DC Drive Fundamentals (1)

UNDERSTANDING DC DRIVES

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 Rowand 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:

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.

From this discussion and Figure 3 a general statement can be made that for PM and Shunt Wound motors load torque determines armature amperage.

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.

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