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Replacing a DC motor with an AC 2
- Thread starter giselle
- Start date
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1
- #2
The only answer possible with this level of information is "maybe". It is necessary to consider, among other things, load characteristics, and the present configuration of the DC drive/motor combo.
For instance, does the existing DC motor operate at full field at base speed and below, or does it use shunt field weakening to operate at above base (nameplate) speed?
How is the DC motor cooled?
What is the application?
What are the load's power requirements? This is relatively easy to determine for a DC motor - measure armature voltage, and armature current, then multiply them (Power= Voltage x Current). Do this at operating speed, and, highly suggested, at various points during a start-up.
When you select an AC motor/drive verify the drive can source enough current at the lowest start-up speed (which will be roughly analogous to motor terminal voltage) to produce the power you need. Keep in mind that simple scalar (Volt/Hz) drives don't perform as well at low speeds as do vector drives.
If the load must be run for extended periods at low speeds it may be necessary to select a motor with a higher full load amp rating to reliably provide that level of power.
This is also where cooling comes into play ... if the DC motor enclosure is forced air (blower) cooled then it may be necessary to use a (more expensive) forced air cooled AC motor of the same rating, or, if a physically larger motor will fit, use a motor in a more available and cheaper TEFC enclosure with separately derived 'air over' cooling.
For instance, does the existing DC motor operate at full field at base speed and below, or does it use shunt field weakening to operate at above base (nameplate) speed?
How is the DC motor cooled?
What is the application?
What are the load's power requirements? This is relatively easy to determine for a DC motor - measure armature voltage, and armature current, then multiply them (Power= Voltage x Current). Do this at operating speed, and, highly suggested, at various points during a start-up.
When you select an AC motor/drive verify the drive can source enough current at the lowest start-up speed (which will be roughly analogous to motor terminal voltage) to produce the power you need. Keep in mind that simple scalar (Volt/Hz) drives don't perform as well at low speeds as do vector drives.
If the load must be run for extended periods at low speeds it may be necessary to select a motor with a higher full load amp rating to reliably provide that level of power.
This is also where cooling comes into play ... if the DC motor enclosure is forced air (blower) cooled then it may be necessary to use a (more expensive) forced air cooled AC motor of the same rating, or, if a physically larger motor will fit, use a motor in a more available and cheaper TEFC enclosure with separately derived 'air over' cooling.
Is the DC motor intended for emergency operation from a battery? If so I would not be looking at battery - inverter - AC motor solutions. Emergency systems work best when they are simple and strongly built.
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If we learn from our mistakes I'm getting a great education!
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If we learn from our mistakes I'm getting a great education!
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1
- #4
giselle, rawelk has provided you with much good advice. Probably the most important piece is that you must understand how your load requires torque over its operating speed range. Again, its not so much about the old motor as the load. After all, the old motor may have been sized incorrectly for the load. You wouldn't want to duplicate the error with the AC system.
One other piece of advice, it is very easy to use AC motor overspeed to make your new system work better. As a general rule, at this hp level, figure on running your motor at 90hz when the load machine is at maximum speed. This may require that you change some sprockets or sheaves to get the extra mechanical reduction or, if that's not possible, pick a six pole motor instead of a four pole. Motor cooling, speed regulation, any braking, and starting torque will all be better if you do this.
One other piece of advice, it is very easy to use AC motor overspeed to make your new system work better. As a general rule, at this hp level, figure on running your motor at 90hz when the load machine is at maximum speed. This may require that you change some sprockets or sheaves to get the extra mechanical reduction or, if that's not possible, pick a six pole motor instead of a four pole. Motor cooling, speed regulation, any braking, and starting torque will all be better if you do this.
- Thread starter
- #5
Thanks to all for the advice. The application is a plastic extrusion machine. The main motor is coupled to a reduction gear box (2422:43 ratio; 25kW). It is typically operated between 60-100% of the rated speed. The DC motor is blower cooled.
Skogsgurra
Electrical
An extruder is a good example of a load that an AC motor can handle easily. Torque is more or less quadratic, so you will have maximum load at highest speed and something like one third at 60% speed. This is somewhat dependent on the temperature of the plastic material and its temperature. You may get away without external fan. But if you already have it - why not keep it?
Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
It has been my experience that plastic extrusion machines are more in the class of constant torque. In most of these machines, the extrusion of the plastic is performed by a screw. The idead is that the machine operate with a constant or set pressure at the extruder head. This back pressure is the cause of most of the torque used. Ther is some increase in torque with speed, but it is not even linear with respect to speed. In the machines I have seen, low speed or starting torque is around 70% of full load torque. You might be able to get an AC motor/drive to work in this application, but I would advise that the AC motor be forced cooled.
Skogsgurra
Electrical
Thanks for that insight djs.
Are the extruders you work with "bag extruders" or profile extruders?
Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
Are the extruders you work with "bag extruders" or profile extruders?
Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
2422:43 (is this a planetary gearbox? European manufacturer?) represents a ratio of 56.326:1, and yields these final screw speeds at 60 Hz (actually, a bit lower once slip is factored in).
2 pole, 3600 RPM motor, 63.9 RPM screw
4 pole, 1800 RPM motor, 32.0 RPM screw
6 pole, 1200 RPM motor, 21.3 RPM
40 HP is fairly small for an extruder, so my guess would be it is driving something on the order of a 1-1/2" to 2" diameter screw.
A 32 RPM final screw speed seems low for full output with the screw designs I'm familiar with, and am guessing you might have a 3600 RPM motor - 64 RPM final output is closer to what I've seen on such machines.
There are places where a forced air ventilated AC motor is the only option, typically on larger extruders where the motor is located directly under the barrel, and forces a hard limit on motor size, but my preference is the same as DickDV's - whenever possible use a TEFC motor and select motor RPM (in my case, usually going from a 1750 RPM DC motor to 1150 RPM AC motor) so the lower limit of normal processing motor speed is 60 to 70% of this, but more typically have it running close to or above base speed.
A second consideration in favor of TEFC motors is their ubiquity. Even if you can't get an exact replacement inverter duty motor during an emergency you'll be able to get something that will bolt into place, and operate successfully until the failed motor can be repaired or replaced. AO "air over" cooled inverter duty AC motors are also becoming very common, but forced air AC motors (at least in the larger sizes) are usually quoted on a build-to-order basis, and tend to incur greater lead times.
There isn't anything wrong with the concept of extruder motor forced air cooling (especially in smaller motors like yours), but most plastics plants I've been in have dust, volatiles from die burn-off, venting, and other such crap in abundance.
Intake filter servicing tends to take a back seat, so, unless you duct in clean air from elsewhere this stuff eventually builds up in the motor blocking cooling passages and softening winding insulation. At 40 HP this isn't too bad because the volume of air through the motor is at most only a couple of hundred CFM, but becomes more of a consideration in higher power motors where cooling requirements may be on the order of 1200-2500 CFM.
Even when filter maintenance is performed regularly I'd just as soon eliminate the need for it in the design stage, and enjoy the reduction in ongoing maintenance costs.
Incidentally, I've found an article from Control Magazine which goes over many of the points of DC to AC extruder drive conversion,
and a terse, yet useful overview of extrusion fundamentals at
2 pole, 3600 RPM motor, 63.9 RPM screw
4 pole, 1800 RPM motor, 32.0 RPM screw
6 pole, 1200 RPM motor, 21.3 RPM
40 HP is fairly small for an extruder, so my guess would be it is driving something on the order of a 1-1/2" to 2" diameter screw.
A 32 RPM final screw speed seems low for full output with the screw designs I'm familiar with, and am guessing you might have a 3600 RPM motor - 64 RPM final output is closer to what I've seen on such machines.
There are places where a forced air ventilated AC motor is the only option, typically on larger extruders where the motor is located directly under the barrel, and forces a hard limit on motor size, but my preference is the same as DickDV's - whenever possible use a TEFC motor and select motor RPM (in my case, usually going from a 1750 RPM DC motor to 1150 RPM AC motor) so the lower limit of normal processing motor speed is 60 to 70% of this, but more typically have it running close to or above base speed.
A second consideration in favor of TEFC motors is their ubiquity. Even if you can't get an exact replacement inverter duty motor during an emergency you'll be able to get something that will bolt into place, and operate successfully until the failed motor can be repaired or replaced. AO "air over" cooled inverter duty AC motors are also becoming very common, but forced air AC motors (at least in the larger sizes) are usually quoted on a build-to-order basis, and tend to incur greater lead times.
There isn't anything wrong with the concept of extruder motor forced air cooling (especially in smaller motors like yours), but most plastics plants I've been in have dust, volatiles from die burn-off, venting, and other such crap in abundance.
Intake filter servicing tends to take a back seat, so, unless you duct in clean air from elsewhere this stuff eventually builds up in the motor blocking cooling passages and softening winding insulation. At 40 HP this isn't too bad because the volume of air through the motor is at most only a couple of hundred CFM, but becomes more of a consideration in higher power motors where cooling requirements may be on the order of 1200-2500 CFM.
Even when filter maintenance is performed regularly I'd just as soon eliminate the need for it in the design stage, and enjoy the reduction in ongoing maintenance costs.
Incidentally, I've found an article from Control Magazine which goes over many of the points of DC to AC extruder drive conversion,
and a terse, yet useful overview of extrusion fundamentals at
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