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Problem when feeding a motor with high frequency source

Problem when feeding a motor with high frequency source

Hello there!

I'm in a project that we're designing a rolling table to transport a bundle of steel rod. To improve the productivity, I want to increase the speed of the gearmotor with an inverter.
The plate info of the gearmotor shows that the nominal rotation is 1780 rpm. But in contact with the manufacturer, it shows that it can run safety with 2800 rpm.

My question is. What a high frequency feeding source can cause to my motor? It will reduce it lifespan?


RE: Problem when feeding a motor with high frequency source

Some more info.

The duty cicle is about 10 seconds in every 5 minutes. So it only run for 10 seconds, then it stays down for 5 minutes, then run again for 10 seconds.

The gearmotor is of 1,5 HP, and if fed with 440 V.
The normal currently of work, with load, is about 1,6 A.


RE: Problem when feeding a motor with high frequency source

Presumably you're talking about feeding it from a VFD. There're a few issues that I can think of:
The addition speed will maybe wear out the bearings sooner, but I doubt it will be noticeable.
The extra speed will require more HP than the motor can produce.
The motor isn't rated for inverter duty and the insulation will fail. An output reactor may help with this, but if the motor isn't inverter duty then you may well have trouble.I've got a press running an OLD non-inverter motor and it's been ok, but we've only run it a few hundred hours and we don't run over 60 HZ.
There're people in the Motors and Generators forum that are better informed than I am, so you may want to ask there.

RE: Problem when feeding a motor with high frequency source

You need to maintain the volts/frequency ratio when you operate above base speed. A 400V / 50Hz motor has a V/Hz ratio of 8, so at 100Hz you would theoretically need an 800V supply. Whether this is possible is a different question because the insulation system would be operating well above design level and 800V VFDs aren't a commonplace item.

One technique is to reconnect a star-wound motor in delta, then use the VFD to set a new base speed at 1.73x the original base speed, e.g. a 50Hz motor would effectively become an 83Hz motor with the same rated voltage and same output torque, but with a 1.73x increase in power output due to the higher speed. You need to consult the motor manufacturer before doing this because it places stresses on the motor which it wasn't designed for: factors like rotor balancing become more significant and bearings may need to be changed to handle the higher shaft speed.

RE: Problem when feeding a motor with high frequency source

I read your question different than the others who replied: i may be wrong.

You have a 1.5hp gearmotor rated 440v, 1780 rpm (60hz). You contacted the mfgr and they say it can run to 2800rpm ok.

THAT tells me it IS an inverter duty motor - or why would the mfgr say it can go to 2800rpm (93hz).

Your load is not a pump; it is a simple conveyor, so I would assume the additional load friction to go 2800rpm instead of 1780rpm is insignificant, so not much additional load to the motor. You did not say what your motor nameplate current rating is, but 1.6 amps for a 1.5hp motor seems less than 1.5hp load (maybe 1.9amp nameplate?)

So yes, an inverter (VFD) will allow your motor to go to 2800rpm with the same 440v input voltage.

Will the motor be overloaded running faster like this? Without morre info, no clue, but my guess is no. Especially since you have such a low duty cycle.

You would simply set the VFD up for 440v @ 60hz, then go into constant HP to 93hz max speed.


RE: Problem when feeding a motor with high frequency source

Hi Gustavo

Increasing the speed might give you problems of the rollers just slipping against the bar stock due to in sufficient friction between load and rollers.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Problem when feeding a motor with high frequency source

Thank you for all reply.

The current did not get higher with the increase of the frequency. It just have a pick with 1,8 A (as I'm from Brazil, "," is used on a decimal number).
The plate current is 2,2 A.

I'm worried about the temperature of the wound on the motor, but as the duty cycle is low, it seems to not be an issue.

RE: Problem when feeding a motor with high frequency source

Gustavo, stop worrying about motor heat! It is simple: as long as you put nameplate current rating or less into your motor, you will not overheat it! Period.


RE: Problem when feeding a motor with high frequency source

KIlroy is correct, with a 10s run every 5 minutes, the motor will stay cool.thumbsup2
John2025's recommendation of installing a dv/dt filter on the drive output is always a good idea, it's cheap insurance.

The pessimist sees difficulty in every opportunity. The optimist sees the opportunity in every difficulty.
Winston Churchill

RE: Problem when feeding a motor with high frequency source

If it works, it works.

The problem WAS, as ScottyUK pointed out, that you could not maintain the ratio of voltage and frequency that the motor was designed for. So once you went over the 1780RPM, the motor began operating in "constant power" mode, rather than constant torque. That means the torque output of the motor was DECREASING as the speed increased. So at 2800RPM you had reduced the V/Hz ratio to around 64%, so your motor was not capable of any more than 64% of it's rated torque, and worse yet, only about 40% of it's PEAK torque, that which is used to accelerate or re-accelerate the load after a change.

But, if it worked, then your motor was providing more torque than needed for the task and the reduction did not matter.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

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