AC 3 phase electric motor 4

[atvhead]

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

 

[CJCPE]

With AC motors, it is the frequency that determines the speed. Synchronous RPM = 120 x frequency / number of poles in the motor. A 1750 RPM motor is a 4-pole motor operating at 60 Hz with an 1800 RPM synchronous speed and 50 RPM slip. If you connect a motor to a voltage other than what is on the nameplate, it will not operate properly.

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.

 

[atvhead]

yes, the motor is not operating "properly". Obviously, many other factors change with the operation of 575volts in the 460volt motor. If it is a VFD, which i believe it is, would it be possible at all to run 575volt in the 460volt motor and expect a motor base speed increase? thanks for the quick response!

 

[eromlignod]

No. The AC input sets up a rotating magnetic field and the frequency of the input voltage decides how fast this field will rotate. Minus slip, this is the speed you will get.

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

 

[asekhar]

The motor should not be operated at 575 Volts. If the motor is designed for 460 volts, that is the voltage at which it should be run. VFD is a variable frequency drive, which will vary the frequency of the supply so that the speed of the drive can be changed, as CJCPE explained above. Hope it si clear. What is your requirement, is it that you want to run the motor at higher rpm??

Anand Sekhar

 

[atvhead]

Thanks so much everyone for all the information.

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_

 

[eromlignod]

The rpm is the only way to affect the CFM of the fan. More voltage or horsepower gets you nowhere. The fan is going to require a certain torque at a given rpm.

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

 

[atvhead]

Thanks Dan, that is a big help. i think we are going to advise the customer to let us size the motor and fan properly based on 575volts. sizing the fan and motor is no major task - but when the customer asked us some questions about this blower and told us what was on the nameplate, I couldn't believe it was a 460 running 575. i will try and get the amp reading from the customer. thanks everyone for your help, you have proven extremely valuable.

Bob
_innov8_

 

[CJCPE]

If there is a VFD between the 575V supply and the 480V motor it could be supplying only 460V to the motor at 60Hz. If the output of the VFD goes above 60Hz and the motor is lightly loaded, the motor could operate OK either if the voltage is held constant as the frequency increases above 60Hz or if the voltage increases in proportion to the frequency. The motor insulation is probably rated for 600V, but it might have difficulty with high voltage spikes that occur with VFDs.

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).

 

[atvhead]

thank you so much for all the input. i feel much better when dealing with AC constant speed motors. Has anyone ever heard of Cowern's Papers?

Bob
_innov8_

 

[CJCPE]

Cowern's Papers

http://www.motorsanddrives.com/cowern/contents.html

 

[atvhead]

thank you so much!

Bob
_innov8_

 

[LionelHutz]

The motor might be running with slightly less slip but it would be a small change in rpm (10 or 20 rpm maybe).

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.

 

[HCBFlash]

Strictly speaking, you CAN allow the load to pull down the speed of a motor being supplied by a lower voltage than rated more easily, but your motor will quickly burn up.

...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!

 

[itsmoked]

Most VFDs don't allow you to step down the voltage do they? How would they do that without an active front-end?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[DickDV]

itsmoked, you have to be careful but you can step down voltage in a VFD. For example, you can power a VFD with 460V and set the motor data to 230V 60hz and the max output voltage to 230V. The drive will adjust its V/Hz ratio so it reached the equivalent of 230V at 60Hz and then limits to 230V. The motor appears to be running as a 230V 60hz 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.

 

[waross]

I have heard of a few compressor manufacturers Running over voltage and over frequency to motors to increase horsepower.
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

 

[jraef]

So to continue DickDVs thought into the OP's issue, applying 575V to a VFD designed for 480V will do two more things not already brought up:
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.

 

[waross]

Hi jraef;
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.

he has been running 575volts to it since 2001.

if the motor is lighty loaded, could it be able to take 575volts as long as it is not pulling in high amps?

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

 

[itsmoked]

An interesting graduate subject on VFDs/motors going on here.

Thanks DickDV et. al.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com