High Efficiency Motor Starting
High efficiency motors are the
standard in most paper mills,
care must be taken in
specific instances in their application
Mike Kozlowski
Paper and Forest Products Industry Engineer
Baldor Electric
864-281-2191
The Situation
A customer has recently purchased some high efficient
motors for his paper mill. The customer also purchased a new
MCC section with electronic trip units. The function of both of
the electronic trip unit is essentially protective devices for the
motor and wiring circuit, they protect it from short circuits,
long term overloads and ground faults. The motor starts and
stops frequently – sometimes reverses.
He calls you to the mill after a week or two and says he is
having a problem with the motors ----- they are continually
tripping out the circuit breakers! What gives!
Background
One of the attributes of high efficiency induction motor design is
that they have high transient currents for the first few cycles.
This high transient current is not the same as locked rotor
current, which is a steady state current as long as the rotor is
not moving, and which follows the same limits in normal and
high efficiency designs. The high transient in the first few cycles
is mainly due to high initial dc offset which is the result of the
lower resistance design of high efficiency motors.
On the next page is a pictorial diagram of the sequence of
events that occur when an induction motor is started “across
the line”.
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Reference: 2007-08-21-High Efficiency Motor Starting.doc
Baldor Industry Solutions Team
General Motor Starting
Sequence
Inrush depends if the
motor is normal
efficiency or high
efficiency design
LOCKED
ROTOR
CURRENT
CONTACT
CLOSURE
INRUSH
CURRENT
T=0s
T = 0 to 33
milliseconds
(~2 3 cycles)
THE CONTACTOR
IS CLOSED AND
CURRENT IS
PERMITTED TO
FLOW IN THE
MOTOR CIRCUIT
INRUSH CURRENT
PERIOD
CURRENT CAN BE
UP TO 6 TIMES FOR
A NORMAL
INDUCTION MOTOR
OR ~13X FOR A
HIGH EFFICIENCY
MOTOR
Unloaded 5hp
motor starting
Characteristics
(amps vs time)
CIRCUIT BREAKER
CAN TRIP ON
INSTANTANEOUS
SETTING
FULL LOAD
CURRENT
T = 33 milliseconds
to 15 Seconds
T = 15 Seconds - to
30 seconds
LOCKED ROTOR
CURRENT
TRANSITION TO
FILL LOAD
CURRENT PERIOD ROTOR BUILDS UP
LOCKED ROTOR
TORQUE AND
ACCELERATES
LOAD
(LRA ~6 X FLA)
CURRENT DRAW
DECREASES FROM
LOCKED ROTOR
AMPS TO
NAMEPLATE AMPS
MOTOR
ACCELERATES
LOAD AND
TRANSITIONS TO
BREAKDOWN
TORQUE
IF MOTOR CANNOT ACCELERATE THE LOAD IN 15 30 S, (CALLED STALL CONDITION) THE MOTOR
THERMAL LIMIT CURVE MAY BE EXCEEDED - THE
CIRCUIT BREAKER IS TYPICALLY SET TO PROTECT
THE MOTOR IN THIS CASE
Depends on inertia of load
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Reference: 2007-08-21-High Efficiency Motor Starting.doc
Baldor Industry Solutions Team
A view of the Motor
Starting Sequence
with protective
limits- called a Time
Current Curve
Time Current Curve High Efficiency Motor current in Per Unit plotted with
typical electronic trip unit protection curves
Plot of the motor’s starting
and running current
characteristic vs. electronic
trip unit protective limits. If
the motor’s current
exceeds the curve, the trip
unit will cause the motor to
be tripped out of the circuit
Electronic Unit will
trip out motor
= Typical current draw premium
efficient AC motor
= Intermediate trip
= Instantaneous trip
= Long time trip
= Short time trip
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Reference: 2007-08-21-High Efficiency Motor Starting.doc
Baldor Industry Solutions Team
In order to get full load current down and efficiency up, one way
is to take resistance out of the motor stator design. Among the
effects of lower resistance is that it limits the initial dc offset, but
allows for higher inrush currents - in some cases to
impressively high values.
Understand the
application first
For smaller motors which are started or reversed repeatedly, run
for short periods of time, or worse yet, are plugged to stop or to
reverse, normal efficiency motors are the better choice.- In any
case, the applications should be examined for where the motor
HP is placed in the NEMA starter rating range. If it is near or at
the upper borderline, the next larger size of combination starter
should be considered.
Some power companies give rebates to users who specify and
use high efficiency motors, and that there are users who specify
such motor throughout.
It should be stated to the user that this may entail the use and
cost of the larger size combination starter and in some cases of
nuisance trips, as fast acting breakers (and fuses) are used
more and more.
There seems to be a couple of problems caused by this high
initial inrush situation:
1) Breaker instantaneous trip needs to be set high enough to
remain closed on the peak inrush.
Most manufacturer’s standard starters use instantaneous trip
only type breakers, with an adjustment dial for setting a trip
multiple. This is a multiple of breaker rated current, not motor
rated current. The breaker rated current is always higher than
motor rated amps. Usually they ship with the trip set at the
lowest setting.
NEC 430.52 limits the maximum setting of the instantaneous trip
to 13 times the motor rated current. The trips have a
manufacturing tolerance band of about 25%, so even if it is
nominally set at to achieve the 13X maximum, it could trip at
something less than that. So the end user may have to pick a
higher nominal setting to actually get the 13X actual trip value.
Typical motor data for a 100 HP, 1200 RPM motor says it has a
first half cycle inrush of 1410 amps, which is 12.4 times the rated
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Reference: 2007-08-21-High Efficiency Motor Starting.doc
Baldor Industry Solutions Team
amps, so a 13X setting would be marginally ok. Actually, there
may be a little more margin here, because the breaker
instantaneous trips won't respond to a half cycle value they are
typically closer to a 1 cycle trip device.
In any event, if the premium motor inrush currents stay below
13X during their inrush, it appears that the typical breaker trips
can be adjusted to both meet the NEC Code and not trip on
inrush.
2) Contactor opening interrupt capability may not be a problem
on larger size breakers.
The starter automatic trip circuit has to go through an overload
relay – and is a lot slower than the breaker instantaneous trip,
so the breaker would handle any interrupting of current due to
this type of instantaneous overload. The time is too short for
any manually initiated stop to be of concern.
However, an MCC engineer indicated he was concerned that
short cycle inrush currents in excess of the contactor rating
might cause tip welding during closing in of the contactor.
When the contactor closes in, the contacts bounce, so they
could be opening and closing on currents higher than their
capability.
This is probably only of concern on the highest HP rated motors
of each NEMA starter size, for example a 10 HP motor on a size 1
starter. In some cases, we could go to the next larger NEMA
size starter to get a higher make/interrupt rating, but that doesn't
always work.
Application Tip
For this reason, higher efficiency motors are not the best fit for
all applications. They are suitable for motors which are not
started often, which run for long periods of time once started,
and which are of a sufficient rating to make the savings
worthwhile.
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Reference: 2007-08-21-High Efficiency Motor Starting.doc
Baldor Industry Solutions Team
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Reference: 2007-08-21-High Efficiency Motor Starting.doc
Baldor Industry Solutions Team