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DC Motor Questions and Answers

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DC Motor Operation

Define motor and explain the electron flow motor rule.

  • A motor is a machine that converts electrical energy into mechanical energy by means of electromagnetic induction.
  • The electron flow motor rule is used to determine the direction of motion of a current carrying conductor in a magnetic field.
  • The electron flow motor rule states that with the thumb, index finger, and middle finger of the right hand set at right angles to each other, the index finger points in the direction of the magnetic field (N to S), the thumb points in the direction of the induced conductor motion, and the middle finger points in the direction of the electron current flow in the conductor.

Explain torque and magnetic force rotation.

  • Torque is developed on a wire loop in a magnetic field.
  • Electron current flow must be at a right angle to the magnetic field. This is required for induced motion because no force is exerted on a conductor if the direction of electron current flow through the conductor and direction of the magnetic lines of force are the same (parallel).

• The result of the two magnetic fields intersecting creates a rotating force, referred to as torque, on the loop. The magnetic lines of force cause the loop to rotate when they straighten.

DC Motor Construction

List and describe the parts of a DC motor.

  • A DC motor consists of field windings, an armature, a commutator, and brushes.
  • Field windings are the stationary windings or magnets of a DC motor.
  • An armature is the rotating part of a DC motor.
  • A commutator is a ring made of segments that are insulated from one another. The commutator connects each armature winding to the brushes using copper bars (segments) that are insulated from each other with pieces of mica.
  • A brush is the sliding contact that rides against the commutator segments and is used to connect the armature to the external circuit.

State how a commutator and brushes deliver voltage to the armature.

  • The armature coils, commutator, and brushes are arranged so that the flow of current is in one direction in the loop on one side of the armature, and the flow of current is in the opposite direction in the loop on the other side of the armature.
  • The flow of current through the commutator reverses because the flow of current is at the same polarity on the brushes at all times. This allows the commutator to rotate another 180° in the same direction. The armature continues to rotate as long as the commutator winding is supplied with current and there is a magnetic field.
  • Torque is exerted on the armature when it is positioned so that the plane of the armature loop is parallel to the field, and the armature loop sides are at right angles to the magnetic field.

DC Motor Types

List the basic types of DC motors.

  • The four basic types of DC motors are DC series motors, DC shunt motors, DC compound motors, and DC permanent-magnet motors.
  • These DC motors have similar external appearances but are different in their internal construction and output performance.

Describe a DC series motor.

  • A DC series motor is a DC motor that has the series field connected in series with the armature. The field has relatively few turns of heavy-gauge wire.
  • A DC series motor produces high starting torque. Although speed control is poor, a DC series motor produces very high starting torque and is ideal for applications in which the starting load is large.
  • In DC series motors, speed changes rapidly when torque changes. When torque is high, speed is low; and when speed is high, torque is low.
  • The speed of a DC series motor is controlled by varying the applied voltage.

Describe a DC shunt motor.

  • A DC shunt motor is a DC motor that has the field connected in shunt (parallel) with the armature.
  • The field has numerous turns of wire, and the current in the field is independent of the armature, providing the DC shunt motor with excellent speed control.
  • A self-excited shunt field is a shunt field connected to the same power supply as the armature. A separately excited shunt field is a shunt field connected to a different power supply than the armature.
  • To control the speed of a DC shunt motor, the voltage to the armature is varied or the shunt field current is varied.
  • A field rheostat or armature rheostat is used to adjust the speed of a DC shunt motor. The rheostat is used to increase or decrease the strength of the field or armature.
  • A DC shunt motor has relatively high torque at any speed. As armature current is increased, so is motor torque, with only a slight drop in motor speed.

Describe a DC compound motor.

  • A DC compound motor is a DC motor with the field connected in both series and shunt with the armature.
  • The series field is connected in series with the armature. The shunt field is connected in parallel with the series field and armature combination. This arrangement gives the motor the advantages of the DC series motor (high torque) and the DC shunt motor (constant speed).
  • Speed control is obtained in a DC compound motor by changing the shunt field current strength or changing the voltage applied to the armature.

Describe a DC permanent-magnet motor.

  • A DC permanent-magnet motor is a motor that uses magnets, not a winding, for the field poles. DC permanent-magnet motors have molded magnets mounted into a steel shell. The permanent magnets are the field coils. DC power is supplied only to the armature.
  • DC permanent-magnet motors produce relatively high torque at low speeds and provide some self-braking when removed from power.

Stepper Motors

Define stepper motor and state how it operates to produce small incremental steps of the motor shaft.

  • A stepper motor is a motor that divides shaft rotation into discrete distances (steps). Unlike other motors that rotate their shafts when power is applied, stepper motors only move their shafts in small controlled increments called steps.
  • The shaft of a stepper motor rotates at fixed angles when it receives an electrical pulse.

– The electrical pulse magnetizes the motor’s stationary field, called the stator field.

  • The magnetic stator field moves the permanent magnet. The permanent magnet is called the rotor or armature.
  • The rotor steps forward to align with the stator’s magnetic field (N to S and S to N) and stops movement until another stator field is energized.
  • To increase the number of steps the rotor can take per 360° rotation, many individual stator and rotor segments are used. The total number of segments a motor has determines the number of individual steps the motor can take in one revolution.

Describe how switches are used with stepper motors.

  • Switches that turn on and off (pulse) the voltage to the coil control when the individual stator coils are energized to produce the magnetic field.
  • The order (clockwise or counterclockwise) in which the coils are turned on and off determines the direction of rotation of the rotor and shaft of a stepper motor. The control switches do not remain on. They are turned on to move the rotor and then turned off (pulsed).
  • Because the stator coils are turned on and off rapidly, solid-state transistors switches are used instead of mechanical switches. The transistor switches are controlled by an electronic control circuit that sends signals to the transistor to start or stop the flow of current through the coil.

Explain the function of an encoder in a stepper motor application.

  • An encoder is a sensor (transducer) that produces discrete electrical pulses during each increment of shaft rotation.
  • In some stepper motor applications, feedback is important since the motor may not have moved the actual steps the controller told it to. To provide feedback to the control circuit about the position of the motor shaft, an optical encoder with a disk on the motor shaft is typically used.
  • The encoder disk has open sections in which light beams shine through. The pulses of the light beams are sent to the electronic control circuit to compare the instructed movement to the actual movement. Any differences can be compensated for by the design of the control circuit.

DC Motor Load Requirements

Define work and state how it is calculated.

  • Work is the application of force over a distance.
  • Force is any cause that changes the position, motion, direction, or shape of an object.
  • Work is done when a force overcomes a resistance.
  • Resistance is any force that tends to hinder the movement of an object.
  • The amount of work (W) produced is determined by multiplying the force (F) that must be overcome by the distance (D) through which it acts.

Define torque and horsepower.

  • Torque is the force that produces rotation.
  • Torque causes an object to rotate. Torque (T) consists of a force (F) acting on a radius (r).
  • Horsepower (HP) is a unit of power equal to 746 W or 33,000 lb-ft per min (550 lb-ft per sec).
  • Horsepower is used to measure the energy produced by an electric motor while doing work.

Explain the relationship between speed, torque, and horsepower.

  • The operating speed, torque, and horsepower rating of a motor determine the work that the motor can produce.
  • If the torque remains constant, speed and horsepower are proportional.
  • If the speed increases, the horsepower must increase to maintain a constant torque.
  • If the speed decreases, the horsepower must decrease to maintain a constant torque.
  • If speed remains constant, torque and horsepower are proportional.
  • If the torque increases, the horsepower must increase to maintain a constant speed.
  • If the torque decreases, the horsepower must decrease to maintain a constant speed.
  • If torque and speed vary simultaneously but in opposite directions, the horsepower remains constant.
  • If the torque increases and the speed decreases, the horsepower remains constant.
  • If the torque decreases and the speed increases, the horsepower remains constant.

You May Also Read: Types of DC Motors and Their Characteristics

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About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.