400V 50HZ operate on 480V 60 HZ

[td1313]

Thanks everyone who has responded. I wanted to add a little more detail, based on the responses so far. There at 2 condensing fan motors on the condenser unit. The primary motor is controlled by a VFD and the 2nd motor, the bad one, is pressure controlled and cycles as needed. The increased speed caused by the hertz difference should not be a problem because there is a 300 rpm difference anyway, the fan may cycle a little more often but not an issue. And yes I know the motor is at risk of failure but I’m at risk of losing a data room HVAC system. The amp ratings are also similar so I’m not really worried about overloading a breaker but will obviously monitor while running.

I have priced an exact replacement motor and was told 90 weeks and asked about the entire condenser unit and was told 58 weeks. I’ve eve considered replacing the whole system and was told minimum time frame of 1 year. Unfortunately management isn’t happy with either choice. The motor was sent to a repair shop and was too far gone for repairs. We’re considering installing 3-4 ton mini-splits as they seem to be the easiest to obtain. What has happened to the US supply chain?

 

[winnie]

The primary motor is controlled by a VFD and the 2nd motor, the bad one, is pressure controlled and cycles as needed. The increased speed caused by the hertz difference should not be a problem because there is a 300 rpm difference anyway, the fan may cycle a little more often but not an issue. And yes I know the motor is at risk of failure but I’m at risk of losing a data room HVAC system.

Just confirming: you are going to keep the old fan and just replace the motor. The motor has the same pole count as the old motor (eg. 1500 RPM 50 Hz motor to replace a 1800 RPM 60 Hz motor). The motor you intend to use is rated at 400V and 50 Hz, replacing a motor rated at 480V and 60Hz. The motor has a similar current rating as the old motor.

If the above are correct, I give this a good chance of working just fine.

-Jon

 

[retirede]

Right, replacing with a 50hz unit BUT RUNNING AT 60HZ

Then how can you say “speed is not changing”?

 

[winnie]

Then how can you say “speed is not changing”?

A 50Hz motor is being used to replace a 60Hz motor in a 60Hz environment. The motor will be operating 20% faster than its rating, but will end up operating at approximately the speed of the motor that needs to be replaced.

-Jon

 

[zbang]

I think a few people are missing the point that this is a replacement to get a piece of gear back into operation solely because the correct part can't be obtained. The new motor could even be considered expendable, but as long is it keeps other things from failing, I'd call it a good use.

Ever put regular gas in a vehicle that wants premium because there was nothing else and it would get you home?

 

[TwoBlocked]

Might be able to change the fan to reduce the load or maybe block some of the air flow.

 

[winnie]

Might be able to change the fan to reduce the load or maybe block some of the air flow.

Shouldn't be necessary if the fan is being moved to the new motor.

As a very rough rule of thumb, if you keep the same V/Hz ratio and same operating current, then the torque of the motor remains constant. This is the reason for the 'constant torque' region of a motor on a VFD below its base speed.

So to a very rough approximation, this new motor which @td1313 has selected has the same rating as the original motor. It is supposed to be a 400V 50Hz motor with the same current rating as the 480V 60Hz motor being replaced. IMHO it is as close a match as a random different 480V 60Hz motor with the same current rating as the original motor.

-Jon

 

[TwoBlocked]

Shouldn't be necessary if the fan is being moved to the new motor.

As a very rough rule of thumb, if you keep the same V/Hz ratio and same operating current, then the torque of the motor remains constant. This is the reason for the 'constant torque' region of a motor on a VFD below its base speed.

So to a very rough approximation, this new motor which @td1313 has selected has the same rating as the original motor. It is supposed to be a 400V 50Hz motor with the same current rating as the 480V 60Hz motor being replaced. IMHO it is as close a match as a random different 480V 60Hz motor with the same current rating as the original motor.

-Jon

But you are running the fan 20% faster. That takes a lot more torque.

 

[winnie]

But you are running the fan 20% faster. That takes a lot more torque.

If the replacement is a 50Hz motor with an associated 50Hz fan, then I agree.

But if this is a 50Hz motor run at 60 Hz with the original fan for the 60Hz motor, then there isn't a problem.

A 50Hz 4 pole motor operated with 60Hz will run at the speed of a 60Hz 4 pole motor.

Jon

 

[Jraef]

He is just replacing a 60 Hz motor with a 50 Hz motor but plans to run it at same speed as original so the driven load won't see any difference, the question was can the 50Hz motor take this?

That’s exactly right. And yes, the motor can take it.

 

[Jraef]

But you are running the fan 20% faster. That takes a lot more torque.

No. The motor torque is determined by the V/Hz ratio, has nothing to do with speed.

 

[Deltaforce]

If the replacement is a 50Hz motor with an associated 50Hz fan, then I agree.

But if this is a 50Hz motor run at 60 Hz with the original fan for the 60Hz motor, then there isn't a problem.

A 50Hz 4 pole motor operated with 60Hz will run at the speed of a 60Hz 4 pole motor.

Jon

HP output above 50Hz motor operate base speed beyond

Fan laws check

Motor overload liability

 

[GoldDigger]

No. The motor torque is determined by the V/Hz ratio, has nothing to do with speed.

What he meant, I think, is that if a fan were running at 20% above its design speed a higher motor torque would be required. But, as many people have explained, the speed of the mechanical load, in the OP's case, will be exactly the same with the replacement motor as it was with the original motor.

 

[TwoBlocked]

The way I see it, output torque is developed when the motor encounters rotational resistance (performs work). Fan curves are not linear. Increasing the speed by 20% (50hz to 60hz) will require more than 20% of additional torque. More work will be performed, that means more watts, and that means more amps. You can expect the motor to trip its overload if the same fan is used.

 

[GoldDigger]

The way I see it, output torque is developed when the motor encounters rotational resistance (performs work). Fan curves are not linear. Increasing the speed by 20% (50hz to 60hz) will require more than 20% of additional torque. More work will be performed, that means more watts, and that means more amps. You can expect the motor to trip its overload if the same fan is used.

But replacing a 60Hz motor with a 50Hz motor in a 60Hz environment will NOT change the speed of the motor output shaft!
The 50Hz motor does not have a fractional pole construction or gearing to cause it have the same rotation speed at 50Hz as a 60Hz motor has at 60Hz.

 

[Deltaforce]

What he meant, I think, is that if a fan were running at 20% above its design speed a higher motor torque would be required. But, as many people have explained, the speed of the mechanical load, in the OP's case, will be exactly the same with the replacement motor as it was with the original motor.

No a higher motor torque

A higher hp output

Motor overload thus

 

[GoldDigger]

No a higher motor torque

A higher hp output

Motor overload thus

Your reference to a greater than 20% increase in fan load for a 20% speed increase is not applicable however.
IF the 50Hz motor and the 60Hz motor had the same design HP rating, the design torque of the 50Hz motor would have to be greater than that of the 60Hz motor to deliver the same power at the lower speed.

That means that the maximum torque output of the 50Hz motor operating at 60Hz with a constant V/F ratio will be unchanged. BUT the 60Hz application will not require full design output torque, so the power supplied by the 50Hz motor will be identical to the power supplied by the 60Hz motor in the same equipement. This alone will not overload the 50Hz motor. And if the equipment design had any safety factor (for example rounding the required motor power up to the nearest standard HP rating) then the the 50Hz motor can take advantage of that safety factor.

Finally, I trust jraef to be fully aware of the practical details of this kind of application and if he says "The motor can take it," I trust him.

 

[Deltaforce]

Your reference to a greater than 20% increase in fan load for a 20% speed increase is not applicable however.
IF the 50Hz motor and the 60Hz motor had the same design HP rating, the design torque of the 50Hz motor would have to be greater than that of the 60Hz motor to deliver the same power at the lower speed.

That means that the maximum torque output of the 50Hz motor operating at 60Hz with a constant V/F ratio will be unchanged. BUT the 60Hz application will not require full design output torque, so the power supplied by the 50Hz motor will be identical to the power supplied by the 60Hz motor in the same equipement. This alone will not overload the 50Hz motor. And if the equipment design had any safety factor (for example rounding the required motor power up to the nearest standard HP rating) then the the 50Hz motor can take advantage of that safety factor.

Finally, I trust jraef to be fully aware of the practical details of this kind of application and if he says "The motor can take it," I trust him.

A vfd operate 50Hz motor 400 v constant HP mode base speed above

Torque reduce

Overload not it

50Hz 400v motor on 60hz 480v

Above base speed

Constant torque

Motor overload may

 

[winnie]

@Deltaforce :
Motors overload based on _current_ not power.

With the increased voltage available this motor will operate at correct V/Hz and be able to produce full torque at the higher speed without overloading.

Confusing Yodaspeak is

@td1313 One question that I have is 'why a 50Hz replacement motor rather than a generic 60Hz replacement of the appropriate horsepower? 60Hz motors are plentiful.

My guess: this is some particular size or strange mounting HVAC motor, possibly the replacement motor from the same manufacturer but targeting 50 Hz equipment.

Can you explain what is so specific that you selected this particular 50Hz motor rather than a generic 60Hz motor of appropriate capacity?

There are generic HVAC blower and fan motors available.

Jon

 

[Tulsa Electrician]

I have been reading and trying to follow the best I can.
From what I read, I would say you need torque to get it up to a fixed speed and HP to keep the fixed load at that speed.

So I can see where the volt hertz ratio comes into play.

Similar to an engine for a given result.

The rest is just how you do it.

Rude analogy, four cylinder at a higher rpm versus a eight cylinder at a lower rpm.
Engine speed for a given output.

One may get you there quicker however they both get you there.
Based on a fixed load.
The rest is at what cost.
One may require more fuel at first then taper off as it reaches speed.
Then the two at speed require different fuel requirements to maintain a fixed speed at a fixed load.
Then there the life span of the machine for a given speed.

So does this machine run for a long period of time or does it start and stop frequently.
How important is it for the machine to get up to speed at fixed time.

Based on load I would look at the motors torque curve for the fixed HP based on speed for the given load.