17: TYPES OF HVAC ELECTRIC MOTORS

What degree of starting torque do small fans normally need?

Low levels

What degree of starting torque do compressors in refrigeration applications normally require?

High levels

What are the 4 common types of a single-phase motors?

1. Split-phase
2. PSC
3. Shaded-pole
4. ECM

This is what is added to the motor when more starting torque is needed

A start capacitor

This is what improves the running efficiency of a split phase motor after it has been started up

A running capacitor

This is what breaks the circuit from the start capacitor to the motor once the motor is up to speed. It ensures that the high insurge of amps needed to start the motor is not remaining in the circuit and overheating the motor once it is up and ready

A centrifugal switch

What typically determines the rated speed of a single-phase motor

The number of poles or windings in the motor. The more poles there are, the slower the motor will be

What are the two common speeds of single-phase motors? What are their speeds adjusted for slip?

1800 (1725 post slip) and 3600 (3450 post slip).

What is "the load imposed on the motor while operating"

Slip

Are there any three-phase motors in residential applications?

Typically not. Three-phase is only used in larger applications, such as industrial and commercial

Three phase motors have a \______ starting torque and \________ running efficiency

High starting torque, high running efficiency

This takes the start winding (that heavy initial surge load) out of a circuit using BEMF

Potential relay

This breaks the circuit to the start winding (once the motor is up and running) by using the motor's run current

Current relay

This motor is used when high starting torque is not required. It needs no starting device. It uses the run capacitor for both start and running requirements

PSC motors

Occasionally some PSC motors will need an additional amount of initial starting torque. This will help them get that torque

PTC device

This motor does not have any brushes

ECM (electronically commutated motor)

These even out on the load on heating and air conditioning systems by adjusting the equipment speed to the actual load needed at a specific time, rather than providing the same base load

Variable speed motors

Using a VFD, what type of variance of motor speed is possible?

Motor speed with a VFD can be varied from about 10% of rated speed to about 120% using electronics

These convert AC power to DC power

DC converters

After AC power has been converted to DC power, these even out the pulsating DC power to make it more smooth and easier to use

Capacitors

This creates an alternating frequency that can be varied

inverters

T/F: The frequency and voltage of a motor must be changed together in order for that motor to perform efficiently

True

In terms of HVAC, what are electric motors used for?

Electric motors are used to turn the prime movers of air, water, and refrigerant, which are the fans, pumps, and compressors. BASICALLY, MOTORS ARE WHAT POWER THE FANS/PUMPS/COMPRESSORS

There are 5 major parts to a motor. What are they?

1. Stator
2. Rotor
3. Bearings
4. End Bells
5. Housing

This is a winding on the motor that, when energized, will generate a magnetic field because the current will be flowing through it

Stator

This is the rotating portion of the motor and is made of iron or copper bars bound on the end with aluminum. It goes inside the stator and is not connected to the power source

Rotor

This is connected to the rotor but extends beyond the rotor wheel

Motor shaft

These are what ensure that while the rotor is rotating, the friction remains low. They aim to reduce the heat generating by the motor's moving and rubbing parts and surface, hoping to prevent overheating and damage to the motor

Motor bearings

These hold the motor components in place and provide a means for securely mounting the motor itself

Motor housing/ End bells/Base

What two things cause the rotation of a motor?

1. Electricity
2. Magnetism

What is magnetism?

Poles of a magnet that are unlike attract each other, while poles of a manger that are alike repel one another

Explain the relationship between the stator and the rotor in terms of magentism:

The stator (charged with current) causes the rotor to turn via repelling and attraction (magnetism). Remember, the rotor is surrounded by the stator. The stator is constantly changing its pulse, which means the roto is being quickly repelled/attracted and thus spinning rapidly...In a normal 60 hertz operation, the polarity changes 120 times per second. That a lot of movement for the motor!

In this type of motor, the stator has two poles with run windings, which, when electrical current is applied, become highly polarized- in a normal 60 second operation, the polarity changes 120 times per second

Two pole split phase motor

The solid rotor in a single-phase induction motor. A rotor with two end rings connected along their circumference with copper or aluminum bars arranged in slots in an iron core. The magnetic fields of the rotor are different than that of the windings, causing the motor to spin

Squirrel cage rotor

This signifies the cycles per second of an alternating current

hertz

60 hertz means

The alternating current is supplied at a frequency of 60

What is the standard frequency of current in the US?

60 hertz

Explain why 60 hertz means that for every second, the electrical current changes direction 120 times

In a 1-second time interval, 60 complete sine waves are generated. Thus, one cycle is completed in 1/60 of a second. During this very brief instant, the direction of current flow changes twice. This means that every second, the current changes direction 120 times.

Explain why the more poles a motor has, the slower it will turn

Alternating current is moving between poles. This movement is what is behind repelling and attraction, and thus the movement of the motor. If there are more poles, the AC takes longer to complete each circuit. Thus, the repelling and attraction is slower, and the motor spins slower. So the less poles there are, the faster AC can complete a circuit.
This is summarized by the equation: rpm \= (frequency x 120) / number of poles...when we plug this in, we get our two standard rpm's for split phase motor's: 3600 and 1800:

Speed of two pole, split phase motor: (60 x 120) / 2 \= 3600 rpm
Speed of four pole, split-phase motor: (60 x 120)/ 4 \= 1800 rpm

What is the main problem with the RPM generated by the equation RPM \= (frequency x 120) / number of poles?

It does not take into account SLIP, or the conditions in which the motor is operated, which includes friction and the mechanism or drives to which the motor is connected. These factors all add resistance, which consequently decreases motor speed.

The resistance to motor speed caused by conditions in which the motor is operating that cause a decrease in RPM

Motor slip

These are used to help get the motor started and up to its standard speed. Once the rpm is reached, they are disconnected, and the motor operates on the running windings instead

Start windings

This is the twisting power of the motor shaft

Torque

This is the power required to initially turn the shaft from the stopped position

Starting torque

What type of current rating is used to power starting torque?

LRA, or "locked rotor amperes", aka inrush current

What are the two types of current ratings?

1. FLA (full load or rated load amperage AKA what the motor is rated to run at)
2. LRA (locked rotor amperage AKA the amperage required to get the motor unlocked and moving)

What is the average difference in size between the locked rotor amperage and the rated load amperage?

Typically, the LRA is 5 times the size of the RLA

Explain the difference in LRA size needed between a compressor application and a small fan:

Thus in a compressor application, the LRA required is significantly higher than its FLA. This is because the PSI of suction (going into the piston) is only 5, while the discharge is 155. To overcome this and get the piston moving initially, a lot of amps are needed.
But small fans are different from compressors. A motor does not need as much LRA/ starting torque to start a small fan. All the motor needs to do is overcome friction to start the fan moving. While the compressor had a 150 pressure difference, the motor has none. There is no pressure difference because the pressures equalize when the fan is not running. Thus, significantly less amperage is needed to get the motor going.

Who furnishes electricity to different buildings and determines their power supply?

The electrical company

From the power company, power is moved across the country by high voltage transmission lines to transformers. What do transformers do?

Transformers reduce the power to the voltage needed at a specific level. They transform it to required voltage

T/F: Several homes can be furnished by the same power reducing transformer?

True

Explain the common applications for: 115/1/60, 230/1/60, 230/3/60, 460/3/60, 277/1/60, and 560/3/60:

115/1/60: Single phase for common residential appliances
230/1/60: Single phase for heavy-duty residential applications
230/3/60: Three phase for large loads such as electric heat or motors
460/3/60: Three phase for large loads such as electric heat or motors
277/1/60: Odd-ball single phase for lighting circuits (may be obtained between 460 V and neutral with certain systems).
560/3/60: Three phase for special industrial systems

This allows a single power supply to be transformed to meet the different requirements within one building:

Transformers

These single-phase motors have two distinctly different windings (a start winding and a run winding) and are normally used for operating fans in a smaller horsepower range. Normal operating ranges are 1800 rpm and 3600 rpm

Split phase motors

Why are split phases called "split phase"

Because of the two different windings, they are "split-phase", or use two different phases. The medium amount of initial torque required uses a higher voltage than the run phase

T/F: All split-phase motors have a start and a run winding. The start windings must be disconnected from the circuit within a very short period of time or they will overheat the motor

True. A centrifugal switch is what disconnected the start voltage from the motor after initial start up is achieved

This disconnected the start winding from the circuit when the motor reaches approximately 75% of its rated speed. The motor then runs on the running winding and not the starting winding

Centrifugal switch

What is the centrifugal switch made out of?

The switch is a mechanical device attached to the end of the shaft with weights that will sling outward when the motor reaches about 75% speed

What is the most common cause of motor problems?

Centrifugal switches getting worn down. Because they are used so much, and once they go out, the motor is in grave danger of overheating

This is what opens the start winding after the motor has started

Electronic relay

What does CSIR stand for

Capacitor start induction run

These are very similar to split-phase motors, except they have two distinctly different windings for starting and running

Capacitor-start motors

This is wired into capacitor start motors and is what gives the motor more initial starting torque

Start capacitor

What is the biggest difference between induction and capacitor motors?

In an inductive current the current lags the voltage, while in a capacitive circuit the current leads the voltage

The amount by which the current lags or leads the voltage

The phase angle

Explain the relationship between a capacitor and the phase angle and motor running speed

A capacitor is designed to create a phase angle that is the most efficient for starting the motor. The capacitor is not designed to be used while the motor is running and must be switched out of the circuit soon after the motor starts, which is done at the same time the windings are taken out of the circuit.

T/F: In capacitor start, capacitor run motors, the start capacitor is only used to create initial starting torque, and is quickly thereafter taken out of the circut

True

This type of motor is normally used with belt-drive fans and compressors because they offer a great deal of resistance to the motor when it initially starts because the motor must turn the pulley which is connected to a belt which is connected to a belt assembly. But once they are initially moving, their running torque is significantly less

Capacitator start capacitator run motors...The added starting torque (provided by the capacitor start capacitor run) is needed to help overcome this resistance. Once the motor starts, however, keeping it turning does not require the same amount of torque.

These motors have windings very similar to those of a split-phase motor, but don't have a start capacitor. Instead they use on run capacitor for both start and running, which is wired into the circuit

PSC (Permanent split capacitator motor)

This is the simplest split phase motor. It is very efficient and has no moving parts to start to motor; however, the starting torque is very low, so it can be used only in low-starting-torque applications

Permanent split capacitator motors (PSC's)

T/F: Compared with split-phase motors used to start up belt-drive fans (which require so much starting torque to get the belt moving), PSC motors are much quieter and require much less moving parts.

True

Why are PSC motors so quiet?

They don't require a strong surge of amperage for initial start up, but rather slowly increase. Thus they are very quiet and don't need to switch circuits

These motors are economical motors used for lighter residential purposes

Shaded-pole motors (won't touch many of them)

These motors have shading coils at the corner of each pole that help the motor start by providing an induced current and a rotating field.

Shaded pole motors

T/F: Three phase motors have no starting windings or capcitors

True. Three phase motors require no start assist for high-torque applications- they have no starting windings or capacitors.

Single phase motors generally need more initial torque than running torque. What are the two main ways this initial torque is generated?

1. start winding and then a run winding
2. Start capacitor and then a run capacitor

T/F: Three phase motors generally do not have dual speed

True

Explain what three phase power supply really is:

Three phase power supply consists of three single-phase power supplies that are generated 120 electrical degrees out of phase with each other. Never are all 3 power phases in sync with each other

This is a single-phase motor designed to operate in refrigerant vapor atmosphere

Hermetic motors

These assist capacitors start, capacitor run motors that still require more assistance in meeting the initial starting torque

Potential Relays. Their main function is to help with starting the motor

These variate the speed of the motor to keep it running and save energy and ensure longevity of the motor as opposed to start/stop motors.

Variable Speed motors

These change the frequency of the electricity coming in (from DC to AC and vice versa) which allows for changes in the speed of the motor.

Variable frequency drives

T/F: most motors do not need to operate at full speed and load except during the peak temperature of the season (warmest and coldest days of the year) and could easily satisfy the heating or air-conditioning load at other times by operating at a slower speed

True (this is where VFD's come in)

T/F: By limiting the number of times the motor starts, the rate of power consumption is reduced

True (it takes more power and kills the life of the motor quicker to continually start/stop than it does to keep the motor going at a generally stable level (As in VFDs)

What determines the speed of traditional motors and what determines it for VFDs?

The power supply and number of poles determines the speed of traditional motors, while VFDs are able to operate at different speeds by using electronic circuits

What converts AC to DC

Rectifiers

What type of current and frequency is furnished throughout the US?

Alternating current at 60 hertz

Explain the relationship between variable speed motors and variable frequency drives

Traditionally, variable speed motors used DC . This made them much more complicated to work with despite the demand for them in certain applications. It also required they used brushes. Today, types of equipment, such as pumps and fans and fractional horsepower equipment. The two types of motors found in equipment today are the squirrel cage induction motor and the ECM (electronically commutated) DC motor. Instead of brushes rubbing on an armature, the motor is electronically commutated. This motor can run a fan as well as fractional horsepower equipment. Electronic components now make variable-speed AC motor operation possible. The electronic components can switch power on and off in microseconds without creating an arc; therefore, open-type contacts are not necessary for switching purposes.

T/F: Light bulb turns on/off 120 times per second.

True. The voltage goes from 0 V to 120 V to 0 V and back to 120 V 60 times per second; thus it is a 60-cycle (60 hertz current).