Replacing a DC motor with an AC
Replacing a DC motor with an AC
(OP)
I have 40hp DC motor and would like to replace with an AC motor. Can I replace it with a 40hp AC motor with similar rated rpm?
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Replacing a DC motor with an AC
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Replacing a DC motor with an ACReplacing a DC motor with an AC(OP)
I have 40hp DC motor and would like to replace with an AC motor. Can I replace it with a 40hp AC motor with similar rated rpm?
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RE: Replacing a DC motor with an AC
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.
RE: Replacing a DC motor with an AC
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RE: Replacing a DC motor with an AC
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.
RE: Replacing a DC motor with an AC
RE: Replacing a DC motor with an AC
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Replacing a DC motor with an AC
RE: Replacing a DC motor with an AC
RE: Replacing a DC motor with an AC
Are the extruders you work with "bag extruders" or profile extruders?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Replacing a DC motor with an AC
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,
http:
www.
RE: Replacing a DC motor with an AC
RE: Replacing a DC motor with an AC