Electric Motors Quiz

Introduction to Electric Motors

• Electric motors are machines converting electrical energy to mechanical power through rotational motion and torque.

Related Terms

• Ampacity: The continuous current a conductor can carry without exceeding its temperature rating, measured in amperes.
• Disconnecting Means: A switch or system for disconnecting an electric motor from its power supply.
• Duty Rating: Indicates how frequently a motor can be started and how long it will run per start.
• Phase: Number of individual voltages applied to the motor.
  • Three-phase: Has three individual voltages.
  • Single-phase: Uses one voltage in a sine wave form.

Motor Components

• Rotor: The rotating part of the motor, typically made from a laminated steel core with copper wires for current conduction.
• Stator: The stationary part of the motor, consisting of laminated iron core with copper windings.
• Service Factor: The maximum load a motor can continuously handle while staying within a safe temperature range.
• Torque: The twisting force generated by the motor.
  • Breakdown Torque: The maximum torque a motor can produce before stalling during overload.
  • Starting Torque: The torque needed to initiate the motor's rotation.

Applications in Agriculture

• Electric motors are used in agricultural applications.
  • Water pumping
  • Driving size reduction equipment
  • Driving material handling equipment
  • Driving various rice milling equipment

Motor Classifications

• Electric Motor: General classification of motors.
  • AC Motor: Motors operating on alternating current.
    • Three-phase: Squirrel cage or wound rotor
    • Single-phase: Squirrel-cage, synchronous, wound-rotor, split-phase, capacitor-start induction, repulsion-start, induction
  • DC Motor: Motors operating on direct current.
    • Shunt-Wound: Constant speed applications.
    • Series-Wound: Applications requiring high starting torque.
    • Compound-Wound: Adjustable motor to suit load requirements.

Motor Classifications (Based on Starting)

• Split-Phase Motor: Low starting loads, auxiliary phase displaced from main winding.
• Capacitor-Start Induction Motor: Capable of handling heavier loads than split-phase. Uses an external capacitor in the auxiliary winding circuit.
• Repulsion Start Induction Motor: High starting torque. Rotor with winding and commutator and short-circuited brushes for starting.

General Rules for Motor Sizing

• Refer to manufacturers' specifications for the particular application.
• A 1½ horsepower motor can replace a human worker.
• To substitute an engine, multiply the engine horsepower by 2/3.

Motor Ratings and Specifications

• Horsepower Rating: A measure of a motor's ability to generate mechanical power. Higher horsepower ratings mean greater capability.
• Speed Rating: Most common motor speeds are in the range of 1700 to 1750 RPM.
• Full Load Current and Wire/Fuse Size: Tables provide recommended wire gauge and fuse sizes for specific single-phase induction motors at 220 volts for different horsepower (HP) ratings.

Motor Formulas

• Formulas for calculating power (P) in watts for single-phase and three-phase motors based on voltage (E), current (I), and power factor (cos θ).

Shaft Output Power

• Formula for calculating shaft output power (P₀) in kilowatts (kW) using shaft torque (T), and shaft speed (N)

Motor Efficiency

• Data tables including horsepower (HP), efficiency in percentages, and power factor for motors with varying kilowatt (KW) ratings. These are typically at 230 volts and 60 Hz frequency.

Selecting Electric Motors

• Considerations for selecting a motor include the amount of power required, available electrical power, service entrance panel capacity, and equipment speed requirements.

Replacing a Motor

• Motor replacement guidelines for various equipment types, including small portable gas engines, industrial engines, and tractor PTOs.

Motor on New Equipment

• Use equipment manufacturer's recommendations for motor selection.

Installing on Hand-Powered Equipment

• Using a rule of thumb, a 1/3 horsepower motor can serve hand-powered equipment.

Power Supply

• Single Phase: 115 or 230 volts; limited to 7½ hp, common for farms and homes.
• Three Phase: 208, 230, or higher volts; up to 1000 hp; less flickering, lower cost and longer life; requires additional installation costs.

Service Entrance Capacity

• Service entrance panel (SEP) amperage should be at least three times higher than the motor's nameplate rating for sufficient starting current.

Motor Speed Selection

• Match motor speed to equipment speed. If needed, use pulleys, gears, or chains for speed conversion.

Motor Duty

• Types include continuous duty (constant full load) and intermittent duty (loads at specific intervals).

Starting Loads

• Easy Starting: Shaded-pole induction, split-phase, permanent-split capacitor-induction, soft-start
• Difficult Starting: Capacitor-start induction-run, repulsion-start induction-run, three-phase general-purpose.

Other Factors

• Considerations such as direction of rotation, cost, maintenance, and potential radio interference issues.

Bearing Types

• Sleeve Bearings: Brass, bronze, or tin-lined cylinders.
• Ball Bearings: Round steel balls within a cage surrounding the shaft.

Lubrication

• Sleeve Bearings: SAE 20 non-detergent motor oil; avoid over-oiling; wipe off excess; oil wick; refill at least twice a year.
• Ball Bearings: Usually prelubricated and sealed. Hand-packed bearings may require grease every 2-5 years.

Mounting Position

• Sleeve Bearings: Typically mounted parallel to the floor; rotations may be needed for end shields to prevent oil from the reservoir running out.
• Ball Bearings: Can be mounted in any position.

Enclosures

• Motors produce heat; cooling via fans and openings in the motor housing. Protect from dust, water, and other contaminants.
  • Dripproof (Open Type): Open design for adequate air circulation,
  • Totally Enclosed: No openings for external air circulation
  • Explosion Proof: Suitable for potentially hazardous locations.

Mounting Base

• Types include rigid (fixed or adjustable screws) and sliding rails.

Overload Protection

• Excessive current occurs with overloads or too low voltage. This needs protection. Include built-in protections like manual reset, automatic reset, automatic starting switches, Time-Delay Fuses, Magnetic Starting Switch and others.

Motor Drives

• Equipment connections using direct connections, flexible hose couplings, flange couplings, cushion-flange couplings, flexible shafts, and various speed conversion systems. Examples are Gear Drives, Chain-and-sprocket Drives, Pulley-and-belt Drives, V-Belt, Webbed Multi-V-belt, Flat-belt, and V-Flat.

Sizing Drives

• Horsepower increases proportionally with speed increases, especially on fans, blowers, and centrifugal pumps.

Pulley Types

• Standard V-pulleys, V-step pulleys, and adjustable V-pulleys.

Sizing Pulleys

• Charts for pulley selection, typically based on motor size and desired equipment RPMs. The calculation of appropriate pulley sizes for these speeds needs to include the diameter of both pulleys.

Belt Types

• FP (Fractional Power ) belts (lighter duty), A, B, C, D, and potentially E section belts (heavier duty). Key is correct belt width matching the pulley groove.

Factors Affecting Belt Life

• Proper belt tension, maintenance of pulley alignment, keeping belts clean, and correct belt selection will improve belt life.

Test your knowledge of electric motors with this quiz that covers key concepts such as ampacity, stator functions, and differences between motor types. Perfect for students and professionals looking to reinforce their understanding of electric motor operations and characteristics.