AC 3 phase electric motor
AC 3 phase electric motor
(OP)
Hello all, I am new here and this is my first post. I did search through the postings as best I could to find an answer to my question, but no luck. My question is: in a constant speed AC 3 phase, is the voltage & motor RPM proportional? For example: if I have a 460v motor operating at 1750RPM if the voltage is increased to 575v, will the motor speed now be 2188RPM? Thanks everyone.
confused Mech. Eng
Bob
confused Mech. Eng
Bob





RE: AC 3 phase electric motor
A motor can be connected to a variable frequency drive (VFD) that will change both the frequency and voltage in a way that allows speed control.
RE: AC 3 phase electric motor
RE: AC 3 phase electric motor
Think of it like a rotating electromagnet that's pulling the rotor along. If you increase the voltage, you just make the electromagnet pull harder, you don't change its speed. You also force more current through the windings which would probably overheat the coil.
Don
Kansas City
RE: AC 3 phase electric motor
Anand Sekhar
RE: AC 3 phase electric motor
What happened here is the customer has a high pressure blower/fan (FRP) with a 5hp motor rated for 460volt (motor speed is 1750RPM and blower speed is 2256RPM). This will supply a CFM of 960 at 8". he has been running 575volts to it since 2001. what i need to know is the new fan speed and CFM (at what pressure) that is produced by running it at 575 volts. what i gather is i should expect the same CFM? if the motor is lighty loaded, could it be able to take 575volts as long as it is not pulling in high amps?
Bob
_innov8_
RE: AC 3 phase electric motor
Apparently, you're getting by with a voltage that's 25% too high because your motor is oversized for the application and isn't pulling enough current to trip your overloads or burn up the motor...not a good idea since the motor insulation probably isn't rated for voltages that high.
If you want to run more CFM, I'd suggest driving the fan with a belt and step up the rpm with pulleys. I would also suggest using the proper motor voltage! Do you have an Amprobe to measure the current you're pulling now?
Don
Kansas City
RE: AC 3 phase electric motor
Bob
_innov8_
RE: AC 3 phase electric motor
With constant voltage operation above 60Hz, the motor capability would be constant Hp with maximum torque decreasing as speed increases. With voltage increasing in proportion to frequency, the capability would be constant torque with Hp increasing in proportion to speed. A 575V VFD could provide constant torque up to 75Hz, 2200RPM. Note that the blower will need torque proportional to speed squared (156% at 125% speed) and horsepower proportional to speed cubed (195% at 125% speed).
RE: AC 3 phase electric motor
Bob
_innov8_
RE: AC 3 phase electric motor
h
RE: AC 3 phase electric motor
Bob
_innov8_
RE: AC 3 phase electric motor
If you need to run over speed you can install a 575V VFD on the 460V motor. Program the VFD to a 575V, 75hz motor. It's all about keeping the V/hz ratio constant. 460V/60hz = 575V/75hz
In this manner, you can theoretically run up to 75hz and maintain rated torque out of the motor. It's close but not quite where they want which is about 76.6hz (75hz is 50rpm short of the fan rated speed). You need to calculate if the fan needs less than 15lb-ft of torque to spin it at ~2200rpm.
At any rate, sticking a small VFD on the existing motor is probably cheaper than changing the motor from a direct coupled 460V motor to a belt drive 575V motor if it will work.
We actually use a few unloaded 460V motors to test 575V equipment. They run close to their rated current with no load so I would expect your fan can't be putting very much load on that motor for it to hvae survived.
RE: AC 3 phase electric motor
...but that's probably more obvious than helpful.
There's lots of helpful info at sights like Baldor, Bodine, Joliet Equipment, and Wikipedia isn't bad at all for the basics. One of our regulars here has a good website too (flamin systems?)
Directly addressing your initial question; no, not speed/voltage proportional but line frequency/speed proportional.
flash
Me wrong? I'm just fine-tuning my sarcasm!
RE: AC 3 phase electric motor
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: AC 3 phase electric motor
The catch in this is that the VFD bus voltage is determined by the input voltage so, with a 460V input, the DC bus will be around 660V. That's the height of each and every pulse going to the motor even tho the output equivalent voltage is limited to 230V. The motor runs as a 230V motor but the insulation is being battered by 660V pulses instead of 330V pulses (that would be the DC bus voltage if the drive input was actually 230VAC).
If you have a dual voltage motor (ie, 230/460V) the insulation will take the extra stress but a straight 230V motor will not live long running that way. You might be able to protect the motor with a dv/dt filter or sine filter but without those, the motor will not survive long.
RE: AC 3 phase electric motor
A 60HP, 460 volt motor may be configured as follow:
Supply: 575 0r 600 Volts.
Drive: 575V or 600V VFD.
Voltage to the motor: 575V.
Frequency to the motor: 75Hz.
Motor speed approx 2200RPM
Motor torque: About the same.
Motor Hp: 75Hp
respectfully
RE: AC 3 phase electric motor
1) It will likely shorten the life of the VFD components because they will be operating outside of their tested range. With a 575V input, the DC bus voltage is now 825V instead of 660V. That's not to say he may not be lucky and have been provided with transistors capable of the higher voltage, but that would be just dumb luck. Higher rated transistors are a lot more expensive, most manufactures would not go to that expense.
2) The same issue about the motor insulation will also be true. Even though the VFD may be limiting the RMS voltage to 460V, the pulses making up that PWM output will be 850V. Standing wave pulses can be as high as 3x the peak voltage, so instead of 1800V from the 480V supplied drive, they will be almost 2500V spikes. Most of the new "spike resistant" magnet wires available to motor manufacturers for 460V motors is now rated for 2000V; plenty for the 480V supplied drive, but now too low for the 575V supplied drive. The likelihood of a 460V "inverter duty" (which typically means the spike resistant wire) motor being wound with 2500V wire intended for 575V motors is low.
RE: AC 3 phase electric motor
Do I understand correctly;
It is safe to push a 230:460 volt motor to 120 Hz. 460 volts, but not safe to push a 460 volt motor to 75 Hz. 575 volts?
The 230:460 Volt motor will be wired with 2000V wire which is not adequate for PWM 575V.
I am not doubting you, I am asking.
A couple of points.
It may be able to take 480V at 75 Hz. if it is lightly loaded.
Possibly the drive parameters were changed, or the voltage was boosted at some time to increase the CFM.
Possibly we will see compressor manufacturers ordering in bulk, 480 volt motors wound with 2500V wire so as to be able to boost the Hp.
Respectfully
RE: AC 3 phase electric motor
Thanks DickDV et. al.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: AC 3 phase electric motor
I also know one manufacturer that just rivets their VFD rated tag to a standard motor and charges more for it. If you have one of their standard motors and point fingers at them for failing on a VFD they will even come back and tell you it's the same motor as a VFD rated one and therefore the motor construction can't be the problem.
So, I would expect the 460V motor to be as likely to survive on a 600V VFD as a 575V motor will.
But, I think poorly wound 575V motors failing when controlled by VFD's will help keep some motor shops in business (in 600VAC use areas). I know of many cases where it was cheaper in the long run to just pay for a good rewind vs getting another free replacement from the manufacturer.
RE: AC 3 phase electric motor
Bob
_innov8_
RE: AC 3 phase electric motor
(that last part was meant as a gross understatement joke)
I think I know of whom you speak, but if I'm not mistaken, their claim is that they have a unique winding method that increases the Corona Inception Voltage capabilities of their standard design motors. I read their paper, it failed to convince me.
RE: AC 3 phase electric motor
it was more of a suggestion than a direct statement:
respectfully
RE: AC 3 phase electric motor
There is no VFD present on this motor.
Bob
_innov8_
RE: AC 3 phase electric motor
No, they just claim to use special wire which apparently also means they can just mush wind the coils and also not worry about using any extra coil-coil or slot insulation. Oh well, they're not the only manufacturer doing a really poor job of winding motors.
RE: AC 3 phase electric motor
As you can see, VFDs are the present black hole in the industrial AC motor field. If you keep your distance you won't get sucked in. (Oh boy, I feel another thread comin' on!)
As far as my own previous post goes, nothing against Kieth's website, but it's marke's website, LMPhotonics.com that's more educational as far as motors go.
flash
Me wrong? I'm just fine-tuning my sarcasm!
RE: AC 3 phase electric motor
HCBFlash; That's just my tag line so people can contact me if they desire. That way they can send me poison snakes in boxes, etc.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com