 Inrush
is the amp consumption of a
motor during start-up.
Motors are inductive. Inductive loads consume more amps when
starting. Once the motor has started, the amperage drops to normal.
Each motor has a label with a locked-rotor code that is needed for the
calculation. Use chart below.
The locked rotor code is a letter A-V, with each letter corresponding
to a range of KVA per HP (horsepower).
KVA is kilo volt amps. Volt amps is watts. So KVA is a measure of watts
or power or KW ... how much power the motor delivers.
The locked rotor code is used to calculate the inrush or amperage
needed for that specific motor during start up.
The example problem on left shows 50 HP 3-phase motor with locked-rotor
H, which has
KVA range
of 6.3-7.1 KVA per HP.
The KVA range is plugged into the formula:
7.1 x 50 HP = 355 KWA or KW at start up
355 KW = 355,000 watts
P (power or watts) = E (volts) x I (amperage) x .173
1.73 is the square root of 3, used for 3-phase calculations.
355,000 watts = 480 volts x I (unknown amps) x 1.73.
Solve for amps: I = 355,000 ÷ 480 x 1.73
Residential single-phase motors also have inrush. This is generally
not an issue when calculating household wire and breaker size since the
HP rating is relatively small, start-up is brief, and circuit breaker
doesn't
react fast enough to trip with the small surge of amperage (heat) on
the wire. However large swimming pool motors, and other application
that use 3-5 HP motors should have oversized wire and breaker to supply
ample electricity for start.
Resources:
Locked rotor KVA nameplate rating: Calculate
inrush .pdf
Difference between single-phase and 3-phase
Color code for wire
|
"Industrial and commercial
3-phase electric systems differ from residential single-phase: 3-phase
power is more complex, but
more efficient in motor applications, and large area uses. Higher
voltages of 277/480v and 347/600v
distribution systems are more efficient, but considerably more
dangerous, and should only be
maintained and modified by trained and qualified
electricians."
"In
a balanced 3-phase system, the wires can be about 75% the size of
conductors (wires) for a single-phase two-wire system of the same KVA
(power) rating.
This helps offset cost of supplying the third wire required by 3-phase
systems."
Voltage
reaches 0 on each individual line, but at no time does voltage reach 0V
on all 3 lines at same time. This translates to higher average voltage
per rotation of generator for 3-phase compared with single-phase. When
3-phase is connected to a motor, the average higher voltage means less
amperage is needed, which means motor runs cooler. Amperage is the heat
on the wire that trips a circuit breaker. Volts x amps = power (watts),
so 3-phase can deliver more power with less heat loss, making 3-phase
more efficient than single-phase. Lower amps also means that wires can
be smaller, thus lowering cost of installation.
"The
horsepower rating of three-phase motors is about 150% greater than for
single-phase motors with a similar frame size. The power delivered by a
single-phase system pulsates, and the power falls to zero three times
during each cycle. The power delivered by a three-phase circuit
pulsates also, but it never falls to zero. In a three-phase system, the
power delivered to the load is the same at any instant. This
produces superior operating characteristics for three-phase motors."
Resources:
3-phase circuits and basic math .pdf
Difference between single-phase and 3-phase |
