Chapter 3: AC and DC Motors - DC Motors: Control of Speed and Torque

Control of Speed and Torque

The speed of a DC motor is a direct result of the voltage applied. As indicated earlier, the DC motor requires two separate circuits to generate motor torque.

Control of Speed
The field receives voltage from a separate power supply, sometimes referred to as a field exciter. This exciter provides power to the field, which in turn generates current and magnetic flux. In a normal operating state, the field is kept at maximum strength, thereby allowing the field winding to develop maximum current and flux. This condition is known as opera-

tion in the armature range. (The only way to control the speed is through change in armature voltage.)

The armature power supply applies voltage to the armature through the brushes and the commutator. Basically, the greater the amount of voltage applied, the faster the speed of the motor. We can see this relationship in the formula below:

where:

As seen in the formula, if the load on the motor remains constant, the armature current will stay constant, as well as the resistance of the armature. In addition, the motor design constant will remain the same, as well as the strength of the field flux. When all of these components remain constant, the only determining factor in speed is the amount of armature voltage applied.

The above formula will work in determining speed, when at or below the base speed of the motor. The formula will also indicate speed, when operating above base speed. It is possible to operate in an extended speed range, as long as the motor manufacturer is consulted for the maximum safe operating speed.

As shown in the formula, if armature voltage is at maximum and all the other components remain constant, speed can possibly be increased by reducing the field flux (φ). It is necessary to point out, however, that this must be done with caution.

Reduced field flux is the result of reducing the voltage from the field exciter. If voltage is reduced to near zero, the speed of the armature can increase to the point of motor self-destruction. This operation above base speed is known as the field weakening speed range, for apparent reasons. The field exciter will have safeguards in place to avoid excessive speed. Most DC drive systems will allow a field weakening range of no less than 1/3 of the normal voltage. If the voltage drops to less than that amount, pre-programmed safety circuits in the drive shut down the armature supply and bring the motor to a safe stop.

Increased speed is made possible by a reduced amount of field flux, when operating above base speed. In essence, less EMF is available to act as holdback magnetic flux. Torque available from the motor is also a function of speed.

Typical armature voltage ratings in the United States are 90, 180, 240, or 500 VDC. Typical U.S. field voltage ratings are 100, 200, 150, or 300 VDC. As stated earlier, the amount of voltage applied to the armature would dictate the output shaft speed. For example, if a 1750-rpm motor with a 240 VDC armature had 120 VDC applied (1/2 voltage), the shaft speed would be approximately 875 rpm (1/2 speed).

Control of Torque
Under certain conditions, motor torque remains constant when operating below base speed. However, when operating in the field weakening range, torque drops off inversely as 1/speed². The amount of motor torque can also be determined by a formula. The following relationship exists in a DC motor and serves to help determine the motor torque available:

where:

As seen in the formula, if the field flux is held constant, as well as the design constant of the motor, then the torque is proportional to the armature current. The more load the motor sees, the more current is consumed by the armature.

A selling point of DC motors is their ability to provide full torque at zero speed. This is accomplished by the two power supplies, energizing their power structures to supply voltage to the armature and field. When additional load is dropped across the armature, magnetic flux of the armature cuts through the field flux. Once this occurs, more current is drawn through the armature, and the drive's power structure conducts the required amount of current to meet the demand. This phenomenon occurs whether the motor is at any speed, including zero.

Control of Speed and Torque

The speed of a DC motor is a direct result of the voltage applied. As indicated earlier, the DC motor requires two separate circuits to generate motor torque.

Control of Speed
The field receives voltage from a separate power supply, sometimes referred to as a field exciter. This exciter provides power to the field, which in turn generates current and magnetic flux. In a normal operating state, the field is kept at maximum strength, thereby allowing the field winding to develop maximum current and flux. This condition is known as opera-

tion in the armature range. (The only way to control the speed is through change in armature voltage.)

The armature power supply applies voltage to the armature through the brushes and the commutator. Basically, the greater the amount of voltage applied, the faster the speed of the motor. We can see this relationship in the formula below:

where:

As seen in the formula, if the load on the motor remains constant, the armature current...