If that is what you need use a Servo motor, although you may need to get a matching "VFD" for it.
Why are we still using 50/60Hz motors?
- Thread starter strantor
- Start date
Lemming
Member
Depends on the number of poles it was wound with...
S (synch speed) = 120 x f (frequency) / p (# of poles)
So a 4 pole motor will spin 120 x 400 / 4 = 24,000 RPM
But if you want it to spin closer to what a 60Hz motor would, you would have to wind it as a 26 pole motor: 1846 RPM (120 x 400 / 26)
But this is kind of the point I was making earlier. If I want to use a motor on a conveyor, a 24,000 RPM motor is going to be useless to me without a very expensive gearbox or a HUGE sheave / belt system. But if I want it to turn slower on a 400Hz system, now I have a motor that is VERY expensive to make, because it must have 26 poles, or thirdly, even though I don't really want to vary the speed I must now buy a VFD to turn the frequency down to 60Hz.
I understand the ratio hz / poles, but wondered if the aircraft actually used high rpm motors, or more modest speeds with big pole counts...
I thought the reason for 400hz on aircraft was the alternators are driven off high speed turbines.. Is this correct ??
jraef
Member
The reason is that for a given power requirement, transformers and motors can be much smaller and lighter, things that are important in limited space vessels like aircraft and Navy ships. But the speed at which motors run has to do with the load activity it must perform. It's difficult to have a centrifugal fan run at 24,000RPM. But a positive displacement gear type hydraulic pump? You can use a very small pump running at a very high speed to get a lot of hydraulic pressure.
You initial query about 400 Hz motors has everyone focused on the motor. I have learned a lot reading the reply posts about the whys, and wherefores of 50/60 and 400Hz spindle motors.
I read the post as: “Retrofitting Motor/Drive for extruder”
May I assume that your existing drive is DC? Large HP Extruders often used DC motors to achieve hi-torque, and consistent torque, and have variable speed control.
What is the root reason of the drive retrofit? Is the end user trying to eliminate DC drive to reduce maintenance?
With the ever increasing use of AC VFDs and “off-the-shelf”, AC induction motors being readily available, and reasonable low maintenance, I can see the yen for the retrofit. I am no way a motor expert, but I believe that the AC induction motor delivers full torque and HP exactly at its normal advertized operating frequency. I also believe that the torque “slips” away when using a VFD to push beyond the normal advertised frequency. The slip may not have any ill effect on a conveyor, or lightly loaded application, but an extruder probably needs full torque.
We did however recently make an installation, where we pushed the motor beyond 60 Hz, without torque loss. We were led to this solution by an AC VFD vendor. The drive vendor calls the strategy, “Supercharging”.
We had a 5hp AC induction motor running a vertical load. The customer wanted to reduce the time to transport the load. We got approval from the motor/drive manufacturer that they were confident that we could safely operate the motor at 120 Hz – 3500rpm.
It is quite a simple process. First, you size the VFD to run the motor FLA at 240 volts. Second, you connect the motor using the 240volt connections, this requires a leap-of-faith for an electrician. Third, enter the Motors 240volt FLA into the drives parameter file. And… “Bob’s your Uncle!”… off to the races.
Sounds crazy, historically if you miss-wire a 480volt application to 240volts, the motor will be destroyed. (see Who let the smoke out?) But the VFD is a smart animal, and the combination of flux vector control, and current limiting by the VFD protects the motors inductive windings. Our motor runs at room temperature.
Again, our application only needed torque near high end speed, this Supercharging may not be what you need for the extruder.
A cold start on an Extruder probably needs full torque even at near zero speed.
DickDV
Member
Interesting thread. Both Rauland and Electric Apparatus Company make application-specific induction motors in higher frequency/rpm ranges. I routinely use them for test lab applications where speeds in excess of 5400rpm are required.
Most of these motors have been used in aircraft hydraulic pump test cells where the pump shaft speed varies from 14000 to 26500rpm. These are two pole motors so output frequencies in the 400hz range are the norm.
This certainly does not mean that such motors would be a good choice for commodity applications. For those, economics dictate everything and 60hz owns that market for good reason.
But, when needed, there are those that will build such higher speed motors. Bearings, balance, and noise generation are all being taken to the limit when approaching 400hz designs. Noise alone generally requires a water jacket, not just for cooling but for noise suppression as well. At 26000rpm or even 18000rpm, these motors resemble a siren more than a motor!
Most of these motors have been used in aircraft hydraulic pump test cells where the pump shaft speed varies from 14000 to 26500rpm. These are two pole motors so output frequencies in the 400hz range are the norm.
This certainly does not mean that such motors would be a good choice for commodity applications. For those, economics dictate everything and 60hz owns that market for good reason.
But, when needed, there are those that will build such higher speed motors. Bearings, balance, and noise generation are all being taken to the limit when approaching 400hz designs. Noise alone generally requires a water jacket, not just for cooling but for noise suppression as well. At 26000rpm or even 18000rpm, these motors resemble a siren more than a motor!
kamenges
Member
OK, I get it now. At first I thought he was talking about high pole count motors. But he isn't. He is talking about using a standard pole count motor (6-pole for example) and running it at 400Hz.
Theoretically I see your point. But as stated before, it is the other stuff that makes this a little less attractive. Most applications dont need a 12000 RPM shaft speed. That means the need for reducers. It has been my experience that a reducer that can handle a 12000 RPM motor will be VERY expensive relative to a reducer that can handle an 1800 RPM input. Not to mention the losses through the added gear elements.
Keith
Theoretically I see your point. But as stated before, it is the other stuff that makes this a little less attractive. Most applications dont need a 12000 RPM shaft speed. That means the need for reducers. It has been my experience that a reducer that can handle a 12000 RPM motor will be VERY expensive relative to a reducer that can handle an 1800 RPM input. Not to mention the losses through the added gear elements.
Keith
Strantor, here's a pdf you might be interested in perusing...
http://www.secowarwick.com/assets/D.../New-Capabilities-in-HPGQ-Vacuum-Furnaces.pdf
Yes, I do find that interesting. Thank you for the link. I seriously doubt that when they say "inverter," they're talking about a motor control VFD, but in any case I really hadn't considered the effect that phase angle power control has on power factor, and what is said in the document is true. I am surprised they were able to increase efficiency by increasing complexity; usually that doesn't work
(more info). But I go back to the tried & true bang-bang heater control... I assume the designers of this furnace system have their reasons for demanding such tight power control, but as I said before, in almost all applications (in my experience) controlling the frequency at anything greater than 1Hz is unnecessary. I usually consider 50/60Hz phase angle power control overkill. What's wrong with a PID heater controller that switches a contactor (mechanical or solid state) on for seconds or minutes at a time and off for seconds or minutes at a time? I've seen that setup work even for micro-sized benchtop injection mold machines with a mold block small enough to fit in a coffee cup, and it held temp within 1degF even as cold medium is loaded in, and hot part is removed. And with this type of control, power factor IS 1.0 (or as close to 1.0 as you can get in the real world), since it's 100% resistive load (or as close to 100% as you can get in the real world), and is more efficient than phase angle control OR an inverter since the only dissipation that isn't in the element is in the wires leading to it, and across contacts (or a sub-volt diode drop for SSR). I can't see this furnace having a thermal time constant less than a mold-in-a-coffee-cup, but like I said, I'm sure they have their reasons, and they're most likely a lot smarter than I am.The original motor is 25HP AC with a VFD. The customer is using a rubber medium that is much harder to extrude than what the machine was designed for. They have burned up 5 motors in 8 years and now the drive exploded. I replaced the drive with one rated for 40HP and now looking for a 40HP AC motor that fits the size of 25/30HP IEC frame motor. Customer has been cautioned that exceeding the designed input torque will likely cause mechanical problems down the road, and they don't care.
This is true of a motor operating across the line, but a VFD can increase voltage at low Hz to produce rated (or >rated) torque almost all the way down to zero speed.
I believe this is true as well; I think of it as being analogous to field weaking in a DC drive. I believe that the reason is that since the output voltage is limited to the input voltage, once the motor reaches base rated speed and voltage is maxed out, it becomes like a "constant HP" machine, and for any more increase in speed, there is a inversely proportional decrease in torque
That is definitely something to look into, thank you for the information. I've never heard of that. That knowledge may come in handy for other applications, but for this one I think probably not. If they've already burned many motors without "supercharging" I can only imagine what a spectacular light & smoke show it would be!
Thank you, I will keep that in mind, but in this case I'm not looking for a high speed motor.
No, you had it right the first time
. I am talking about getting same speed, but higher power, out of a smaller package, by increasing frequency and pole count. Sorry for being unclear.
Why focussing on frequency? High power motors have been using high-voltage for years because of reduction in size, weight and cost. In maritime applications they use voltages up to 11kV for diesel-electric propulsion, where power output (thus speed of the ship) is regulated by high-voltage VFD's.
Kind regards,
Kind regards,
jraef
Member
There is a MAJOR flaw in that article! They directly equivocate changes in power factor (cos theta) with changes in power (kW). There is no such correlation in the manner they describe. This is a really common tactic of scam artists in the "energy saver" industry because they are taking advantage of the fact that 99% of people reading their blurb will not have a deep enough understanding of power issues to recognize that.
Mark Snodgrass
Member
Lancie, I have to correct you on one thing. The Navy uses primarily 60hz for generation and distribution. The only places we used 400hz was for the navigational gyro, it had a motor/generator set to supply its voltage. At least as late as the 1990s when I got out.
They are indeed talking about a VFD for a motor. They talk about the common choices of using either a soft-start or a blower-motor frequency inverter in the gas cooling process. After that, they refer to the control used as an inverter, implying a frequency inverter (VFD). I admit that it is a somewhat obtuse reference. The patent mentioned in the article is more explicit.Yes, I do find that interesting. Thank you for the link. I seriously doubt that when they say "inverter," they're talking about a motor control VFD, but in any case I really hadn't considered the effect that phase angle power control has on power factor, and what is said in the document is true. I am surprised they were able to increase efficiency by increasing complexity; usually that doesn't work
Also, there are very good reasons for applying a variable amount of power in a vacuum furnace rather than going with time purportioning. First, several common heater designs for vacuum furnaces cannot have full voltage applied to them when cold. These heating systems are designed to work up to 2400 degrees F or more. The resistance/temperature coefficients of the heating elements chosen need to be taken into account. Second, heat moves around in a furnace faster the higher in temperature it is. You might have a reasonable time constant in your injection molder, but you are only operating at, what, a few hundred degrees? Not to mention that the thermocouple you are using is likely buried in a large (relative to the system) thermal mass which evens out temp swings from the heaters. I'm not necessarily calling this an easy system to control; I'm just pointing out that the experience and assumptions that work in that system don't necessarily transfer to a large vacuum furnace.
I personally wondered if they went the route they did to lower capital costs. One expensive 'dimmer switch' with contactors to switch the loads out is likely to be cheaper than two expensive 'dimmer switches'. The power factor improvements may have been a happy coincidence. I seriously doubt that they were trying to control power at greater than line frequency for better control.
They specifically say that the kWh difference between the VFD and SCR control methods are 0. In fact they claim that that fact proves they were using the same load for the tests (didn't use a lighter load to cheat.)
I agree that advertising power factor improvements is a common tactic of scam artists. However, it is only a scam because most people don't pay surcharges on household power for bad power factors. IE the scammers are selling a solution to an actual problem that their victims don't actually need. However, for large power customers (the kind that own vacuum furnaces and pull significant fractions of a megawatt from their wall socket on a regular basis) are commonly presented with a surcharge on their bill for bad power factors. It is often a multiplier that applies to their overall bill. For these people, improvements in power factor for these large loads can translate to non-negligible savings. It can often be enough to pay for the controls necessary to improve the power factor.
Brian
kamenges
Member
But that isn't what they are doing with the motor you reference in your first post. If you look at the specs page the motor is rated at 12000 RPM. That would be a 4-pole motor run at 400 Hz.
Keith
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