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Chapter 16: Problem 57

In examining a single-phase ac motor, what features could we look for to identify it as a universal motor?

Short Answer

Expert verified
Identify series winding, commutator, and brushes; check for AC/DC compatibility and usage in high-speed appliances.

Step by step solution

01

Understand the Characteristics of a Universal Motor

A universal motor is a type of electric motor that can operate on either AC or DC power. It is known for having high starting torque and variable speed control. Unlike other motors, the universal motor's construction allows it to run efficiently on both types of power.
02

Identify the Components of the Motor

Check if the motor has a series-wound configuration where both the field winding and the armature winding are in series with the power source. This design characteristic is crucial for universal motors, as it allows them to run on both alternating and direct currents.
03

Look for Physical Features

Universal motors typically have a commutator and brushes. The commutator is used to reverse the direction of current through the windings, which is necessary for their operation on AC power. The brushes make electrical contact with the rotating commutator.
04

Examine the Motor Rating Plate

Check the motor rating plate for voltage and current specifications. A universal motor usually has a wide voltage range it can operate on, indicating its capability to function on both AC and DC supplies.
05

Consider the Usage Applications

Note if the motor is designed for appliances requiring high speed and power like vacuum cleaners or drills. Universal motors are preferred in these applications due to their compact size and efficiency at high speeds.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Single-Phase AC Motor
A single-phase AC motor is a type of electric motor that runs on single-phase alternating current (AC). It is commonly used in household appliances and small machinery. An intriguing aspect of these motors is how they utilize a single sinusoidal voltage to produce a rotating magnetic field.
Single-phase AC motors generally offer simpler construction and lower cost than polyphase motors. However, they typically have lower efficiency and torque.
  • Simple design makes them cost-effective.
  • Lower efficiency compared to three-phase motors.
  • Sufficient starting torque for small appliances.
These motors are prevalent in domestic and light industrial settings, where the ease of use and maintenance makes them a preferred option even if they do not offer the high performance seen in more complex motor types.
Electric Motor Characteristics
Electric motor characteristics help us understand how motors operate and where to use them effectively. These characteristics include starting torque, running efficiency, speed control, and power delivery. Universal motors, which can run on either AC or DC, are distinct because they have:
  • High starting torque: Ideal for applications needing a fast startup.
  • Variable speed: Enables easy control over motor speed by adjusting voltage or current.
  • Compact size: Supports smaller, high-speed appliances like mixers and hand drills.
Knowing these characteristics is crucial when selecting the right motor for a specific application, considering factors like load, environment, and desired performance.
Commutator in Motors
The commutator is a vital component in motors, especially in universal motors. Its primary role is to reverse the direction of electric current through the motor windings, a function necessary for maintaining consistent motor rotation on AC power. The commutator works in tandem with carbon brushes to ensure a smooth electrical connection as the armature spins.
  • Reverses current direction to maintain consistent rotation.
  • Vital for AC operation in universal motors.
  • Works with brushes to ensure effective contact.
Commutators are especially common in universal motors due to their versatile power capabilities. This design feature also contributes to noise, wear, and maintenance needs, highlighting the trade-offs between simplicity and performance.
Motor Components Identification
Identifying motor components is key in understanding how motors work and diagnose issues. For universal motors, these components include field windings, armature windings, commutators, and brushes. Understanding the function of each part is crucial.
Field and armature windings are placed in series, enabling the motor to operate on either AC or DC.
  • Field Winding: Generates a magnetic field that interacts with the armature winding.
  • Armature Winding: Carry the current that drives motor rotation through electromagnetic interaction.
  • Commutator: Acts as a switch to reverse current direction, crucial for AC operation.
  • Brushes: Provide electrical contact with the rotating commutator, allowing current to pass.
Recognizing these parts helps in identifying and verifying a motor as a universal motor and facilitates maintenance and repair.

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Most popular questions from this chapter

We want a four-pole synchronous motor that operates at a speed of \(1000 \mathrm{rpm}\). Determine the frequency of the ac source. List several other speeds that can be achieved by using synchronous motors operating from this ac source. What is the highest speed achievable?

A certain 25 -hp three-phase induction motor operates from a 440 -V-rms (line- toline) three-phase source. The full-load speed is \(1750 \mathrm{rpm}\). The motor has a starting torque equal to 200 percent of its full-load torque when started at rated voltage. For an engineering estimate, assume that the starting torque of an induction motor is proportional to the square of the applied voltage. To reduce the starting current of the motor, we decide to start it with a line-to-line voltage of \(220 \mathrm{~V}\). Estimate the starting torque with this reduced line voltage.

A series-connected dc motor has \(R_{A}=0.5 \Omega\) and \(R_{F}=1.5 \Omega\). In driving a certain load at \(1500 \mathrm{rpm}\), the current is \(I_{A}=20 \mathrm{~A}\) from a source voltage of \(V_{T}=220 \mathrm{~V}\). The efficiency is \(90 \%\). Find the output power and rotational loss.

A shunt-connected dc motor has zero rotational losses and \(R_{A}=0\). Assume that \(R_{F}+R_{\text {adj }}\) is constant [except in part (d)] and that \(\phi\) is directly proportional to field current. For \(V_{T}=200 \mathrm{~V}\) and \(P_{\text {out }}=2 \mathrm{hp}\), the speed is \(1200 \mathrm{rpm}\). What is the effect on \(I_{A}\) and speed if: a. the load torque doubles; b. the load power doubles; \(\mathbf{c} . V_{T}\) is changed to \(100 \mathrm{~V}\) and \(P_{\text {out }}\) remains constant; d. \(R_{F}+\) \(R_{\text {adj }}\) is doubled in value and \(P_{\text {out }}\) remains constant?

A certain de motor produces a back emf of \(E_{A}=240 \mathrm{~V}\) at a speed of \(1200 \mathrm{rpm}\). Assume that the field current remains constant. Find the back emf for a speed of \(600 \mathrm{rpm}\) and for a speed of \(1500 \mathrm{rpm}\).
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