HVAC Equipment Power Rating Calculations – Part Two

In Article "HVAC Equipment Power Rating Calculations – Part One", we explain the following points:
  1. Summary of heating and cooling systems,
  2. Parts Consuming Power in HVAC Systems,
  3. Types of motors used in HVAC Systems,
  4. Types of pumps used in HVAC/refrigeration,
  5. Parts consuming power as per used unit/system,
  6. Motor Nameplate for Air Conditioner Motor Applications.

Today, we will explain the common types of motors used in HVAC industry and the used HVAC system units/ratings.

The Common Types Of Motors Used In HVAC Industry

First: the Common Types of Motors Used in HVAC Industry to drive Compressors

The Common Types of Motors Used in HVAC Industry to drive Compressors can be listed under (2) main types:

  1. Single-Phase Hermetic Motors,
  2. Poly-Phase Hermetic Motors.

1- Single-Phase Hermetic Motors

Basically, there are four types of single-phase induction motors used in hermetic assemblies

  1. Split Phase (SP),
  2. Capacitor-Start, Induction-Run (CSIR),
  3. Capacitor-Start, Capacitor-Run (CSCR),
  4. Permanent Split Capacitor (PSC),

1.1- Split Phase (SP)

Split Phase Motor Wiring Diagram

  • This is the simplest design where the RUN winding and START winding are connected in parallel. It is usually used in small pumps, fans and blowers where the capacity is below (1) horsepower. It has a low starting torque but high starting current. Since the torque is low, the ability to start the motor is only practical for low load condition.
  • The RUN winding is made from bigger diameter wire and shorter turn for lower resistance and high inductance properties. The START winding is made from smaller diameter wire for higher resistance and low inductance properties. 
  • When power is connected to the motor, both the windings will be energized with the current in the RUN winding lags the current in the START winding by about 30° electrically. This out-of-phase effect on the stator produces a starting torque and causes the rotor to start rotating.
  • Typically the speed of the motor is 1800 rpm or 3600 rpm when running without any load. When the load is connected, the speed can go down to 1725 rpm and 3450 rpm respectively.
  • There is a switch known as centrifugal switch which is connected in series with the START winding. This mechanical switch will open when the motor speed reaches 75% of the rated speed typically within 2 seconds. Once the switch opened, the START winding in circuit is disconnected. This is to protect the START winding from overheating. When the motor is powered off, the switch will close the circuit to get ready for the next starting of the motor.
  • These days, electronic relay is also being widely used to disconnect the START winding. This relay may be thermal, current or potential type.

1.2 Capacitor-Start, Induction-Run (CSIR)

Capacitor-Start, Induction-Run (CSIR) wiring Diagram

  • This motor is similar to the split-phase motor except that there is an external capacitor that is connected in series with the START winding. This capacitor will cause the current in the START winding to lead the voltage.
  • The current in the RUN winding lags the voltage. When this happens, the phase difference between the two windings is 90° electrically hence a true two-phase starting is achieved.
  • The starting torque of this motor is very high making it suitable to drive small compressor which needs to start under full load. The capacity of this motor can go up to (1) horsepower.
  • Once the motor has reached 75% of the rated speed, the capacitor and the START winding will be automatically disconnected from the circuit by using the centrifugal switch, potential or current relay. 
  • After the capacitor and START winding have been removed from the circuit, the magnetic field being generated continuously will cause the motor to continue running.

1.3 Capacitor-Start, Capacitor-Run (CSCR)

Capacitor-Start, Capacitor-Run (CSCR) Wiring diagram

  • This motor design is similar to the capacitor-start, Induction-Run design except that there is a second capacitor known as the RUN capacitor which is connected in parallel with the START capacitor and the switch.
  • These capacitors are effectively connected in series with the START winding. During the starting of the motor, both the capacitors are connected in the circuit. The START winding and the RUN winding will remain connected to the circuit at all times.
  • Usually the capacitance of the RUN capacitor is lower than the START capacitor. During starting, the effective capacitance is the combination of both capacitors causing a greater phase angle shift between the windings. This provides a higher starting torque and can be used to drive the compressor as well as in belt-driven motors.
  • As the rotor speed reaches 75% of the rated speed, the switch will be automatically open to disconnect the START capacitor from the circuit. The START winding remains in the circuit.
  • The RUN capacitor helps to correct the power factor of the circuit making it more efficient. The capacity of this type of motor can go up to (10) horsepower and is one of the most efficient motors used in the HVAC industry.

1.4 Permanent Split Capacitor (PSC)

Permanent Split Capacitor (PSC) Wiring diagram

  • This motor has similar design to the phase-split motor except that there is a RUN capacitor connected to the START and RUN windings. This motor does not have any switch and the START winding, RUN winding and the RUN capacitor are active whenever the motor is ON.
  • This type of motor has low starting torque and is suitable to be used in small fan motors such as the fan coil unit of a split air conditioning system.
  • Multi-speed PSC is achieved by changing the winding resistance. If high speed is required, the terminal is connected to the least winding resistance. If low speed is required, the terminal is connected to the highest winding resistance. By utilizing relays to choose the terminal to be connected to the line voltage, different speed of fan motor can be achieved.
Motor with 4 speed

In the diagram above, there are 4 motor speed that can be selected. Super High (SH), High (H), Medium (M) and Low (L). The selection can be done by using electronic relays to connect L2 to one of the four terminals depending on the speed required.

2- Poly-Phase Hermetic Motor

  • Large hermetic compressors usually are driven by three-phase motors. The surges of current in these motors are closer together than with a single phase current supply. Therefore, they are more efficient power sources.

  • These motors are usually of the 220V or 440 V type. The dome terminal block has nine terminals. The technician may wire the motor for either 220V or 440V See Figure 7-30. Some of these motors use a 550 V supply.

Schematic wiring diagram showing circuit and connections for three-phase hermetic motor

  • A Circuit as connected for 220 V B-Circuit as connected for 440V. Note that L1, L2,and L3 are the three-phase line connections. Numbers1-2-3, 4-5-6,7-8-9 are the connections to the motor windings. Each motor winding coil is designed for 220 V; for example, the coil between terminals 1 and 4 is rated for 220 V.
  • Three-phase motors are available from 1/2 hP size and up. The building in which the unit is to be placed must be wired for three-phase service. Very few residences have three-phase electrical Power However, most industries and some commercial buildings are wired for three-phase.
  • Three-phase motors use contactors or motor starters .They don't have the usual starting relays. below figure shows a three-phase motor wiring circuit with its starting and protection circuit. it is always best to have an electrical journeyman do the electrical work on these units. Since each unit may have certain differences, it is important to use the manufacture wiring diagram when servicing the system.
Three-phase hermetic motor circuit L1, L2, andL3 are three-phase line connections Overload protection is shown in L1 and L3 circuit and operates magnetic coil of starter switch (top part of drawing). Special magnetic starter is required.
  • Occasionally, a three-phase motor may blow a fuse or open a circuit breaker on one Phase only. The motor will attempt to operate on the remaining two phases. The motor will quickly overheat and may burn out if there is too much Load on it. This is because the remaining two windings must carry all the load. Each one will need 1.5 times the current to compensate for the lost phase.
  • Phase loss monitors are sometimes used to shut down a motor to prevent it from damage. Each Phase of a three phase motor must be tested individually using a voltmeter. There will be about 50 volts difference between the open line and one of the other Lines. The circuit having the "blown" fuse will indicate below normal voltage.
  • The direction of rotation of a three-phase motor may be reversed. This is done by changing any two of the power leads to the motor.

Second: the Common Types of Motors Used in HVAC Industry to drive Fans
  • Generally, the condenser fan motor leads are connected to the common terminal and the running winding terminal of the compressor motor. This connection puts the fan motor in parallel with the compressor motor and allows it to be controlled by the thermostat.
  • The safety overload cutout is also put in the circuit ahead of the fan. It will also cutout the fan motor. Some fan motors have their own thermal safety controls.
  • Many of these motors are of the two- or three-speed type as shown in below table. The variation in speed may be obtained by using extra poles in the stator or by using a solid state control.
Schematic of some common fan motor circuit

  • The speed of a fan motor is quite sensitive to the applied voltage. As the voltage drops, so will the fan speed.  Fig. is a schematic of some of the common fan motor circuits. One-, two-, and three-speed motor circuits are shown.

voltage affects fan speed for six and twelve-pole motors. A-Twelve-pole. B-Low speed for a high efficiency twelve-pole motor.  C-Medium speed twelve-pole. D-High speed, twelve-pole. E- six pole.

Shaded-Pole Motors

  • Shaded-Pole motor construction is different than that of the motor previously described. The shaded-pole produces a moving magnetic field Perpendicular to the field pole.
  • Approximately half of each pole face has a small copper plate insert, A, with a small winding. The insert slows down the build-up of the magnetic field through the copper plate. It is slowed down just enough to cause a magnetic motion toward the copper plate.

Shaded-pole fan motor. S-South polarity. N- North polarity. A-Shaded-pole plate (copper)

  • This action produces a lag for induced magnetism in the rotor (opposite magnetism). The rotor turns as it is attracted by the magnetism. Movement of the rotor will continue as the alternating current changes the polarity of the poles and the rotor.
  • Shaded-pole motors have less starting torque than other types of motors. Nevertheless, the shaded-Pole motor is very successful for small motors 1/6 to 1/100 hp.

HVAC System Units And Ratings

Here we will list some of the HVAC System Units and Ratings that you can find in the brochures or specifications of the air conditioner or heat pump units as follows:

  1. Cooling Capacity,
  2. COP (Coefficient of Performance),
  3. EER (Energy Efficiency Ratio),
  4. SEER (Seasonal Energy Efficiency Ratio),
  5. HSPF (Heating Seasonal Performance Factor),
  6. Energy Star.

1. Cooling Capacity
  • Cooling capacity for a room is defined as the heat load in a room that has to be removed in order to achieve a certain room temperature and humidity. The typical design is set to 24°C temperature and 55% Relative Humidity.
  • The amount of cooling needed by the space will be used to determine the capacity of the air conditioner needed. The cooling capacity can be expressed in many units as follows:

1.1 BTUH – “British Thermal Units per Hour”:
  • 1 BTU/hr is the heat energy needed to increase 1 pound of water by 1°F.

1.2 Ton of cooling:

  • One ton of cooling is the heat extraction rate of 12000 Btu per hour. Theoretically it is energy required to melt one ton of ice in 24 hour. 

1.3 Ton of Refrigeration Effect:
  • The cooling capacity of older Refrigeration units is often indicated in "tons of Refrigeration" A ton of Refrigeration represents the heat energy absorbed when a ton (2000lb.) of ice melts during one 24-hour day.
  • The Btu equivalent of one ton of refrigeration is easy to calculate. Multiply the weight of one ton of ice (2000lb.) by the latent heat of fusion (melting) of ice (144 Btu/lb). Then divide by 24hours to obtain Btu/hr.

      One ton of Refrigeration effect= 2000 (lb) x 144 (Btu/lb) /24 (hours) =  288,000Btu/24 hours = 12,000Btu/hr
  • A refrigerating or air conditioning mechanism capable of absorbing heat can be rated in tons per 24 hours by its heat-absorbing ability (HA) in Btu divided by (24 hr x 12000 Btu = 288,000).

T = HA / 288,000
T = tons of refrigeration effect
HA = heat-absorbing ability in Btu

The heat-absorbing ability of a refrigerator unit is 1,440,000 Btu per 24 hours. What is its ton rating?

T =  1,440,000/ (24 x 12,000) =  1,440,000/ 288,000 = 5 tons of refrigeration effect

2. COP (Coefficient of Performance)
  • This coefficient is the ratio of the cooling capacity (W) as the output power (in form of removed heat load) versus power consumption (W) as the input power.

COP= Cooling Capacity (W)/Power Consumption (W)
  • The higher the COP, the higher the efficiency of the air conditioner. Usually the value range from 2-4 but in recent years, the use of inverter compressors have enabled this coefficient to go higher than 4.

3. EER (Energy Efficiency Ratio)
  • This rating was established by ARI or Air Conditioning and Refrigeration Institute in 1975 for manufacturers to rate their equipment so that consumers or consultants can tell the cooling efficiency of the air conditioner by just looking at the specifications provided.
  • This rating is obtained by dividing the cooling capacity (Btu/h) with the input power (Watt). The rating points are at 80 °F dry bulb/67 °F wet bulb indoor temperature and 95 °F dry bulb/75 °F wet bulb outdoor temperatures. 
  • For instance, if you look at the brochure and the unit has a Cooling Capacity = 25,000 Btu/h and Input Power = 2,400 W,

EER = (25,000 Btu/h)/2400W = 10.42
  • The larger the value of EER, the more efficient the air conditioner is. Therefore, choose a bigger EER if you are comparing between two equipment.
  • The EER has a limitation in that it is measured only when the unit is in steady-state condition. The starting up and shutdown cycles are not included in the calculation. Therefore, this rating does not give a complete picture of the efficiency of the unit. A better efficiency ratio known as Seasonal Energy Efficiency Ratio or SEER is developed.

4. SEER (Seasonal Energy Efficiency Ratio)
  • This ratio is rated by AHRI (Air Conditioning, Heating and Refrigeration Institute) and manufacturers' equipment ratings are published in their catalogs. This ratio is more accurate as it takes into consideration non steady state conditions such as the start-up and shutdown cycles of the air conditioner.
  • This ratio is obtained by dividing the total cooling that the equipment is able to provide over the entire season (Btu) over the total energy in Watt-hours it will consume (Wh).

SEER = [Total of Btu/h cooling outputs at all test conditions]/[Total of all Watt inputs at all test conditions]
The unit of SEER is Btu/W.h.

  • The COP of a machine can be found by multiplying the SEER by a factor of 0.293

COP = SEER X 0.293

  • In choosing the SEER, the choice is always to go for a higher SEER as it is more efficient equipment. The trade-off in choosing a higher SEER is that usually the initial cost of the equipment will be higher.
  • All central split-cooling system produced in the US must have this ratio effective Jan 23, 2006. In order to ensure more efficient units are produced, a minimum SEER of 13 has been stipulated except for window units which has a minimum SEER of 10.
  • You can find the Watts Input, Cooling Capacity as well as the SEER information on the label which is usually a metal plate that is attached to the heat pump unit. If SEER is not provided, a simple calculation can be done.

If the label displayed Cooling Capacity = 5000 Btu/hr and Watts Input = 600 W.
SEER = [5000 Btu/hr]/[600W] = 8.3 
If another 2.5-ton air conditioner requires 2 kW of electrical power, what is the SEER?
1 ton of refrigeration is 12,000 Btu/hr.
SEER = [2.5 X 12,000 Btu/hr]/[2,0000 W] = 15

5. HSPF (Heating Seasonal Performance Factor)
  • This ratio is used to determine the efficiency of air source heat pumps equipment. It applies to the heating mode by which the total heating used during the entire season is divided by the energy in Watt-hours that it consumed.

HSPF =the total heating used during the entire season in Btu / the energy in Watt-hours that it consumed in KWH
  • A ratio of greater than 8 is considered efficient equipment. However, the advancement of better control and inverter compressor has enabled units to have HSPF up to 13. The higher the HSPF, the better the unit is.

A ductless split unit heat pump delivering 100,000,000 Btu during the entire season and consuming 12,000 kWh. Calculate its HSPF?
HSPF = 100,000,000 Btu/12,000 kWh  = 8.33

6. Energy Star
  • This rating for an equipment shows that the equipment is designed to save energy hence reducing your electricity bills as well as protecting our environment. The below figures show the SEER and EER equivalent to each star rating.

Energy Star SEER Rating

Energy Star EER Rating

Energy Conversions In Air Conditioning / Refrigeration Systems

In a compression refrigeration unit, there are different types of energy conversions as follows:
  1. Electrical energy flows into an electric motor,
  2. Then, this electrical energy is turned into mechanical energy,
  3. Finally, the mechanical energy is used to turn a compressor. The compressor, in turn, compresses the vapor to a high pressure and high temperature. This process transforms mechanical energy into heat energy.

Various units are used for measuring mechanical, heat, and electrical energy. Energy conversion units are expressed as follows:
Electrical to mechanical

746 watts = 1 hp
Mechanical to electrical

 1 hp = 746 watts
Mechanical to heat

1 hp=2546 Btu/hr
778 ft-lb = 1 Btu
Heat to mechanical

1 Btu/hr = 0.000393 hp
Electrical to heat

1 watt (1 joule/sec.)= 3.412 Btu/h
1 kilowatt (kW) = 3412 Btu/hr
Heat to electrical

1 Btu/hr = 0.293 watts
Other conversion units are used in calculating loads and determining the capacity of equipment required for specific refrigeration applications as follows:

In the next article, we will explain in details the NEC Rules for HVAC System Power Sizing Calculations. So, please keep following.

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