DC motors have been available for nearly 100 years. In fact the first electric motors were designed and built for operation from direct current power.

AC motors are now and will of course remain the basic prime movers for the fixed speed requirements of industry. Their basic simplicity, dependability and ruggedness make AC motors the natural choice for the vast majority of industrial drive applications.

Then where do DC drives fit into the industrial drive picture of the future?

In order to supply the answer, it is necessary to examine some of the basic characteristics obtainable from DC motors and their associated solid state controls.

1. Wide speed range.
2. Good speed regulation.
3. Compact size and light weight (relative to mechanical variable speed).
4. Ease of control.
5. Low maintenance.
6. Low cost.

In order to realize how a DC drive has the capability to provide the above characteristics, the DC drive has to be analyzed as two elements that make up the package. These two elements are of course the motor and the control. (The “control” is more accurately called the “regulator”).

DC MOTORS

Basic DC motors as used on nearly all packaged drives have a very simple performance characteristic — the shaft turns at a speed almost directly proportional to the voltage applied to the armature. Figure 1 shows a typical voltage/speed curve for a motor operating from a 115 volt control.

From the above curve you can see that with 9 volts applied to the armature, this motor would be operating at Point 1 and turn at approximately 175 RPM. Similarly with 45 volts applied, the motor would

be operating at Point 2 on the curve or 875 RPM. With 90 volts applied, the motor would reach its full speed of 1750 RPM at point 3.

From this example a general statement can be made that DC motors have “no load” characteristics that are nearly a perfect match for the curve indicated in Figure 1.

However, when operated at a fixed applied voltage but a gradually increasing torque load, they exhibit a speed droop as indicated in Figure 2.

This speed droop is very similar to what would occur if an automobile accelerator pedal was held in a fixed position with the car running on level ground. Upon starting up an incline where more driving torque would be needed, the car would slow down to a speed related to the steepness of the hill. In a real situation, the driver would respond by depressing the accelerator pedal to compensate for the speed loss to maintain a nearly constant speed up the incline.

In the DC drive a similar type of “compensation” is employed in the control to assist in maintaining a nearly constant speed under varying load (torque) conditions.

The measurement of this tendency to slow down is called Regulation and is calculated with the following equation:
% Regulation = (No Load Speed – Full Load Speed)/No Load Speed X 100

In DC drives the regulation is generally expressed as a percentage of motor base speed.

If the control (regulator) did not have the capability of responding to and compensating for changing motor loads, regulation of typical motors might be as follows:

HP	% MOTOR REGULATION	HP	% MOTOR REGULATION
1/4	13.6	1.5	8.0
1/3	12.9	2	7.2
1/2	13.3	3	4.2
3/4	10.8	5	2.9
1	6.7	7.5	2.3

One other very important characteristic of a DC motor should be noted. Armature amperage is almost directly proportional to output torque regardless of speed. This characteristic is shown by Figure 3. Point 1 indicates that a small fixed amount of current is required to turn the motor even when there is no output torque. This is due to the friction of the bearings, electrical losses in the motor materials and load imposed by the air in the motor (windage).

Beyond Point 1 through Point 2 and 3, the current increases in direct proportion to the torque required by the load.

In summary, two general statements can be made relative to DC motor performance.

1. Motor Speed is primarily determined by Applied Armature Voltage.
2. Motor Torque is controlled by Armature Current (amperes).

Understanding these two concepts of DC motors provides the key to understanding total drive performance.

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.