3 phase motor wired for low voltage, controlled by VFD at 480vac

[drummin89]

At the plant I work at we have a piece of equipment that came from the manufacture with a 50HP 3 phase motor wired for low voltage (I need to verify this, not sure on the voltage) that is controlled by a VFD that is supplied with 480vac. The motor nameplate parameters in the VFD list the motor as being 45kW, 84 FLA, 460V, 104Hz, 3016 rpm. I am waiting for the machine to shut down so I can grab the actual nameplate data off of the motor.

I really didn't understand a whole lot about this and how it is done. I ran across the following post by Jraef in an older thread that helped me to understand things a little better. http://forums.mikeholt.com/showthread.php?t=115549&page=2&p=1074268#post1074268 Post 18 if it doesn't take you to his post. The manufacture is based out of Germany (equipment for chocolate refining) and the tech that flew over here tried explaining this to me a few months ago but he/what is was explaining was difficult to understand.

What Jraef described matches the setup I believe I am dealing with. The manufacture uses a motor that is undersized for the application, not 100% sure why they do it this way, I know a larger motor and gearbox probably wouldn't fit in the machine so that might be a reason. I believe they also do this to gain a higher speed. We constantly run this motor between 85 and 104Hz

Anyways my question is, what affect does this have on the current that the motor pulls? How would you size the VFD for this application? The VFD that is used is I believe rated for 150-200HP.

We have 1000+ motors and a couple hundred VFDs at my plant but this is the only one setup like this.

Thank you for your time!

 

[rlundsrud]

It sounds like the motor is wired for 460 volts. Why do you believe it is wired for 240 volts?

 

[drummin89]

Correction: 60HP, not 50.

Something out of the ordinary was done here, or at least that is what we were told. We just recently had a company switch over the control system on this machine from siemens to allen bradley and they had spec'd out a 60HP drive for this motor because they were going off of the nameplate data. first startup it smoked the drive. Manufacture said we need a drive rated for 125HP.

I attached the nameplate and a picture of the terminations at the motor. Looks to me like its wired Delta?

 

[Besoeker]

At the plant I work at we have a piece of equipment that came from the manufacture with a 50HP 3 phase motor wired for low voltage (I need to verify this, not sure on the voltage) that is controlled by a VFD that is supplied with 480vac. The motor nameplate parameters in the VFD list the motor as being 45kW, 84 FLA, 460V, 104Hz, 3016 rpm. I am waiting for the machine to shut down so I can grab the actual nameplate data off of the motor.

I really didn't understand a whole lot about this and how it is done. I ran across the following post by Jraef in an older thread that helped me to understand things a little better. http://forums.mikeholt.com/showthread.php?t=115549&page=2&p=1074268#post1074268 Post 18 if it doesn't take you to his post. The manufacture is based out of Germany (equipment for chocolate refining) and the tech that flew over here tried explaining this to me a few months ago but he/what is was explaining was difficult to understand.

What Jraef described matches the setup I believe I am dealing with. The manufacture uses a motor that is undersized for the application, not 100% sure why they do it this way, I know a larger motor and gearbox probably wouldn't fit in the machine so that might be a reason. I believe they also do this to gain a higher speed. We constantly run this motor between 85 and 104Hz

Anyways my question is, what affect does this have on the current that the motor pulls? How would you size the VFD for this application? The VFD that is used is I believe rated for 150-200HP.

We have 1000+ motors and a couple hundred VFDs at my plant but this is the only one setup like this.

Thank you for your time!

Sounds like a 50Hz base set up.
But I'm not sure what you are asking.

 

[Ingenieur]

60 Hz
can be wired
266 delta 121 A
460 wye as in 266 - n per winding gives a total of 460 ph-ph 70 A
can't wire 460 delta...smoke

45 kw ~ 60 hp

 

[drummin89]

Thanks guys. This has me really confused. I need the manufacture to explain this to me again.

So is the post by Jraef not possible at all or just not for my application? http://forums.mikeholt.com/showthread.php?t=115549&page=2&p=1074268#post1074268

I tracked down the motor that was originally used. Something happened a few years ago and it had to be replaced. It was sent out to be rebuilt. This data plate is a little different. It shows for 230v its wired as double delta. The manufacture had put a label on the motor connection box that said it was wired parallel.

 

[GeorgeB]

Your 3rd motor plate photo is quite different from the 1st, and in not in agreement with the terminals.

You have a 460Y266delta motor at 60 Hz. We usually think of the 480 supply which corresponds to 277 phase-to-neutral ... which is of no consequence, just confusing.

It is wired delta. That is 4.43 volts/hz. At 460V on the delta wiring, the frequency would be 104 Hz maintaining that relationship, for a synchronous speed of about 3120 rpm.

Most likely, SEW was told to use a 4 pole motor balanced for operation at 3600 rpm, not difficult as many 2 pole motors are probably made in that frame.

As to why ??? you'd have to ask the engineers who designed the machine. Perhaps gearbox size and design matches another product in their line and this needed to run faster ... but it is correct as operated. The only issue MIGHT be cooling.

 

[drummin89]

Thank you.

What happens to the current in a situation like this? Is the VFD sized off the 266 delta ratings?

 

[Jraef]

When a motor needs to run at higher than base speeds, the problem becomes that once you get to the full motor design voltage and frequency, further increasing the frequency causes a loss of torque. So the trick of starting off with a lower motor base voltage than the available line voltage allows the speed to increase above base frequency while maintaining motor design torque. So why do people need to run at higher than base motor speed? That’s a mechanical engineering decision, this is just the electrical accommodation of it.

Current follows torque, so because the purpose of this exercise is to maintain the DESIGN torque of the motor, current remains the same as well. Now, that is the current at the newly reconfigured motor voltage. So if the motor is connected as 266V, that’s the current at 266V, not the 460V nameplate current. That means then that everything else in that circuit, OCPD, conductors etc., must be based on that current value as well. A US based NEMA motor will not state that value because it’s not a connection we normally use, but the current will be 1.732x the 460V current.

 

[Ingenieur]

Thank you.

What happens to the current in a situation like this? Is the VFD sized off the 266 delta ratings?

wire the motor delta
get a 240 drive rated at >140 A

you could wire it wye
480 drive > 70 A output

I prefer delta config

 

[GeorgeB]

you could wire it wye
480 drive > 70 A output

but then he would need something over 800V line (832 by my calculations based on the 480V) to maintain full torque at 104 Hz.

 

[Ingenieur]

but then he would need something over 800V line (832 by my calculations based on the 480V) to maintain full torque at 104 Hz.

nameplate says
266 delta
or
460 wye
either will produce rated output

 

[drummin89]

At 460v and 60Hz it wouldn't be at the rpm the machine requires. If we increase the Hz to get the rpm that is needed it would probably stall.

The machine is a 2 roll refiner. 2 large rolls are pressed together with up to 30 bar of pressure. The back roll is powered by a 40hp motor that is wired for 460v on a soft start. It runs at a constant speed. The front roll is powered by the 60HP motor in question which is supposedly wired Delta 266v to a 200HP VFD that can supply up to 460v at 104Hz. It's ran between 85 and 104Hz.

I am still questioning how this is setup. I checked on Friday and at 100Hz it was only drawing 65amps. Maybe it is correct. Since the back roll is powered and essentially driving the front roll, maybe that's helping to reduce the current draw.

 

[drummin89]

Forgot to add the front roll is supposed to spin at the same or greater speed than the back roll in order to get a shearing action.

 

[winnie]

There has already been some very good info posted.

The motor is physically capable of being run at higher speed than you would get with a 60Hz supply. Not all motors are, and doing this increases bearing wear.

By connecting the motor for low voltage you _increase_ the current required for any given torque, but you _reduce_ the voltage required for any given operating frequency. The net result is that for a given torque and speed the total power is the same, but at lower voltage (and higher current).

This lets a standard 480V inverter to drive a standard motor at greater than 60Hz while maintaining full magnetic flux and full available torque.

When you connect the motor in this fashion you essentially halve the voltage and double the required current. Since inverters are sized by their output current, when you connect the motor this way you essentially double the required horsepower rating of the inverter. So you would be running a nominal 50Hp motor with a nominal 100Hp inverter...but because you are now capable of doubling the speed of the motor your nominal 50Hp motor can produce almost 100Hp of mechanical output.

You never get something for nothing; the motor needs to be capable of operating at this higher speed, and bearing wear will be substantially increased.

-Jon

 

[Ingenieur]

the motor is rated ~ 1760 rpm
if you put 460 across a coil rated 266 it will be destroyed

delta 266 line i is 121
phase is 70
z ~ 266/70 = 3.8 ohm

wye 460 line is 70 = phase
delta and wye ph/coil i are both 70 a
460/(0.866(3.8 + 3.8)) = 70 a

if you put 460 delta or per coil
460/3.8 = 121 a
the coil will likely be damaged if sustained

z is mostly inductive
the above is at rated 60 hz
assume 100 hz z ~doubles

460/(100/60 x 3.8) = 72 a
line 125 a

so as long as run at increased freq the current will be reduced below the coil rating

 

[kwired]

Keeping motor windings at current level they are designed for is one of most important things you must have happen regardless if you run it at 1/4 rated speed or twice rated speed. Voltage, frequency and total power output will definitey be different at each of those extremes, but high current in windings lets the smoke out of them pretty quickly.

For what reason they decided to overdrive this motor to get the speed they wanted was a design decision. How common is a high speed motor over about 15 HP? I can't recall ever seeing any, maybe they are rare enough it was decided to overdrive this one, though simple mechanical speed increaser/reducer would probably cost less.

 

[Ingenieur]

rated operating points
60 hz 1760 rpm

delta
266 x 121 x sqrt3 = 55.8 kva
60 hp = 44.8 kw
eff x pf = 0.80
T = 180 lb ft

wye
460 x 70 x sqrt3 = 55.8 kva
60 hp
T = 180 lb ft

assume running 100 hz 2930 rpm 460 v delta
from my previous post ~ 125 A
kva ~ 99.4 kva
~ 107 hp
~ 192 lb ft
these are likely 5-10% lower
I assumed a pure inductive load
but with an x/r range of 6-8 close enough to illustrate a point

rated vs 100 hz 460 delta
more hp (hence the required 125 hp drive on a 60 hp motor)
same torque
same coil current
100 hp performance from a 60 hp motor
a 60 hp /3600 rpm T = 90 lb ft, half as much
clever

 

[Besoeker]

For what reason they decided to overdrive this motor to get the speed they wanted was a design decision. How common is a high speed motor over about 15 HP? I can't recall ever seeing any, maybe they are rare enough it was decided to overdrive this one, though simple mechanical speed increaser/reducer would probably cost less.

We did quite a few in the 56kW to 200kW typically capable of 20,000 to 30,000 rpm for machine tool applications but I agree with you - these are special purpose and relatively rare applications in the great scheme of things.

I also wonder why a gearbox/v-belt/morse pulley arrangement wasn't used. Space maybe? Perhaps the OP knows the reasoning behind that.

 

[Ingenieur]

even if a 60 hp / 3600 rpm was slowed down to ~3000 and P kept constant
T is only 105 lb ft <<< 180 developed from over driving the 60/1800