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3.6
Intermittent Duty Operation
If a motor operates intermittently, it is not necessary that the peak load torque fall within the
motor's continuous torque capabilities. However, it is important that the RMS (root mean
square) load torque be inside the continuous duty zone. Note that a sufficient OFF time
follows each On time of the motor.
Equation 1 (EQ-1) gives the RMS torque:
(EQ-1)
T
= Torque at Time i
i
t
= Duration of time i
i
This equation assumes t
motor (TCTP) for torque values significantly larger than the continuous torque (Tc). This is
not always a good assumption. For cases where torque values significantly exceed Tc, the
following equation is used:
T
OUT
T
C
where, duty cycle = t
T
Tc
t
ON
TCTP = thermal time constant for peak power of motor
This equation expresses operating torque as a function of ON time. It also breaks the
operating cycle down into individual periods of ON time and OFF time. Substituting for duty
cycle and solving for t
(EQ-2)
For a specific output torque and a given ON time, the required OFF time is known. This OFF
time is required so the motor cools sufficiently and does not exceed its thermal limits. The
calculated OFF time should proceed the initial ON time to ensure that the ultimate
temperature is not surpassed on the first cycle.
It may also be useful to calculate a time to ultimate experience temperature based on a one-
time excursion from ambient temperature. Consider the following pair of equations:
(EQ-3) T Actual Above Ambient =
(EQ-4)
33
Kollmorgen | kdn.kollmorgen.com | October 2020
1 1 +
2 t
2
T
T
2
t
T RMS =
t + t + t
2 ... i
1
is small compared to the thermal time constant for peak power of
i
1-e
-t
/Duty Cycle * TCTP
ON
=
1-e
-t
/TCTP
ON
/(t
+ t
)
ON
ON
OFF
= output torque
OUT
= continuous torque at operating speed
= time on
), the above equation yields Equation 2 (EQ-2):
OFF
(1-e
-t
= -TCTP 1n 1-
T
OFF
R
T
R Rated Above Ambient
T
Rated = T
Ultimate (1 - e
R
R
EB-10X Installation & Service Manual | Motor Data Package
T
2 + ... i t
2
i
/TCTP) T
ON
2
OUT
-t
ON
T
2
C
2
T OUT
Tc
-t
)
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