Hard start compressor motor applications

[tree_75]

I need someone to shed some light on the priciples of hard start kits commonly applied to HVAC installations. I deal with industrial three phase motors on a daily basis. For isolated, off-grid installations (mostly offshore), we often utilize soft starters on large motors to help reduce the required starting current capacity of generators. While chatting with one of my neighbors who is an HVAC tech (unsure of his competency though) I asked if there were any common modifications to AC circuits that allowed for reduced current starting. He mentioned "hard start kits" but could not explain the concepts involved. I'm trying to size a generator for home use and would like to consider means of reducing the required starting current for my AC units (4 &

1.5 ton units).

Thanks in advance for any valuable knowledge! tree

[Dido]

From my experience I have found that are big variations in starting of refrigeration compressors. Hard start and soft start amperage will very but only for short period, well how on the soft start amperage may be little smaller but for longer time, in hard start amperage will be higher but for shorter period of time, so the average current it will most likely be very close, you will have to choose what you can with stand more, with these phenomena you may had to experiment your self to find what works better, remember time is milliseconds and all compressors are not same including that some compressor may run without starting cap. or hard start (device) and yet it could be use with both. It depend on OEM design and how often machine start and stops. Hard start kit takes place of starting relay and starting cap. so if your system using starting relay and cap. you will not use hard start kit (device) their will not be any benefit from it. I work with all setup above and I can not see that anyone can tell you what will work for you in your situation. Good luck from Dido

[Dido]

From my experience I have found that are big variations in starting of refrigeration compressors. Hard start and soft start amperage will very but only for short period, well how on the soft start amperage may be little smaller but for longer time, in hard start amperage will be higher but for shorter period of time, so the average current it will most likely be very close, you will have to choose what you can with stand more, with these phenomena you may had to experiment your self to find what works better, remember time is milliseconds and all compressors are not same including that some compressor may run without starting cap. or hard start (device) and yet it could be use with both. It depend on OEM design and how often machine start and stops. Hard start kit takes place of starting relay and starting cap. so if your system using starting relay and cap. you will not use hard start kit (device) their will not be any benefit from it. I work with all setup above and I can not see that anyone can tell you what will work for you in your situation. Good luck from Dido

[Jake]

As Paul said... a Hard-Start is nothing like a soft-start.

Hard-starts are simply stored energy capacitors that inject energy (sometimes out-of-phase) into a circuit for a very short period of time. This gives single phase motors a little more -ommph- for staring but has little bearing on starting current.

Soft starts a frequently 'frequency choppers', which vary the frequency/voltage curve of three phase motors in applications like pumps and fans... when the current draw is nearly linear to how fast the connected load is moving.

Most AC units list something called LRA... which means 'locked rotor amps' and is a very good indication of the current required to start the equipment. If you have a compressor/condenser with a LRA of 42, for example... and the supply voltage is 240 volts... then 42 times 240 =

10,080... which is roughly the watts required to start the load. Some generator makers will supply 'surge ratings' this high in a 7500 Watt generator. Look around.

There is no magic cure for the current requirements in starting an induction motor. Scrimp on the capacity and you'll be sorry.

Jake

[jbgreig]

I had one of these installed to help start my 5 ton AC on a generator, and it helped quite a bit.

I also notice that the lights still dim a bit, but it is for a much shorter period of time. Just a quick blip.

Brad