variable speed drives 2

[AT1000]

can you explain in general terms, say for the eurotherm 690 ac drive the functions required to control the drive output from an outside source 0-10 volts and how to use the digital outputs when the drive is enabled. AS you can see I'm new to the use of ac drives but have installed quite a few as long as they only use the speed pot. I need to understand the meaning of analog 1, analog 2, and how and what is 4-20 ma used for control. Deseperate for some good info. to understand general usage. Also to replace a dc drive with an ac motor is there any general rating formula when all the info. you have is on the motor plate. Please assist where possible.
thanks
tom

 

[1x1x1x]

The motor speed (the drive output frequency and voltage) is a function of the speed reference.
The speed reference is normally connected to the drive as analog input, which is normally 0-10 V DC or 4-20 mA DC. The 0-volt or 4- mA equal the minimum speed (this parameter set in the drive software), the 10 volt or 20 mA is equal the maximum speed.
The 4-20 mA signal is mostly used because this signal is less sensitive for the electrical noise, and in some applications less than 4 mA can indicate the loss of speed reference. The 4-20 mA signal is converted to 1- 5 volt DC or 2 -10 volt DC signal using either a 250 ohm or 500 ohm burden resistor at the analog input.
The analog 1 and analog 2 are independent analog inputs which can be used for speed or torque control (based an the configuration and application).
To replace the DC drive with an AC the application shall be studied particularly the torque requirements at low speed and any regenerative or breaking capability of the drives speed control accuracy (static and dynamic) etc.

 

[jraef]

Wow, I can't believe that you are buying drives from someone who can't answer these simple questions! I just goes to show how "lowest pricing" is affecting the marketplace. When an end user has to go to anonymous strangers for advice on how to use the most basic functions of his VFD we are all in trouble. Oh well, I guess it has come to this.

When you used drives in the past with pots for speed control, you were giving the drive a 0 - 10VDC signal through that potentiometer. As you moved the wiper of the pot you were changing the amount of resistance, increasing or decreasing that 10VDC signal generated in the drive. Now that you are not using the pot, that signal is just coming from somewhere else and another device is changing resistance in order to cange the voltage going to the VFD.

Analog 1 vs analog 2 is refering to the fact that the VFD can be told to respond to either of those two signals, usually by programming a digital input to respopnd to a switch telling it to do so. For example, lets say you have Lucile Ball standing at a conveyor run by a VFD moving chocolate candy. The speed of the conveyor is being controlled from another room by an unseen operator using "Analog 1". In the classic TV episode, Lucy resorts to eating the candy as it is moving too fast for her to package. Now she could just flip a switch that says "Load Out Speed Control" which energizes a Digital Input of the drive programmed to switch speed reference to Analog 2. A pot next to her would then take control of the drive speed instead of the guy off camera. She could now slow the belt down to a workable speed using "Analog 2". No longer humorous, but effective.

4-20ma is an industry standard process signal that is more imune to electrical noise and interference than a voltage signal would be. It is also good for long distances where a 0-10VDC signal my loose voltage due to wire resistance. It works by having a "current loop" on the twisted pair of signal wires, with various devices capable of altering the current passing through them in response to changes in whatever they measure. The VFD simply responds to that same current change by changing speed.

Swapping out a DC drive for an AC drive is something best left to professionals that will look at all of the available information. It can be very problematic to simply look at motor nameplate information. The latest and reatest AC drives and motors on the market can duplicate all of the functions of a DC Drive and motor, but some of them are cost prohibitive in some cases. I would suggest finding a good systems integrator with DC drive swap experience.

Good luck. Subvert the dominant paradigm... Think first, then act!