DC Drive Fundamentals (2)

REGULATORS (CONTROLS)

The control provides two basic functions:

  1. It rectifies AC power converting it to DC for the DC motor.
  2. It controls the DC output voltage and amperage in response to various control and feedback signals thereby regulating the motor’s performance, both in speed and torque.


RECTIFYING FUNCTION

The basic rectifying function of the control is accomplished by a combination of power semiconductors (Silicon Controlled Rectifiers and Diodes) that make up the “power bridge” assembly.

REGULATING FUNCTION

The regulating function is provided by a relatively simple electronic circuit that monitors a number of inputs and sums these signals to produce a so called “error” signal. This error signal is processed and transformed into precisely timed pulses (bursts of electrical energy). These pulses are applied to the gates of the SCR’s in the power bridge thereby regulating the power output to the DC motor.
For most purposes it is not necessary to understand the electronic details of the regulator, however, in order to appreciate the regulator function it is good to understand some of the input signals that are required to give the regulator its capabilities, these are shown diagrammatically in Figure 4.
The AC to DC power flow is a relatively simple straight through process with the power being converted from AC to DC by the action of the solid state power devices that form the power bridge assembly.
The input and feedback signals need to be studied in more detail.

SET POINT INPUT

In most packaged drives this signal is derived from a closely regulated fixed voltage source applied to a potentiometer. 10 volts is a very common reference.

The potentiometer has the capability of accepting the fixed voltage and dividing it down to any value of from, for example, 10 to zero volts, depending on where it is set. A 10 volt input to the regulator from the speed adjustment control (potentiometer) corresponds to maximum motor speed and zero volts corresponds to zero speed. Similarly any speed between zero and maximum can be obtained by adjusting the speed control to the appropriate setting.

SPEED FEEDBACK INFORMATION

In order to “close the loop” and control motor speed accurately it is necessary to provide the control with a feedback signal related to motor speed.
The standard method of doing this in a simple control is by monitoring the armature voltage and feeding it back into the regulator for comparison with the input “set point” signal.
When armature voltage becomes high, relative to the set point, established by the speed potentiometer setting, an “error” is detected and the output voltage from the power bridge is reduced to lower the motor’s speed back to the “set point”. Similarly when the armature voltage drops an error of opposite polarity is sensed and the control output voltage is automatically increased in an attempt to re-establish the desired speed.
The “Armature Voltage Feedback System” which is standard in most packaged drives is generally called a “Voltage Regulated Drive”.
A second and more accurate method of obtaining the motor speed feedback information is called “Tachometer Feedback”. In this case the speed feedback signal is obtained from a motor mounted tachometer. The output of this tachometer is directly related to the speed of the motor. Using Tachometer Feedback generally gives a drive improved regulation characteristics. When “tach feedback” is used the drive is referred to as a “Speed Regulated Drive”. Most controls are capable of being modified to accept tachometer signals for operation in the tachometer feedback mode.
In some newer high performance “digital drives” the feedback can come from a motor mounted encoder that feeds back voltage pulses at a rate related to motor speed. These (counts) are processed digitally being compared to the “set point” and error signals are produced to regulate the armature voltage and speed.

CURRENT FEEDBACK

The second source of feedback information is obtained by monitoring the motor armature current. As discussed previously, this is an accurate indication of the torque required by the load.

The current feedback signal is used for two purposes:

  1. As positive feedback to eliminate the speed droop that occurs with increased torque load on the motor. It accomplishes this by making a slight corrective increase in armature voltage as the armature current increases.
  2. As negative feedback with a “threshold” type of control that limits the current to a value that will protect the power semiconductors from damage. By making this function adjustable it can be used to control the maximum torque the motor can deliver to the load.

The current limiting action of most controls is adjustable and is usually called “Current Limit” or “Torque Limit”.

In summary, the Regulator accomplishes two basic functions:

  1. It converts the alternating Current to Direct Current.
  2. It regulates the armature voltage and current to control the speed and torque of the DC Motor.
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