Date post: | 11-Jan-2016 |
Category: |
Documents |
Upload: | august-barker |
View: | 215 times |
Download: | 1 times |
1
HVACR214 – Electrical for Oil
Motors
2
Motors
• Motors consist of a stationary part called the stator that surrounds a rotating part called the rotor.
• One or more windings are mounted on the stator.
• There are no windings usually on the rotor.
3
Parts of a motor
4
Motors
• The rotor, or the center part of the motor is not connected to any external source of voltage.
• When the winding on the stator is powered it generates a magnetic field.
• As the A/C changes from a positive to a negative this magnetic field will rotate.
5
Magnets
Like poles of a magnet will repel each other. Unlike poles will attract each other.
6
Motors
• As you remember like poles of a magnet repel each other.
• As the magnetic fields of the stator windings change from positive to negative it “chases” the rotor as the poles on the rotor are positive and negative.
7
Motor
• This constant changing of the magnetic field will keep a motor running.
• The problem comes when you are trying to start a motor. If the magnets are lined up evenly between poles the motor would not start.
• This is the purpose of the start winding.
8
Start and Run Windings
9
Start Winding
• This is an additional winding in a single phase motor that allows the motor to start when the poles of the stator and rotor are not in the correct position.
• The current applied to the start winding is slightly out of phase with that applied to the run winding.
10
Run winding
• This is the winding on the stator that allows the motor to continue to operate.
• The run winding gets constant voltage until the motor shuts off.
11
Shaded Pole Motor
• The shaded pole motor is the simplest motor.
• It has one winding surrounding a laminated core.
• It uses a shaded pole to determine its rotation.
12
Shaded Pole Motor
• The shaded pole motor consists of a squirrel care rotor, an iron core and two or more rings (poles) with a coil and small copper inserts.
• The insert slows the buildup of the magnetic field just enough to cause rotation towards the magnetic field.
13
Shaded Pole Motor
• The action provides for induced magnetism into the rotor.
• The rotor will then turn as if it is attracted by the magnetism.
• Movement of the rotor will continue as long as there is change of the polarity of the poles and the rotor.
14
Shaded Pole Motors
• If the motor stalls or if the bearings freeze the motor windings will usually not burn out.
• When the motor is in a locked condition the windings draw very little current.
• The direction of a shaded pole motor is towards the shaded pole.
15
Shaded Pole Motor
• Shaded pole motors can be made multiple speed by inserting voltage taps into the motor winding.
• Shaded pole motors are low in cost but have a very low starting torque and a very low power factor.
16
Shaded Pole Motor Example
17
Shaded Pole Motor
• Shaded pole motors are built for very small sizes and are used for light duty in small appliances.
• In refrigeration shaded pole motors are frequently used for Evaporator fan motors.
18
Split Phase Motors
• Split phase motors are also called induction start - induction run motors.
• These motors use a current relay or a centrifugal switch to remove the start winding from the circuit.
• The start windings work on a phase shift principle.
19
Split Phase Motor
• The voltage from the start winding to the run winding is 10 degrees out of phase.
• Starting torque and running torque are relatively low in split phase motors because the phase shift is so low.
20
Split phase motors
• These motors are used for belt drive blower applications such as small appliances like:– Washers– Dryers– Dish washers– Some older furnace blowers
21
Split Phase Motors
22
Split Phase Motor
• The centrifugal switch removes the start winding from the motor circuit once the motor reaches around 75% of its rated speed.
• This is the part that most often fails when a motor is started and stopped many times.
23
Centrifugal Switch
• The switch is normally closed when the motor is not operating.
24
Centrifugal Switch
25
Permanent Split Capacitor Motors
• In order to increase the starting torque of a motor you can insert a capacitor in series with the start winding circuit.
• In a PSC motor this capacitor remains in series while the motor is operating. There is no switch or relay that removes this capacitor from the circuit.
26
PSC Motor
• Residential Condenser fan motors and residential furnace blower motors are the primary use of PSC motors.
27
PSC Motor
• PSC motors have moderate starting torque and intermediate running efficiency.
• These motors are used primarily for small compressors and direct-drive blower and fan applications.
28
PSC Motor Example
29
CSIR Motors
• CSIR are capacitor start and induction run motors.
• They use a start cap and a start relay.• Once the motor has reached 75% of its
rated speed the start capacitor is removed from the circuit.
30
CSIR motors
• CSIR motors have a high starting torque and are used in residential and small commercial compressors.
• May see them in beverage coolers or small reach ins.
31
CSCR Motors
• CSCR (Capacitor Start Capacitor Run) motors are the same as Permanent Split Capacitor motors except they have a start capacitor wired to them.
• A start capacitor is a high capacitance capacitor that will provide the motor with additional torque when starting.
32
CSCR Motors
• Once the motor has been started it, the start capacitor must be taken out of the circuit using a switch or a relay.
• These devices are called start components and this is covered in the next section.
33
CSCR Motors
• This motor can be abbreviated to CSR. (Capacitor Start and Run).
34
CSCR Motor Example
35
Hermetic Motors
• Hermetic motors are single phase up to about 5 hp (horsepower).
• Hermetic motors are those found inside the compressor shells.
• They are wired the same way as any other split phase motor, but the start components must remain outside the compressor shell.
36
Hermetic Motors
37
Hermetic Motors
• On the outside of the compressor shell there are three terminals. Most of the time these have some markings on them.
• They are labeled C, R, S– S = Start winding– C = Common– R = Run Winding
38
Hermetic Motors
39
Motors
• If the windings are not marked use your ohm meter and check resistance.
• Check between all three windings.• The winding measurement with the highest
resistance is between start and Run (S and R). The remaining terminal is Common ( or C).
40
Motors
• Once you know C check between C and the other two terminals the one with the highest resistance is S (or Start).
• The one with the lower resistance is Run (or R).
41
Motors
• Best way to do this is to write down C, S, R in a triangle on a piece of paper and make sure you write down each measurement.
• If you have any doubt - go back and do it again.
42
Motor Speeds
• Motor speeds are shown in RPM’s• RPM is revolutions per minute. This is a
measure of how many times per minute the shaft rotates.
• Sometimes you will need to figure out RPM’s for a motor.
43
Motor Speeds
• To figure motor speed first use the motor labels
• If no label is found use the formula:– Speed (rpm) = (frequency x 120) / Poles
44
Shaded Pole Diagram
CR
45
CSCR Diagram
C
S
R
46
Split Phase Diagram
C
S
R
47
PSC Diagram
C
S
R
48