I have an application where I'll be using a 5hp 3 phase motor driving a series of jack screws. The motor will turn on for 2 seconds, turn off for four seconds, reverse direction and turn on for another 2 seconds, then stop for 35 seconds. This cycle will be repeated over and over 24 hours a day. What kind of motor will be able to handle this cycle?

 

Unless the motor is under a heavy load ( >50 %), it
should a reasonable task. I would suggest an inverter
duty motor with TENV cooling, fan cooling won't help
if the motor doesn't turn much. If you have a heavy
load, then consider a blower cooled motor.

Also, you should use at least an inverter or a soft
start to limit the inrush current when the motor
starts. A vector drive would probably be even better.

 

You have about a 10% duty cycle there.

I suspect most good (as opposed to low end) motors would take this without too much trouble. Especially if you put a VFD on it to take a bit of the startup stress off the motor. You might need an external cooling fan though.

Talk to the motor manufacturer's tech support people direct though. The sales people probably won't have much of a clue and would either tell you their low end junk is fine for this app or you need an real expensive one.

Bob Peterson

 

There are a few questions I would have in regards to this:

How many RPM are you going to be turning, and will this vary?

Does the position in which the jack screw reaches during the driving time matter?

How much torque (%) will the motor be turning during the driving portions of the system?

Regards

Jason

 

Any type of 5hp 3 phase should do, Since your duty cycle is so low and these motors are made for continuous operation.

I would be more concerned with the control (relay) for a 5 hp motor as at startup they draw a lot more power than at normal operation and you will be doing a lot of starts.

 

Responding to Matt Hafele's April 11 query:

Your description of the motor's duty cycle is only one part of the quest to determine the equivalent horse power needed. Some other salient parameters to investigate are:

a) Load torque.
b) Load inertia.
c) Define "reversal"... is motor reverse-plugged? That is, reversed
while running in one direction, and then immediately reversed?
d) Define "off"... power removed? or running without load?
e) Define "stopped"... braked) or plugged? or just allowed to decelerate
for 35 seconds?
f) Cooling method? Ambient temperature? Motor enclosure?

The above are only a "pointer" for you to use. They are necessary to determine if the motor's heat dissipation is adequate to so as to not destroy the motor.

It all boils down to your own capability. If you perceive it as just a learning experience, then, there are many List members, myself included, that can provide details. But, if safety, costly assets, or products are paramount, then, I recommend you seek the help of a Motion specialist.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

You don't say how quickly you need to accelerate the motor, nor to what speed it will be accelerating to.

T(lb-ft) = (Delta RPM x WK^2)/(308 x Accel Time (sec))

You can do an RMS calculation of your accel, decel and running torques combined with idle time. If RMS torque value is less than 60% of rated value, my experience is that TEFC motors work fine.

Since TEFC motors have by design considerably less inertia than TENV motors, the actual torque consumed in accelerating and decelerating the motor is much less - my hunch is that the jack screws have negligeable inertia reflected to the motor and are more of a friction load.

So, if after running your RMS calcs you find that your required torque is less than 60% of motor rated torque, go with the TEFC design.

I agree with Bill that ACVector (especially closed loop) is the way to go, the stator will provide maximum torque for a given amount of current under closed loop vector control and motor heating will be minimized.

 

Responding to several comments regarding the effects of soft-start in the application noted. While it is true that the driven-machinery might benefit, there is no such benefit to the motor.

Neglect for a moment torque requirements. Then, the heat produced during acceleration is proportional to:
a) the total system inertia;
b) the difference between the motor's initial-slip squared and its final-slip squared; and
c) the stator-to-rotor resistance ratio. Stated another way, all of the heat producing factors are constants, therefore precluding soft-start advantage(s).

The second factor is the relative heat produced for functions other than acceleration. It can be shown, that, if heat produced by acceleration equals 1-unit, then, 3-units will be produced for a plug-stop, and 4-units for a plug-reverse.

The third factor is the rate of heat removal. It is highly unlikely the duty-cycle proposed will effect any adequate cooling.

Now I realize that Matt omitted key data, but my point is... it is futile to propose fixes when you don't know what's broke.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])