DIY Soft Starter circuit

[AntronX]

After looking at the prices for compressor soft starters I wonder how hard would it be to make one yourself. After some research I found this circuit diagram of Triac based soft starter. I know there has to be start winding phase delay capacitor that needs to phase shift it by 90 deg. Does any one know more info about these soft starters and how they work?

 

[ricardocello]

Can't help with the circuit, but the EasyStart and others all switch in a relay to handle the running current after a second or two.
I assume this allows the triacs to cool down before the next startup event.

 

[AntronX]

I assume this allows the triacs to cool down before the next startup event.

To bypass them so they don't waste power after compressor starts.

 

[Quattrohead]

Big arse thermistor is a method I have seen but not yet used.

 

[ricardocello]

They cost so much because of all of the bells and whistles: Bluetooth, phone apps, learning modes, microcontroller with nonvolatile settings and bluetooth module, pretty clear cases, etc.

I wish someone would sell soft starters with simple dipswitch adjustable ramp speed. But no one would probably buy it.

 

[AntronX]

I wish someone would sell soft starters with simple dipswitch adjustable ramp speed.

They have basic ones for $220 and up. Even cheap ones on Aliexpress are $65 of who knows what kind of quality. I think this is too much for what essentially is a fancy light dimmer.

 

[ricardocello]

Here are some circuit ideas that may be useful, at least he gives component values for the BoM.
https://www.homemade-circuits.com/adding-soft-start-to-water-pump-motors/

 

[RCinFLA]

Easystart uses dual SCR's in parallel reversed polarity. Triac's are a little tricky in avoiding parasitic re-triggering when dealing with highly inductive motor startup loads.

The SCR's do not have heat sinking so they have a relay to bypass them after start up, to avoid the heating due to their voltage drop and run current.

I like the Easystart's but there is a dirty little secret in the BT current reporting. It is only the Run winding current so the auxiliary (start winding) current is not included in the reported AC current numbers.

Auxilary winding current is about 70% of run winding current with 90 deg current phase offset. This results in the Easystart reported current being about 20% below the compressor's common wire current when you add together the two complex phase currents of aux and start windings.

You can verify by measuring run and auxiliary winding currents during run period with clamp on AC current meter, compared to what Easystart is reporting. The compressor's common wire is total compressor current. Sum has some variance based on run cap value and load on compressor. Typical compressor run current for 85-95 deg F outside temp is about 60-65% of compressor nameplate rated max run current. The more dirt clogging the condenser coil the higher the compressor head pressure and greater the run current. Condensing temp has to get up to 140-150 degs F to come close to compressor max rated run current. Not good for compressor.

If you play with a soft starter, tweaking initial ramp starting voltage and ramp up rate, be careful of short run cycles. Even if you allow off time to relieve head pressure, every time compressor starts up, a teaspoon of oil is injected into compressor head. You need to allow a few minutes of run time to circulate this oil out of compressor head. If you fail to do this you will eventually build up oil lock on the compressor, potentially damaging compressor. A very expensive mistake.

I would not advise training an Easystart, or other start capacitor boosted softstarter, when outside temperatures is below 75 degrees F. It may select a too aggressive ramp that may result in a compressor stall during startup when temps get hotter outside.

 

[ricardocello]

I've got the EasyStart 368 on two 3-ton heat pumps (one Carrier, one York).

I thought the run current number from the bluetooth app looked low, and now I know why!!
I'll try to get a clamp meter around one of the input lines next time I have the cover off.

 

[AntronX]

@RCinFLA Thanks I was hoping you would chime in. Good tip on SCR vs. TRIAC re-triggering with inductive loads. This explains why this 3ph soft starter from Aliexpress uses two SCR's per phase. Some good clues in this picture:

Looks like it has 8bit ATMega micro controller and MOC3052 Triac driver opto coupler which right there tells me how to interface this to the MCU. Big blue MOVs are for clamping inductive voltage spikes and yellow cap might be RC snubber.

Great tip on compressor oil lock issue from short cycling. I was going to test the circuit on my cheap 5000 btu window AC first before moving onto my 3.5 ton house AC.

 

[RCinFLA]

Yes, just a quick look inside my 368.

Of part numbers I can easily read:

SCR's -Vishay 50amp 50TPS12L
SCR's -STM 40 amp TYN840-SCR (probably on start cap)
MOC3081M opto isolators
Atmel-ATmega328P uP
ZETTLER AZ2501P2-1A-12DE Latching Relays, 50A 12vdc coil (little surprised they are using latching relays)

Their recommend start cap values:
72-86MFD - Up to 12K BTU, 1 ton
88-106MFD - 13-23K BTU, 1 ton to 2 ton
108-130MFD - 24-36K BTU, 2 ton to 3 ton
189-227MFD - 37-48K BTU, 3 ton to 4 ton
270-324MFD - 49-72K BTU, 4 ton to 6 ton
230VAC applications must use 330V rated start capacitors.

Problem is unit BTU rating does not mean compressor is same BTU rating. My 16 SEER 48kbtu Trane with large condenser has 38 kbtu rated Copeland compressor. Copeland recommends 88-108 mfd start cap value for that model compressor. You want cap value to give the 90 deg current shift during start up.

 

[AntronX]

Thanks! That helps a lot. I now have a circuit I can try.

 

[RCinFLA]

Just a voltage ramp softstarter will reduce startup surge current but it will also lengthen the motor startup time. The current reduction may be enough to stay within the momentary surge current limit of the inverter, but the lengthened startup often exceeds what the inverter will tolerate for surge current duration.

A hard start kit is just an extra, high mfd. start capacitor temporarily put in parallel with run capacitor to increase startup torque of motor. It uses a normally closed contact with its relay coil fed from starter winding. When motor reaches run speed, the start winding back EMF causes the voltage across start winding to increase from about 70vac to about 300 vac (for 240vac motor) pulling in the relay coil and disconnecting the start capacitor.

A hardstart kit does not reduce the peak magnitude of startup surge current. It does reduce the startup duration by increasing startup motor torque. You end up with situation of nice short startup surge current duration, but the peak value of surge current is still too high for inverter surge current ability. You may read a lower current on a clamp-on amp meter only because the clamp-on amp meter doesn't get a long enough current burst duration. Most clamp-on amp meter peak current measurement function need 0.4 to 0.5 second current burst duration to average for their read out. An AC current burst of shorter duration will result in a lower reading on meter.

Easystart and other like units are a combination of softstart voltage ramping and hardstart capacitor. They add in a large mfd. electrolytic start capacitor to auxiliary (start) winding to give a closer to 90 degree current offset into the auxiliary (start) winding during the highly inductive startup period when the motor has a lot of rotor slip. This increases the motor startup torque during startup period, reducing the startup time back to a more normal 0.4-0.5 seconds. They apply the same softstart voltage ramp to run winding and start capacitor/start winding to get the surge current reduction. The parallelled start capacitor is disconnected after startup as it is with a hardstart kit.

 

[kanelr]

I ran across this in my notes this weekend......COULD THIS WORK?? Light bulbs were once used in the day to keep PA drivers from blowing. The resisted soft start is bypassed by a delay timer right after a "diffused' surge through the light bulb

 

[kanelr]

Was also looking for soft start but for sump pump motor surges. Ended up going to a much smaller pump. Runs longer though- no free lunch with Mr Newton.