50hz motor 60hz supply

[jimmy2times]

I have read thread faq237-1224 as a starter

someone has authorised up grading a vent fan motor on a ship. existing motor was 3kw 60Hz 440V

I was called today as they are setting to work and existing overload is tripping after 2 mins (thankfully).

The new motor selected was 4kW 50Hz 400V. Deliberated selected I am told so as to achieve greater air flow to ships internal compartments, but without a radical redesign of the exhaust air system. The nominal current draw is 10 amps.

The electricians advised that the existing 6.5A overload was tripping when set to work. This tripped after 2 mins. In this time current draw was measured and it was 12.5amps.

The question I have is I am safe to upgrade the overload given that motor will run 12.5A (whereas the nominal is 10A, so 25% over). I guess this translates into 56% additional heat loss using I squared relationship. Is the fact that the motor is running 20% faster going to aid cooling.

My initial thoughts are no they should have picked a bigger motor. There should have been a thermistor connected as part of original design but this was never wired up, I'd feel more comfortable in increasing overload size and monitoring the motor over a couple of days if this was fitted.

Is it a case that motor life will be severely shortened? We basically need 1-2years before the ship is decommissioned. Or is it an outright fire hazard in which case reject the move to increase the overload and have a rethink on options. If so what could be done, a VFD?

appreciate your help.

 

[jraef]

"Nominal current draw" is not really relevant here, what is the motor nameplate Full Load Current? That is the only determining factor with regard to the expected motor life.

By the way:

... Deliberated selected I am told so as to achieve greater air flow to ships internal compartments, but without a radical redesign of the exhaust air system. ...

Someone is not thinking correctly on this. The original fan is designed for 60Hz operation (presumably). You have installed a 50Hz RATED motor, but you are (presumably) giving it the same 60Hz that the previous motor was supplied with. Nothing then will change as far as the fan air delivery, that is based on the SPEED of the fan, not the kW rating of the motor. Now, if someone also changed the BELT ratio, that's another story and if so, might be the root of your problem. You need to do more investigation here before we can offer any meaningful help.


"Will work for (the memory of) salami"

 

[jimmy2times]

I was fed a bit of duff information, I have been given the design proposal and it states "A fan motor increase to ATU-7 is also proposed, due to the extended ductwork run. It is proposed to replace the existing 3 kW motor with a nominal 4 kW or equal motor."

No mention that a 50Hz 400V motor would be selected.

Datasheet for new motor attached.

 

[jimmy2times]

sorry attached datasheet attached now

 

[waross]

Check the pulley ratio. The 4kW @50Hz/400v should develop 4.8 kw @ 60Hz/480V. Derating for the lower voltage should produce 4.4 kW @ 440V.
Assuming a centrifugal fan, either the pulley ratio has been changed, the ductwork has been reworked to reduce back pressure or both.
It is common to find a variable ratio pulley on a fan motor. If someone has used the ratio adjustment to tighten the drive belt instead of moving the motor base that would cause your problem.
A 3 kW / 50 Hz motor should have been adequate at 60 Hz / 440 Volts. A change of pulley ratio will do the same as a VFD a lot cheaper. It may only need adjustment.
You need to slow the fan down until the motor draw is reduced to rated current.

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

 

[waross]

By the way, if the pulley on the motor is a fixed diameter or pitch, consider changing it to a pulley with an adjustable pitch. Then adjust it for full rated current. That should be your best performance without changing motors.

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

 

[jraef]

The operative point being, fan load = air flow. If something in the mechanical side of this equation increased the air flow, that will increase the load on the motor. Just changing the capacity of the motor does NOT, in and of itself, change the air flow. Something else may have done that, such as a change in the pulley ratio or, as Bill reminded me, a change in the static duct pressure.

If whatever else was changed, increased the required motor load to more that 4.4kW, that must be fixed first. Your motor is only capable of handling 10A continuously, if it is running at 12.5A, it is in a continual state of overload. If you increase the overload protection to allow that over current to flow, expect that it will not last more than a few months, if that. There is no way to predict it exactly, but a really old old rule of thumb we used to use here in North America nicknamed the "Dumb Horse Chart" (because we use Horsepower) dictated that for every 1% increase in current beyond rated service factor, you cut the motor life by 25%. Being that this is an IEC design motor, there is no "service factor" and you will be pulling 25% more current. Do the math...

"Will work for (the memory of) salami"

 

[Parchie]

Check the pulley ratio. The 4kW @50Hz/400v should develop 4.8 kw @ 60Hz/480V. Derating for the lower voltage should produce 4.4 kW @ 440V.

As we can see, the power draw of the replacement motor that is running at around 1150 rpm(1200 rpm synchronous speed of 6-poles at 60 Hz, instead of its rated 960rpm at 400V/50Hz) was around 5.6kW (12.5A as posted and assumed PF =0.7; eff =.84). That would indicate that they altered the pulley ratio and overloaded the replacement motor in the process.