Heat @ 0 hz.

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ptonsparky

ptonsparky

Tom
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Drag motor with a commanded speed of 0 Hz during part of the process. Coasting or turning backwards Is not an issue. I do not know what the current is at 0 hz. Could it be significant enough to shorten the life of the motor? 2hp 480v. Hz is between 43 & 58 at all other times. Excluding ramp.
 
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winnie

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Springfield, MA, USA
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Electric motor research
It could be. It all depends on how much current is flowing and how much kinetic energy is being dissipated each time the motor stops.

So you could just have normal heating, or you could have significant increased heating.

-Jon
 
kwired

kwired

Electron manager
Location
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petersonra said:
You could also get a motor with an external cooling fan.
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Which may be needed anyway if you have a situation where the motor can run below 15 Hz or even higher in some cases, kind of depends on the application conditions.

I guess OP said what range it normally runs and is not that low other then the zero Hz at some points.
 
Jraef

Jraef

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It's going to depend on what it's doing at 0Hz, and why. On a drag line, 0Hz could just be a way of telling the motor "do nothing, but be ready", or it could be a way of applying the brakes, as in "give me rated full load rotor torque against a load that WANTS to move". If it is braking, the amount of torque the motor is expected to develop against that overhauling load to keep it from moving will be directly equivalent to the percentage of FLA the motor is rated for. So if, for example, you have to put out maximum FLT to hold the motor shaft still, then the motor will pull maximum FLC. Given that the motor is not going to be moving, any convection cooled design scheme for the motor will not be working, hence the need for the separately powered blower. Without the separately powered blower, the current will overheat the motor in a manner similar to having a locked rotor condition, but without the locked rotor amps (because the VFD will be limiting it). I'd give that no more than about 20 seconds before you start damaging the insulation.

But if you are using a TENV (Totally Enclosed Non Ventilated) motor, which is possible at 2HP, then the motor is designed to NOT need the fan forced cooling, it uses radiant heat dissipation. In that case, you should be able to pull FLC virtually forever.
 
kwired

kwired

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Location
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Jraef said:
It's going to depend on what it's doing at 0Hz, and why. On a drag line, 0Hz could just be a way of telling the motor "do nothing, but be ready", or it could be a way of applying the brakes, as in "give me rated full load rotor torque against a load that WANTS to move". If it is braking, the amount of torque the motor is expected to develop against that overhauling load to keep it from moving will be directly equivalent to the percentage of FLA the motor is rated for. So if, for example, you have to put out maximum FLT to hold the motor shaft still, then the motor will pull maximum FLC. Given that the motor is not going to be moving, any convection cooled design scheme for the motor will not be working, hence the need for the separately powered blower. Without the separately powered blower, the current will overheat the motor in a manner similar to having a locked rotor condition, but without the locked rotor amps (because the VFD will be limiting it). I'd give that no more than about 20 seconds before you start damaging the insulation.

But if you are using a TENV (Totally Enclosed Non Ventilated) motor, which is possible at 2HP, then the motor is designed to NOT need the fan forced cooling, it uses radiant heat dissipation. In that case, you should be able to pull FLC virtually forever.
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Given his location something tells me when he said drag motor he didn't mean "dragline" but meant a "drag conveyor" commonly used in grain handling operations. They typically need no braking even with no load on them.

But good information anyway.:thumbsup:
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
kwired said:
Given his location something tells me when he said drag motor he didn't mean "dragline" but meant a "drag conveyor" commonly used in grain handling operations. They typically need no braking even with no load on them.

But good information anyway.:thumbsup:
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Always good information.

The vfd would give me the amps at 0 hz, if I had a chance to look. Missed the chance today. Hopefully tomorrow.
 
kwired

kwired

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Location
NE Nebraska
Not a feature I have had to mess with before, but I imagine it is possible with most drives to select no output when frequency reference is zero, or the ability to brake if the frequency reference is zero. Of course deceleration times are going to fit into this somehow as well, at least when first changing frequency reference from non zero to zero.
 
kwired

kwired

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ptonsparky said:
0 hz = 2.48 amps. TEFC.

To long.
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No idea if it is too long, but would think that one needs to factor in voltage as well as consider what percentage of full load VA this current is. If it is @ 0 hz it about has to be DC current. and must be braking current. Maybe you can get into braking settings and lower this current or even eliminate it altogether.
 
kwired

kwired

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Location
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mike_kilroy said:
Your 2hp motor probably has a thermal time constant of about 20 minutes. There is no way 1 min at flc is going to harm it once a day.
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Even when there is no cooling air flowing because the rotor isn't turning? May still not harm it but heat management is different at full rated speed then at no speed I would think.
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
mikekilroy said:
Your 2hp motor probably has a thermal time constant of about 20 minutes. There is no way 1 min at flc is going to harm it once a day.
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Why I asked, IDK.

It was faulting on overcurrent even with no load on the motor. Less than a year old so I get to eat it regardless. Connections were good in the peckerhead. Bearings good. Guessing a winding to winding fault because it did not take long to let the smoke out once I took the drive out of the equation.
 
kwired

kwired

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Location
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ptonsparky said:
Why I asked, IDK.

It was faulting on overcurrent even with no load on the motor. Less than a year old so I get to eat it regardless. Connections were good in the peckerhead. Bearings good. Guessing a winding to winding fault because it did not take long to let the smoke out once I took the drive out of the equation.
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How long is run between drive and motor, and do you have a load side reactor installed? may or may not have anything to do with carrying current at 0 Hz but may also be IGBT reflected wave problem. I've seen motor that fail from reflected wave problems - they look and test fine, but will not even rotate the shaft before drive trips, connect across the line and you remove all doubt on the condition of the motor though.
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
kwired said:
How long is run between drive and motor, and do you have a load side reactor installed? may or may not have anything to do with carrying current at 0 Hz but may also be IGBT reflected wave problem. I've seen motor that fail from reflected wave problems - they look and test fine, but will not even rotate the shaft before drive trips, connect across the line and you remove all doubt on the condition of the motor though.
Click to expand...
50' to motor, maybe a bit more. Rated for vfd. Line reactor, not load.

Hindsight says send motor in for autopsy before cremating it.
 
kwired

kwired

Electron manager
Location
NE Nebraska
ptonsparky said:
50' to motor, maybe a bit more. Rated for vfd. Line reactor, not load.

Hindsight says send motor in for autopsy before cremating it.
Click to expand...
I'd put a reactor on the load side of the drive, especially with 480 volt system.

Also did you mention what frequency the motor typically runs at (other then the one minute no speed period occasionally)? I though you did but am not finding it. More important is does it run for long periods at low speeds, you can get away with more on fans and pumps as they are variable torque, but constant torque loads don't like the reduced cooling effects from the slower fan speed when the torque level is still high. I had to put a VFD rated motor with external cooling fan on a high pressure pump because we only were running that thing at about 30 Hz a lot of the time. Rewound a 50 Hp motor on that one 3 or 4 times, each time about a year had passed, before switching to the new motor - haven't had a failure yet and it has been there maybe close to 10 years now. Never have had a load side reactor on that one either - though the distance was probably in the 50 foot range. Think I would put one on it if I were doing it today though.
 
Besoeker

Besoeker

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Jraef said:
It's going to depend on what it's doing at 0Hz, and why. On a drag line, 0Hz could just be a way of telling the motor "do nothing, but be ready", or it could be a way of applying the brakes, as in "give me rated full load rotor torque against a load that WANTS to move".
Click to expand...
0Hz is not the same as zero speed.
You need slip to get torque.
 
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