Control, the AC system without feedback (Open loop vector) will perform at least as well as the DC with tach feedback and AC with Encoder feedback can outperform DC with tach feedback.
- I acknowledge that DC can perform as well as the AC if the DC system can use encoder feedback.
Extrusion system reliability comparison between AC and DC drives
- DC is inherently more reliable than AC since it has fewer parts to fail (until you take the Motor into account).
- AC is inherently less reliable than DC, until you take the Motor into account, since it has more parts than DC.
- However, the DC system is less reliable than AC in that the major failure point of the DC system is the DC motor.
- The DC motor brush system is a high maintenance item giving rise to an inherently weak link in the system.
- A DC motor failure has a high probability of causing a DC drive failure as collateral damage.
- The DC drive system can also be less reliable than AC since it depends upon the AC utility line switched by SCRs to control the current to the DC motor.
- In the event of a motor failure, the SCR will continue to conduct till:
The AC line commutates to the opposite polarity or,
The AC line fuse blows whichever comes first.
The latter is many times the result of the former.
- With the AC system, the transistor is switched off much faster than the SCR.
- The AC drive can detect the high current of a motor failure and switch off before blowing a fuse or damaging an AC drive component
- Many AC drive manufacturers’ systems still cannot detect a destructive fault and will fail as a result of the motor failure.
Why Yaskawa Matters in Extrusion
- Yaskawa has broken the reliability curve (better stated that Yaskawa has SMASHED the reliability curve) of all other solutions.
- Yaskawa has, by nature of their quality process, their design criteria, and their excessive attention to detail, made the most reliable drive on the planet
- Yaskawa’s patented transistor system does in most cases turn off before a catastrophic failure can occur.
- Here is an application paper on Yaskawa and extrusion machine applications.
- When planning what to do with your extruder's DC drive system, consider the following: AC beats DC for performance; AC beats DC for energy savings; Yaskawa AC trumps DC for system reliability; chose an systems integrator who has experience with DC to AC retrofits.
Steve Lyons is a member of the Houston engineered solutions team and is widely known as an industry authority on AC drive applications in control systems, including extrusion, centrifuge and many others. You can reach him at firstname.lastname@example.org