I have fan ( 1/3 hp 3 ph 460 vac) for a fluid cooler which repeatedly
cycles on 1 minute and then off 1 minute. I am afraid it might run hot
and burn out prematurely. How can I determine a safe cycle period and
duty cycle for this motor? Do I need more info than is available from
its name plate?
in article 0DFTb.20115$ email@example.com, D.Miller at
firstname.lastname@example.org wrote on 2/2/04 8:40 PM:
My guess is that you are ok. You do not describe the motor well. I presume
that it is an induction motor. I also presume there are no tricky starting
Except for starting transients and power factor, just monitoring the
temperature of the windings to make sure you do not overheat them should be
Thirty starts an hour seems to be rather a lot. You're unlikely to be able
to adequately instrument the motor for temperature rise to confidently know
whether this number of starts is acceptable. Generally 6-8 starts an hour
is acceptable but 5 times that rate should get some advice from the motor
manufacturer to see what the motor is actually designed to take.
Some center pivot irrigation motors operate on a one minute cycle. They
start and stop once per minute. Some others run on a 30 second cycle so
they start and stop twice a minute. The pivots in the U.S. operate on 3ø
480VAC and the motors last for years. The motors are rated for continuous
You need to know 2 things; the motor speed (actually the number of
poles, but speed will tell you that) and the intertia of the load.
There is actually a formula for this. NEMA publishes a chart for motor
duty cycle design. The determination is the LESSER of the following 2
choices, assuming that it is a 4 pole (1800rpm) motor design,
A) 38 starts per hour. This is essentially an UNCOUPLED rating
B) 5.8 starts per hour x Wk2 (the load inertia)
If you do not know the load inertia value it is NOT safe to assume
that 38 starts per hour is OK. I.e. if the Wk2 is 2.5, the calculation
for method B is 2.5 x 5.8 = 14.5 starts per hour. So if you do not
know the load inertia, you can't safely answer that question.
BTW, In either case, the MINIMUM rest time between starts is 38
seconds on 4 pole motors up to 1 HP. If it is a 2 pole (3600rpm)
motor, that goes up to 75 seconds, a violation of design criterea for
your application right there.
It must be a 6 pole motor as rpms are approx 1200. The unit is sealed
under a sheet metal hood which makes it difficult to access the motor
name plate info. As a result I don't have a manufacturer or model number
available. I am speaking with the vendor who sold us the unit. I would
much rather speak directly with the motor manufacturer. There is a
lesson here... and that is to document name plate info on all equipment
I can increase the cycle period by getting a new thermostat with an
adjustable differential. Instead of a 4°F differential I can probably
live with an 8°F one. Ultimately I would like to get a firm answer from
I thank everyone for their comments.