AC Motors Flashcards
Two types of electric motors
AC (single phase and three phase) and DC
Single phase voltage ratings
Low- 110 to 120
High- 220-240
Three phase voltage ratings
Low- 208, 220-240
High- 480
Two digit frame size (ex: 48,56)
Shaft center height in 16ths of an inch.
48/16=3 in, 56/16=3.5 in
Three digit frame sizes (ex: 245)
First 2 digits indicate the shaft center height in fourths of an in. Third digit tells us about the bolt pattern.
(24/4=6in, with 5 mounting holes)
Frame suffixes for ac motors
T- long shafts
TS- short shaft
C- c face motor
TC- c face motor with long shaft
Factors that can cause a motor to overheat
High or low voltage
Frequent or repeated starts
High temperatures
Blocked ventilation holes or cooling fins
Voltage imbalances in 3 phase motors
Motor insulation/motor class
Letter indicating how much temp increase the windings can take without overheating. (A, B, F are the most common)
Why do we insulate motors
So the motor doesn’t go phase to phase and to coat the motor windings to prevent them from shorting to ground
Service factor
The amount of overload a motor can deliver without overheating
SF 1.0- the rated load on the nameplate
SF 1.15- 115% of rated load
SF 1.25 125% of rated load
Motor Class Chart
A- 220 degrees
B- 266 degrees
F- 311 degrees
H- 356 degrees
Speed and torque
They are inversely proportional. One goes up the other goes down. Also torque and current are always equal
C face motors
The motor supports the driven unit
D flange motors
The driven unit supports the motor
Advantage of c face and d flange motors
They are always aligned once you get the bolts in and tight
Disadvantage of c face and d flange motors
There’s limited space to get the coupling half’s attached
Devices to protect AC motors
MTOs and ATOs
MTO- manual thermal overloads
ATO- automatic thermal overloads (make sure power is off before working because is starts up automatically when it cools off)
AC motor parts
Coupling side end bracket, rotor, stator, lifting lug, fan side end bracket, fan, fan shroud
Rotor and stator are the main parts
Rectifier
Goes from AC to DC
Inverters
Goes from DC-AC
Transformers
AC to AC
Number of poles (windings)
60Hz
2 3600
4 1800
6 1200
8 900
Motor poles
The number of windings. The more poles the motor has the lower the rpm is but the torque is proportionally higher
Slip
The difference between the sync speed and the actual speed. ( 1%-5%)
3 phase motors
Most common because of low cost and high efficiency. They’re the work horse of industry
Single phase motors
Major problem is the no or low start up torque available. Manufacturers have 4-5 designs
Shaded pole motor
Has a copper loop called shading ring. Surrounds a portion of the stator pole. Used in small appliances.
Split phase
Start windings positioned 90 degrees from run windings. Start winding drops out at 3/4 the sync speed. Used for Light duty applications
Capacitor start
Most common in industry. Has a compactor in series with start winding. Over 200% more starting torque
Permanent split capacitor
Has identical main and auxiliary windings with a capacitor. Have very little starting torque. Used for fans and pumps for hvac. For 1hp or below
