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Soft, Hard, or Easy? Where does one possibly start?

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Well let's start back 20+ years ago when I was actively servicing marine air conditioning and refrigeration systems.

In the heat of summer, I'd often get calls mid-week asking to check air conditioning that wouldn't start during the weekend and that was popping breakers. When tested during the week, these systems inevitably worked flawlessly, but a quick look at the vessel's docking situation usually gave a clue to the problem.

Older docks with older wiring, plus "senior" boaters that need lots of air conditioning, equals voltage reduction issues on sunny summer Sunday afternoons, especially for boats at the far end of the dock. The more occupied boats there are with air conditioning on, the lower the voltage will be at the end of the dock.

Once the voltage is reduced below a certain point, the electrical load required to start an air conditioning compressor, especially older types of compressors, is often too much for weak dock power supplies. Small, often portable, generators also often cannot provide enough power to start a large air conditioner, although it will happily power it once the compressor is up and running. So, what can we do to help a compressor get started on a weak power supply?

Let's first look at why the compressor needs a big kick to get it started. The very nature of alternating current (AC) means that it's going backwards and forwards, positive to negative and back again, 60 times a second (50 in most of the rest of the world).

If we simply had just one winding (a very long piece of thin wire wrapped around a metal core to make an electro-magnet), the rotor would simply sit still and vibrate, going first one way then the other, 60 times a second, backwards and forwards, and the thing would never actually rotate.

CSCR diagramSo these types of motors have a second winding, a Start Winding, in addition to the main Run Winding, that, when energized, yanks the rotor in the correct direction, and once rotation starts, the motor will hum along happily. Older, reciprocating type compressors with Capacitor Start Capacitor Run (CSCR) motors (see diag) require a big kick, and so a Start Capacitor is connected to the Start Winding to act as a sort of booster.

But a Start Capacitor must be disconnected once the motor is up to speed otherwise it will soon have a melt-down resulting in a nasty, gooey mess. This disconnection is accomplished by using a Start Relay that senses electrically when the compressor is up to speed and then disconnects the Start Capacitor.

Now, although the compressor motor will run OK with the Start Winding disconnected, there are efficiency benefits to be had by keeping it in the circuit, so a second, much smaller capacitor, a Run Capacitor, is also connected to the Start Winding, and this is left in the circuit after the Start Capacitor is disconnected.

PSC diagramThe newer breed of rotary compressors found almost universally in smaller marine units, do not require anywhere near as big a starting kick as their reciprocating ancestors, and although there are still the two windings, Start and Run, plus a Run Capacitor, there is typically no Start Capacitor or Start Relay. These are known as Permanent Split-Capacitor (PSC) motors.

So let's say you have a modern-ish air conditioning unit with a rotary compressor that will not start on a portable generator or on ratty docks with highly questionable power. What options do you have?

1. Buy a bigger generator and stay away from ratty docks.

2. Try installing a Hard Start Kit.

3. Install an EasyStart "soft start" device.

Ignoring 1 as being totally frivolous, options 2 and 3 are add-on items for all types and sizes of compressors.

A Hard Start Kit is essentially a plug-and-play Start Capacitor and Start Relay all in one. This simply plugs on to the existing Run Capacitor with two wires, and takes a few minutes to install with just basic tools. If there is already some sort of "start assist" device installed, this gets removed and discarded.

Essentially, you'll then have the same sort of high-torque starting system that was commonplace on older-style reciprocating compressors. You can get these for less than $10 online, so well worth a try, but don't come to Coastal for help. You're on your own with this one, or hire a marine electrical guru if you feel it's beyond your capabilities.

Hard Start Kits can also be used to replace existing Start Relays and Start Capacitors on older reciprocating compressors.

A Soft Start, like the EasyStart offered by Coastal, is essentially a Hard Start Kit on steroids. It too contains a Start Capacitor and some type of Start Relay, electronic or otherwise, but also employs comprehensive electronics with sophisticated algorithms that sense and record multiple aspects of the start routine, and which constantly adjust the settings in order to produce the best result. After a dozen or so starts, the EasyStart will have learned the most efficient way to start that particular compressor,

The EasyStart requires some moderately complicated wiring modifications that may be beyond the scope of the average boat operator, so consultation with a marine electrician or air conditioning expert is highly advised.

See a demonstration with a 16,000 Btu compressor on a Honda 2000i portable generator here.

For the ultimate in soft/hard/easy compressor starts you'll need a three-phase compressor and a variable-frequency drive. Then you can start the compressor at 0 RPM and slowly wind it up to speed, incurring absolutely no start-up surge along the way.

This impressive technology is mainly for chiller systems, and is incorporated in the Climma ECO DC 40, 50, and 65 Variable Speed chiller on display at upcoming boat shows in Newport, Annapolis, Tampa, and Fort Lauderdale.


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Phone: (301) 352-5738
Office | Warehouse:
4831 Tesla Dr., Suite H
Bowie, Maryland 20715
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Coastal Climate Control
Cooling, Monitoring & Solar Solutions


Office | Warehouse:
Coastal Climate Control
4831 Tesla Drive
Suite H
Bowie, Maryland 20715
Phone: (301) 352-5738

Request Information
Click here for directions.