How Motors Operate
Presented by John Freeland
Reference: Audel Electric Motors
By Rex Miller & Mark Miller
6th Edition
Midcontinent Library Call Number: 621.46 M617
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1
Voltage
+
-
time
Direct Current (DC) e.g. Battery
V=A sin (360ft)
A
45
1/60 sec
120
240
RMS
120 volt
Average 0 volt
Peak (A) 170 volt
360
Three Phase Alternating Current
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Series Connection
Parallel Connection
2
Basic Motor Operation
N
S
N
S
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DC Motors - General
• The speed is directly proportional to voltage
•The DC motors we see are usually in vehicles, toys and cordless tools
•They are fractional horsepower motors
•Reversible by reversing the supply voltage polarity
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DC Motors
1. Wound rotor (armature) and wound field coil
Lower efficiency because energy required to energize field coils
Relatively high maintenance because of brushes and commutator
Example: First generation cordless drills
2. Permanent magnet field motor
Has wound rotor (armature) with commutator and brushes
Example: Some newer cordless drills and car door lock motor
3. Permanent magnet rotor
Has a wound field with electronics
Advantages: More efficient
Lower maintenance
Longer battery life
Example: New brushless drill motors
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Universal Motors
Universal motors can operate on AC or DC power
Has a wound field coil, wound rotor with brushes and a commutator
Speed is proportional to applied voltage
Series wound universal motor (field coils and armature connected in series):
• Can operate up to 20,000 rpm
• Has poor speed regulation with varying load
• Needs feedback circuitry to regulate speed
Example: Vacuum cleaner, router
Parallel wound universal motor (field coils and armature connected in parallel):
• Operates at lower rpm
• Has good speed regulation with varying load
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Single Phase AC Motors
One common type is a split phase induction motor
Example: Table saw, planer, jointer, drill press…
The split phase stator has two windings: starting (auxiliary) and running
The starting winding:
• Provides starting torque
• Is disconnected by a centrifugal switch between 2/3 and 3/4 of the synchronous
speed. 15% of synchronous speed is usually sufficient to start the motor.
• This switch is a reliability issue. On a quiet machine/motor you can usually
hear the switch open as the motor starts up and close as the motor turns off
• Is offset from the running winding ninety degrees for a two pole motor and fortyfive degrees for a four pole motor.
• Has fewer turns of smaller gauge wire.
• Occupies the outer portion of the stator slots.
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Single Phase AC Motors
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•
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The starting and running windings are connected in parallel. Their relative
connection determines if the motor runs clockwise or counterclockwise
If both ends of the starting and running windings are accessible, the motor
can be reversed
“Induction” in a split phase induction motor means a current and therefore a
magnetic field is induced in the rotor by the stator magnetic field
(transformer action.)
The rotor usually has aluminum cast into the rotor laminations which create
one turn coils. This arrangement gives rise to calling the motor a squirrel
cage motor.
The speed of a squirrel cage, split phase, induction motor is nearly constant
with load and depends on the line frequency, number of poles and slip.
The starting torque is 1.5 to 2.0 times the full-load torque
The starting current is 6 to 8 times the full load current.
Capacitors may be connected in series with the starting and/or running
windings to increase torque and reduce starting current. These capacitors
are a reliability issue.
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Single Phase AC Motors
•
Synchronous speed
Ns = RPM = 120 x frequency (f) = 7200
number of poles (p)
(p)
Ns
f
p
3600
60
2
1800
60
4
1200
60
6
900
60
8
3000
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50 (England)
2
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Single Phase Motors
•
Slip is usually 1-4%of the synchronous speed and can be expressed three
ways:
1. As a percent of synchronous speed
2. As a decimal fraction of synchronous speed
3. Directly in RPM
P
Ns
Actual Speed
Slip as %
Slip as decimal
Slip as RPM
2
3600
3450
4.2
0.042
150
4
1800
1725
4.2
0.042
75
6
1200
1150
4.2
0.042
50
8
900
860
4.4
0.044
40
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Single Phase Motors
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Three Phase AC Motors
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Three phase power is “industrial power.”
Three phase power is how power is generated and distributed from the
power plants
Three phase motors are very reliable because they don’t use brushes, a
commutator, a starting centrifugal switch or a starting capacitor
Motor speed is a function of the number of poles, the line frequency and slip
Reversible by swapping any two of the three phase supply wires
ABCABCABCABCABC
CBACBACBACBACBA
•
Relatively inexpensive solid state controls are available to convert single
phase power to three phase power with variable frequency and voltage
(speed controls)
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VFD Variable Frequency Drive
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Refer to www.VFDs.com
A VFD is a motor controller that drives a motor by varying the frequency and
voltage supply
Voltage to frequency ratio (V/F) remains constant from the VFD
The strength of the motor’s magnetic poles is proportional to V/F. If the ratio
is too low the magnetic pole strength is too weak to create sufficient torque.
If the ratio is too high the iron in the poles will magnetically saturate and
cause overheating.
Maximum motor speed is 120% of rated speed
Minimum speed is 20% of maximum rated speed
Motor bearings have a higher failure rate when the motor is powered by a
VFD
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VFD Block Diagram
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