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Motor voltage 380-480v

Motor voltage 380-480v

(OP)
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

RE: Motor voltage 380-480v

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

RE: Motor voltage 380-480v

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

RE: Motor voltage 380-480v

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

RE: Motor voltage 380-480v

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

RE: Motor voltage 380-480v

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

RE: Motor voltage 380-480v

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.

RE: Motor voltage 380-480v

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[blush]

RE: Motor voltage 380-480v

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.

RE: Motor voltage 380-480v

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:

Quote (ScottyUK)

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

RE: Motor voltage 380-480v

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?

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