860RPM motor

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ptonsparky

ptonsparky

Tom
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NE (9.06 miles @5.9 Degrees from Winged Horses)
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EC - retired
Customer replaced 3 ph 3/4 hp 860 rpm motor yesterday without looking for the cause of failure. Typical. They burnt up the second motor within a short time. These motors may be common for some of you, but not here. Short term solution would be VFD on a 1750 rpm but once that is installed they will run it until death do us part. Will the VFD give me the constant torqe I want at that speed for long term? Assume drive rated motor because they all seem to be that way anymore.
 
A VFD could be a solution if the panel is locked out from production employees. I have done it on a couple of machines where a VFD was necessary to control the input/output of the production on a machine. Although, if the panel is not locked out the production employees will normally max out the VFD to get more out of the machine especially if they get a bonus for meeting quotas.

With the new motor burning out in such a short time it may have just been a bad motor from the manufacturer. Although I couldn't know that without seeing the motor up close and taking a few voltage/current measurements while the machine is operating.
 
mcclary's electrical said:
1750 rpm motor ran at 850 rpm is going to be about 1/2 torque.
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That's right. To replace it you need to know if torque is an issue, or just speed.

A 3/4HP motor at 860RPM is going to put out 4.58 ft-lbs of torque (.75 x 5252/860)

To get the same torque from a 1750 RPM motor, you would go backwards with HP = tq x rpm/5252, so 4.58 x 1750 / 5252, = 1.5HP
 
090913-1355 EST

Why would the torque be 1/2?

The torque capability might not drop too much other than it might need to be restricted because of the lower speed and less cooling. If torque capability was constant, then dropping the speed in half would reduce the HP to 1/2.

In turn this would mean that a 1750 motor run at half speed would need to have an HP rating twice that of the original 875 motor to supply the required torque or HP at 875. Depending upon the motor and drive characteristics a 2 HP 1750 motor might be required instead of the 3/4 HP unit.

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But wouldn't expecting max torque from a 1750 motor running at 860 tend to overheat it due to the low speed of the fan on the motor?

Wouldn't you have to go higher with the HP?
 
Bob's right , expecting maximum torque from a 60 htz motor running at 30 htz is just wrong, in addition, running a 60 htz motor much past 90 htz and you start losing torque.
 
090913-1948 EST

Bob I did indicate you would probably have to use a somewhat larger motor. You can also put a blower on the motor to remove the heat. A blower is a common technique on large DC motors that have to operate over a large dynamic range.

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mcclary's electrical said:
Bob's right , expecting maximum torque from a 60 htz motor running at 30 htz is just wrong, in addition, running a 60 htz motor much past 90 htz and you start losing torque.
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I have used drives on augers more than once and have had pretty good luck running them at 20 hz and above. Below that cooling is a major problem without additional fans. Many VFDs have a constant torque vs fan/pump option in their setup. We have worked on 15 hp 3ph motors that are used in a plastic recyling plant that molds plastic sheets. Those motors operate from 1 rpm on up to 1750 at the same torque and have been for years. Thicker sheets weigh more and require more cooling time so the operation is slower.

How well will a VFD maintain the torque of typcial vfd rated motor through out its range of 0-60hz?

Obviously I need to at least up my hp to use a VFD.
 
gar said:
Bob I did indicate you would probably have to use a somewhat larger motor.
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I had not read your post when I composed mine, I was responding to ones above yours.:)

You can also put a blower on the motor to remove the heat. A blower is a common technique on large DC motors that have to operate over a large dynamic range.

.
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Yeah you could, but making the simple more complicated is something I try to avoid.
 
ptonsparky said:
Customer replaced 3 ph 3/4 hp 860 rpm motor yesterday without looking for the cause of failure. Typical. They burnt up the second motor within a short time. These motors may be common for some of you, but not here.
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I would call the manufacturer and find where it is stocked and than ...

UPS red
 
ptonsparky said:
I have used drives on augers more than once and have had pretty good luck running them at 20 hz and above. Below that cooling is a major problem without additional fans. Many VFDs have a constant torque vs fan/pump option in their setup. We have worked on 15 hp 3ph motors that are used in a plastic recyling plant that molds plastic sheets. Those motors operate from 1 rpm on up to 1750 at the same torque and have been for years. Thicker sheets weigh more and require more cooling time so the operation is slower.

How well will a VFD maintain the torque of typcial vfd rated motor through out its range of 0-60hz?

Obviously I need to at least up my hp to use a VFD.
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...and make sure that the motor is rated for inverter service.

When and if operated at constant speed ASD should NOT be the solution, normally.

Having said the above, I had an application where the process equipment parameters were diffcult to calculate and small RPM changes would result in significant process yield change, so we installed an ASD and experimentally proved the optimum speed, which then was set as the constant speed.
 
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