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Effects of 50Hz on a 60Hz motors. 2
- Thread starter mikepz
- Start date
-
1
- #3
mikepz (Electrical) 3-phases motors ranging from 220V 60Hz to 480V 60Hz.
The flux that the stator winding creates is proportional to the line Voltage and inversely proportional to the number of turns and frequency ( flux/p= 4.44 Vn/Kp*Kd*N*F).
Leaving the actual winding, the voltage must be reduced in proportion to the frequency change.
For 480V- 60 HZ --? 440*50/60 = 400 Volts at 50 HZ ( hopefully it will work at 380 Volts with 5% flux reduction test it). You try to keep the flux to avoid saturation of the magnetic circuit. If saturation is reached, your no load current (magnetizing mainly) will be too high, the PF decreases and the motor will overheat or eventually burn ( even under no load).
The voltage reduction keeps the flux then the torque produced is close to constant, but the HP output will be reduced due to the speed reduction.
T=5252*HP/rpm or HP= T*rpm/5252
For instance: 10 HP, 480 V, 60 HZ, 1750 rpm. IT CAN WORK AT 400 Volts, 50 HZ.
T=5252*10/1750 =30 Lb*Ft
New HP at 400 V, 50 HZ (1458 rpm)
HP=30*1458/5252 = 8.33 HP
The reduction of speed will reduce the cooling air flow, but the reduction of HP will result in lower losses and the temperature rise is almost the same as the 60 HZ, 440 Volts temperature operation.
If the operating voltage at 50 HZ is different, the stator must be re-winded keeping the flux densities of the original 60HZ design and the power de-rated proportional.
The flux that the stator winding creates is proportional to the line Voltage and inversely proportional to the number of turns and frequency ( flux/p= 4.44 Vn/Kp*Kd*N*F).
Leaving the actual winding, the voltage must be reduced in proportion to the frequency change.
For 480V- 60 HZ --? 440*50/60 = 400 Volts at 50 HZ ( hopefully it will work at 380 Volts with 5% flux reduction test it). You try to keep the flux to avoid saturation of the magnetic circuit. If saturation is reached, your no load current (magnetizing mainly) will be too high, the PF decreases and the motor will overheat or eventually burn ( even under no load).
The voltage reduction keeps the flux then the torque produced is close to constant, but the HP output will be reduced due to the speed reduction.
T=5252*HP/rpm or HP= T*rpm/5252
For instance: 10 HP, 480 V, 60 HZ, 1750 rpm. IT CAN WORK AT 400 Volts, 50 HZ.
T=5252*10/1750 =30 Lb*Ft
New HP at 400 V, 50 HZ (1458 rpm)
HP=30*1458/5252 = 8.33 HP
The reduction of speed will reduce the cooling air flow, but the reduction of HP will result in lower losses and the temperature rise is almost the same as the 60 HZ, 440 Volts temperature operation.
If the operating voltage at 50 HZ is different, the stator must be re-winded keeping the flux densities of the original 60HZ design and the power de-rated proportional.
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1
- #4
I disagree with aolalde's view that motor at 50 HZ and the same voltage will burn out. Sure, the flux density will increase and whether it will hit saturation depends on the original flux density at 60 Hz. Even if it hits the saturation, the possibility of winding burn out is remote.
I remember some post (by jraef or skosgurra, sorry I do not recall the thread no) where he had actually tested some 60 Hz motor at 50 Hz with the same volatge and found no abnormal temp rise.
I remember some post (by jraef or skosgurra, sorry I do not recall the thread no) where he had actually tested some 60 Hz motor at 50 Hz with the same volatge and found no abnormal temp rise.
jbartos
Electrical
- Jan 15, 2000
- 6,440
Comment:
When you lower the frequency to 50 Hz and the voltage stays the same, X Vac, you change the V/f ratio. If the motor operates at X Vac, 60 Hz, the V/f ratio will be X/60Hz. At 50Hz, the V/f ratio is X/50 in Vac/Hz. This increase in V/f ratio can cause more current to flow to maintain the same torque.
Output power reduces by 5/6 ths as shaft power is the product of torque (similar at both frequencies) and speed. If the speed reduces by 5/6 ths so does the shaft power.
When you lower the frequency to 50 Hz and the voltage stays the same, X Vac, you change the V/f ratio. If the motor operates at X Vac, 60 Hz, the V/f ratio will be X/60Hz. At 50Hz, the V/f ratio is X/50 in Vac/Hz. This increase in V/f ratio can cause more current to flow to maintain the same torque.
Output power reduces by 5/6 ths as shaft power is the product of torque (similar at both frequencies) and speed. If the speed reduces by 5/6 ths so does the shaft power.
I concur with aolalde response.
- V/f ratio should remain the same while changing frequency
- Rated HP should be decreased.
The thread edison123 is refering was posted by me and i gave practical values myself at the end. Unfortunately my memory is the same as edison123 (no offence edison123) and so i also cannot remember the thread. The thread concerned a motor rated for 400V, 50 Hz and running at 480V, 60 Hz.
- V/f ratio should remain the same while changing frequency
- Rated HP should be decreased.
The thread edison123 is refering was posted by me and i gave practical values myself at the end. Unfortunately my memory is the same as edison123 (no offence edison123) and so i also cannot remember the thread. The thread concerned a motor rated for 400V, 50 Hz and running at 480V, 60 Hz.
Thx kassad for jogging mty memeory (which is poor btw). I clicked on member profile under your name and located the thread you started. But, unfortunately that was not the thread I was referring.
My reffered thread had posts about actual tests done by jraef or skosgurra (and No, I don't intend to go thru their profiles and start searching for their vast ocean of their much appreciated posts).
Anyway, I stick by my earlier post about winding burnout. While the flux density will go up at 50 HZ and at the original voltage, the eddy current loss (proportional to squares of both frequency and flux density) will not be drastically affected due to fall in frequency (60 to 50 Hz).
My reffered thread had posts about actual tests done by jraef or skosgurra (and No, I don't intend to go thru their profiles and start searching for their vast ocean of their much appreciated posts).
Anyway, I stick by my earlier post about winding burnout. While the flux density will go up at 50 HZ and at the original voltage, the eddy current loss (proportional to squares of both frequency and flux density) will not be drastically affected due to fall in frequency (60 to 50 Hz).
Edison123
Your statement “Even if it hits the saturation, the possibility of winding burn out is remote.” is too daring. Have you seen what happens to a motor connected for 230 Volts energized with 460 Volts (hitting saturation)? It will explode!
Some times when a 460V, 60 HZ motor is used in a country with 50 HZ the voltage has lower levels like 380V, 400V or 415V. If the motor moves the very same load the speed reduction will release some power demand from the motor. That particular condition could work.
I don’t think you are recommending to use those 60 HZ motors as equivalent in HP to operate on 50 HZ, 460 Volts, do you?.
Your statement “Even if it hits the saturation, the possibility of winding burn out is remote.” is too daring. Have you seen what happens to a motor connected for 230 Volts energized with 460 Volts (hitting saturation)? It will explode!
Some times when a 460V, 60 HZ motor is used in a country with 50 HZ the voltage has lower levels like 380V, 400V or 415V. If the motor moves the very same load the speed reduction will release some power demand from the motor. That particular condition could work.
I don’t think you are recommending to use those 60 HZ motors as equivalent in HP to operate on 50 HZ, 460 Volts, do you?.
aolalde,
I am puzzled by your "explosion". Where did I mention that a 230 V motor can be run at 460 V ? I mentioned 50 Hz and original voltage. Also, I didn't say about anything same HP being retained at lower frequency.
I only contested last part of your statement "If saturation is reached, your no load current (magnetizing mainly) will be too high, the PF decreases and the motor will overheat or eventually burn ( even under no load).", with which I still disagree.
I have advised many of my clients to run 380 V/60 Hz motors at 415 V/50 Hz without any rewinding and they are all running well without any excessive temp rise. How could a no-load current equal to 100% load current could burn the stator beats me ?
I am puzzled by your "explosion". Where did I mention that a 230 V motor can be run at 460 V ? I mentioned 50 Hz and original voltage. Also, I didn't say about anything same HP being retained at lower frequency.
I only contested last part of your statement "If saturation is reached, your no load current (magnetizing mainly) will be too high, the PF decreases and the motor will overheat or eventually burn ( even under no load).", with which I still disagree.
I have advised many of my clients to run 380 V/60 Hz motors at 415 V/50 Hz without any rewinding and they are all running well without any excessive temp rise. How could a no-load current equal to 100% load current could burn the stator beats me ?
RalphChristie
Electrical
Edison123
I think this is the thread you are refering to:
thread237-86353
Ralph
I think this is the thread you are refering to:
thread237-86353
Ralph
electricpete
Electrical
I vote a star to both edison and aolalde for good thought-provoking comments.
What followsis nothing new, already known to aolalde and edison but I will post anyway an excerpt from NEMA MG-1 2003:
“14.34 OPERATION OF GENERAL-PURPOSE ALTERNATING-CURRENT POLYPHASE, 2-, 4-, 6-,
AND 8-POLE, 60-HERTZ MEDIUM INDUCTION MOTORS OPERATED ON 50 HERTZ
While general-purpose alternating-current polyphase, 2-, 4-, 6-, and 8-pole, 60-hertz medium induction motors are not designed to operate at their 60-hertz ratings on 50-hertz circuits, they are capable of being operated satisfactorily on 50-hertz circuits if their voltage and horsepower ratings are appropriately reduced. When such 60-hertz motors are operated on 50-hertz circuits, the applied voltage at 50 hertz should be reduced to 5/6 of the 60-hertz voltage rating of the motor, and the horsepower load at 50 hertz should be reduced to 5/6 of the 60-hertz horsepower rating of the motor. When a 60-hertz motor is operated on 50 hertz at 5/6 of the 60-hertz voltage and horsepower ratings, the other performance characteristics for 50-hertz operation are as follows:
14.34.1 Speed The synchronous speed will be 5/6 of the 60-hertz synchronous speed, and the slip will be 5/6 of the 60- hertz slip.
14.34.2 Torques The rated load torque in pound-feet will be approximately the same as the 60-hertz rated load torque in pound-feet. The locked-rotor and breakdown torques in pound-feet of 50-hertz motors will be approximately the same as the 60-hertz locked-rotor and breakdown torques in pound-feet.
14.34.3 Locked-Rotor Current The locked-rotor current (amperes) will be approximately 5 percent less than the 60-hertz locked-rotor current (amperes). The code letter appearing on the motor nameplate to indicate locked-rotor kVA per horsepower applies only to the 60-hertz rating of the motor.
14.34.4 Service Factor The service factor will be 1.0.
14.34.5 Temperature Rise The temperature rise will not exceed 90°C (see 14.30).”
What is stated by NEMA is the same approach as suggested by aolalde. It also stands to reason that many motors have some unknown amount of margin built in and we can often be successful in pushing motors beyond the limits identified in the standards. I was very interested to hear edison’s comments regarding actual experience of operating motors beyond rated volts per hertz. Knowing edison, I place a lot of weight on his experience and judgement regarding motors. Being perhaps an overly cautious person, I personally would be hesitant to specify operation of a motor outside of recommendations of the standards.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
What followsis nothing new, already known to aolalde and edison but I will post anyway an excerpt from NEMA MG-1 2003:
“14.34 OPERATION OF GENERAL-PURPOSE ALTERNATING-CURRENT POLYPHASE, 2-, 4-, 6-,
AND 8-POLE, 60-HERTZ MEDIUM INDUCTION MOTORS OPERATED ON 50 HERTZ
While general-purpose alternating-current polyphase, 2-, 4-, 6-, and 8-pole, 60-hertz medium induction motors are not designed to operate at their 60-hertz ratings on 50-hertz circuits, they are capable of being operated satisfactorily on 50-hertz circuits if their voltage and horsepower ratings are appropriately reduced. When such 60-hertz motors are operated on 50-hertz circuits, the applied voltage at 50 hertz should be reduced to 5/6 of the 60-hertz voltage rating of the motor, and the horsepower load at 50 hertz should be reduced to 5/6 of the 60-hertz horsepower rating of the motor. When a 60-hertz motor is operated on 50 hertz at 5/6 of the 60-hertz voltage and horsepower ratings, the other performance characteristics for 50-hertz operation are as follows:
14.34.1 Speed The synchronous speed will be 5/6 of the 60-hertz synchronous speed, and the slip will be 5/6 of the 60- hertz slip.
14.34.2 Torques The rated load torque in pound-feet will be approximately the same as the 60-hertz rated load torque in pound-feet. The locked-rotor and breakdown torques in pound-feet of 50-hertz motors will be approximately the same as the 60-hertz locked-rotor and breakdown torques in pound-feet.
14.34.3 Locked-Rotor Current The locked-rotor current (amperes) will be approximately 5 percent less than the 60-hertz locked-rotor current (amperes). The code letter appearing on the motor nameplate to indicate locked-rotor kVA per horsepower applies only to the 60-hertz rating of the motor.
14.34.4 Service Factor The service factor will be 1.0.
14.34.5 Temperature Rise The temperature rise will not exceed 90°C (see 14.30).”
What is stated by NEMA is the same approach as suggested by aolalde. It also stands to reason that many motors have some unknown amount of margin built in and we can often be successful in pushing motors beyond the limits identified in the standards. I was very interested to hear edison’s comments regarding actual experience of operating motors beyond rated volts per hertz. Knowing edison, I place a lot of weight on his experience and judgement regarding motors. Being perhaps an overly cautious person, I personally would be hesitant to specify operation of a motor outside of recommendations of the standards.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
electricpete
Electrical
Some discussion on what happens to core loss if we decrease frequency and leave voltage constant.
Bmax increase in proportion to frequency within the linear range.
I see a variety of formulations for eddy and hysteresis losses including the following
Pe = Vol * Pi^2* Tau^2 * f^ 2 * (Bmax)^2 / (6*rho)
Ph = k Vol* f* (Bmax)^1.1
Where:
Pe = eddty current losses
Ph = hysteresis losses
f: Frequency [Hz].
Vol: Volume of iron specimen [m ].
Tau_: Thickness of lamination [m].
resistivity [_ m].
k: A constant for a given iron type and given range of flux density.
Bmax : Maximum flux density [Tesla].
The above formulas would suggest Pe is constant and Ph increases when we decrease frequency with votlage constant. I’m sure there are other formulations. It will get more complex if Bmax is limited by saturation effects. As mentioned above there are also increase in no-load current. May be difficult to measure due to high harmonic currents.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
Bmax increase in proportion to frequency within the linear range.
I see a variety of formulations for eddy and hysteresis losses including the following
Pe = Vol * Pi^2* Tau^2 * f^ 2 * (Bmax)^2 / (6*rho)
Ph = k Vol* f* (Bmax)^1.1
Where:
Pe = eddty current losses
Ph = hysteresis losses
f: Frequency [Hz].
Vol: Volume of iron specimen [m ].
Tau_: Thickness of lamination [m].
resistivity [_ m].
k: A constant for a given iron type and given range of flux density.
Bmax : Maximum flux density [Tesla].
The above formulas would suggest Pe is constant and Ph increases when we decrease frequency with votlage constant. I’m sure there are other formulations. It will get more complex if Bmax is limited by saturation effects. As mentioned above there are also increase in no-load current. May be difficult to measure due to high harmonic currents.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
Hi pete,
Your formulae for eddy current and hysteresis looses are universally accepted. Besides having practically seen the worst possible scenario (higher voltage/lower frequency) of 380 V, 60 Hz motors running at 415 V, 50 Hz satisfactorily over the years, I also have a theoretical explanation.
For the same voltages at 50 and 60 Hz (though in practice they seldom are) eddy current loss, as deduced from your formula remains the same due to the same amount of increase/decrease in Bm and F respectively.
As for the hysteresis loss, which is proportional to frequency and Bm^x, (the exponent x, as a rule, is 1.5 to 1.6 at 1 Tesla, 2.2 at 1.5 T and falls to 0.8 at 2T as per design books) may increase or even decrease (!). Depending on the original Bm at 60 Hz, the exponent x can be at worst 2.2 and at best 0.8. Taken in conjunction with the fall in frequency, the hysteresis loss may be 4% less (best case with x = 0.8) or 25% more (worst case with x = 2.2).
Out of typical 10% total loss, I figure hysteresis loss may be around 1%. So, an increase of 25% in this 1% loss is not going to start any fireworks. Also, with lower I^2R loss at 50 Hz (due to capacity reduction and due to original winding copper area being retained), one may actually end up even in cooler motor - at least theoretically - even with reduced cooling at 50 Hz.
Apart from all the theory, my opinion is still based on actual experience of seeing these motors operating well at higher V/Hz.
Of course, others may disagree (especially will all the above rambling)
Your formulae for eddy current and hysteresis looses are universally accepted. Besides having practically seen the worst possible scenario (higher voltage/lower frequency) of 380 V, 60 Hz motors running at 415 V, 50 Hz satisfactorily over the years, I also have a theoretical explanation.
For the same voltages at 50 and 60 Hz (though in practice they seldom are) eddy current loss, as deduced from your formula remains the same due to the same amount of increase/decrease in Bm and F respectively.
As for the hysteresis loss, which is proportional to frequency and Bm^x, (the exponent x, as a rule, is 1.5 to 1.6 at 1 Tesla, 2.2 at 1.5 T and falls to 0.8 at 2T as per design books) may increase or even decrease (!). Depending on the original Bm at 60 Hz, the exponent x can be at worst 2.2 and at best 0.8. Taken in conjunction with the fall in frequency, the hysteresis loss may be 4% less (best case with x = 0.8) or 25% more (worst case with x = 2.2).
Out of typical 10% total loss, I figure hysteresis loss may be around 1%. So, an increase of 25% in this 1% loss is not going to start any fireworks. Also, with lower I^2R loss at 50 Hz (due to capacity reduction and due to original winding copper area being retained), one may actually end up even in cooler motor - at least theoretically - even with reduced cooling at 50 Hz.
Apart from all the theory, my opinion is still based on actual experience of seeing these motors operating well at higher V/Hz.
Of course, others may disagree (especially will all the above rambling)
jbartos
Electrical
- Jan 15, 2000
- 6,440
Suggestion to aolalde (Electrical) Apr 28, 2004 marked ///\\JBartos The original posting does not specify motors designed for 50/60 Hz but 60 HZ.
///I am aware of it. So what?
Have you visited
for:
These highly efficient 3-phase TEFC motors make pumps, fans, blowers and other industrial equipment operate at peak performance levels. Uses: Pumps, blowers, machine tools, air compressors, and other industrial applications where 3-phase power is available.
Mounting: Rigid welded base
Ambient: 40°C
Thermal protection: None
Bearings: Double-shielded ball
rotation: CW/CCW
NEMA Design: B
NEMA Frame: 184T
Volts 60Hz: 230/460
====================
Service Factor: 1.15
NEMA Nominal Efficiency: 89.5
Frame Material: Cast Iron
Insulation Class: F
50Hz operation on 190/380Volts at 5/6 of 60Hz horsepower
========================================================
and RPM at 1.0SF
2 year warranty
Inverter-Duty, meets NEMA MG 1 Part 31: 10:1 variable and 4:1 constant torque
NEMA Premium™ efficient product.
Now in this context the original posting states:
"""mikepz (Electrical) Apr 27, 2004
I have a numerous 3-phases motors ranging from 220V 60Hz to 480V 60Hz. I understand that these motors would overheat if I run them at 50Hz.
What can I do to run these motors at 50Hz ? """
===========================================
Simply: Refer to the above for:
50Hz operation on 190/380Volts at 5/6 of 60Hz horsepower
========================================================
and perform similar adjustments for different voltages.
Now, what seems to be at a question?\\=======================================
///I am aware of it. So what?
Have you visited
for:
These highly efficient 3-phase TEFC motors make pumps, fans, blowers and other industrial equipment operate at peak performance levels. Uses: Pumps, blowers, machine tools, air compressors, and other industrial applications where 3-phase power is available.
Mounting: Rigid welded base
Ambient: 40°C
Thermal protection: None
Bearings: Double-shielded ball
rotation: CW/CCW
NEMA Design: B
NEMA Frame: 184T
Volts 60Hz: 230/460
====================
Service Factor: 1.15
NEMA Nominal Efficiency: 89.5
Frame Material: Cast Iron
Insulation Class: F
50Hz operation on 190/380Volts at 5/6 of 60Hz horsepower
========================================================
and RPM at 1.0SF
2 year warranty
Inverter-Duty, meets NEMA MG 1 Part 31: 10:1 variable and 4:1 constant torque
NEMA Premium™ efficient product.
Now in this context the original posting states:
"""mikepz (Electrical) Apr 27, 2004
I have a numerous 3-phases motors ranging from 220V 60Hz to 480V 60Hz. I understand that these motors would overheat if I run them at 50Hz.
What can I do to run these motors at 50Hz ? """
===========================================
Simply: Refer to the above for:
50Hz operation on 190/380Volts at 5/6 of 60Hz horsepower
========================================================
and perform similar adjustments for different voltages.
Now, what seems to be at a question?\\=======================================
Hello edison123
As a member of the 50Hz world, I can confirm that I have seen amny 60Hz motors run on 50Hz at the same rated voltage and fail due to overheating.
If the motor is specifically designed for maximum efficiency at rated voltage on 60Hz, then you can expect a dramatic increase in iron loss at 50Hz and this will cause an early failure.
Motors tend to be wound to minimise losses. As you increase the turns for a geven voltage, the flux will reduce and the copper loss will increase. Similarly, as you reduce the turns, the copperloss will reduce and the flux increase, increasing the iron loss. The optimum point is usually just below saturation flux, so if you increase the flux beyond this point (60Hz -> 50Hz), you will drive the fux into saturation and this will result in a major increase in iron loss and premature failure. If the motor has been well overwound, then this is not a problem.
As a rule, keep the V/F ratio constant and there will not be a problem. Raise the V/F ratio and you could cause overheating, even at no load.
Best regards,
Mark Empson
As a member of the 50Hz world, I can confirm that I have seen amny 60Hz motors run on 50Hz at the same rated voltage and fail due to overheating.
If the motor is specifically designed for maximum efficiency at rated voltage on 60Hz, then you can expect a dramatic increase in iron loss at 50Hz and this will cause an early failure.
Motors tend to be wound to minimise losses. As you increase the turns for a geven voltage, the flux will reduce and the copper loss will increase. Similarly, as you reduce the turns, the copperloss will reduce and the flux increase, increasing the iron loss. The optimum point is usually just below saturation flux, so if you increase the flux beyond this point (60Hz -> 50Hz), you will drive the fux into saturation and this will result in a major increase in iron loss and premature failure. If the motor has been well overwound, then this is not a problem.
As a rule, keep the V/F ratio constant and there will not be a problem. Raise the V/F ratio and you could cause overheating, even at no load.
Best regards,
Mark Empson
Some manufacturers design their motors very close to the magnetic circuit saturation. When such a 60 HZ motor is operated at 50 HZ it will saturate. If a motor is designed working with very low flux densities it will not saturate when the frequency is reduced or the voltage is increased.
In my opinion, Engineering recommendations should not be based on special cases but be consistent for every application.
As a conclusion: May I expect satisfactory operation of a 60 HZ motor operating at 50 HZ?
Yes; if the V/HZ ratio is kept and the power output is reduced to 5/6 of original.
In my opinion, Engineering recommendations should not be based on special cases but be consistent for every application.
As a conclusion: May I expect satisfactory operation of a 60 HZ motor operating at 50 HZ?
Yes; if the V/HZ ratio is kept and the power output is reduced to 5/6 of original.
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