over running load on electric motor

[iaguy]

I have an application where I want to start a large inertial load, run it for a few minutes then stop it using the electric motor.

The startup cycle is a constant torque cycle going from 0 to 220 hp in six seconds. The torque goes from 0 to 9202 in-lbf in the same six seconds.

The load then drops to 88-115 hp for five + minutes.

Then I then want to stop the rotating load by back driving the electric motor. The torque would go from 8,305 in-lbf to 0 as the load stopped over a six second cycle. The maximum hp the motor would have to absorb would be 182 hp. It would go to 0 as the load decreased its speed.

The motor would then idle for 10 minutes while other operations are executed.

I would like to use a 150 hp electric motor to drive this arrangement. I assume it could handle the starting cycle torque & hp ( the motor would not start against this load) it is less than 200% of the rated power. Is there any thing to worry about?

What about the stopping cycle? Can I back drive the electric motor with this over-running load without problems?

 

[DickDV]

I think you will be ok with the 150hp motor as far as the motoring portion is concerned. I would suggest a NEMA C design, however to limit inrush current and further enhance the starting and accelerating torque.

As to stopping, my recommendation would be a DC injection brake. It will have to sized carefully to maximize the braking torque in the motor. I would review with the motor manufacturer the maximum allowable DC injection current and any thermal issues that could come up.

My gut tells me the thermal side of this, both motoring and braking, is not a problem but I'm not sure you can get enough braking torque with DC injection. Every other option at 150hp is way more expensive.

Let us know how this works out. I, for one, could learn a bit from your experience.

 

[waross]

I wouldn't do this with a "T" frame motor, and the biggest "U" frame motor I have seen plugged was a 50HP. Mind you, it had a start button and a jog reverse button. Every stop was a manual plug stop.
Plugging is throwing a motor into reverse when it is running forward. A special speed switch is used to cut the current just as the motor comes to a stop.
You may consider series resistors to limit the reverse current.
Many of the speed switches way back then were quite sensitive and it was possible to close the zero speed switch in some applications by manually turning the shaft. When the switch closed, the reverse contactor closed and gave the motor a shot of power. If the shaft had turned because a mechanic had just leaned on a wrench attached to a coupling bolt the wrench may be thrown back at him with all the force of a starting motor.
A plugging switch was a zero speed switch with a restraint that trapped the contacts and prevented them from operating. When the control circuit called for plugging, a solenoid would release the restraining plate and allow the contacts to operate.
Plugging is the quickest way that I know of to stop a motor electrically.
If you need faster stopping or don't want to submit your motor to the abuse of plugging, consider an air brake.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Each braking cycle must be counted as a starting cycle. So first determine the Starts-per-Hour rating f the motor in question, then use 1/2 that value for your application. For example, if your 150HP motor is rated for 10 SPH, then for your application you can only do 5SPH because of the braking. DCIB would be better than plug reversing in my opinion, because it can be adjusted to a level that does the job without undue stress on the mechanical components.

But the best method would be to use a Line Regenerative Brake; feed the braking energy back into the grid. Not only do you recover the energy, but you remove the thermal stress from the motor and maybe you can get away with a smaller system. There are VFDs that have line regenerative braking capability, there are also companies that make stand-alone Line Regen Brake units, Bonitron is one that comes to mind, although they seem to stop at 100HP as a standard product. Maybe they will build larger versions as specials though.

http://www.bonitron.com/m3345.html.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[electricpete]

I have an application where I want to start a large inertial load

How large is the inertia?

The startup cycle is a constant torque cycle going from 0 to 220 hp in six seconds. The torque goes from 0 to 9202 in-lbf in the same six seconds.

I don't know how to interpret this. Is it constant load torque (the first sentence) or ramp change in load torque (the 2nd sentence). And what are you assuming about speed during this period of time? Are you assuming linear ramp of speed? Are you prepared to take what you get?

And what is the rated speed of the motor you intend to use?


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

"Are you prepared to take what you get? " was referring to DOL start.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[waross]

Hi Jraef;
Will the VFD option possibly allow more starts per hour?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

There is no doubt vfd start allows more starts than DOL start.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[LionelHutz]

Yes, what is this high inertia?

When you say the load goes from 0 to 9202 in-lbf in 6 seconds do you mean you want it to accelerate in 6 seconds?

The torque the motor produces does 2 things. First, the amount of torque required to just spin the load is used. Then, any excess torque gets used to accelerate the load. You need lots of motor torque if you have a high load torque and also require fairly quick acceleration of a high inertia.

To me, it sounds like a good VFD might be your best chance of having this work. It might still require a motor with a very large breakdown torque and an oversized VFD.

Still, I think you need to investigate this much further. You need to create a load torque vs speed curve and know the inertia. Then, you can pick a motor and model the starting profile.

US Drives also manufacturers regen units. We use one here for testing the last 3 or 4 years and it's been good.

 

[iaguy]

I think I need to clarify.

I want to use the electric motor to drive a hydrostatic transmission. This transmission would be a variable speed drive to my load. It has two states: "ON" which would accelerate my rotating load to speed and hold the speed for a few minutes; and "OFF" which would stop the load. Stopping the load does not stop the electric motor. I want the electric motor to hold a constant speed while I try to back drive the electric motor. It is like going down a hill with your car. The car's engine tries to keep the car from accelerating. Removing your foot from the gas pedal tries to further slow the car down. This is all limited by the cars engine's ability to absorb power. I don't want to slow the electric motors operating speed, just keep it constant.

The question about the starting torque is correct. The torque on the electric motor varies from zero to full torque as the hydrostatic transmission moves from a large "gear ratio" to a small "gear ratio". The torque on the inertial load (from the hydraulic motor) remains constant during this cycle(constant hydraulic motor displacement, constant pressure).

When the inertial load reaches operating speed, the hydraulic system pressure drops to an operating pressure based on the work done. This lowers the torque and power draw from the electric motor to approximately 88-115 hp. Torque is dependent on normal electric motor operating speed. (Five minute duty cycle.)

In stopping, I am using the hydrostatic transmission to go from a small "gear ratio" to a large "gear ratio." This tries to speed up the electric motor and I want the motor's ability to hold a constant rotating speed to slow the inertial load to zero rpm while the electric motor remains at its normal operating speed. The torque placed on the motor and the hp generated by stopping the inertial load are listed in the first email. The torque listed in the first email is based on an electric motor running at 1800 rpm. If the motor was to run at 3600 rpm, the torque would be half that. The hp draw would be the same regardless of normal motor running speed. The ability to put power back into the grid may be an advantage. I will have to raise this point with the customer.

The electric motor operating speed is not set at this point. I have pumps that can run at 1800 rpm and at 3600 rpm. Which would be better?

 

[electricpete]

That’s a good clarification and changes the picture quite a bit (simpler I think). One more clarification: what happens when you initially turn on the motor... is there load while the motor itself comes up to speed?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[iaguy]

No, electricpete, there is no load per se on the electric motor. The inertia of the hydraulic pump (minimal), and pumping of small amounts of hydraulic fluid (20 gpm) under very low pressure (300 psig) are all the loads that would be present. A 150 hp motor probably wouldn't even know there was a load.

 

[LionelHutz]

Well that helps.

So, the cycle is;
6 second peaking at about 150%
5 minutes under about 75% load
another 6 second peaking at about 120% load
10 minutes idle

I can't see that being a problem for a typical 150hp motor. Just make sure the motor you chose has a breakdown torque well above the maximum torque you want the motor to provide so the motor does not stall when you reach these peak overloads.

 

[electricpete]

I think Lionel summed it up well.

Also the motor will feed power back to the power system during the period when it is acting as a generator. I have a hard time thinking of any scenario where that would cause a problem.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.