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May 14th, 2004, 09:51 AM  #1 
Member

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? 
May 14th, 2004, 10:30 AM  #2 
Member

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 noload to full load motor speed regulation. Keith 
May 14th, 2004, 10:42 AM  #3 
Member

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? 
May 14th, 2004, 11:19 AM  #4 
Member

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 
May 14th, 2004, 12:01 PM  #5 
Member

Thanks for the answer.I understand now.

May 14th, 2004, 12:19 PM  #6 
Member

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. 
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