Someone explain "SUPERCHARGING" an AC Motor to me ??? 460 VFD / 230 MTR

[milmat1]

Using a 460 VAC Drive (VFD). and we wire the motor for 230V rather than 460V, And the VFD then must be twice the size of the motor.

Example I have a 2 HP Motor with a 460V - 5HP VFD, and motor wired for 230V. The claimed reason is that they can get the speed needed for this conveyor application.

I know that the motor speed is a function of frequency and number of poles in the motor so I'm having a hard time grasping this concept.

I realize that this so called "Supercharging" is sometimes done but I fail to understand the logic or the engineering behind it. To me I calculate the required HP / speeds etc. and choose a motor / gearbox / VFD for that application.

Can you explain what happens in the motor when they do this ?

 

[winnie]

The mechanical output power of a motor is given by the product of torque and speed. The _torque_ is limited by the physical size of the motor, but you can adjust the speed with a VFD. So for a given motor the peak output power is proportional to speed.

The voltage needed to drive the motor varies linearly with speed. To double the speed you need to double the voltage. The output voltage of the inverter is limited, so you can only increase the speed so far.

If you connect the motor for half voltage, then you essentially double the speed you can reach before you reach the inverter output voltage limit. But by connecting the motor for half voltage you are also double the motors current requirements. This double current requirement is what forces you to have an inverter with double the output current capacity.

But since you can now drive the motor to double speed (presuming the motor is rated for that), you can get double the output _power_ from the motor. You are not increasing the motor torque capability, but doubling speed, doubling power, and doubling the required inverter capacity.

-Jon

 

[junkhound]

NO VFD ?

There is a 2nd 'supercharging' concept - just for fun, don't try this at the plant!

Since torque proportional to K*I, all you need to do is keep the motor cool enough to pull twice the current without overheating.

Needed 9 HP to run a 4 GPM, 4000 psi pressure washer pump: solution = 'supercharge' (in the IC engine concept terminology) a 184T frame 5 HP motor with a big fan. Have used this for a few years at 5-6 hours at a time, no problems. The motor is covered in the shroud and 2000 CFM air forced thru and around. Motor pulls 48A at 240 Vac 60 Hz single phase, no VFD

 

[dfmischler]

Winnie - Thanks for an excellent explanation.

 

[iwire]

Using a 460 VAC Drive (VFD). and we wire the motor for 230V rather than 460V,

This seems like an NEC violation to me.

 

[iceworm]

Using a 460 VAC Drive (VFD). and we wire the motor for 230V rather than 460V, And the VFD then must be twice the size of the motor. ...

... I know that the motor speed is a function of frequency and number of poles in the motor so I'm having a hard time grasping this concept.

I fail to understand the logic or the engineering behind it. To me I calculate the required HP / speeds etc. and choose a motor / gearbox / VFD for that application.

Can you explain what happens in the motor when they do this ?

Disclaimer: I don't have any experience, so this is just based on what I have read. I hear poeple talking but I have not seen an application.

Following on jon's post:

The trick is to keep the volts/hertz ratio the same all the way up.

The motor speed is porportional to the frequency. So if one wants a 7200 rpm (7000 with slip) motor, one must use a 2pole at 120Hz. To keep the V/F ratio, the motor is wired for 230V, and the drive is set to ramp to 230V at 60 hz and to 120Hz at 460V.
Drawing on what motor/VFD experience I do have:
At 7000rpm the motor cooling fan is going to sound like a siren and could well be drawing 8X the power it would draw at 3500rpm. I suspect one must trim the cooling fan down and I also suspect that means the motor will no longer cool at 3500rpm.

As for the current, the heat rejection is porportional to I^2. I don't think the motor will operate at any more than nameplate current.

One other issue is bearing life. I worked on a few high speed pump applications. It is imperative to carefully balance the impeller/shaft and the motor stator/shaft as well. Just the least vibration and the motors bearings will not last.

The physics is sound. But I have not seen any applications -- Yet

ice

This seems like an NEC violation to me.

 

[Aleman]

Can you explain what happens in the motor when they do this ?

Probably gets really hot since it's not running to design and probably will die at some point.

This sounds like it would be a good system to weed out the weaker motors, like Darwinism of the motor world :lol:

I have never heard of this, is it legit?

 

[Jraef]

Perfectly legit, done all the time. The purpose is for applications where you need a really wide speed range adjustment but don't want to do it mechanically. So for instance if I want a conveyor to run at speeds between 10Hz and 120Hz but need full torque at both ends without basing the motor size on the upper speed range and expecting good performance at the low end, that's the only way to do it.

Where it gets used most though is on high reduction gearbox applications. For example I once did a system for ice makers where there was a 1600:1 gearbox turning a drum that was plumbed with refrigerant into a tank of water, then scraping the ice that formed on the drum. At low speeds for production, it took upwards of 4 minutes to get the drum to make one turn, so the user wanted to over speed the motor for setup and testing, but still needed to maintain the finite control at lower speeds because that's what determined the nature of the ice flakes. So we set up a 1/2HP 230/460V motor as 230V, then fed it with a 480V drive so that above 60Hz, we could maintain the proper V/Hz ratio and get full torque from the motor. In essence, the motor became 1HP at 120Hz, but it is the torque we were after, the HP was inconsequential.

 

[Aleman]

I'll be damned. Never have seen this done or even heard of it. Mostly we use Allen Bradley PF40's and 70's. These
have a ton of settings we never use. Can the voltage to frequency ratio be adjusted on these? I don't think something
like this would be of use to me in the current job but it is definitely interesting.

 

[milmat1]

WINNIE,
Thats the best simple explanation I have seen so far. THANK YOU !!

 

[Jraef]

I'll be damned. Never have seen this done or even heard of it. Mostly we use Allen Bradley PF40's and 70's. These
have a ton of settings we never use. Can the voltage to frequency ratio be adjusted on these? I don't think something
like this would be of use to me in the current job but it is definitely interesting.

Yes, it can be done with the PF40s and 70s, in fact pretty much all drives. You don't really need to program anything special as far as the V/Hz ratio (although you can), all you do it to tell the drive it is a 230V 60Hz motor, which establishes the V/Hz ratio, then tell it the maximum speed is 120Hz, it will do the rest.

By the way, never do this without consulting the motor mfr as to the motors ability to run over design speed, they are the ONLY ones who can tell you if it is possible. Most "inverter duty" motors will be able to but there is no official definition of that term, so do not assume.