Electrical Sciences DC Motors
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The EMF that is induced in the armature producing a current in the opposite direction. The induced EMF in a motor.
Counterelectromotive force (CEMF)
The CEMF produced in the armature as it rotates in the magnetic field.
When the forefinger is pointed in the direction of the magnetic field lines (North to South), and the center finger is pointed in the direction of current flow, the thumb will point in the direction of force.
Right-hand rule for motors
Incorporated into the motor design to limit starting current to 125 to 200 percent of full load current.
Force which tends to produce and maintain rotation (ft lbf).
Given the magnetic field and current flow directions, determine the direction of force on a conductor using the right-hand rule.
Describe the relationship between armature current and torque produced in a DC motor
If the armature current was reversed but the field was the same, torque would be developed in the opposite direction. Likewise, if the field polarity was reversed and the armature remained the same, torque would also be developed in the opposite direction.
State the function of torque in a DC motor and how it is developed
The function of torque in a DC motor is to provide the mechanical output needed to drive the piece of equipment that the DC motor is attached to.
Describe how voltage is determined in a DC motor
The effective voltage acting in the armature of a motor is the applied voltage, minus the counter EMF.
State the function of the CEMF that is developed in a DC motor
The CEMF opposes the applied voltage and functions to lower armature current.
Describe the relationship between field current and magnetic field size in a DC motor
An increase in field current will cause field flux to increase.
Describe how the speed of a DC motor is adjusted
- If the field flux of a DC motor is decreased, the motor speed will increase.
- Increased torque causes the motor to increase in speed.
Describe the differences in construction between a shunt-wound and a series-wound DC motor with respect to the relationship between the field and the armature windings
- The motor is called a shunt motor because the field is in parallel, or shunts the armature.
- The motor field windings for a series motor are in series with the armature.
Describe the construction of a compound DC motor
The compound motor is desirable for a variety of applications because it combines the characteristics of a series-wound motor and a shunt-wound motor. The compound motor has a greater torque than a shunt motor due to the series field; however, it has a fairly constant speed due to the shunt field winding.
Describe the torque versus speed characteristics for a shunt-wound and a series-wound DC motor
- A shunt-wound DC motor has a decreasing torque when speed increases
- The torque versus speed characteristics of a series-wound motor as the speed decreases, the torque for a series-wound motor increases sharply. As load is removed from a series motor, the speed will increase sharply.
Explain why starting resistors are necessary for large DC motors
In order to reduce this very high starting current, an external resistance must be placed in series with the armature during the starting period.
List the four nameplate ratings for a DC motor.
Voltage, current, speed, and power
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