Hvac

Question 1

Voltmeter meters range from 600v

Answer 1

to 1mv

Question 2

The ohmmeter battery produces its own voltage

Answer 2

– The meter measures the resistance its voltage encounters

Question 3

• Therefore, main power must be off

– Example: Switch open, broken wire, etc.
• “0” means no resistance (closed)
– Example: Switch closed, wires connected, or a direct short
• “Measurable resistance” is between “0” and “OL”
– Examples: Checking motor windings, heater wires, etc.
• Meters in following slides will show “999” for measurable resistance

Answer 3

• “OL” means “Open Line” or infinite resistance

Question 4

Most common ammeter is a “Clamp-on” type

Answer 4

– Ammeter jaws must encircle only one wire

Question 5

True RMS Meters

Answer 5

• Standard meters read standard amperage
• Solid state devices (computers) affect readings
• True RMS meters give accurate amperage

Question 6

The more loads in a series circuit,

•

Answer 6

– the more resistance in the circuit

Question 7

Total resistance is:

Answer 7

– the sum of all the resistances in the circuit

Question 8

amperage will be the same

Answer 8

– everywhere in the circuit

Question 9

The left side:
–
•

Answer 9

Usually considered the “main power”

Question 10

The right side:

Answer 10

– Usually considered “common”

Question 11

Controls and switches are in series with loads

Answer 11

– Opening the switch stops current to the load

Question 12

• Tstats and safety controls are always in series with loads

Answer 12

• A disconnect switch is in series with the circuit

Question 13

– Used to “disconnect” power for equipment service

Question 14

They have two windings:

• Lower resistance winding
• Check them by measuring the resistances
– Start (S) to Common (C), plus
– Run (R) to Common (C), should equal:
– Run (R) to Start (S)

Answer 14

– A start winding to help it start rotating

Question 15

• High resistance winding
– A run winding to keep it rotating

Question 16

RSIR: Resistance start, induction run
– Uses a start relay only
• CSIR: Capacitor start, induction run
– Uses a start relay and
– Start capacitor
• CSCR: Capacitor start, capacitor run
– Uses a start relay and
– Start capacitor and
– Run capacitor

Answer 16

Current Relay
• Potential Relay
• PTC (Positive Temperature Coefficient) Relay

Question 17

Used on small compressors
–
•

Answer 17

Usually under 1HP

Switch contacts are normally open (NO)
• Relay coil energized by high starting
current (amps)

Question 18

Used on fixed metering device systems

Answer 18

– Pressures equalize when compressor is off
– Starting is easier under low head pressure

Question 19

Used on fixed metering device systems

Answer 19

– Pressures equalize when compressor is off
– Starting is easier under low head pressure

Question 20

• Relay contacts close on high starting current
• Then, relay contacts open as amperage drops

Answer 20

A start capacitor adds starting torque
• Required on most TEV systems, because
– Pressures may not equalize in off-cycle

Question 21

Increase phase shift for more starting torque

• Some have a resistor across its terminals
– Quickly “Bleeds” off excess charge when de-
energized
• Prevents arcing damage to relay contacts on restart

Answer 21

• In series with start relay,
– Energized for only a few seconds
– Excess charge disperses during off-cycle

Question 22

Replacement Start Capacitors

Answer 22

• Must be the same microfarads as the original
• The same or greater volt amp capacity (VAC)

Question 23

Electricity is sent to the motor on start up

Answer 23

– Power goes through relay switch to start windings

Question 24

• When motor rotates it produces back EMF

•

Answer 24

– This voltage energizes the relay coil

Question 25

The relay contacts open

Answer 25

– The start winding circuit is now open
– It remains open as long as the motor runs

Question 26

When the potential relay opens
– The start capacitor is out of the circuit
– But the run capacitor is still in the circuit

Answer 26

Provide a phase shift to the start winding
– By sending partial voltage to the start winding
• Helps motor start and remains in the circuit
– Motor runs more efficiently drawing less amps

Question 27

How Do Run & Start Capacitors Differ?
• Color
– Start capacitors are black,
– Run capacitors are gray
• Microfarads
– Start caps have a range of MFD,
• Run caps a single MFD

Answer 27

Positive Temperature Coefficient (PTC)
– Resistance increases on temperature rise
– A solid state relay
• No moving parts
• A ceramic disc acts like a switch
– Starting amps cause disc to heat up
– Heat increases resistance in disc
– Current flow stops when resistance is high
– Disc stays hot as long as compressor runs

Question 28

Wired in parallel with the run capacitor
• It allows full voltage starting,
– When cold it acts like a jumper wire
• Bypassing the run capacitor

Answer 28

29

“Hard Start” Kit
• Used on air conditioning PTC compressors
– When compressor has trouble starting
• Usually from low incoming voltage
• Contains a solid state relay
– And a start capacitor
• Installed parallel to the run capacitor
– Creates more starting torque

Question 29

On start up relay passes full voltage to start
terminal
• The disk heats up quickly,
– Heat raises resistance, stops electric flow
• Power takes path of least resistance
– Goes through run cap to start terminal
• The disk stays hot while compressor runs
– When compressor stops the disc will cool down
– After 3 to 5 minutes it is ready to start again
• The next two slides show the sequence in action

Question 30

Shaded pole motors are only 30% efficient

Answer 30

Each pole has a copper band attached
– The shaded-pole provides the phase shift
• Usually “impedance protected” *
– *A stalled blade will not cause burned
windings

Question 31

Shaded pole motors are only 30% efficient

Answer 31

Each pole has a copper band attached
– The shaded-pole provides the phase shift
• Usually “impedance protected” *
– *A stalled blade will not cause burned
windings

Question 32

Speed depends on winding resistance
– Low speed: most resistance
– High speed: least resistance
• Motor speed is based on where power is
connected into the winding

Answer 32

Two separate windings:
– A run winding, and
– A start winding
• The start winding provides the phase shift
– More resistance
– Energized during start
• Split phase compared to shaded pole have:
– More torque
– More efficient (about 60% efficiency)
– More expensive

Question 33

Permanent Split Capacitor (PSC) Motors
• It is just a Split Phase motor
– With a run capacitor
• The capacitor provides a phase shift to
– Help start the motor, and also
– Help it run efficiently (lower amps)
• PSC motors are about 60% efficient
– Remember, shaded pole motors are only 30%

Answer 33

PSC (Permanent Split Capacitor) motor
with a bad run capacitor will act like an
overloaded motor (high amps & overheating

Question 34

The “nominal” speed is determined by:
– The number of poles
• A “pole” is the number of windings
• The more poles, the lower the speed

Answer 34

Slippage is the loss of speed from motor load

Question 35

One cycle has two current flow reversals

Answer 35

The windings are “tapped” in different spots
• The more windings, the more resistance
• The more resistance, the less power (torque) it has
• The less power, the slower it runs under load

Question 36

Fan Affinity Law”:
– Power to run a fan varies as the cube of its speed

Answer 36

20% reduced speed dropped the HP by 50%
– So, lowering fan speed saves a lot of energy

Question 37

High starting torque
• High efficiency (90-95%)
• Medium to high cost