You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

---------->>>>>Get FREE PLC Programming Tips

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

PLC training tools sale

Reply
 
Thread Tools Display Modes
Old May 14th, 2004, 08:51 AM   #1
atatari
Member
Canada

atatari is offline
 
atatari's Avatar
 
Join Date: Sep 2002
Posts: 92
Speed and Torque control with DC Motors

Hello everybody,

I understand that DC motor adjusts the speed by increasing/decreasing Av.But if the load torque increases, Av should be increased in order to provide sufficient Ia .Now speed increases because Av has increased so "N" should go up again and it goes to a loop .I don't understand how the torque can be controlled without thouching motor speed.
I think for variable speed applications torque must stay constant otherwise we won't be able to adjust the speed.Is this right?
  Reply With Quote
Old May 14th, 2004, 09:30 AM   #2
kamenges
Member
United States

kamenges is offline
 
kamenges's Avatar
 
Join Date: Nov 2002
Location: Brillion, WI
Posts: 3,643
Motor windings have a finite resistance. As you increase the current through this finite resistance you get a voltage drop across the motor windings. The sum of this voltage drop plus the voltage to overcome the motor back EMF is equal to the motor terminal voltage. As an equation:
Vm = (Kv * Nm) + (Im * Rm)
If you hold your motor terminal voltage constant and you increase the motor load the (Im * Rm) part of the equation will increase. Since Kv is a constant (well, pretty much anyway) Nm (motor speed) needs to decrease to keep the equation balanced. To get back to the desired speed you need to increase the voltage by an amount equal to the change in (Im * Rm). Some drives have an adjustment called IR Compensation. This is used when running in armature feedback to try and model the (Im * Rm) part of the motor voltage equation. This tends to help with no-load to full load motor speed regulation.

Keith
  Reply With Quote
Old May 14th, 2004, 09:42 AM   #3
atatari
Member
Canada

atatari is offline
 
atatari's Avatar
 
Join Date: Sep 2002
Posts: 92
I know these concepts.What I am trying to understant is by increasing the voltage speed goes up also.then if I want a fixed speed voltage has to be reduced which in turn reduces the Av which means less Ia and then motor can't keep up with the required torque.
is this right?
  Reply With Quote
Old May 14th, 2004, 10:19 AM   #4
kamenges
Member
United States

kamenges is offline
 
kamenges's Avatar
 
Join Date: Nov 2002
Location: Brillion, WI
Posts: 3,643
Not exactly.
I am assuming you are increasing your output voltage becasue the motor has already decreased in speed due to a load increase.

Let's use some actual (if not real) numbers with the above equation.

Kv = 100 V/1000 RPM
Rm = 5 Ohms

We will assume a perfect world to start with. There is no friction in the drive system. This means we require no current to run the motor (remember I said perfect world). The motor will run a speed based completely on the motor back EMF.
Our base equation is:
Vm = (Kv * Nm) + (Im * Rm)

We move things around to get motor speed:
Nm = (Vm - (Im * Rm)) / Kv

We apply 100 VDC to the motor leads and we get:

Nm = (100 - (0 * 5)) / 100
Nm = 1000 RPM

Now lets apply a load to the motor that required 5 amps to generate the needed torque to overcome the load. The motor speed will drop based on the following:
Nm = (100 - (5 * 5))/100
Nm = 750 RPM
To get my speed back to where I want to run I need to add a voltage equal to the voltage lost due to motor resistance. So I increase my motor terminal voltage by 25 volts to compensate for the IR loss. So we get:
Nm = (125 - (5 * 5)) / 100
Nm = 1000 RPM

I think the thing you need to keep in mind is that as the motor requires current to produce torque the armature resistance robs voltage that used to be available to counteract the motor back EMF. Another way to look at this is you need a voltage difference between the motor terminal voltage and the motor back EMF to get current to flow through the motor windings. If you hold the motor terminal voltage constant and yo u load the motor, the only way to get the voltage difference to drive the current is for the motor speed to decrease. This will decrease the back EMF allowing the current to flow. The motor speed will decrease until the difference between the back EMF and the terminal voltage is great enough to get the current level required for the torque needed to overcome the load.

Keith
  Reply With Quote
Old May 14th, 2004, 11:01 AM   #5
atatari
Member
Canada

atatari is offline
 
atatari's Avatar
 
Join Date: Sep 2002
Posts: 92
Thanks for the answer.I understand now.
  Reply With Quote
Old May 14th, 2004, 11:19 AM   #6
steve708
Member
United States

steve708 is offline
 
steve708's Avatar
 
Join Date: Sep 2003
Location: Ypsilanti, MI
Posts: 30
This is why you need some kind of feedback on DC drive systems.

The simplest and least accurate feedback for speed control is "armature" feedback where the drive package reads the armature current and adjusts the armature voltage to try and keep the speed constant. The best you can get from this is about 2% speed regulation.

Most drives will include a tachometer feedback input so you can sense the actual speed of the shaft and use this to adjust the input command to maintain constant speed. This will get you down to about 0.5% regualtion.

In either case as the load increases the armature voltage will be automatically increased to counteract the IR loss in the winding but the speed will remain constant, and current will be provided to drive the load up to the limits of the drive package.
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
control torque and speed necyil LIVE PLC Questions And Answers 5 November 15th, 2004 05:00 PM
50HP DC motor to AC !!! Pierre LIVE PLC Questions And Answers 26 October 18th, 2004 02:47 PM
120 volt speed control cciblazer LIVE PLC Questions And Answers 5 October 4th, 2003 02:45 PM
Pumps Alan Case LIVE PLC Questions And Answers 14 June 9th, 2003 04:30 PM
Motor guru's I need your help... Russ LIVE PLC Questions And Answers 50 July 17th, 2002 05:11 PM


All times are GMT -5. The time now is 09:48 PM.


.