Frequent motor start-up 5

[bulkhandling]

A 4 pole motor, 200hp, will have to start-stop for 6 times per hour (or 12 depends on the control philosophy). I know that 4 cycles are normally allowed for this motor size, but we have VFD so we can start at low speed. The load in each cycle will be 81 brake horse power for 1.5 minutes and 159 bhp for 10 seconds.
Will this be any problem?

 

[sreid]

You may be OK as long as you can significantly increase the acceleration time. The limititation to the number of across the line starts per hour is the winding temperature rise due to the high start currents.

Doubling the accel time will half the energy input to the motor as a first approximation. For example (since accel time is approximately inversly proportional to current)

10 amps for five seconds = I^2 x t = 10^2 x 5 = 500

5 amps for 10 seconds = I^2 x t = 5^2 x 10 = 250

 

[electricpete]

NEMA limits for repetitive D.O.L. starting are here:

http://www.joliet-equipment.com/allowable_starts.htm

4 starts per hours is the number listed there... so this is apparently the source of the limit you quote.

You are not much above that and with vfd as sreid points out you have the potential to significantly reduce the heating. I'm guessing if you can get ramp up to speed over 5 sec or more it should be no problem. (maybe someone else can comment). Of course if your application permits to ramp up even slower, that will be even better.


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[aolalde]

It seems that the 200 Hp motor combined with a VFD will successfully handle the proposed cyclic load.

The more critical part is the acceleration from zero speed to operating speed. If the VFD could keep a reduced current (100% to 200% of full load current) during the starting period, you will not have any problem.

The total energy (E) required to accelerate a load inertia (wk^2) to reach a working speed is a constant. The only thing that you are able to modify is the Power (P) demand (P= E/time). Extended accelerating time allows heat transfer to the cooling system and the temperature rise of the motor is reduced.

Calculation of the average RMS power demand per hour including the acceleration and load BHP will give you the basis of decision.

 

[jraef]

Most constant torque VFDs will have a maximum current limit of 150% if they are scalar, 200% if they are vector, and in either case they can only deliver that for 60 and 30 seconds respectively, so be wary of that issue in your scheme. If however, you can accelerate the load slow enough without exceeding FLA + service factor, you can start it as many times as you like.

"Venditori de oleum-vipera non vigere excordis populi"

 

[DickDV]

Assuming a NEMA Design B motor, across-the-line starting will result in about 160% of nameplate torque and 600% inrush current. On an inverter, the same 160% torque can be produced with only 160% of nameplate current. Thus the same acceleration time can be acheived with much lower currents and winding heat.

And, even tho constant torque or heavy duty drives are usually rated 150% current for 1 minute, it is important to note that this is 150% of continuous drive current, not motor nameplate current. Since drives are usually rated to NEMA standard efficiency currents and the more common high efficiency or premium efficiency nameplate currents are lower, the resulting motor 1 minute overload rating is more than 150%. And, if that isn't enough current, you can always oversize the drive to take advantage of more of the motor short-term rating (typically 220%).

At any rate, my judgement is that the starting scenario in the opening post will most likely be possible without motor heating problems, assuming accel ramps of more than three seconds and a drive that can produce the current to drive the motor fully at these accel times.

 

[bulkhandling]

Thank you all for your input! It sounds positive, but I need more time to study your comments.

 

[light1]

Original posting does not indicate the motor duty type. The manufacturer tech support could be contacted for this information. Also, the VFD may have a rated number of motor starts, which should not be overlooked.

 

[Farkel]

What enclosure do you have on the motor, and what are the deceleration time, and rest time?

 

[bulkhandling]

TECO NEMA B TEFC Optim HE, 200hp/1800rpm, 447T, 460/3/60. Do not know decel time yet, rest time 3 or 6 minutes.

 

[Farkel]

The best drive would be a vector, for one can regulate the torque independently of the speed, or another way of saying the same is one regulates the amps. Your motor looks good for the application applying a RMS-Torque method of computing. As follows:
Cycle time cooling factor Eqt Torque TSq TSqt
Accel 10 75% 7.5 463.75 215064 2150640
Run 90 100% 90 236.25 55814 5023265
Decel 20 75% 15 46 2116 42320
Rest 360 25% 90 0 0 0
480 202.5Sec 7216226

7216226/202.5=35635, of which the square root is 188.77ft/pds
188.77x1784RPM/5250=64.14HP The motor is listed as being able to produce 841.6ft/pds of torque across the line,and running torque at full load of 588.56 ft/pds. My figures are somewhat hypothetical, since I do not know for certain the exact perameters; however, I believe they are close. Hope this helps.

 

[Marke]

Hello bulkhandling

Motors are rated in starts per hour and maximum starting time due to the very high power dissipation in the motor while it is operating under high slip conditions.
There is a very high dissipation as the motor acellerates up to rated speed, and the dissipation in the rotor is proportional to the full speed kinetic energy of the driven load.

In the case where the motor is driven by a VFD, this high dissipatio does not accur because the motor is never operated under high slip conditions. In simplistic terms, I describe the control of a motor on a VFD as "the motor is never started, it is RUN from zero speed to full speed"

If the motor is ramped up by the VSD at say rated torue from zero speed to rated speed, then the energy dissipated in the rotor is the same as it would be under normal full load conditions. i.e. as far as the rotor is concerned a start never happened. The temper of the rotor will be elevated due to the reduced cooling at low speed, but otherwise there will be no problems.
Analysis of the stator will yeild a similar result, so consider the shaft torque during the operational cycle and if the motor can operate safely within that envelope, you will be OK. The only additional considerations are that the cooling will be reuced at reduced speeds, so you will want to reduce that average shaft torque accordingly, and there will be additional harmonic losses in the motor which will result in a small amount of additional heating.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[GGOSS]

Hello Marke,

That deserves a star for 2 reasons;

1. It's right on the money
2. I didn't have to crunch it out myself.

Keep up the good work.

Regards,
GGOSS

 

[light1]

When there are more motor starts per hour, e.g. 10 or more, it will be better to consider a different solution, e.g. clutch or electromagnetic coupling while keeping the motor running continuously. This way it is ensured that the motor has a proper cooling and proper shaft loading.

 

[electricpete]

even with vfd?

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[DickDV]

With vfd and properly set accel parameters, there would be no limit on starts per hour since, as was mentioned so nicely above in this thread, since the motor is ramped up from zero speed synchronously, there is no "start" as far as the motor is concerned. And, of course, no inrush current either.