3 phase motor rating.

[Rampage]

Firstly, I'm not doing this, I'm getting someone else to. I'm just interested.

I have a machine arriving which has a largish motor, the plate on it is the one below.
The horsepower is 0.6 but the current stated is 2.4A. This to me doesn't work out, is this the in-rush current?
The motor is approximately 14" diameter and 16" long (complete guess). I'm surprised it's only 0.6hp?

For anyone interested it's from an Aciera F3.

 

[Marvo]

Motor inrush currents are generally calculated using a rule of thumb. The inrush current is over so quickly it shouldn't trip the MCB and especially for smaller motors it doesn't really figure in the circuit design. The current stated on the plate will always be the maximum running current the motor is capable of handling and all the control components would be specified around this figure, not the inrush current. It's worth noting that your motor is wired in star and is rated at 875 RPM so it's possibly 6-pole.

 

[peterb]

...... dont forget about power factor. Esp if its a multipole motor. It may physically that big so it has some more inertia given its for a machine tool.

 

[Darkwood]

I take it rampage you don't do a lot with motors as this is a baby .... when you said largish motor I was expecting 30Kw + ...... did make me chuckle when I saw your pic lol
Horsepower is the work the motor can do and with most induction motors can be related as such to running current but motors are designed and built in many forms and efficiencies so not always as you expect. Sometimes efficiency is sacrificed for durability but I wouldn't concern yourself you got the current rating and that's all you need.
Id expect a standard induction motor of 0.6hp(not a standard rating) at 400v to have a running current of 1.44amps but as I said its not always reliable to use HP to calculate current.

Tony's the man who might be able to expand on this with ref' to your particular motor... im sure he'll drop a comment in.

 

[Knobhead]

0.6HP @ 400V .8PF 80% efficiency comes to 0.96A by my reckoning. 110V gives figures near what you give.

 

[Rampage]

I realised as soon as I posted it that it would be a small motor to some people on here!

I have been given a 4kW Rotary Phase Converter to run this mill and a medium(!) size lathe. I want to change to a digital phase converter as the rotary is so noisy.
So should I use the current rating on the motor or the horsepower on the motor!
Its probably obvious I don't know anything about motors!

 

[clematis432]

I realised as soon as I posted it that it would be a small motor to some people on here!

I have been given a 4kW Rotary Phase Converter to run this mill and a medium(!) size lathe. I want to change to a digital phase converter as the rotary is so noisy.
So should I use the current rating on the motor or the horsepower on the motor!
Its probably obvious I don't know anything about motors!

I would use the current on the motor, that,s what you use when setting motor protection or overloads.

 

[ipf]

I realised as soon as I posted it that it would be a small motor to some people on here!

I have been given a 4kW Rotary Phase Converter to run this mill and a medium(!) size lathe. I want to change to a digital phase converter as the rotary is so noisy.


So should I use the current rating on the motor or the horsepower on the motor!
Its probably obvious I don't know anything about motors!

Did they not go through all this during your training. lol.

 

[t-s]

you would normally use an inverter, the sizes will be in kw or hp just google conversion table if you need it.

 

[Rampage]

Did they not go through all this during your training. lol.

lol, I think this was a 1 day add-on that I didn't take!

 

[Knobhead]

Don’t worry about it.
An inverter would be the best bet, but if you’re stuck with the converter then use it. I hope you’re not paying the leccy bill. A converter is hellishly inefficient.
As has been said the O/L’s must be set to the rating plate.
At the end of the day you can only go by what the manufacturers say.

 

[Rampage]

Don’t worry about it.
An inverter would be the best bet, but if you’re stuck with the converter then use it. I hope you’re not paying the leccy bill. A converter is hellishly inefficient.
As has been said the O/L’s must be set to the rating plate.
At the end of the day you can only go by what the manufacturers say.

Is it inefficient because the motor is always running?
I'm going to try and get a digital one, these are presumably more efficient?

Thanks.

 

[44ALLAN]

My old engineer used to say..............flc + 10% for o/load setting.
Worked fine until someone put hydraulic oil in the o/load dash pots

Just curious but what do other folk set their motor overloads at. ?
Does anyone actually think about it or just accept what their set at !

 

[Knobhead]

You will have mechanical losses as well as electrical. And something called windage (nothing to do with flatulence). It’s the power consumed by the motor/alternator cooling fan/s.
Motor generator sets are by nature inefficient. Much as I like old technology they’ve had their day.

 

[Silly Sausage]

Much as I like old technology they’ve had their day.

A bit like the wife! :-(

 

[44ALLAN]

Who's wife?

 

[Knobhead]

Can’t be my wife, I’m divorced! But the ex is still one of my best friends. Archy must be referring to his own.

 

[Darkwood]

My old engineer used to say..............flc + 10% for o/load setting.
Worked fine until someone put hydraulic oil in the o/load dash pots

Just curious but what do other folk set their motor overloads at. ?
Does anyone actually think about it or just accept what their set at !

I set the O/L up to the motor duty in normal running and not the rating + whatever % of the motor this way issues are picked up early doors without major down time as such.....when it trips you know something is making the motor work harder usually the gears haven't been topped up with the slippy stuff.

 

[Knobhead]

O/L's always set to 100% unless the manufacturers say different.
Setting to normal + a bit running current may be OK in light industry. Try it where the motor is expected to work for it's living!

 

[Darkwood]

Yes Tony agree nothing wrong with doing that but some of the machines I do have an advantage of using the O/L setting to pre-empt issues, you then increase it to normal and arrange a service day long before it can cause an issue like a stiff bearing or dry gears .... well it works for me anywhere but each to their own.

 

[clematis432]

My old engineer used to say..............flc + 10% for o/load setting.
Worked fine until someone put hydraulic oil in the o/load dash pots

Just curious but what do other folk set their motor overloads at. ?
Does anyone actually think about it or just accept what their set at !

I,ve worked in industry quite a lot, and within various factories, and motor overloads have been set from 100% FLC to 130% FLC depending on circumstances but I would say 110% is about the most common I,ve experienced. Trying to get a balance I suppose between protection and nuisance tripping.
Motors are more efficient running around 80% their FLC

 

[Marvo]

If the motor is under constant load we set the overload according to the normal running current and not the FLA marked on the info plate. For example if a motor has an FLA rating of 33Amps but under normal operating conditions it only draws 26.5Amps we'd set the overload at 26.5 + 10%.

 

[Darkwood]

If the motor is under constant load we set the overload according to the normal running current and not the FLA marked on the info plate. For example if a motor has an FLA rating of 33Amps but under normal operating conditions it only draws 26.5Amps we'd set the overload at 26.5 + 10%.

Exactly what I said and do ...this has saved my customers countless down time as I work in the cotton industry a lot and it can wrap around the roll bars and beariings, when it trips I usually have extra available motor power to increase the O/L setting in the short term to overcome the resistance and schedule in a shutdown at convenience.

 

[Knobhead]

DW, Marvo,
Please don’t go in to anything that’s safety critical if that’s you’re method. You wouldn’t survive 5 minutes in the chemical or steel industries.

Oh, look, the plants blown up. The motors OK though so that’s alright, isn’t it?

I’ve shorted O/L’s out, I didn’t give a flying f*** what happened to the motor. It had to keep going to get the process stable to do a controlled shut down. Anything critical had a back up, in some cases two back ups. They had to run no matter what.

O/L’s are set to 100% end of!

Anything critical would be oversized, electrically and mechanically. I had an 800HP blower motor with smoke coming out of it, the little beauty kept slogging on though. By this point all ancillary staff had been told to leg it out of harms way. The plant controller, charge hand fitter and myself as shift engineer had to stay. Common sense would say leg it, try explaining that in an enquiry in to a plant explosion! “You saved yourself, now there’s the body count to tally.”

The motor wasn’t very well afterwards. To rub salt in the wounds, my shift changed it!

 

[Darkwood]

I work in a very different industry which does show from our various sometime conflicting answers but at the end of the day I realise what I do on one machine isn't good on the next its just a judgement call, I also realise your in a very different industry and understand when we have differing views .... i learned all i know from the safety engineer at the Chernobyl nuclear power station ....well on some occasions you could be led to believe that.

 

[Knobhead]

My watch words. "Remember Flixbough."
That was a monumental cock up that circumvented safety to keep production going.

The things I’ve done haven’t been without a lot of soul searching first, but that has time constraints when things are going bang.

 

[Marvo]

Anything critical would be oversized, electrically and mechanically. I had an 800HP blower motor with smoke coming out of it, the little beauty kept slogging on though. By this point all ancillary staff had been told to leg it out of harms way. The plant controller, charge hand fitter and myself as shift engineer had to stay. Common sense would say leg it, try explaining that in an enquiry in to a plant explosion! “You saved yourself, now there’s the body count to tally.”

The motor wasn’t very well afterwards. To rub salt in the wounds, my shift changed it!

The scenarios you're talking about are industrial plant with auto reclosers and first fault override, the OP had a motor that was under a kilowatt in size and smaller than most household appliances, protection considerations will vary drastically between these environments. With very small motors like this there's a valid argument that protection could even outweigh the cost of the motor itself and isn't actually worth installing at all. If he is going to install protection and if the motor is under a predictably constant load range then I would still advise he sets the trip current according to the actual load current rather than at the FLA on the info plate.

 

[GLENNSPARK]

Exactly what I said and do ...this has saved my customers countless down time as I work in the cotton industry a lot and it can wrap around the roll bars and beariings, when it trips I usually have extra available motor power to increase the O/L setting in the short term to overcome the resistance and schedule in a shutdown at convenience.

and if left.....cotton and wool can shear through shafts as well.....

 

[clematis432]

Can,t help thinking this threads getting a little complex. Every individual motor is set to suit it,s OWN use, purpose and conditions. These are many, i.e. plant conditions, flameproof/non flameproof environment,type of motor, type of starting of motor, classification of o/l relay, ambient temperature, etc. Whether the o/l is set by reference to running current or stated FLC is arguable and again individual as to motor duty and purpose. Also the comment made in an earlier thread that "O/l,s are set to 100% end of" is I,m afraid too simplistic and generalized. We could debate all night settings of o/ls for induction motors and nobody would be 100% correct.