Control loop for DC drives

rejoe.koshy

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Join Date
Dec 2011
Location
kolkata
Posts
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Hey guys,
I need help in understanding the control loop of DC drives.I went through some articles on it but could fully comprehend reason behind controlling the current with a inner current loop, when ideally they should be controlling voltage.

The firing angle is varied to control the speed of motor , which is basically control of voltage not current.Does the drive convert current to voltage output ?

Can anyone direct me to a good technical write up .
 
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I don't know about a 'technical write up' . . .

But DC drives with SCR's converting the heap big AC into DC as a general rule one
is talking ohms law - the more voltage the more current.

During the positive half cycle of the AC sine wave, the gate on the SCR is pulsed and
it comes on (starts passing current) for the rest of the positive half cycle. When the
sine wave goes negative the SCR turns off again. On the next half cycle the SCR is
fired again - repeat over and over and over again every half cycle.

Fire the SCR earlier in the half cycle, and one gets more current and more voltage.
Meaning the motor goes faster. Later in the cycle and less volts and less current
and the motor goes slower.

As far as the motor is concerned it is getting straight DC. In reality it is a pulsating
DC but with three phases being pulsed in sequence it pretty well smooths things out.

But it really is more like a straightforward ohms law - the more volts you shove at it
the more current one gets through it. None of that AC impedance business.

(Not to mention series fields and shunt fields - shunt fields complicate things. Hey,
I just watch the darned things go round and round and fix whatever goes wrong with
them.)
Poet.
 
I don't know about a 'technical write up' . . .

But DC drives with SCR's converting the heap big AC into DC as a general rule one
is talking ohms law - the more voltage the more current.

During the positive half cycle of the AC sine wave, the gate on the SCR is pulsed and
it comes on (starts passing current) for the rest of the positive half cycle. When the
sine wave goes negative the SCR turns off again. On the next half cycle the SCR is
fired again - repeat over and over and over again every half cycle.

Fire the SCR earlier in the half cycle, and one gets more current and more voltage.
Meaning the motor goes faster. Later in the cycle and less volts and less current
and the motor goes slower.

As far as the motor is concerned it is getting straight DC. In reality it is a pulsating
DC but with three phases being pulsed in sequence it pretty well smooths things out.

But it really is more like a straightforward ohms law - the more volts you shove at it
the more current one gets through it. None of that AC impedance business.

(Not to mention series fields and shunt fields - shunt fields complicate things. Hey,
I just watch the darned things go round and round and fix whatever goes wrong with
them.)
Poet.

I am aware of this principle but I guess it will be Kirschoff's law if I need to convert from current to voltage ie. V=E +IR, where E is the back emf, R= armature resistance.
 
Think of a DC motor as a torque device (based on current). An AC motor is a speed device (based on frequency).

Within reasonable limits, current and torque in a DC motor are directly proportional. Same with frequency and speed on an AC motor.

As you apply current to a DC motor, it spins. It generates a 'counter emf' based on speed which limits the incoming current well below the I*R (ohms law) that would be present at locked-rotor. As torque is applied, the I*R voltage drop is against the incoming power, and thus the motor slows down (if the current is not adjusted).

While DC motors are the simplest (electro-magnetic rotating) machines, the fancier electronics for the other motors keep most people from having to worry about how they work :)

Thus, to control the speed of a DC motor, you have to control the current and thus the resulting voltage. So the current loop is inside, the voltage loop is next and then the speed loop is outside.
 
Think of a DC motor as a torque device (based on current). An AC motor is a speed device (based on frequency).

Within reasonable limits, current and torque in a DC motor are directly proportional. Same with frequency and speed on an AC motor.

As you apply current to a DC motor, it spins. It generates a 'counter emf' based on speed which limits the incoming current well below the I*R (ohms law) that would be present at locked-rotor. As torque is applied, the I*R voltage drop is against the incoming power, and thus the motor slows down (if the current is not adjusted).

While DC motors are the simplest (electro-magnetic rotating) machines, the fancier electronics for the other motors keep most people from having to worry about how they work :)

Thus, to control the speed of a DC motor, you have to control the current and thus the resulting voltage. So the current loop is inside, the voltage loop is next and then the speed loop is outside.

Thank you for the explanation.But how does drive arrive at or calculate the voltage which it needs to feed the motor from a current feedback.
 
The voltage loop compares the speed command to the speed feedback to produce a speed error. The speed feedback can come from a tachometer (encoder) or the armature voltage. The speed error signal passes through the current loop which does nothing to the speed error signal until the current reaches the current limit setting. When the current tries to exceed the current limit setting, the current loop reduces the signal to the SCR firing circuit. This reduces the drive output voltage to reduce the current.
When a DC drive is connected to a short circuit, current loop will reduce the output voltage to near zero.
 
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