locked rotor amps (LRA)

Hi,

There is any conversion formula for LRA to Ton.

regards
vinod

Hi Vinod,

I don't believe you can convert amps to Tons. Certainly not LRA. This is the amperage when the motor starts and has nothing to do with the compressor capacity.

FLA (full load amps) may give you an "indication" of the motor size (HP or kW) but this does not provide useful information about capacity.

Even comparing motor HP (or kW) to cooling capacity is not very useful since different compressors may have different pumping capacity with the same size motor. This is determined by the operating conditions of the system (low temperature or high temperature evaporating).

Hallo attached are some convention

1 To = 3,5 kW
1 Btu/h = 0,293 Watt

1 kW = 1,33 HP

LRA is nothing to do with TR.

http://www.digitaldutch.com/unitconverter/
try this for converting N to Tons

Quote:

---

Originally Posted by US Iceman

Hi Vinod,
This is the amperage when the motor starts and has nothing to do with the compressor capacity.

---

Hi, guys. It may have a little mistake. LRA is the current when the motor is locked(cannt rotor), which iceman said is Start Current.:)

well he is correct as the instance power hits it is locked untill it starts to rotate, so for that instant it will be consumming full LRA basicly (This is in milliseconds if not less)

Quote:

---

Originally Posted by The MG Pony

well he is correct as the instance power hits it is locked untill it starts to rotate, so for that instant it will be consumming full LRA basicly (This is in milliseconds if not less)

---

Hi, pony. As I know, they are different. Start current usually is lower than LRA cause it has a smaller load. you can find this in any compressor's technical specification. :rolleyes:

LRA is the amps needed to accelerate the motor from a dead stop, to it's operating speed under load. This is considerably higher than the normal running current. This is the same as the inrush amps.

FLA is the normal running current that the motor will use at it's maximum capacity. This is usually much lower than LRA.

If I remember this right, for full voltage start, the LRA is about 7 times greater than the FLA. Or something like that.;)

exactly US. Ernstillin you did not understand my post, read it again a fiew times, you aut to get it.

I'm sorry for careless..u're right, pony..

There isn’t correlation between motors electrical data and refrigeration effect; you can use the name plate info to get a close idea about the heat out put of the motors (heat loss)

Here is a quick example pulling out some rating sheets I find that at 75F condensing a compessor at -40F is rated at 6.3 tons and 15.2 BHP or 2.41 BHP/ton, take that same compressor at 75F and 40F and it is rated at 65.0 tons and 39.8 BHP or 0.61 BHP/ton. The reason mainly is the volumetric efficiency of the compressor, the re-expansion of gas at high pressure ratios result in a lower amount of work being done because of essentially shortening the stroke of the compressor because of the gas left over in the cylinder re-expanding. This works on a recip. On a screw or other oil flooded compressor it is basically ruled by the cascading effect of the gas around the rotors (sealing tips and oil film), the lower the pressure difference the greater the efficiency of the compressor. Now this does not even touch on motor effieciency.

There used to be a rule of thumb though for airconditioning equipement and I cannot remember it right now but I thought is was a 1/2 hp per ton for a ball park estimate.

Quote:

---

Originally Posted by refteach

Here is a quick example pulling out some rating sheets I find that at 75F condensing a compessor at -40F is rated at 6.3 tons and 15.2 BHP or 2.41 BHP/ton, take that same compressor at 75F and 40F and it is rated at 65.0 tons and 39.8 BHP or 0.61 BHP/ton. The reason mainly is the volumetric efficiency of the compressor, the re-expansion of gas at high pressure ratios result in a lower amount of work being done because of essentially shortening the stroke of the compressor because of the gas left over in the cylinder re-expanding. This works on a recip. On a screw or other oil flooded compressor it is basically ruled by the cascading effect of the gas around the rotors (sealing tips and oil film), the lower the pressure difference the greater the efficiency of the compressor. Now this does not even touch on motor effieciency.

There used to be a rule of thumb though for airconditioning equipement and I cannot remember it right now but I thought is was a 1/2 hp per ton for a ball park estimate.

---

I do also believe that refrigerants mass flow rates are greatly affected by pressure, at higher pressure the refrigerant molecules are tightly packed (dense) increasing the amount of refrigerant vapor per given cylinders areas this turns into an increase of compressor pumping capabilities plus lower compression ratios. In low temperature applications the suction pressure drifts away from the discharge pressure increasing the compressors ratios and the lower pressure allows the refrigerants to expand and become less dense.

Just a note on electric motors
If a motor is running completely unloaded it will turn at nearly synchronous speed. This speed depends upon the way in which the motor is constructed. A two pole motor running with a supply frequency of 60 hz would in theory turn at 3600 rpm. And a 4 pole motor would run at 1800 rpm. A 50 hz supply would reduce these speeds in proportion to the supply frequency. In these hypothetical conditions the motor would absorb no current from the supply.
However as we don’t yet live in a perfect world, the motor is not 100% efficient. The windings have resistance, the bearings have friction and there are windage losses due to the rotor velocity. These inefficiencies tend to slow the motor slightly. As this happens power is taken from the supply to overcome these losses until the motor speed stabilises at what is termed is ‘free running speed’. As energy is a constant this power is ultimately converted in to heat which in this free running state is dispersed into the atmosphere. For this to happen there must be a temperature difference and therefore the internals of the motor heat up to a temperature at which this heat can be conducted from the motor.

An increasing load applied to the output shaft of the motor results in further slowing of the motor, more current is absorbed to overcome the applied load plus the increased internal inefficiencies of the motor which result in higher internal temperatures. This effect will continue until the load is sufficient to stall the output shaft. The current drawn from the supply at this condition is termed the motor ‘Locked rotor amps’.

However our compressor motors are not designed to operate at this stalled condition. We require them to turn our compressors at a speed to meet with our cooling requirements. Motors are therefore usually designed to operate with a slip of approx. 4 % of full load speed. (Slip is the percentage of synchronous speed the motor is running at). This condition is termed the ‘Full load speed’ and the ‘Rated full load power’ of the motor. The power absorbed at this condition is termed the ‘Full load amps or fla’ This is usually in the region of one sixth of the locked rotor amps. The motor being designed to dissipate the internal heat at this condition without causing damage to the insulation of the motor windings. Allowance being made for short time overload and for starting.

Remember it’s the temperature limitations of the motor windings that determine the maximum power of the motor. That’s why we fit klixons and other temperature sensors as the fuses won’t necessarily protect against overload

So if you switch on a motor direct on line (DOL) it will initially take up to 6 / 7 times the flc of the motor as this is in effect a stalled condition, admittedly normally for only for a very short time.

This starting current in most applications can be reduced by various means depending upon the torque / speed characteristics of the compressor or whatever the motor is connected to.

Some small motors are designed to operate at a stalled condition indefinitely, think of car door window winder motors and some control motors.

Hope this helps

mick

Quote:

---

Originally Posted by johny*****

I do also believe that refrigerants mass flow rates are greatly affected by pressure, at higher pressure the refrigerant molecules are tightly packed (dense) increasing the amount of refrigerant vapor per given cylinders areas this turns into an increase of compressor pumping capabilities plus lower compression ratios.

---

That is exactly right. At higher pressures the gas density increases, so you have more mass per unit of volume. Since the compressor is only pumping volume (it doesn't know about Tons, kW, kcal, etc). the mass flow increases as the suction pressure increases.

That's why the capacity increases so drastically. The volumetric efficiency is only a small part of the overall increase. The compressor pumping capacity increases incrementally as the pressure ratio decreases, since there is less re-expansion effect.

Hi Vinod,
Maybe you mean a check figure or a ballpark figure and not a conversion formula from LRA to Ton of Refrigeration.

Some of the figure i have been using since the 80's:

For aircon: 40F(evap temp)at 100Fct(condnsng temp) approx 0.87 HP per Ton Ref
For 0F et/100F ct approx 1.7 HP/TR
For -40F et/100F ct approx 3.25 HP/TR

You can then figure out HP from LRA and then TR capacity using above BALLPARK figures.

Usually i use all those figures ONLY
(1) To have a feel of the capacity when in the field and cannot access your reference books/manuals or
(2) Use the figure to check the capacity after i have made the DETAILED computation.

I always tell my engineers: there are no short cuts, there are no magic figures.