Procedure & Important Terms for Selecting AC Motors

 

Procedure & Important Terms for Selecting AC Motors

The motor type selected for a given application is determined by the features required, which include:

·         The energy source, 

·         System specifications, 

·         Motor type, 

·         Kind of motor insulation, 

·         The duty cycle of a motor 

·         Kind of bearing, 

·         The motor mounting technique, 

·         The motor's price and size, 

·         Method of regulating Speed, 

·         Environmental circumstances.

Important Terms for Selecting AC Motors

 The Energy Source

The power supply can be identified by its rated voltage, frequency, and number of phases.

A poly-phase power system consists of two or more alternating currents, all of which have the same frequency and amplitude but are rotated to one another at different phase angles. 

Three-phase power has the advantage of requiring less maintenance due to its simpler structure for motors.

Motor Nameplate Voltage

The available power supply determines the motor nameplate voltage. For a voltage drop in the system connecting the power source & the motor leads, the nameplate voltage will typically be lower than the actual nominal distribution system voltage. Dual voltage motors ratings for poly-phase and single-phase motors:

The ratio of the two voltages is either 1:2 or 1:3 (for example, 230/460 at 60 Hz; 2300/4000 at 60 Hz; 220/380 at 50 Hz). 

Voltage ratios are only 1:2 for single-phase power. 

Voltage imbalance

Any voltage imbalance will lead to a larger current imbalance, which will increase temperature.

Calculating percent voltage imbalance is done as follows:

(100 times Maximum Voltage Deviation above Average Voltage)/Average Voltage is the percent formula unbalanced. Note: Using a motor with a voltage imbalance of more than 5% is not advised.

Full-Load Speed: When the voltage is unbalanced, full-load Speed is slightly slowed down. 

Locked-rotor KVA will only marginally rise. At an unbalanced voltage, full-load current will be unbalanced by a factor of six to ten greater than the unbalance voltage. 

Temperature Increase: A 3.5 percent voltage unbalance boosts a temperature rise of about 25%. 

Standard Frequency 

In the US, 60 hertz is the most common frequency. However, different nations frequently use 50 hertz systems.

The nameplate horsepower multiplied by the de rate factor gives you the rated horsepower at 50 Hz.

The variation in voltage that is permitted at de rated HP is 5%. For motor overload protection, use 1.0 Service Factor and 60 Hz Amps.

Motor speed is equal to 5/6 of nameplate speed.

 Service Factor Motors designed for usage at 60 hertz should be supplied at 60 Hz motor without consideration for 50 Hz operation. 

Dual Frequency 

Motors that must operate at both fifty and sixty Hz are non-NEMA specified motors and will be name-plated accordingly. When a motor is required, this must be noted in the order.

Variation in Voltage and Frequency

Variations in voltage are likely to result in the following situations:

·         A voltage increase or decrease at the rated horsepower load may cause more heating. It could limit the life of the motor insulation. 

·         The power factor will typically decrease as voltage rises. Breakdown and locked-rotor torque will be inversely proportional to voltage squared. As a result, the available torque will decrease as the voltage drops. 

·         A 10% increase in voltage will cause the slip to decrease by roughly 17%. A 10% voltage reduction would result in a 21% increase in slip. 

The following circumstances are probably to happen with varying frequency:

·         Over-rated frequency improves power factor while lowering maximum torque and locked rotor. Additionally, this circumstance raises Speed. 

·         In contrast, a drop in frequency usually increases locked-rotor maximum torque & locked-rotor current while decreasing power factor and Speed. 

Operation with Variable Frequency

On variable frequency inverters, motors are an option. Inverters can be broadly classified into three categories:

·         In a square wave inverter called a VVI, the voltage and frequency change proportionately, producing a consistent number of volts per hertz. 

·         The PWI kind of pulse width modulated inverters is similar to the VVI type except that time-varying pulses are used to approximate sine waves. 

·         The constant current inverter, or CCI, uses a square wave current source rather than a voltage supply. 

System prerequisites

 It will consist of the following:

·         Rated Speed (Speed expressed in shaft rotations per minute (RPM)).

·         The torque.

·         Power in horses.

·         A motor's torque-speed performance.

·         Motor torque, speed speed, and current relationship.

Maximum Speed 

The input power frequency combined with the total amount of electric magnetic poles the motor is wound for determines the Speed. The motor operates more quickly the higher the frequency. The motor operates more slowly the more poles it has.

The equation that follows is used to calculate an induction motor's synchronous Speed:

Synchronous Speed (rpm) is calculated as (60 x 2 x Frequency) / the number of poles.

Actual full-load Speed, or how fast the induction motor will run at nameplate-rated load, will be slower than synchronous Speed.

Slip is the distinction between synchronous Speed & full-load Speed. Percent Slip equals Synchronous Speed multiplied by the full load speed multiplied by 100.

Force 

Put, torque is a force or turning effort that acts along a radius. Horsepower accounts for how quickly the motor shaft rotates. The shaft needs more horsepower to turn quickly. Therefore, horsepower is a measurement of the pace of work. According to the definition, torque and horsepower are related in the following ways:

(Hp x 5252)/Full-Load rpm = Full Load torque in lb-ft

 

 

Locked Rotor Torque 

When the rated V and rated f are applied, the motor will produce locked-rotor torque, which will do in all rotor angular positions.

Pull up Torque 

It has the least force that builds up from when the rotor is locked to when it breaks down.

Breakdown Torque 

The greatest torque a motor can produce at rated voltage and rated frequency without experiencing a sudden decrease in Speed is known as the breakdown torque.

Full Load Torque 

The torque required to generate estimated horsepower with full-load Speed .Speed is known as full-load torque. Hp x 5252 / Full-Load revolutions per minute is the formula for full-load torque in lb-ft.

Full-Load Current.

An induction motor's steady-state current drawn from the power line during full-load operation at rated voltage and specified frequencies is known as the full-load current.

Locked-Rotor Current 

The steady-state current for a motor with its rotor locked and its rated voltage delivered at a rated frequency is known as the locked-rotor current the formula for KVA / horsepower is as follows:

KVA/HP is calculated for three-phase motors as three times the discharge (in amperes) x volts per (1000 x Hp).

For single-phase motors, KVA/Hp equals current (in amperes) times volts / (1000 x Horsepower).

The following equation for determining locked-rotor current can be derived by modifying the equation for KVA/Hp with the three-phase motors previously mentioned:

The formula for calculating LRA is as follows: LRA = (1000 x Hp x Locked-Rotor KVA/Hp) / (3 x Volts).

The starting current of a 7 1/2 Hp, 230V motor would be approximate:

LRA = (3 x 230) Divided (one thousand x 7.5 times 6.0) = 113 Amps

Motor Temperature

The impacts of excessive heat on motors include accelerated insulation breakdown and early insulation failure. 

Cause the degradation of bearing grease. Two things determine a motor's maximum operating temperature:

·         The temperature is outside or ambient.

·         A rise in internal or motor temperature. 

The majority of motors are made to function at temperatures up to 40°C. The heat produced by motor inefficiencies during operation is what causes the temperature to rise .Temperature increase occurs with no load due to friction within the ball bearings, losses to the core (eddy current or hysteresis losses), and stator losses from I²R.

At full load, rotor I²R loss and wandering load losses are additional losses that contribute to heating. (I stands for current (amps) and R for stator or rotor resistance (ohms)).

The increase in temperature will be much higher in these circumstances since the current rises with increasing motor load and with a locked rotor.

 

  Motor cooling 

The heat produced while motor operations will be transported to the ambient air because the overall temperature of a motor is higher than the surroundings. The pace of heat transfer impacts the maximum load capacity or duty cycle of a particular motor design.

Applications for duty cycles can be categorized into one of three categories:

1. Constant work

It is a need for the service to operate for an extended period at a practically constant load. About 90% of motor operations fall under this.

2. Continuity of duty

It is a condition of service that operations be performed for alternately specified periods of load and no load, load and rest, or load, no load, and rest. Typically, 30 and 60-minute motors were given.

3. Changing duties

A horsepower against time curve will allow for the assessment of the highest horsepower needed for this type of duty cycle, & an estimation of its root-mean-square (RMS) power will show the appropriate motor rating from the perspective of heating.

Configurations for Motor Mounting

There are different mounting configurations for motors

A: Positions for Floor Mounting

The position of the pipe box can be adjusted according to the mounting configuration and position on both sides of the framework.

B: Positions for Ceiling Mounting

A foot-mounted motor may be put on a ceiling with certain modifications.

C- Positions for Wall Mounting,

A foot-mounted motor may be put on a wall with some modification. Wall mounting locations begin with the letter W.

Mounting Faces for Motors

When a motor is linked straight to a gearbox, it is occasionally essential to link it immediately for the equipment it drives

C-face

A C-face motor features threading bolt holes on the face, usually the end. Bolts utilized to attach the motor go through matching holes.

 D-flange

The bolts are inserted into the equipment's threaded mating holes after passing through the holes in a D-flange motor's flange.

The cost and size of the motor

Three main steps can be used to size motors for a given application:

·         Examining the load operating properties

·         Taking the operational environment into account

·         Preparing for the available power supply.

The price of an AC motor is determined by the motor's type, size, voltage, and speed specifications for your application. Most of the time, a high-power gear motor is the most cost-effective.

Backlash is a crucial consideration when choosing a Gear motor. It is important if load reversals are needed for precise applications.

 

Device speed controls that are passive

  Modify the strength of the magnetic field, the voltage levels, or other motor features. Control other motor properties; passive device controls comprise constant or variable resistors and transformers. Passive components like resistors make the motor circuit more resistant.

Solid state movement controls to manage motor voltage and power supply frequency, either to provide electronic commuting and hence control motor speed, solid state controls use increasingly complicated circuits made up of active devices such as integrating circuits.

Switching amplifiers alter the length of time the entire line voltage is supplied to the armature rather than the amount of resistance. The overall result is an average voltage similar to the voltage level.

Half-Wave SCR Controls are an example of solid-state control.

Motor housings

The type of enclosure needed depends on the environment where the motor is installed, the level of mechanical protection needed, and the level of corrosion resistance needed.

The open machine is a device that has ventilation apertures that allow outside air to circulate all around the motor's winding.

A completely enclosed machine is built to prohibit the free flow of air between the interior & exterior of the motor. However, it is not completely enclosed.

Height

Standard motor ratings presumptively operate at level with the sea in a 40°C environment. The density of the air affects how chilly it can be when it is ventilated. Higher elevations have lower atmospheric pressure and density.

 

Ambient Temperatures  

When the usual 40°C ambient temperature is added, standard motors ensure that the internal temperature rise does not exceed the winding insulation temperature limit.