Question on replacing DC motors and drives with AC motors and drives

[ctolbert]

Does anyone have guidelines or online resources used in retro-fitting dc motors and drives in general with ac motors and drives.

Common pitfalls?
Mistakes?
Good practices?
Opinions?

Thanks,
Carl

 

[jbartos]

Suggestion: Visit

http://www.serd.ait.ac.th/reric/Variable Speed Drives - TFS 2[1].pdf

etc. for more info
Rather than to try to reinvent a wheel, an ac motor manufacturer may be contacted for a substantial technical support in this area. Contact a suitable or preferred larger ac motor manufacturer for information.

 

[amptramp]

Look closely at the starting torque requirements of the application. An AC motor has difffernt starting torque characteristics than a DC motor. Futher, an AC Drive has different starting torque curves than a DC Drive.

An AC Drive's (and hence the motor's) starting torque is limited by the current limit of the drive, typically 250 - 300% of full load current for some short interval of time.

For example, I once saw 30 hp Reliance DC Drives replaced with 30 hp Allen-Bradley AC Drives. The application was driving gearbox driven augers conveying "raw" wood chips.

The AC Drive could not start the auger. It would trip out on overcurrent. The drive was "optimized" by the factory, but it still could not handle the application. Eventually, a larger, 40 hp I believe, AC Drive drive was installed to provide the required starting torque.

 

[jbartos]

Suggestions to ctolbert (Electrical) Jan 16, 2004 marked ///\\Does anyone have guidelines or online resources used in retro-fitting dc motors and drives in general with ac motors and drives.
Visit

http://www.jarscopublishing.com/ASDsample.pdf

for a reference, section 3.4
Common pitfalls?
///There is a wider variety of AC Motor Drives. It is necessary to be more careful in the selection of AC Motor Drive.\\Mistakes?
///It easy to make a mistake in filter requirements on the output side and harmonic suppression on the input side.\\Good practices?
///Testing and measurements to ensure proper application of the AC Motor Drive.\\Opinions?
///What was the reason for DC Motor Drives to be selected in the first place? In some applications, the DC Motor Drives are required, e.g. high rpm, smooth speed control in wide rpm range, somewhat less filtering required, load torque-speed characteristics, etc.\\

 

[jbartos]

Suggestion: Visit

http://www.acev.fi/theelect.htm

for comparisons of Electric Vehicles with DC drives to AC drive

 

[jraef]

Common pitfalls:
1) Trying to do it on the cheap. To do it right, you must look at all of the things you need to do, then make absolutely sure the AC system will do them. These include starting torque, running torque, breakdown torque (maximum torque), load step change response, operation at zero-speed, operation at high speeds, audible moise, ambient temperature to name a few. Don't try to get out of buying the right AC combination by ASSuming that any of these issues don't matter. Know for sure before making a decision.

2) Running an AC motor at low speed continuously. They are not designed for it. DC motors are. If that is your operating criteria, buy an AC motor with a separately powered fan. Oversizing the motor a bit helps too.

3) Use reactors, especially on the load side. Don't listen to the vendors who will try to get the order by convincing you that their drive doesn't need them. DC motors and drives are very forgiving of transients, mainly because of the rectified power going right to the motor. The added transistors on the load side of an AC drive are susceptable to being damaged by transients in the load connections or motor windings (think of water getting into the conduit for instance). Reactors are cheap insurance.

Mistakes:
1) Not making sure that mechanical advantages are accounted for. In general, DC motors turn slower than AC. If the speed was changed with pulleys, sheaves or gears, do not assume that buying a faster AC motor will do the job right. Do your due dilligence on torque and speed calculations throughout the system.

Good practices: (aside from those mentioned above)
1) Use a true Vector drive. Be careful that some vendors will call their drive a Vector, but cannot really perform as such. Choose a drive that can be easilly changed from "Open Loop" mode to "Closed Loop" if needs be. The best ones are capable of doing that by a slight change in programming and the addition of a shaft encoder on the motor or load. Be wary of VFDs that claim DC performance without the need for a shaft encoder. They are typically just a little bit shy of really duplicating DC performance and that may end up being the little bit you needed (see pitfall #1).

2) Use a good quality motor from a reputable manufacturer. "Inverter Duty" motors are now available from just about everyone and they are a good idea. But don't ASSume that any inverter duty motor is as good as the next. Never forget that "You Get What You Pay For". Cheap motors are usually cheap for a reason. Sometimes it may just be that you are buying from the actual manufacturer instead of a brand-label of a good name, and that is a legitimate bargain. Ask around for references in your area. A good supplier is just as important as a good manufacturer.

Quando Omni Flunkus Moritati