Replacing DC Motor to AC Motor

Hi...
I have Bollina cutter machine (cut to length machine for metal application) using dc motor complete with analog drive and ac tacho. I want to replace this dc motor using AC Motor (Inverter Duty, Baldor) and PowerFlex 700 Vector Control (complete with encoder and braking resistor).
Could it works? Any idea or suggestion?
Thank you.
Charly

Replace DC Motor with AC Motor

Complete machine data:
machine speed: 30 meter/min
Existing DC motor speed: 1750 RPM
New AC Motor: 400 VAC, 60 Hz, 1750 RPM, Inverter Duty, Brand: Baldor.
Thank you
Charly

Whats the hP of the motors, what about shaft size?

Eugen makes a good point--check out the motor frame size. You may need an adaptor coupling. Otherwise, the AC motor will likely be smaller, especially shorter than the DC but you will still need adaptor rails to position it properly.

And why spend the money for an encoder. Use the sensorless vector capabilities to the max without an encoder. Performance should be plenty good enough. Remember, with that analog tach on the DC system, the best you could hope for was 1% speed error. Any good sensorless system with a premium efficient 2% slip or tighter motor should be able to do better than that.

Existing DC motor 5 HP. Shaft size 41.5 mm, using pulley.
AC motor 5 HP, shaft size 43 mm, modify pulley.

What presently commands the DC drive? (control)
What control type (name type model #), output voltage (±10)?

I don't understand (AC tacho), do you mean tach or pulsecoder/encoder or ???

I see feed back as your main concern

Rod (The CNC dude)

Two things: first, if you can replace the pulley with a smaller one sized so the motor must run at 90Hz (2700rpm) when the machine is running at max speed.

Second, forget the encoder and choose a sensorless vector drive.

Regarding the first point, if the pulley can't be reduced in size, substitute a six pole (1200rpm) inverter duty motor for the four pole (1800rpm) unit and run it up to 90 hz to get 1800 rpm. System performance will be greatly improved probably exceeding the old DC system and for sure without a tach or encoder.

I dont understand, wht switch to AC motor. Mr personally prefer DC motors for this type of application. DC seems to give you more precise control.

But again this is just my opinion

Charlie maybe you should consider registering.

Anyway for maximum plcs.net experience I recommend reading this thread clik me.

I wold like to hear more about an AC tach also. The big advantage is you get freq output which could be converted to a voltage or mA signal. Sounds kinda neat.

There is nothing wrong as such about DC. Great for speed control
IF
1. YOu like changing and seating brushes
2. Cleaning out all that carbon dust
3. Undercutting mica
4. Neutral plane setting
5. All that arcing and sparking when you forget about 1 2 3 4
6. Cleaning the reostats

Had two 300 KW DC/AC mg sets on submarine. Stbd was OK, port ate brushes - replaced em every two or three months. Got real saturated with replacing brushes.

VFD from what I have seen of it looks like a good replacement for DC. Course your electrician may not like this but there is always something else breaking.

Are they now rating motors up to 150% full rated speed -- ie OK at 90 Hx when rated for 60?? IF replacing a 1750 why not use a 3600 RPM w reduction instead of 90 Hz. This is another learning thing for me but until I understand more I am going to be leery about running motors at 150%. I know they run em up pretty fast for balancing on test stands but the test stands have a scatter shield on em.

Dan Bentler

AC tachs were quite common on DC drive systems that did not reverse. The AC was simply rectified to DC and used as a speed feedback device with lower maintenance than a DC generator.

No need to be leery about 150% overspeed on cast rotor induction motors. Simply look at the torque-speed curves in the manufacturer's motor catalog. I have a Reliance catalog right here that shows 90Hz operation on most of their motors with 120 hz on the inverter-duty versions. Don't take my word for it.

The benefits of using a lower base speed motor are mainly because the available torque is higher at starting and up to the motor base speed. At the same hp rating, a 3600rpm motor develops 1.5ft-lbs of torque, an 1800rpm motor develops 3ft-lbs, a 1200--4.5ft-lbs, and a 900rpm motor--6ft-lbs per hp. Of course, the torque drops off as the motor goes over its base speed but below base speed you have the benefit of extra torque. At no time will the lower base speed motor develop less torque than the higher base speed version.

Dickdv,
Is AC tach the same as resolver? It's something new to me.

Rod (The CNC dude)

The application is cutter machine, using DC Motor as main drive. This motor will pull material from uncoiler section and use the encoder to measure length of material and motor will stop in length setpoint and cutter will cut the material. Machine speed is 30 meter/min with cutting length variation is maximum 0.2 mm. We need high precision control of ac motor and why we choose PowerFlex 700 vector control. We need close loop for accurate speed control. Any comment?
Thank you.

Originally posted by Rod:

Is AC tach the same as resolver? It's something new to me.

Click to expand...

An AC tach and a resolver are two different things. Think of an AC tach as a small three-phase AC servo motor running as a generator. It has permanent magnets on the rotor and windings around the stator. This will give you htree sine waves spaced 120 degrees electrical apart, with the frequency and amplitude dependent on speed.

A resolver is a rotaty transformer with variable flux coupling. It has one excitation coil and two signal coils placed 90 degrees apart. The excitation gets a fixed voltage and frequency. The signal coils will produce a signal at the excitation frequency but with an amplitude depenndent on the rotational position of the rotor.

Originally posted by Charly{
We need high precision control of ac motor and why we choose PowerFlex 700 vector control. We need close loop for accurate speed control.

Click to expand...

I think DickDV's point is that you currently have a DC system that works to your expectations. The best that a DC system with an analog tach can do is 1% speed regulation. So by that you have defined your level of performance. 1% speed regulation is good enough for you. According to DickDV you can expect that or better with a sensorless vector drive and a properly selected motor. So why spend the money on the encoder, cabling and extra wiring to get performance you don't need? Now, if you are actually looking to IMPROVE your performance relative to the DC system then you need to do a little more evaluation.

Keith

I believe the drive may not need the encoder but the application does. He stated it was a cut to length system. Read this thread if you have time, it pertains to building a machine using an AC drive/motor on a cut to length system for steel wire, which seems to be what you are doing. NOTE this is probably the longest, most informative thread this site has created so far.
http://www.plctalk.net/qanda/showthread.php?t=11015

If you need an accurate count, in this case for length, then I would think the drive would need added input. I know a drive can do alot with speed and torque but position is another issue. I believe a plain incremental encoder would be fine for this application.