Motor voltage 380-480v

[Indy]

We have a motor that has the following spec
3kw
380-480v
4.6A
My question is will the motor be rated 3kw 4.6A at 415v if we use it with a VFD?


Thanks

 

[waross]

The simple answer is yes.
4.6 A x 415 V x 1.73 = 3303 VA
3 kW x 746 W/HP = 2238 VA
2238 VA / 3303 VA = 68%
The long answer is:
If the combined efficiency and power factor is above 68%,
AND if the motor is running at rated frequency and speed the motor will be capable of 3kW.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iop95]

3kW is valid at 480V / 4.6A.
3kW at 415V/4.6A may be true for 95% efficiency and 0.95 cos fi.

 

[ScottyUK]

Is the 380 / 480V rating based on 50Hz and 60Hz operation respectively? That is an unusually broad voltage range for single-frequency operation.

 

[Sazirul]

Can you please explain this equation @waross
4.6 A x 415 V x 1.73 = 3303 VA
3 kW x 746 W/HP = 2238 VA
2238 VA / 3303 VA = 68%
Thanks!

https://www.anelectricalengineer.com

 

[waross]

Sorry
4.6 A x 415 V x 1.73 = 3303 VA
3 kW / 3303 VA = 90.8%
Efficiency x PF = 90.8%

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

If the motor stator is provided with 6 separated windings [9 terminals] 380 to 480[460 V] it is possible.
a single one winding rated current is 4.6 A [connected series] and 480/2/sqrt(3)=138.6 V. In parallel for 380 V the motor could draw double current but the voltage will be 380/sqrt(3)=220 V per bobbin.
A bobbin is then rated for 220 V and 4.6 A.
At 415 V you have to connect series the bobbins since in parallel the voltage per bobbin will be 10% more than rated.
The current will decrease only by 13.6% but the torque 25.3% .Then the slip has to rise proportionally If at rated voltage was 0.5% now will be 0.5/.253=~2%.
As the maximum torque slip it is about 6*srat=3% it will be still o.k.

 

[7anoter4]

Sorry. Wrong numbers. On an actual motor of 3 kW 460 V Tm/tr=2.9 , rated slip about 3% and the maximum torque slip was 16.8%
At 415 V the slip will be 3%/.253=11.86% and the sm=48.6%
The old rpm was n460V=nsyn*(1-3%) and the new n415V=nsyn*(1-11.86%).
The power required by the motor-for the same rated torque- will be 3*(1-11.86%)/(1-3%)=2.73 kW

 

[7anoter4]

However, something is still wrong here. This calculation does not match the experience, yet.
Let's start from the induction motor diagram.
The power transferred from stator to rotor it is:
Pi=m2*R2*I2^2/s where 2 it means "rotor" and m2=rotor number of phases.
s=(nsyn-n)/nsyn the slip where:
nsyn=synchronous rpm ; n=the actual rotor rpm.
I2=Vstator/SQRT(R1+R2/s)^2+X2^2) neglecting Io and X1[stator leakage reactance]. For simplification's sake I'll take the R1=0 as well- where 1 it means “stator".
The torque it is the power divided by velocity in rad/sec.
T=m2*R2*V^2/(R1+R2/s)^2+X2^2)/s/2/pi()/nsyn/(1-s)
If K=m2*R2/(2*pi()*nsyn) and R2^2=A X2^2=B Neglecting R1
T=K*V^2*s/(A+B*s^2-A*s-B*s^3)
Since s is very low we can say s^3=s^2=0
T=K*V^2*s/A(1-s)
For different V[Vi initial and Vf after change]:
Ti/Tf=(Vi/Vf)^2*si/sf/(1-si)*(1-sf)
We may consider (1-s1)/(1-s2)=1 and for a same required torque in different supply voltages Ti/Tf=1 then:
sf=(Vi/Vf)^2*si
s(415V)=(480/415)^2*3%=4%!!!
That means it is not 11.86% but only 4%. This result it is more possible.

 

[waross]

One possibility:
3 kW x 380V/480V = 3.8 kW The motor is designed as a 3.8 kW motor at 480 Volts. It may be safely run at 480 Volts saturation.
On 380 Volts there is enough iron to develop 3 kW without overheating.

Another possibility:

Is the 380 / 480V rating based on 50Hz and 60Hz operation respectively? That is an unusually broad voltage range for single-frequency operation.

Could this be a harmonization issue?
It is common in North America to run 460 Volt rated motors on 480 Volt supplies.
480 Volts three phase is a common voltage. 240 Volt three phase is not a common voltage. 120/208 Volts is a common supply voltage.
At one time 230-460 Volt rated motors were run on 208 volts with slightly reduced capacity. This is still the case but in addition triple voltage motors are becomming common.
A common rating is 208-230/460 Volts. These motors are suitable for use in shopping centers, malls and large apartment buildings where 120/208 Volts is a common supply voltage and for use in industrial applications where 480 Volts is a common supply.
Refrigeration and air conditioning equipment is generally rated for 208-230/460 volts.

A picture of the nameplate will be helpful.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Is this 50Hz vs 60Hz?

In North America. a motor running on a 480V power system is typically nameplated as 460V and 380V/460V =~ 50Hz/60Hz.

Even if it is 480V nameplated, I say that 380V/480V =~ 50Hz/60Hz is still close enough to be possible.

So, did the frequency of each voltage get left out of the question?