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4hp single phase motor problem

4hp single phase motor problem

4hp single phase motor problem

(OP)
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 .7Ω and the running windings have a 60 µf condenser and  the windings are about 1.9 Ω.

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

Jemjack
 

RE: 4hp single phase motor problem

Hi jemjack

I'm no expert however its of no surprise to me that the motors getting hot if the start windings are still in circuit, can you alter the centrifugal switch to cut out at the lower speed?
One thing you haven't mentioned in your post is the torque required for your system, its possible that the motor cannot produce the torque required to operate the load at the speed you require.
Remember its the load demand which determines the power output of the motor and you haven't told us what the load demand is.
One other possibility is that you have a faulty centrifugal switch but I doubt that if it opens correctly under no load.

desertfox

RE: 4hp single phase motor problem

At the motor (while starting) you read 180V, but what is the voltage on the other end of your cable nearer the meter?   

RE: 4hp single phase motor problem

Is the compressor unloaded during start-up, if not that may be the problem. Are there unloader valves on it?

rasevskii

RE: 4hp single phase motor problem

Are you trying to start the compressor against back pressure?
Does the compressor have a faulty unloading system?
The compressor depends on flywheel inertia to complete the compression cycle. If any load comes on the compressor before the compressor is running fast enough to develop adequate flywheel inertia it will stall or at the least stop accelerating.
Check the 400 uf capacitor by replacing it with a new 400 uf capacitor. Then if it won't start try both 400 uf capacitors in parallel.  

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

RE: 4hp single phase motor problem

As was mentioned your compressor might have an unloader problem.  Your compressor can NOT start under any load.

Disconnect the compressor's output air line so the air is free to vent to atmosphere and start it.  If it starts correctly you have an unloader problem.

Your system may have other problems too, but one thing at a time.

Keith Cress
kcress - http://www.flaminsystems.com

RE: 4hp single phase motor problem

(OP)
Thank you very much for all your help, I will check the voltage at the meter when starting up, though I think that it may well be the same, as well over 50 amps is asking a lot.  

The compressor defiantly does not have an unloader problem, as it worked with the 3 HP motor before, (which was an English motor rated at 240 volts so was running way below its rated voltage) and the air line was unconnected. There is no back pressure. The compressor turns over quite easily. I don't have any details about the torque required for the compressor, only that it should be supplied with a 3 kw  single or 3 phase motor or a 3.7 kw gasoline motor.

I would not think that there was any way to change the centrifugal switch to a slower speed is there?

I will certainly check the 400 µf condenser, I have all ready checked it for resistance, leakage. And try doubling the capacitance, this seems a good idea, although the manufacture specified the 400 µf. Do you think that this would affect the guarantee?

Once again thank you for all your help

Jemjack
 

RE: 4hp single phase motor problem

Is the motor wired for the correct voltage?

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

RE: 4hp single phase motor problem

Have you tried powering the motor from a different power supply?
Take it to a suitable repair shop & get them to test it.
You might have a problem with your building wiring system.
Bad or loose connection??   

RE: 4hp single phase motor problem

Hi jemjack

The torque rating of the motor is most important in motor selection, I don't suppose you have a motor torque curve for the motor you purchased?

desertfox  

RE: 4hp single phase motor problem

I don't know what is typical for Spain, but if 50 or so amps of starting current is "asking a lot" of your incoming service, there is a good chance that you have just found the issue.  

RE: 4hp single phase motor problem

Hello jemjack

I am agree with desertfox, the problem coul be maybe you buy an standard torque motor and really you need a high torque motor

In Spain the single-phase motors are clasified in standard , medium and high torque,if you don´t know about the hi-torq motor and  the supplier  sold you whats he think or what he have in the moment.(Inmediate delivery) then this could be the failure.

Also I will check the belt tension and pulleys alignment this two issues could cause problems like overloadding.

Attached you will find some good information.

Good Luck(Suerte tío)

Carlos  

RE: 4hp single phase motor problem

(OP)
Hi everyone thanks once again,

yes the motor is wired for the correct voltage, thanks.

I haven't got a torque curve for the motor, but the specification is,3 kw 4 hp 18.2 Amp 2820 rpm,  fp cosф 0.95, rend 79% ,ppc (nm) 79,  Pa 2,  PMa 1.7  1a ipc 7 wt 23.2 Kg,

Is this any help?

Carlos, the motor is "SERIE MONOFÁSICA DE ALTO PAR DE ARRANQUE" so I suppose that it is high torque, and the belt is correct and in line, so that is not the problem. But thank you for that information.

I am wondering is it is correctly wired up in the connection box? Its z wirings are 1.9 Ω and permanently in circuit through the 60 µf condenser and the 400 µf condenser which should switch out after startup, Its u windings are .7Ω permanently in circuit  in parallel with the z windings.

I would be very grateful if someone could give me a wiring dia for this type of motor.

Thank you once again for all your help.

Jemjack
 

RE: 4hp single phase motor problem

If the motor rpm is less than 2900*.75=2175 rpm the centrifugal switch does not work and the current is elevated for longer than permitted. Since the motor alone can reach the rated speed the motor could not be the problem.
At the start, if the compressor is still not loaded the only load is the inertia moment [I think it is not so elevated].
If the supply source is not changed and the voltage drop is indeed = (215-180)/220=16% 4hp motor can start well.
Usually, if the compressor is rotating at low speed the cause could be a slipping belt. But in this case the motor speed is low and the transmitting ratio seems to be the rated.
I think the cause could be the compressor it self. The load is rising up when still in starting process-problem of valve, oil pump, filter and other.
 

RE: 4hp single phase motor problem

The start winding should be in series with the 400 uf cap. Both the winding and the cap should be disconnected by the centrifugal switch in the designs that I am familiar with.

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

RE: 4hp single phase motor problem

I think this is similar to what I have had but in that case I experienced, the unloader valve was the culprit! The unit I got relies on a timer and the timer was inadvertently turned earlier causing the motor to be loaded even when it has not reached the required speed. Tweaked the timer longer and the motor-compressor unit worked fine.

BTW, granting your unloader valve is okay, I did some quick calculations based on your data:

Your actual pulley ratio = 2.76

Required rpm for compressor = 1300 rpm

@a full-load speed of 2820 rpm, your compressor shaft speed is:
Compressor speed = 2820/2.76 = 1021.74 rpm.

The derated output = 1021.74/1300 = 78.6% of compressor CFM rating

Derated power required = 4 X (1021.74/1300)^3 = 1.94 hp; say 2 hp (95% efficiency)

Assuming you are filling air bottles with rated pressures of 3000 psi (207 bars) and you are using a compressor with a max pressure of 300 bars (4350 psi), your final pressure head at 1021 rpm is:

final pressure head = 3000 X (1021.74/3000)^2 = 1853 psi (127.76 bars)  - definitely not a filled cylinder; a far cry from the 3000 psi you wanted!

Also, assuming your bottles have initial pressures of 50 psi before filling and you wanted it filled-up in 5 minutes (3000 psi, 3 gallons(11.2-liter bottles), you probably needed 352 liters per minute compressor (without air banks). If your compressor is of lower air capacity, expect a longer filling-up time, IMO.

Either you decrease the diameter of the driven pulley or increase the motor side pulley to hit a pulley ratio of 2.17.

RE: 4hp single phase motor problem

Torque motor don't overpass load required torque and speed curve is limited at 1950rpm with high current from grid because low back-emf. I think you need to increase pulley ratio to 3.3 - 3.5 to overpass load torque demand even if in such way speed will be lower.

RE: 4hp single phase motor problem

(OP)
Hi waross, that's what I thought, but this defiantly is both coils left in circuit. Thanks burnt2x for all your information, luckily I am only filling to 150 ats most of the time, and I can do that in about 15 mins which is ok for me. The motor ran today, see below at 2500 rpm, so the compressor ran at 1000 rpm, by changing the motor pulley to a slightly bigger one I could have the compressor running at 1300 which is its correct speed, but perhaps it is better to leave things as they are ?

I have now got the compressor working, by overriding the centrifugal switch, I disconnected one wire of the 400µf compressor and touched it on the terminal for a couple of seconds, and then disconnected it, and the motor worked perfectly. 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 when I switched it off and the temperature then rose to a maximum of 63ºc.

This is a bit of a crude way of running the compressor, but there we are. I realise that the wiring would reach a much higher temperature than the 54ºc  that I measured on the case, but is this reasonable? I presume that since the motor only drew 13 amps, it was not working very hard, this 4 hp motor should draw 18 amps! Also why did it only reach 2500 rpm it should reach about 2850 rpm shouldn't it ?

Thanks once again for all your help

Jemjack.
 

RE: 4hp single phase motor problem

Typical slip for that size of motor should be around 3-5%. Since the motor ran at 2500 rpm (16%), something is wrong with the running windings.

One more question here. Was the drive belt on when you took rpm measurement of 2500? Motor should be running at near synchronous speed (3000) when free. If it did not, then we can be sure something is wrong with your motor's running windings.

Good that your compressor works for now. Thanks for the feedback.

RE: 4hp single phase motor problem

(OP)
Hi Burnt2x and everyone else who has been so helpful,

The motor was under load from the compressor when the rpm reading of the motor was 2500 rpm. A few days ago I tried the motor without the belts, and it ran perfectly, starting correctly and running at 2900, at 215 volts.

I suspect the windings or some other fault, as the compressor started perfectly before using the same pulley ratio and a uk 240volt 3hp motor, the trouble with this motor was that it got very hot after quite a short while. And the compressor specified a 4 hp motor.

Is 54 ºc reasonable for the case temperature of this type of motor? Please.

Thanks to everyone, best wishes

Jemjack.

 

RE: 4hp single phase motor problem

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

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

RE: 4hp single phase motor problem

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.

 

RE: 4hp single phase motor problem

(OP)
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
 

RE: 4hp single phase motor problem

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!
 

RE: 4hp single phase motor problem

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

RE: 4hp single phase motor problem

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

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

RE: 4hp single phase motor problem

(OP)
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
 

RE: 4hp single phase motor problem

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

RE: 4hp single phase motor problem

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.
 

RE: 4hp single phase motor problem

Good point Lionel. lps

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

RE: 4hp single phase motor problem

(OP)
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.
 

RE: 4hp single phase motor problem

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

RE: 4hp single phase motor problem

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

RE: 4hp single phase motor problem

(OP)
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

 

RE: 4hp single phase motor problem

Quote (jemjack):

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

RE: 4hp single phase motor problem

(OP)
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.
 

RE: 4hp single phase motor problem

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).

RE: 4hp single phase motor problem

(OP)
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.
 

RE: 4hp single phase motor problem

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

RE: 4hp single phase motor problem

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.

RE: 4hp single phase motor problem

@Jemjack,
It really is very complicated with that cap-start, cap-run single-phase motor you got! That is an induction machine where you get to separate the rotor circuit calculations because there is a forward and reverse mmf significantly making motor circuit modeling a bit bloody.

BTW, you posted the nameplate specs of FLA = 18.2 amps; Volts = 220V and the pf = 0.95. But if you make a quick calc: P = 220 X 18.2 X 0.95 = 3,804 watts or 3.8 kW! If you use pf=0.749, then you get the rated output of 3kW. If indeed the FLA and pf is correct, it is safe to say your vendor gave you a 5HP motor instead of a 4 HP (3.8kW ~ 3.7kW ~5HP).

RE: 4hp single phase motor problem

burnt2x,

IEC motors are rated based on their mechanical output. Our motors also have losses. You need to factor in the efficiency, which in a small machine is relatively low, and in a single phase machine is lower still.
  

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

RE: 4hp single phase motor problem

ScottyUK,

If I will factor-in the motor efficiency in my calcs, I get 78.87% efficiency! (0.95/0.749 = 0.7887 or 78.87%)! Is that within the normal range of efficiencies of IEC motors?

RE: 4hp single phase motor problem

Good grief.. PF times efficiency... To get efficiency... I think it is better close this thread - or just delete it.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: 4hp single phase motor problem

(OP)
Oh dear have I stirred up a hornets nest here? Thanks anyway.

RE: 4hp single phase motor problem

No, not a hornet's nest - just a bumble-bee.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: 4hp single phase motor problem

It may be that the problem is not electrical but mechanical nature.  Assuming for the moment that the centrifugal switch is not at fault it could be a slight misalignment of the pulleys or excess tension in the belt that is causing the bearings to bind enough to create an additional load on the motor thereby limiting its ability to accelerate properly.  Considering that the motor performs properly when the belt is removed, I would be looking to make sure that:

1. the centerline of the pulley grooves are in the same plane,

2. the belt tension is not excessive,

3. the motor is intended for this use as opposed to direct drive; i.e., are there ball bearings, roller bearings, and or thrust bearings, since each is intended for a different loading.

4. there's enough of the right grease in the bearings and lubricant in the compressor (assuming this isn't an oil-less system).

 

RE: 4hp single phase motor problem

There needs to be a good clearing-up job there. Sorry for the inadvertent typo in last post. Here:

Pout = V x I x (PF x eff)

The initial formula used was:

    Pout = V x I x (pf), the result of my initial calc was 0.749.

If one wants to know the efficiency, the Pout, V and I cancels out leaving:
    pf x eff = PF
PF, the power factor arrived at neglecting the motor efficiency and pf = the nameplate PF rating of the motor. The resulting equation should have been:

eff = PF/pf = 0.749/0.95 = 0.788 or 78.8%

Now, that should absolve me of whatever mistake I made!
 

RE: 4hp single phase motor problem

I'll buy you a coffee in the Pub, Gunnar. Time to change the subject.
Yours
Bill (grin)

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

RE: 4hp single phase motor problem

(OP)
Thank you RamConsult  I am afraid that it is an electrical problem, as all your suggestions that you have made have been attended to and all is in order mechanically. Hi Burnt2x now I know.

Thanks once again. Who wants a coffee in a pub?

Jemjack.
 

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