hard starting load basics?

[archibaldtuttle]

Need to replace a compressor motor that has always struggled with cold starts and melted down when the overload failed on a cold start.

I could have this motor rebuilt again. theoretically it was matched to the load in the day (1950s refrigeration compressor) Its a massive Westinghouse 5 HP single phase. In normal operation in refrigeration,it would, ironically never be cold and would have had short cycle timers to prevent starting against pressure, but i read in various threads elsewhere, e.g., that folks would add soft starter to refrigerator compressors that didn't come that way OEM to ease struggling on start and high start current.

this is Westinghouse 456113A Sser. # 4803 - 1710 RPM on a 254 frame (which these days by NEMA standards would be on a 15HP motor)and it draws 28 running load amps. Don't know if that indicates the efficiency of modern motors that draw 21 amps for 5HP rating or it indicates conservative rating in the old days -- or a little of both. I have some motor guys look at it and say I'd need a 10 HP motor to do equivalent work.

don't know if that is knowing homage to these antiques or just the size of the motor is so much bigger than equivalent HP today it begs comparison to much higher HP motors.

Using contemporary tables the assumed torque for horsepower is about linear with 5 HP at 2000 RPMs (guess they picked 2000 cause its even #. motors are really in the 1700 to 1800 range) 13.1 ft. pounds and 7.5 HP is 20 ft. pounds and 10 HP is 26 ft. pounds.

So theoretically if I want more torque i just replace with higher horsepower, but those figures are torque at speed not locked rotor or starting torque which seem more relevant to me. obviously i don't have a really good idea of the theoretical load of the compressor i'm running and can't find any specs on the original motor - maybe there is some ole book somewhere but the model doesn't come up on the internet anywhere.

In nibbling around the edges on the starting problem, i always thought the other hard starting solution was a motor manufactured with extra start windings that dropped out on a centrifugal switch. But i don't really seem to find any offerings that indicate this design. Maybe I'm not using the right terminology. Assuming i could use the soft starter linked above, that theoretically can handle 7 HP. its RLA load is up to 32 and none of the 7.5 HP motors i've looked at go above 31 so don't know if i could get away with it on 7.5 HP.

The obvious other answer is that, for the price of a motor or rebuild, i could buy an entire chinese compressor set up that wouldn't have the same capacity as this unit but would have enough capacity for most of my uses. But this old Compressor runs absolutely smooth as a cucumber at speed and it really has a lot of capacity and i've already got it paired with 150 gallon tank so I'm at least trying to understand approaches to hard starting loads before ditching it.

thanks for any thoughts if you happen to be sitting around on the 4th thinking abstractly about motors like me.

brian

 

[jraef]

HP is just a shorthand expression of torque at a particular speed. So if the motor is rated 5HP, it has the same torque as another 5HP motor if running at the same speed. What IS a little different about your motor is the 1710 RPM, as opposed to a typical motor being 1750+. This indicates that your motor is higher slip than normal, which also generally results in higher peak torque, which is useful in re-acceleration or starting from a stop. Both conditions are good for reciprocating compressors, although likely at the cost of having lower efficiency. A commercial off the shelf design B motor would be more efficient, but would not provide quite as much peak torque, so you might have to over size the motor, which tosses the efficiency issue out the window anyway. I'd stick with what you have.

Putting a soft starter on a recip compressor is fine, as long as you make sure the unload valve is ALWAYS open to atmosphere on startup. A soft starter reduces current, yes, but it is really about reducing TORQUE. So if you motor struggles now, that actually will get worse, not better. Also, soft starters and single phase motors are often not a great combination. It depends on the type of single phase motor that you have. Taking too long to accelerate can cause damage to the starting capacitors, if any.

Best place for you to start on this then, in my opinion, is to figure out why your motor "melted down". How did your OL fail that allowed that to take place? That's definitely not normal. You need better protection.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[itsmoked]

Thoughts...
Skip the soft starter nonsense. Your system is struggling to start up, reducing the motor's available torque with a SS is probably not going to help.

The amperage read on a meter or nameplate on motors can be a little misleading because that current includes the current used to deliver the HP and the current used to excite the motor, the reactive current. If a motor was designed with a lousy power factor (pf) then it will consume a bunch more of the reactive current leading to a higher amperage.

Only single phase motors have "start windings" and it's not a case of some do and some don't (at least standard squirrel cage induction motors as we're discussing) , they all require them.

No one bothers with 1 phase motors over about 5HP because they are oodles more expensive and get really HUGE. I hope you're not making comparisons between 1ph and 3ph motors, it's not real clear to me.

If you have 3ph power I'd suggest you change to a 3ph 5hp motor for your compressor and call it a day.

If not, I'd contemplate several possible problems that took-out your old motor. Replace the old motor with another of the same horsepower and make sure it's a capacitor start motor which will have more starting torque than a conventional 1ph motor. Replace the motorstarter with a new one with electronic overloads so the failure prone crappy heater overloads don't fail and destroy yet another motor. They often fail in refrigeration service causing havoc and costing thousands of extra dollars damage.

Keep in mind that using a too-big motor will cost you a little more because the motor will run with worst efficiency and a lower pf - which could affect your power bill.

Make sure your refer system is working correctly. Stuff like, does the compressor have an unloader? If it does but it's not working it could be a 5hp motor will never get it running. Or, is something causing short cycling which will fry the motor quickly. Is something allowing liquid back to the compressor? Is the head icing up? All the basic refrigeration checks are in order.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[archibaldtuttle]

thanks for these replies.

itsmoked

seems to capture the problem. i had no idea that thermal overloads had a flakey reputation. i don't run a lot of large motors and didn't realize i could have installed electronic over current protection.

this reefer is no longer in refrigeration service. i'm using it as an air compressor. it has an automatic unloader for normal op and i manually unload it with a full port valve on the manifold when cold starting after long off periods.

i kind of wondered how starting lower voltage and ramping up - from what i could see the claimed method for soft starter - would really help. i assume this is all supposed to happen fairly quickly because if you sustained lower voltage it is going to draw more amps which is going to heat the motor unless this goes quick. maybe there is some kind of quench on the amps through the soft starter. you especially crystalized my concern by pointing out that there is no magic here, it reduces torque.

unfortunately i do not have access to 3-phase. i have friends who make three phase by running a three phase motor on two phase and hooking up service for their actual work motors to the full 3 phase output. He starts the 'converter' motor with a rope wrapped around its pulley like an outboard motor start.

That is a plan when i bring various 3 phase tools online but this compressor came in at single phase and i left it that way so as not to need to resort to 'converter' for the time being. of course 3 phase motors are a dime a dozen relatively speaking, so maybe i should bite the bullet here.

jraef

i always wondered what the little variations in rpms around target speeds meant. For the amount i run this, i'm not vastly worried about efficiency. i have seen the torque curves presented in abstract fashion but it didn't occur to me to read what was happening.

the motor tales off toward no torque and no current if it free wheels over nameplate speed with only its own bearing load, kind of thing. as load is applied current consumption rises and assuming the load is balanced to the motor it hits the RPM target at about the rated Run Load AMps.

efficiency is not a big concern for me, given the amount i run this, so i almost wish i could get a motor that slips to 1500. the replacement i've been offered is a 7.5 HP at 1800 RPMs. conceivably, if the actual run load is 5 HP it would run faster than that.

Now I see my preference would be a motor with a lot of slip rated even slower at 5 HP. I don't know if there is a category of motor aimed at 1200 RPM for instance. It really wouldn't much affect operation as an air compressor if this ran at 2/3 speed. I've got plenty of capacity and the operating pressures are lower than it experienced in refrigeration service so it is easier to keep up.

is there a line of motors designed with a lot of slip or simply aimed at even lower RPM service that displays more locked rotor torque? is the B motor you describe standard. are there As or Cs or . . . ?

finally back to the start winding/capacitor start question. not sure from an electrical engineering perspective quite why start windings are a single phase device vs. three phase. it is pointed out that large HP singe phase motors get huge. i guess this is to get the needed space / separation for all these windings. are the capacitors just in service of the start windings or do they boost voltage or simply energize the start windings on discharge during starting. are they de-energized by centrifugal call over a certain RPMS or how are they engaged. If some capcitors is good, are more better?

thanks,

brian

 

[ScottyUK]

Brian,


Motor speeds are determined by the supply frequency in your part of the world. In mine it is 50Hz, in yours it is 60Hz. Synchronous speeds in the 50Hz worlds are 3000, 1500, 1000, 750 rpm and theoretically lower still although rare in practice. In the 60Hz world the equivalent speeds are 3600, 1800, 1200, 900 rpm, etc. In all cases induction motors run a little slower than the synchronrous speed. You could probably find one which runs at a nominal 1200 rpm, although low speed single-phase machines are relatively uncommon.

On the capacitor question...

A three phase winding creates a rotating magnetic field, so the motor is inherently self-starting. A single-phase winding creates an oscillating field, not a rotating one: it can put energy into a rotating motor, but it won't initially set the rotor in motion. The starting winding uses the capacitor to introduce a phase shift in the current to that winding, which in turn creates a poor imitation of the third phase which creates a rotational torque. The 'poor imitation' is the reason why single-phase motors have relatively poor starting characteristics compared to a three-phase type of similar power output.

 

[LionelHutz]

You could always use a VFD as a phase converter and then use a 3-phase motor. It's not terribly difficult to do, you just size the VFD to about 2x the motor. For example, 5hp motor vs 10hp VFD.

 

[itsmoked]

All good stuff above.

The main reason compressors need to start quickly is because it's often a race between the motor making it to speed and the load caused by the pressure building against the compressor piston(s). If you do anything to slow the start progression the pressure can beat out the acceleration and stall the motor. Usually a very ugly scene results that includes bad smells and cursing. A head unloader that keeps the load off the pistons until released avoids this and allows more starting possibilities.

Loaded compressor starts can be seen with domestic refrigerators occasionally. You will hear the refrigerator start humming but not start. This will go on about 2 seconds before a thermal overload/delay trips. The delay is there to allow the compressor to cool and to allow the gas pressure pinned between the compressor piston and the capillary tube (restriction) to bleed away allowing a start(hopefully) during the next attempt.

I've used the 3ph synthesis LionelHutz suggested many many times with good results. A 10HP VFD costs about $550.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The frame size doesn't tell the whole story.
Current motors use "T" frames.
Old motors used "U" frames.
Very old motors used NEMA frames.
If you can find a comparison chart you will see that A "T" frame size will produce about twice the HP of an old Nema motor of the same frame size. We were horrified when the "T" frame motors came on the market and we were told that we had about 14 seconds locked rotor to burnout. The old "U" frame motors would take a lot more abuse than the "T" frames. A LOT more.
Another option for an occasional air compressor is a smaller drive pulley. Just don't go too small.

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

 

[itsmoked]

14 seconds!!?? Our 5HP 1ph starts emitting visible smoke after 2 seconds of stall when the head unloader is um, ah, badly, managed.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[KuanYau]

To Mr archibaldtuttle

1. Your observations
a) [single-phase] compressor motor always struggle with cold starts],
b) motor melt down when overload failed on a cold start.

2. Assuming the unloader, centrifugal switch or starting current relay are in order; the likely problem is the starting capacitor which had been aged resulting to lowered in its capacity {micro-farad} value. This resulted to lowered its starting torque.
Replace the starting capacitor with one of equal or slightly higher in capacity (micro-farad) or voltage (Vac) rating.
Attention: 1. The starting capacitor is always wired {in series} with the starting winding. For capacitor-start motors, the capacitor is cut off by the centrifugal or starting current switch. For capacitor-run motors, the capacitor remained permanently connected.
2. There may or may not be another power-factor correction capacitor. Its capacity is irrelevant on the starting torque.

3. Replace the old over-load with a modern thermal or electronic thermal over-load. Select a type with (tripping time around 8-10s at 6x the set current), from cold.
Attention: If you are replacing the existing (single-phase) over-load with a (three-phase); wire {all three sensing elements} in series.

 

[archibaldtuttle]

thanks again for more food for thought.

so i happened to find brand new 575V 10 HP 6 pole motors on offer from ebay but local pickup. who knew.

i'm a little confused on 3 phase options. so called 208/480 ? is the standard? i've got new 575 V 6 pole motors 10 HP on offer right down the street on close to we'll pay you to take 'em away. they were surplus out of textile industry.

and these 6 pole motors are touted as 1770 RPM which i thought according to frequency that a 6 pole motor would be 1200 rpm range.
but maybe i mistake the frequency that these 55 Volt motors expected to see.

and found another thread that said they will run finr but at reduced HP on 480,but would the VFD really show 480? and purportedly this 480 is relaly 300V standard . . . ??? and the 575 is 600 V standard. If such a thing might work, of course these are nominally bigger motors so i'd have to find a nominally bigger VFD given the recommendation of double the capacity of the motor, so maybe i'm barking up the wrong tree. was just surprised to find something on ebay right around the corner.

what are the sensitivity of VFDs to lightening issues? one thing i'm always concerned about is putting electronics where they are not truly needed. my other alternative although i have resisted but it is the old simple approach is to use another motor as 3 phase convertor. not sure of recommended sizing of convertor motor compared to load. does it work similar to VFD , double size or . . .

Of course i could easily afford several of those 575 volt motors but i'm not sure they would run on 240 even if started by hand (we used to use a rope on the pulley of the convertor motor like starting a lawnmower.

I guess yet another nother option would be adding more capictance in series with the start winding of the motor i have. i'm going to struggle it into the car and drop by a rebuilder just to walk through those options as well.

and/or keep looking. will i ever find a 6 or 8 pole single phase motor if I bide my time? think that would help. maybe it is possible to stick with a 4 pole and reduce pulle size a relevant amount. current pulley is 4 sheeve 5" OD. I have to scratch my head a little thinking of what diameter you actually do your calcs at for effective RPM reduction. outside of belt, middle of belt, inside of belt. but i could get a 20% reduction in RPM is i went down to a 4" pulley. can't see going a lot smaller with the bend of the belts over smaller pulleys and the shaft size being 1.25 or 1.375. maybe i could go a little less than that. i can look at my options.

think that covers about all the suggestions and i definitely think the old NEMA standards kcik butt as far as what the motor can take before melting. i even tried to start this motor at 6 times rated current for maybe 10 seconds and cool as a cucumber.although i sympathize with itsmoked on the poor unloarder management problem.

keep those cards and letters coming.

thanks

brian

 

[waross]

Sounds like some surplus motors out of Canada.
Standard motor voltages are multiples of 115 Volts. Standard supply voltages are multiples of 120 Volts (with root three factors thrown in on four wire three phase systems.
So 115 Volt motors run on 120 Volt systems.
And 230 Volt motors run on 240 Volt systems.
And 460 Volt motors run on 480 Volt systems.
and in Canada,
575 Volt motors run on 600 Volt systems.
At 230 Volts and 460 Volts most motors for these voltages are re-connectable for either voltage. 575 Volt motors will have three leads and are only suitable for a single voltage.
And for use on 120/208 Volt systems, motors are available rated at 208/230:460 Volts and some motors are rated at 200 Volts.

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

 

[itsmoked]

Bill, do you want to dive in and use the new editing feature on your post?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Thanks Keith.
Bill

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

 

[LionelHutz]

Well, that is a 1175rpm motor. You can clearly see the real rpm on the rating plate picture. You would need to create 1-phase or 3-phase 600V power to be able to use that motor with a VFD. Good news is you'd have no issue with the motor size if you double the HP and also cut the speed to 2/3.

Torque reduces by the square of the voltage reduction. So, using 480V instead of 600V reduces the possible torque the motor can produce to 64%.

 

[waross]

The most bullet proof, indestructible motor I have ever seen was a repulsion-induction motor on an old refrigeration compressor. I don't know the history or if this was an original motor. The drive belts were very loose but not slipping. The belts seemed to be over sized for the application. The belts were tightened to normal tension. Several days later it was discovered that running the belts tight caused too much drag or load on the motor. The motor would accelerate as a repulsion motor until it transferred to induction mode. Then the load would pull the speed down until the motor transferred back to repulsion mode.
Any other type of single phase motor that was loaded to the point that it was cycling on the start winding could be expected to let the smoke out within minutes. This motor had been cycling between run mode and start mode for days. The belts were loosened and normal operation resumed.
Too bad these motors are no longer available.

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

 

[itsmoked]

I can concur Bill. I saw one taken out of refer service and pressed into air compressor service. They'd powered one of its lines via a ground rod. On days it struggled they watered the ground to increase its supply.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Remember the story about the dog that always whined and peed just before the phone rang. GRIN

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

 

[itsmoked]

Oh yeah.... LOL

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

To share the allusions;

jraef (Electrical)
22 Nov 05 02:08
An elderly lady phoned her telephone company to report that her telephone failed to ring when her friends called -- and that on the few occasions when it did ring, her pet dog always moaned right before the phone rang.

The telephone repairman proceeded to the scene, curious to see this psychic dog .... or the senile elderly lady. He climbed a nearby telephone pole, hooked in his test set, and dialed the subscriber's house. The phone didn't ring right away, but then the dog moaned loudly and the telephone began to ring.

Climbing down from the pole, the telephone repairman found:
1. The dog was tied to the telephone system's ground wire via a steel chain and collar.
2. The wire connection to the ground rod was loose.
3. The dog was receiving 90 volts of signaling current when the phone number was called.
4. After a couple of such jolts, the dog would start moaning and then urinate on himself and the ground.
5. The wet ground would complete the circuit, thus causing the phone to ring.

This goes to show that some problems can be fixed by pissing and moaning.

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