4hp single phase motor problem 1

[jemjack]

I have a high pressure compressor for filling air cylinders for diving, and a little while ago I asked about 5 hp motors for it, and got many helpful replies from you , thanks very much. I did have a mental block, as when I read the blurb a bit more carefully I saw that it really only required a 4 hp motor. Sorry about that.

The compressor at startup runs very freely and in the past I was using it with a 3 hp motor which worked well but after 40 minutes or so the motor got very hot, so I had to stop it. As time went on the motor would cut out after less and less time until it would only run for 15 minutes which was not enough time to fill a bottle, and I felt that soon I would burn out the motor! I had it connected through a DOL and it was this that cut out, presumably because the amperage rose to high?

I have just bought a new 4hp motor 220 volts ( I live in Spain) single phase 3000 rpm, which is the required motor for this compressor. Also new DOL for it.

The compressor is supposed to run at 1300 rpm but my pulleys are letting it run at rather less, So to my reckoning it should not need 4 hp to drive it!

All connected up and when I press the green button it starts up and runs for about 5 seconds or so and blows the 25 amp fuse at the mains. By the sound of the motor, it is up to the speed that it will run at, i.e. it has stopped accelerating, but has not tripped out the centrifugal switch . This is the problem . which I would very much like some help on please.
The motor runs at 2900 rpm when it has nothing attached to it, the voltage is 215 volts and the centrifugal trips out with no problem.
When the compressor is attached to it by the belt and pulleys the motor runs up to 1950 rpm but the centrifugal switch does not trip out at this speed. The amperage is way off the scale of my meter which only reads up to 50 amps the voltage drops to 180 volts. and the compressor is running at 725 rpm. The pulley ratio is 1: 2.76 I realize that this is so because the started windings are still in circuit.

So it would seem to me that
1 the motor is not producing 4 hp or
2 the centrifugal switch should trip out , then the voltage would rise, and the currant would drop to somewhere around 20 amps. and the motor would produce more power, and accelerate to somewhere near 2800 rpm
3 Or am I missing something here.

I have no way of having 3 phase, the cables from motor to meter are heavy duty about 6 mm so there should be no volt drop in them at 20 amps or so. The start windings have a 400 µf condenser and the windings have a resistance of about .7? and the running windings have a 60 µf condenser and the windings are about 1.9 ?.

Any help or advice would be very gratefully received,
Best wishes

Jemjack

 

[waross]

Is this a new motor or a motor of uncertain history?

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

 

[LionelHutz]

Something is wrong. It's not starting correctly and it's not reaching the proper running speed. You shouldn't just ignore these problems.

I believe you are using the same pulleys and belt as the 3hp motor used. So, the pulley ratio and belt drive should not be the issue. So, I can think of 3 likely issues;
- The motor is connected for the wrong voltage.
- The line voltage is dropping which lowers the motor torque and then the motor doesn't produce enough torque to accelerate to full speed.
- The motor is defective.

 

[jemjack]

Hi Waross, and Lionelhutz,

Thank you for answering my questions, the motor is brand new.

I am using the same pulleys as the 3 hp motor, so the 4 hp motor should run with no problem. The motor voltage is 220 volts on the plate, and the supply voltage is 220 volts sometimes it drops to 215 volts. It does drop to 180 volts whilst the 400 µf starter capacitor is in circuit as mentioned earlier. The motor works well if I manually disconnect the 400 µf capacitor but only at 2500 rpm as mentioned.
This does give a big spark, should I put a high resistance in series with it and if so, what wattage?)
I do think that the motor should accelerate to 2800 rpm or so as there is not much torque required when the valves are open on the compressor at startup.( It was never a problem with the 3 hp motor) It is only drawing 14 amps at 220 volts whilst filling a bottle .

The motor does reach 54ºc on the case, is this reasonable?

Thank you all for your very great help, this now puts me in a very good position to argue that the motor might be faulty.

Best wishes

Jemjack

 

[ScottyUK]

Jemjack,

You may find a temperature code letter on the nameplate, likely Class F or perhaps Class H. This letter refers to the temperature rating of the winding insultation, which is 155°C and 180°C respectively. It is unlikely you could even touch a Class F motor at full load, and you could likely simmer a pan on a Class H motor. 54°C casing temperature is nothing to be alarmed about, the motor sounds like it is operating well within rating.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

A couple of observations that point to a problem with your speed measurement:
1) "I tried the motor without the belts, and it ran perfectly, starting correctly and running at 2900, at 215 volts"

Comment: A two-pole motor that runs with no load (no belts) should run very close to 3000 RPM when on 50 Hz. If you read 2900 RPM, then your RPM measurement is probably wrong.

and

2) "It drew 13 amps at 220 volts and ran at 2500 rpm I ran it for about an hour and a half and at the end of this period the temperature of the casing of the motor rose from ambient 27ºc to 54ºc"

Comment: If your motor runs for 90 minutes with surface temperature increase from 27 to 54 degrees, it is not working near rated power at all. That is what I would expect from an idling motor.
Still, your speed measurement (2500 RPM) indicates severe overload.


So, something doesn't compute here. Either you mix numbers up or your speed measurement is wrong. What do you use for speed measurements?





Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Further to Gunnar's observations, it sounds like a flakey tachometer.

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

 

[jemjack]

Jemjack

Thanks very much everyone, I have not mixed up my numbers, but the tachometer is an old mechanical one. And reads on the 1 to 10000scale so I agree that it could be a bit out. Also when I said that the motor was off load reading 2900 rpm, it did in fact have a rather heavy cast iron 3 vee pulley on it . It’s a gash pulley I had in the workshop to tide me over until I get the right pulley for the unit. Would this make some difference to the no load reading?

Thanks once again everyone,

Best wishes to you all

Jemjack

 

[Skogsgurra]

The pulley as such makes no difference. It acts as a fly-wheel.

But that mechanical tach is something I doubt very much. Taking a speed reading is also an art in itself. Especially if you don't have a rubber tipped tach. Or if the rubber is dry. Or if there's grease present.

I think your motor is OK. The temperatures and currents say so.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

The speed switch NOT opening when the motor has a load VERY CLEARLY says something is NOT OK.

You claim you are using the same pulleys as the 3hp motor. You also claim the speed switch opens when the belts are removed but does not open when the belts are installed. To me, this is indicating that the motor is not reaching the proper running speed.

There is something wrong, and it's not the tach measurement. Well, the tach measurement may also be wrong, but your misreading of a tach isn't causing the speed switch to stay closed.

 

[waross]

Good point Lionel. lps

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

 

[jemjack]

Once again thanks for all your help, I am now in a good position to tell the company that I bought the motor from that there is most likely a problem with the centrifugal switch of the motor. Or at least that the motor is not working as it should. I have checked the tachometer with various other motors, and if it is out it is not much.

Many thanks

Jemjack.

 

[petronila]

jemjack

You could find single phase motor with ot centrifugal switchs and this one use an electronic starter

Try with this option,many workshops can remove the centrifugal switch and install the electronic starter device so maybe you don´t need to change the motor.

Regards(Saludos)

Carlos

 

[MAGTiger]

Sorry, I just scanned the thread so excuse if this has been suggested but why not use a 3 phase motor and VFD but supply the VFD with single phase. This is a fairly common solution. Just upsize the VFD 175 to 200%. so a 7.5hp VFD would drive the 4hp 3ph motor. Plenty starting torque and speed control to boot.

Neil

 

[jemjack]

I am hoping not to change the motor, and the supplier is sending me a new 400µf condenser in case that is the problem, if not I will send the motor back, as I think that it could be faulty from all the advice that everyone has sent to me. The VFD idea is certainly the best, but very much more expensive. And at the moment just detaching the condenser after startup works very well even if it is rather crude.

Thanks very much

Jemjack

 

[burnt2x]

. . .The start windings have a 400 µf condenser and the windings have a resistance of about .7? and the running windings have a 60 µf condenser and the windings are about 1.9 ?.

Is this really the actual connection on your motor? Something is wrong there, IMO. The starting winding will have a larger resistance value compared to the running winding. That run capacitor (60uf) should be in series with the 1.9-ohm winding and the 0.7-ohm winding ends tapped across the supply. I didn't pay attention to your connections in your OP.

 

[jemjack]

Hi burnt2x,

Sorry, your jpg. Is exactly what my circuit is, I got very muddled up before. But when I traced your circuit to mine, I found that they were exactly the same, so thank you very much for your help, at least now I know that the motor is correctly wired up.

Best wishes

Jemjack.

 

[burnt2x]

I tried some simple calculations and I found out your nameplate full-load power factor of 0.95 is not attainable. the most this motor of yours can perform is at PF = 0.749.

Sorry for beating this horse,(if this horse is already dead).

 

[jemjack]

Hi Burnt2x,

Thank you for taking such an interest in my problem. I am afraid that my electrics are a bit too rusty to remember how to calculate power factor or even how important it is. So would be most grateful if you could enlighten me a bit and tell me what affect this has on the motor not starting up without my disconnecting the 400µf capacitor just after startup. Also I get quite a flash across the contacts when I disconnect it. Would a high resistance ( what size resistance please) across the contacts stop this and what harm am I doing if any by the flash? Or this method of starting up the motor? Please.

Best wishes and thanks once again,

Jemjack.

 

[waross]

I would be wary of getting too close to the cap. It may be failing and may explode.

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

 

[LionelHutz]

I believe the point was that if the pf number is not even close to correct then the manufacturer is producing a motor where the true capabilities are likely not known and the catalog numbers don't mean anything.