motor selection basic

8/8/2019 Motor Selection Basic

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Electric Motor Definitions and TerminologyAngular accuracy

The measure of shaft positioning accuracy on a servo or stepping motor.

Back EMF

The voltage generated when a permanent magnet motor is rotated. This voltage is proportional to

motor speed and is present regardless of whether the motor winding(s) are energized or de-energized.

Bipolar chopper driverA class of step motor driver which uses a switch mode (chopper) technique to control motorcurrent and polarity. Bipolar indicates the capability of providing motor phase current of either

polarity (+ or -).

Breakaway torque

The torque required to start a machine in motion. Almost always greater than the running torque.

Brushless motor

Class of motors that operate using electronic commutation of phase currents, rather than

electromechanical (brush-type) commutation. Brushless motors typically have a permanentmagnet rotor and a wound stator.

C-face mounting

A standard NEMA mounting design, where the mounting holes in the face are threaded toreceive the mating mount.

Class B insulation

A NEMA insulation specification. Class B insulation is rated to an operating (internal)

temperature of 130C.

Class F insulation

A NEMA insulation specification. Class F insulation is rated to an operating (internal)

temperature of 155C

Class H insulation

A NEMA insulation specification. Class H insulation is rated to an operating (internal)

temperature of 180C.

Closed loopA broadly applied term, relating to any system in which the output is measured and compared to

the input. The output is then adjusted to reach the desired condition. In motion control, the termtypically describes a system utilizing a velocity and/or position transducer to generate correction

signals in relation to desired parameters.

Cogging (Cogging torque)

A term used to describe non-uniform angular velocity. Cogging appears as a jerkiness, especiallyat low speeds.

Commutation

1. A term which refers to the action of steering currents or voltages to the proper motor phases soas to produce optimum motor torque. In brush type motors, commutation is done

electromechanically via the brushes and commutator. In brushless motors, commutation is doneby the switching electronics using rotor position information obtained by Hall sensors, aTachsyn, or a resolver.

2. Commutation of step motors is normally done open loop. Feedback from the motor is not

required to hold rotor position precisely.

Continuous rated current (ICR) (Amperes)The maximum allowable continuous current a motor can handle without exceeding the motor

temperature limits

Continuous rated torque (TCR) (lb-in.)The maximum allowable continuous torque a motor can handle without exceeding the motor

temperature limits

Continuous stall current (ICS) (Amperes)


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Amount of current applied to a motor (at locked rotor conditions), which results in rated

temperature rise. Refer also to definition of "Continuous stall torque"

Continuous stall torque (TCS) (lb-in.)The amount of torque at zero speed, which a motor can continuously deliver without exceeding

its thermal rating. Determined by applying DC current through two windings with rotor locked,

while monitoring temperature. Specified with motor windings at maximum rated temperature,with motor in 25 degrees C ambient, mounted to a heat sink. Refer to individual specs for heat

sink size.Current at peak torque (IPK) (Amperes)

The amount of input current required to develop "peak torque". This is often outside the lineartorque/current relationship.

Current, Rated

The maximum allowable continuous current a motor can handle without exceeding motortemperature limits.

D-flange mounting

This type of mount has clearance holes on the flange, and the mounting bolts stick out throughthe flange from the motor side. This mount is common in cases where the motor is integral to the

machine.

Demag currentThe current level at which the motor magnets will start to be demagnetized. This is an

irreversible effect, which will alter the motor characteristics and degrade performance. Also

known as peak current.

Detent torqueThe maximum torque that can be applied to an unenergized step motor without causing

continuous rotating motion.

DPBV - Dripproof Blower VentilatedType of motor cooled by blowing air through the inside of the motor using an attached blower.

Drive

An electronic device that controls torque, speed and/or position of an AC or brushless motor.

Typically a feedback device is mounted on the motor for closed-loop control of current, velocityand position.

Driver

Electronics which convert step and direction inputs to high power currents and voltages to drivea step motor. The step motor driver is analogous to the servomotor amplifier's logic.

Duty cycle

For a repetitive cycle, the ratio of on time to total cycle time.Duty cycle (%) = [On time / (On time + Off time)] x 100%

Dynamic braking

A passive technique for stopping a permanent magnet brush or brushless motor. The motorwindings are shorted together through a resistor which results in motor braking with an

exponential decrease in speed.

EfficiencyThe ratio of power output to power input.

Electrical time constant (te) (Seconds)

The time required for current to reach 63.2% of its final value for a fixed voltage level. Can be

calculated from the relationship te=L/R where L is inductance (henries) and R is resistance(ohms).

Encoder

A feedback device which converts mechanical motion into electronic signals. The mostcommonly used, rotary encoders, output digital pulses corresponding to incremental angular

motion. For example, a 1000-line encoder produces 1000 pulses every mechanical revolution.


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The encoder consists of a glass or metal wheel with alternating transparent and opaque stripes,

detected by optical sensors to produce the digital outputs.

FeedbackA signal which is transferred from the output back to the input for use in a closed loop system.

Ferrite

A type of permanent magnet consisting of ceramic compounds made up of oxides of iron, bariumand strontium.

Form factorThe ratio of RMS current to average current. This number is a measure of the current ripple in a

SCR or other switch-mode type of drive. Since motor heating is a function of RMS current whilemotor torque is a function of average current, a form factor greater than 1.00 means some

fraction of motor current is producing heat but not torque.

Four quadrantRefers to a motion system which can operate in all four quadrants; i.e., velocity in either

direction and torque in either direction. This means that the motor can accelerate, run, and

decelerate in either direction.

Friction

A resistance to motion caused by contact with a surface. Friction can be constant with varying

speed (Coulomb friction) or proportional to speed (viscous friction).Hall sensor

A feedback device which is used in a brushless servo system to provide information for the

amplifier to electronically commutate the motor. The device uses a magnetized wheel and hall

effect sensors to generate the commutation signals.

Holding torque

Sometimes called static torque, holding torque specifies the maximum external torque that can be

applied to a stopped, energized motor without causing the rotor to rotate. Generally used as afigure of merit when comparing motors.

Horsepower

An index of the amount of work a machine or motor can perform. One horsepower is equal to

746 watts. Since power is equal to torque multiplied by speed, horsepower is a measure of amotor's torque and speed capability; e.g., a 1 HP motor will produce 36 lb-in. at 1,750 rpm.

Formula:

HP = Torque (lb-in.) x Speed (RPM)/63,025or

HP = Torque (lb-ft.) x Speed (RPM)/5,252

orHP = Volts x Amps x Efficiency/746

Hybrid step motor

A motor designed to move in discrete increments of steps. The motor has a permanent magnetrotor and a wound stator. Such motors are brushless. Phase currents are commutated as a

function of time to produce motion.

Idle current reductionA step motor driver feature that reduce the phase current to the motor when no motor motion is

commanded (idle condition) for a specified period of time. Idle current reduction reduces motor

heating and allows high machine throughputs from a given motor.

IndexerElectronics which convert high level motion commands from a host computer, PLC or operator

panel into step and direction pulse streams for use by the step motor driver. Indexers can be

broadly divided into two classes. A preset indexer typically accepts distance, velocity and ramptime inputs only. The more sophisticated programmable indexer is capable of complex motion

control and includes program memory.

Inductance (L) (mH - millihenries line-to-line)


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The electrical equivalent to mechanical inertia; that is, the property of a circuit, which has a

tendency to resist current flow when no current is flowing, and when current is flowing has a

tendency to maintain that current flow. Pacific Scientific measures inductance (line-to-line) witha bridge at 1000 Hz and with the rotor positioned so the back-EMF waveform is at the peak of

the sinusoid.

Inductance (mutual)Mutual inductance is the property that exists between two current carrying conductors or coils

when magnetic lines of force from one link with those of the other.Inertial match

For most efficient operation, the system coupling ratio should be selected so that the reflectedinertia of the load is equal to the rotor inertia of the motor.

Insulation Class

The rating assigned to the maximum temperature capability of the insulating components in amotor or other piece of equipment.

Mechanical time constant (tm) (Seconds)

In a simple first order system, the time required for the motor's speed to attain 63.2% of its finalvalue for a fixed voltage level. Can be calculated from:

where:

J is inertia in lb-in./s2R is resistance in ohms

KT is torque constant in lb-in./amp.

8.87 is a conversion factor

tM is calculated in seconds

Microstepping

An electronic technique for increasing a step motor's position resolution and velocity smoothness

by appropriately scaling the phase currents. Microstepping is also a technique used to reduce oreliminate the effects of system resonance at low speeds.

Mid-range instability

A phenomenon in which a step motor can fall out of synchronism due to a loss of torque at mid-

range speeds. The torque loss is due to the interaction of the motor's electrical characteristics andthe driver's electronics. Some drivers have circuitry to eliminate or reduce the effects of mid-

range instability.

NEMA - National Electrical Manufacturer's AssociationAcronym for an organization which sets standards for motors and other industrial electrical

equipment.

NTC - Negative Temperature CoefficientA negative temperature coefficient thermistor is used to detect and protect a motor winding from

exceeding its maximum temperature rating. Resistance of the device decreases with an increase

in temperature.

Open-loop

A system in which there is no feedback. Motor motion is expected to faithfully follow the input

command. Stepping motor systems are an example of open-loop control.

Overload capacity

The ability of a drive to withstand currents above its continuous rating. It is defined by NEMA as

150% of the rated full-load current for "standard industrial DC motors" for one minute.

Peak torque (Tpk) (lb-in.)The maximum torque a brushless motor can deliver for short periods of time. Operating PacTorq

motors above the maximum torque value can cause demagnetization of the rare-earth magnets.

This is an irreversible effect that will alter the motor characteristics and degrade performance.This is also known as peak current.

Not to be confused with system peak torque, which is often determined by amplifier peak current

limitations, where peak current is typically two times continuous current.


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Poles

Refers to the number of magnetic poles arranged on the rotor of the brushless motor. Unlike an

AC motor, the number of poles has no direct relationship to the base speed of the motor.

Power factor

Ratio of true power (kW) to apparent power (kVA).

PTC - Positive Temperature CoefficientA positive temperature coefficient thermistor is used to detect and protect a motor winding from

exceeding its maximum temperature rating. Resistance of the device increases with an increasein temperature.

Pull-out torqueThe maximum friction load, at a particular inertial load, that can be applied to the shaft of a

synchronous motor (running at constant speed) and not cause it to lose synchronism.

Pulse rateThe frequency of the step pulses applied to a step motor driver. The pulse rate, multiplied by the

resolution of the motor/driver combination (in steps per revolution), yields the rotational speed in

revolutions per second.

Pulse Width Modulation (PWM)

1. A PWM controller (amplifier) switches DC supply voltage on and off at fixed frequencies.

The length of the on/off interval or voltage waveform is variable.2. Pulse width modulation (PWM), describes a switch-mode (as opposed to linear) control

technique used in amplifiers and drivers to control motor voltage and current. PWM offers

greatly improved efficiency compared to linear techniques.

RegenerationThe action during motor braking, in which the motor acts as a generator and takes kinetic energy

from the load, converts it to electrical energy, and returns it to the amplifier.

RepeatabilityThe degree to which a parameter such as position or velocity can be duplicated.

Resistance, Hot (RH)(Ohms line-to-line)

The motor's terminal resistance value specified at the hot winding temperature, which is at the

motor's maximum rated temperature.Resolution

The smallest increment into which a parameter can be broken down. For example, a 1000 line

encoder has a resolution of 1/1000 of a revolution.

Resolver

An electromagnetic feedback device which converts angular shaft position into analog signals.

These signals can be processed in various ways, such as with an RDC (resolver-to-digitalconverter) to produce digital position information. There are two basic types of resolvers;

transmitter and receiver. A transmitter-type is designed for rotor primary excitation and stator

secondary outputs. Position is determined by the ratio of the sine output amplitude to cosineoutput amplitude. A receiver-type is designed for stator primary excitations and rotor secondary

output. Position is determined by the phase shift between the rotor output signal and one of the

primary excitation signals.

Resonance

Oscillatory behavior caused by mechanical limitations.

Restart torque

The maximum friction load, at a particular inertial load, that can be applied to the shaft of asynchronous motor without causing it to lose synchronism when accelerating to a constant speed

from standstill.

RingingOscillation of a system following a sudden change in state.

RMS Current - Root Mean Square Current


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In an intermittent duty cycle application, the RMS current is equal to the value of steady state

current which would produce the equivalent motor heating over a period of time.

RMS Torque - Root Mean Square Torque.In an intermittent duty cycle application, the RMS torque is equal to the value of steady state

torque which would produce the equivalent motor heating over a period of time.

RotorThe moving part of the motor, consisting of the shaft and magnets. These magnets are analogous

to the field winding of a brush-type DC motor.Settling time

The time required for a parameter to stop oscillating or ringing and reach its final value.

Shock loading

A load that produces extremely high peak torques for very short durations. This type of load is

associated with conveyorized grinding, crushing and separation processes.

Stall Torque

The amount of torque developed with voltage applied and shaft locked, or not rotating. Also

known as locked-rotor torque.

Stator

The non-moving part of the motor. Specifically, it is the iron core with the wire winding in it that

is pressed into the frame shell. The winding pattern determines the voltage constant of the motor.Step angle

The angular distance the shaft rotates upon receipt of a single step command.

Stiffness

The ability to resist movement induced by an applied torque. Stiffness is often specified as atorque displacement curve, indicating the amount a motor shaft will rotate upon application of a

known external force when stopped.

SynchronismA motor rotating at a speed corresponding correctly to the applied step pulse frequency is said to

be in synchronism. Load torques in excess of the motor's capacity (rated torque) will cause a loss

of synchronism. This condition is not damaging to a step motor.

TENV - Totally Enclosed Non-VentilatedAcronym describing a type of motor enclosure, which has no outside air going into it. It is cooled

only by convection to the frame, which is usually finned.

Thermal protectionA thermal sensing device mounted to the motor to protect it from overheating. This is

accomplished by disconnecting the motor phases from the drive in an over temperature

condition.

Thermal resistance (Rth) (C/watt)

An indication of how effectively a unit rids itself of heat; a measure of temperature rise per watts

lost. In Pacific Scientific literature, it is the specified value from the motor windings to theambient, under locked rotor conditions.

Thermal time constant (tth) (minutes)

The time required for a motor to attain 63.2% of its final temperature for a fixed power input.

Torque Constant (KT = lb-ft./A)

An expression of the relationship between input current and output torque. For each ampere of

current, a fixed amount of torque is produced.

Torque-to-inertia ratioDefined as the motor's holding torque divided by the inertia of its rotor. The higher the ratio, the

higher a motor's maximum acceleration capability will be.

Unipolar driverA step motor driver configuration that uses a unipolar power supply and is capable of driving

phase current in only one direction. The motor phase winding must be center tapped (6 or 8 lead)


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to operate with a unipolar driver. The center tap is used instead of providing the current reversal

of a bipolar driver.

Viscous Damping (KDV) (lb-in./kRPM)Inherent losses are present in all motors which result in lower torque delivered at the output shaft

than developed at the rotor. Losses which are proportional to speed (i.e. speed dependent terms

such as windage, friction, eddy current) are related through the motor's "viscous damping"constant, measured as the slope of the damping curve.

Voltage constant (KE) (V/kRPM peak, line-to-line)May also be termed back-EMF constant. When a motor is operated, it generates a voltage

proportional to speed, but opposing the applied voltage. The shape of the voltage waveformdepends upon the specific motor design. For example, in a brushless motor, the waveshape may

be trapezoidal or sinusoidal in nature. All Pacific Scientific brushless motor designs have a

sinusoidal voltage constant. For a sine waveform, the voltage constant can be measured fromline-to-neutral or line-to-line and expressed as a peak value or "RMS" value.

Start and Running Torque Requirements, Calculations for Electric Motors

When determining the torque requirements for a electric motor, consideration should be given to

the load and start time demands during the start duration, operating torque, and peak load torque.The starting torque is dependant on the number of times an electric motor will have to start in a

given time, as well as, the duration of the start cycle. The actual start torque applied should bemany times greater than the actual start torque required by the application. The greater difference

in torque applied by the motor and the start torque required by the application, the faster the

applied acceleration of the electric motor.

The time duration required to accelerate a application from a dead stop to operating speed

is given by the following:

T = [ N x WR2 ] / [ Ta x 308 ]

Where:

T = Time ( seconds )

N = Velocity at load ( rpm )Ta = Average Torque During start ( ft-lbs )

WR2 = Rotating Inertia (lbs-ft3)

W = Weight (lbs)R = Radius of Gyration (ft2)

308 = Constant derived converting minutes to seconds, mass from weight, and radius to

circumference

Running or operating torque is determined by the following equation:

To = [ 5250 x HP ] / N

Where:

To = Operating or running Torque ( ft-lbs )

HP = Horsepower delivered by electric motor

N = Rotational velocity ( rpm, revolutions per minute )5250 = Constant converting horsepower to ft-lbs/minute and work/revolution to torque


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Use the peak horsepower to determine the maximum operating torque.

Basic Electric Motor Torque Equation

Equation: T = FR

Where:

T = Torque, lb-ftF = Force, lb

R = Radius, or distance which force is applied from pivot location,

ft

To determine a fan or blowers horsepower use the following equation.

Equation:

Where:

P = Power, hp

Q = Flow Rate, cfmp = Pressure, lb/in2

= Efficiency coefficient

Hydraulic Pump Horsepower Equation

To determine a pumps horsepower use the following equation.

Equation:

Where:

P = Power, hp

Q = Flow Rate, gpmS = Specific Gravity of fluid

H = Head height, ft

= Efficiency coefficient

Linear Motion to Rotary Motion Equation

To convert the linear motion or velocity of an object into rotary motion (rpm) use the followingequation.


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Equation:

Where:

N = Rotational speed of shaft (rpm)V = Linear velocity of object (ft/min)

D = Diameter of sprocket or pulley (ft)

Linear Object Motion Horsepower Equation

To determine linear objects motion horsepower use the following equation.

Equation:

Where:

P = Power, hp

F = Force, lbV = Velocity, ft/min

Rotating Object Horsepower Equation

To determine the rotary horsepower of an object into rotary motion (rpm) use the following

equation.

Equation:

Where:

P = Power, hpN = Rotational shaft speed, rpm

T = Torque, lb-ft

Bearings and Lubrication Application Generators

Application bearings and lubrication used on generators - Several types of bearings, each

with specific lubrication requirements, are used on the generators. Usually, a generator has two

bearings, one to support each end of the armature shaft. On some generators, one end of the shaft

is supported by the coupling to the prime mover and one bearing is used at the other end. Theselections of bearing type and lubrication are based on generator size, type of coupling to prime

mover, and expected usage. A generator is usually equipped with either sleeve or ball bearings

which are mounted in end shields attached to the generator frame.

Sleeve bearings are usually bronze and are lubricated with oil. Most unit s with sleeve-typebearings have a reservoir for the oil and a sight gauge to verify oil level. Bearings and the


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reservoir are fully enclosed. Distribution of oil to shaft and bearings from the reservoir is by an

oil-slinger ring mounted on the generator shaft. Rotation of the slinger ring throws the oil to the

top of the bearing. Holes in the bearing admit oil for lubrication. Some units with sleeve-typebearings have an absorbent fiber packing, saturated with oil, which surrounds the bearing. Holes

in the bearing admit oil for lubrication.

Ball bearings (or roller-type bearings) are fully enclosed and lubricated with grease. Most units

with ball or roller-type bearings are equipped with a fitting at each bearing to apply fresh grease.Old grease is emitted from a hoie (normally closed by a plug or screw) in the bearing enclosure.

Some units are equipped with prepacked, lifetime lubricated bearings.

Cylinder Inertia Calculations and EquationsSolid Cylinder Inertia Based on Weight and Radius Equation and Calculator

Use this equation and calculator to determine the Inertia of a Cylinder.

Solid Cylinder

Equation:

Where:

J = Inertia, lb-in.-sec2

W = Weight, lbs

R = Radius, inches

g = Gravitation constant 386 in./sec2

Solid Cylinder Inertia Based on Density, Radius and Length Equation and Calculator

Use this equation and calculator to determine the Inertia of a Cylinder.

Solid Cylinder

Equation:


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Where:


L=

Length, in.R = Radius, inches


p = Density, lb/in3

Hollow Cylinder Inertia Based on Weight and Radius Equation and Calculator

se this equation and calculator to determine the Inertia of a Hollow Cylinder.

Hollow Cylinder

Equation:

Where:


W = Weight, lbs

R0 = Radius outside, inches

Ri = Radius inside, inches


Hollow Cylinder Inertia Based on Density, Length and Radius Equation and Calculator

Use this equation and calculator to determine the Inertia of a Hollow Cylinder.


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Jl= Load Inertia, lb-in.-sec2

Jm = Drive Motor Inertia, lb-in.-sec2

Gear Drive Motor Moment of Inertia Equation

Gear Drive Motor Moment of Inertia Equation: Use these equations and calculator to

determine the Inertia of a gear drive system. For any change in rotation speed, the load inertia

will reflect back through the gears to the motor.

Gear Drive System

Equation:

Motor Speed

or

Motor Torque:

Reflected Load Inertia

Total Inertia realized at Motor:

Where:

Sm = Motor Speed, rpm

Sl = Load Speed, rpm

Nl = Number teeth on load gear

Nm = Number teeth on motor gear

N = Gear Ratio


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Tm = Torque of Motor, lb-in

Tl = Torque of Load, lb-in

e = Efficiency

Jl= Load Inertia, lb-in.-sec2

Jm =Drive Motor Inertia, lb-in.-sec2

Jr =Reflected Load inertia, lb-in.-sec2

Jt =Total Inertia, lb-in.-sec2

Tangent Drive Motor Moment of Inertia Equation and Calculator

Use these equations and calculator to determine the Inertia of a tangent drive system. Tangent

drive may be the following: Timing belt and pulley, Chain and sprocket, rack and pinion, etc...

Tangent Drive SystemConveyor System

Equation:

Motor Speed:

Load Torque:

Friction Torque:

Load Inertia:


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Total Inertia

Where:


Vl = Load Speed, rpm

R = Radius, in

Fl = Load Force, lb

Tf = Friction Torque, in-lb

Ff = Friction Force, lb

Tl = Torque of Load, lb-in

Jl = Load Inertia, lb-in.-sec2

Wlb = Weight of load plus belt, lb

g = Gravitational constant, 386 in./sec2

Jp1 =Pulley Inertia, lb-in.-sec2

Jp2 =Pulley inertia, lb-in.-sec2

Jm =Motor inertia, lb-in.-sec2

Jt =Total Inertia, lb-in.-sec2

Lead Screw / Worm Gear Drive Motor Moment of Inertia Equation and Calculator

Use these equations and calculator to determine the Inertia of a lead screw / worm gear drive

system.

Worm Gear / Lead Screw Drive System

Equation:

Motor Speed:


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Load Torque Reflected to Motor:

Friction Force:

Friction Torque:

Total Inertia

Where:


Vl = Load Speed, inches/minuteP = Pitch, rev/in

Tr = Torque reflected to motor, lb-in

Fl = Load force, lb

e = Efficiency

Fpf= Preload Force, lb

= Coefficient of friction of screw

Ff = Friction force, lb

W = Weight, lb

Tf = Friction torque. lb-in

Jl = Load inertia, lb-in-sec2

g = Gravitational constant, 386 in/sec2

Jls = Worm gear / lead screw inertia, lb-in-sec2

Jm = Motor inertia, lb-in-sec2

Electric Motor Torque and Force Equations and Calculations.

Electric Motor Accelerating Torque and Force Equation andCalculator

To determine a fan or blowers horsepower use the following equation.


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Equation:

=Where:

T = Required Torque, lb-ftWK2 = Inertia of load to be accelerated(See moment of inertia calculations)

= Change of speed, rpmt = Time to accelerate the load, seconds

Electric Motor Solid Cylinder Rotating About Own Axis Torque, Force Equation and

Calculator

to determine a fan or blowers horsepower use the following equation.

Equation:

and

Where:

T = Required Torque, lb-ft

WK2 = Inertia of load to be accelerated lb-ft

2

(See moment of inertia calculations)

= Change of speed, rpm

t = Time to accelerate the load, seconds

W = Weight of object, lb

R = Radius of cylinder, ft

Electric Motor Hollow Cylinder Rotating About Own Axis Torque, Force Equation and

Calculator

To determine a fan or blowers horsepower when driving a hollow

cylinder/shaft use the following equation.

Hollow Cylinder


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Equation:

and

Where:


WK2 =Inertia of load to be accelerated lb-ft2





R1 = Outside Radius of cylinder, ft

R2 = Inside Radius of cylinder, ft

Electric Motor Driving Conveyor in Linear Motion Torque, Force Equation and

Calculator

Use this equation and calculator to determine the the torque and force todrive material in linear motion with respect to a continuous fixed relation to arotational drive/velocity. Such as, a material moving system or conveyormachine.

Equation:

and(Moment of Inertia)

Where:


WK

2 = Inertia of load to be accelerated lb-ft2





V = Linear velocity, fpm

N = Rotational speed of shaft, rpm

Electric Motor Driving Speed Reduction ( Gear, Belt, or Chain ) Torque, Force Equation

and Calculator


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Use this equation and calculator to determine the torque and force to drivegears, belt, or chain mechanical systems.

Equation:

and

Where:

T = Required Torque, lb-ft= Reflected inertia, lb-ft

2

= Load inertia, lb-ft2

= Reduction ratio



= Change speed of shaft, rpm


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Electric Motors

There are many types of electric motors, somesmaller than a human hair others large enoughto power a locomotive. For the purpose of thispage we will be discussing induction motorswhich are typically found on most workshopmachinery such as air compressors, drillpresses, table saws, band saws, jointers,

shapers and lathes. These types of motors haveno brushes and work only on alternating current.They may range in size from 1/4 horsepower upto 5 horsepower or more.

To find the specs of an electric motor check the name plate, it will tell you among otherthings:

Manufacturer's type and framedesignation

Horsepower output.Time rating.Maximum ambient temperature forwhich motor is designed.Insulation system designation.RPM at rated load.Frequency.Number of phases.Rated load current.Voltage.

Types of Motors

Split Phase

The split phase motor is mostly used for "medium starting" applications. It has start and runwindings, both are energized when the motor is started. When the motor reaches about 75% of itsrated full load speed, the starting winding is disconnected by an automatic switch.

Uses

This motor is used where stops and starts are somewhat frequent. Common applications of split


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phase motors include: fans, blowers, office machines and tools such as small saws or drill presseswhere the load is applied after the motor has obtained its operating speed.

Capacitor Start

This motor has a capacitor in series with a starting winding and provides more than double thestarting torque with one third less starting current than the split phase motor. Because of this

improved starting ability, the capacitor start motor is used for loads which are hard to start. It hasgood efficiency and requires starting currents of approximately five times full load current. Thecapacitor and starting windings are disconnected from the circuit by an automatic switch when themotor reaches about 75% of its rated full load speed.

Uses

Common uses include: compressors, pumps, machine tools, air conditioners, conveyors, blowers,fans and other hard to start applications.

Horsepower & RPM

Horsepower

Electric motors are rated by horsepower, the home shop will probably utilize motors from 1/4 HP forsmall tools and up to 5 HP on air compressors. Not all motors are rated the same, some are ratedunder load, others as peak horsepower, hence we have 5 HP compressors with huge motors and 5Hp shopvacs with tiny little motors. Unfortunately all 5 HP compressor motors are not equal inactual power either, to judge the true horsepower the easiest way is to look at the amperage of themotor. Electric motors are not efficient, most have a rating of about 50% due to factors such as heatand friction, some may be as high as 70%.

This chart will give you a basic idea of the true horse power rating compared to the ampere rating.Motors with a higher efficiency rating will draw fewer amps, for example a 5 HP motor with a 50%efficiency rating will draw about 32 amps at 230 VAC compared to about 23 amps for a motor with a70% rating.

TRUE HPAMPS at115VAC

AMPS at 230VAC

1/4 3.2 - 2.3 1.6 -1.2

1/3 4.3 - 3.1 2.2 - 1.5

1/2 6.5 - 4.6 3.2 - 2.3

3/4 9.7- 7.0 4.9 - 3.5

1 13.0 - 9.3 6.5 - 4.6

1 1/2 19.5 - 13.9 9.7 - 7.02 25.9 - 18.5 13.0 - 9.3

5 64.9 - 46.3 32.4 - 23.2

A quick general calculation when looking at a motor is 1 HP = 10 amps on 110 volts and 1 HP = 5amps on 220 volts.

RPM

The shaft on a typical shop motor will rotate at either 1725 or 3450 RPM (revolutions perminute).


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The speed of the driven machine will be determined by the size of pulleys used, for example a 3450RPM motor can be replaced by a 1750 RPM motor if the diameter of the pulley on the motor isdoubled. The opposite is true as well but if the pulley on the 1750 RPM motor is small it is notalways possible to replace it with one half the size. It may be possible to double the pulley size onthe driven machine if it uses a standard type of pulley, (not easily done on air compressors forexample).

Electronic speed reducers such as the ones sold for routers will not work on induction type motors.

Phase, Voltage & Rotation

Whether or not you can use a motor will likely depend on these factors.

Single Phase

Ordinary household wiring is single phase, alternating current. Each cycle peaks and dips asshown. To run a three phase motor a phase converter must be used, usually this is not practical, itis often less expensive to change the motor on a machine to a single phase style.

Three Phase

This is used in industrial shops, rather than peaks and valleys the current supply is more evenbecause of the other two cycles each offset by 120 degrees.

Voltage

Many motors are dual voltage, by simply changing the wiring configuration they can be run on 110volts or 220 volts. Motors usually run better on 220 volts, especially if there is any line loss becauseof having to use a long wire to reach the power supply.

Motors are available for both AC and DC current, your typical home wiring will be AC, there are DCconverters available which are used in applications where the speed of the motor is controlled.

Rotation

The direction the shaft rotates can be changed on most motors by switching the right wires, there is

usually a diagram on the motor.

The direction of rotation is usually determined by viewing the motor from the shaft end and isdesignated as CW (clockwise) or CCW (counter-clockwise). Note: Some manufactures may have adifferent method of determining shaft rotation but will usually make a note of it.


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Back of plate showing voltage and rotation terminals

Frame Style

Motors are built to standard specifications, such as shaft height, shaft diameter, and style ofmounting. The different styles are defined by a number and lettering system developed by Nema(See Reference Chart).

Types of Mounts

The three most common types of mounts you will find are:

Rigid base

Is bolted, welded or cast on main frame and allows motor to be rigidly mounted.

Resilient base

Has isolation or resilient rings between motor mounting hubs and base to absorb vibration andnoise.

NEMA C face mount

Has a machined face which allows direct mounting, bolts pass through mounted part to threadedholes in the motor face.

Enclosures

The two most commonly used styles are:

ODP

An ODP enclosure on a motor means "Open, Drip Proof". They are relatively inexpensive motorsused in normal applications. The construction of an ODP motor consists of a sheet metal enclosurewith vent stamped to allow good air flow. The vents are designed in such a way that water drippingon the motor will not normally flow into the motor. A fan is mounted on the motor's rear shaft to pullair through the motor to keep the motor cool.

TEFC

A TEFC enclosure on a motor means "Totally Enclosed, Fan Cooled". This is probably the mostcommonly used motor in ordinary industrial environments. It costs only a few dollars more than theopen motor, yet offers good protection against common hazards. It is constructed with a small fanon the rear shaft of the motor, usually covered by a housing. This fan draws air over the motor fins,removing excess heat and cooling the motor. The enclosure is "Totally Enclosed". This ordinarily
http://www.motorsanddrives.com/cowern/motorterms2_chart.htmlhttp://www.motorsanddrives.com/cowern/motorterms2_chart.html


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means that the motor is dust tight, and has a moderate water seal as well. Note that TEFC motorsare not secure against high pressure water.

Switching

Low Voltage (110)

Use a "Single Pole/Single Throw" switch since onlythe black wire needs to be switched.

High Voltage (220)

Use a "Double Pole/Single Throw" since both theblack and white wires need to be switched.

This configuration may also be used on a LowVoltage machine if there is a possibility it will bechanged to High Voltage later on.

Troubleshooting

Before you start to work on the motor, MAKE SURE THE POWER IS OFF. Always turn the poweroff at the electrical service entrance breaker box or fuse, to prevent electrical shock.

FAILURE TO START

1. Check for blown line fuse or tripped breaker.2. Allow motor to cool and try to reset.3. One of the most common problems in a woodworking shop is a build-up of sawdust in the motor,especially withopen drip proofmotors. Give the motor a shot of air to blow out the sawdust, ninetimes out of ten this will get the motor to start again, to fix the problem the motor should be replacedwith a totally enclosed, fan cooled model.

MOTOR IS NOISY

Excessive vibration.1. Check for loose mounting.2. Check shaft alignment.3. Check for bent shaft.

Excessive noise.1. Check for damaged bearings. Replace as necessary.2. Check for rotor rub by rotating the shaft slowly by hand.

OVERHEATING

(Motor runs but overload trips)

1. Check for adequate ventilation. Be sure motor vent holes (or fan blades) are free of obstructions.2. Use a shorter or heavier gauge extension cord.

For more detailed information see this page fromReliance Electric
http://www.sawdustmaking.com/ELECTRIC%20MOTORS/electricmotors.html#ODPhttp://www.sawdustmaking.com/ELECTRIC%20MOTORS/electricmotors.html#ODPhttp://www.sawdustmaking.com/ELECTRIC%20MOTORS/electricmotors.html#TEFChttp://www.reliance.com/prodserv/motgen/h7000.htmhttp://www.reliance.com/prodserv/motgen/h7000.htmhttp://www.sawdustmaking.com/ELECTRIC%20MOTORS/electricmotors.html#ODPhttp://www.sawdustmaking.com/ELECTRIC%20MOTORS/electricmotors.html#TEFChttp://www.reliance.com/prodserv/motgen/h7000.htm


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motor selection basic

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