Hello all.
A 2HP / 3500 RPM plain induction single phase motor drives a compressor. Is there some passive circuit/trick to make it start slower ramping up during two or three seconds ?

 

Why do you need the ramp-up?
That is unusual for a 2pole motor on a compressor, usually 4 pole !
What is the nature of the compresor? Vane? Piston?

 

One way, of course, is an NTC thermistor input current limiter rated for the motors running current under load.

Another is a delay relay that momentarily connects a power resistor of appropriate value in series with the motor input.

Love those deciBels!

 

I would be interested to know why the delay is required?

 

I prefer to attempt a 'gentle' start treatment to an antique compressor instead of stressing some components exposing it to a jerky jolt breaking its inertia from zero to 3500+ rpm in less than a second. Concerns about lubrication and cold temperature starts.

 

I don't know whether this would work with your motor:

 

I have seen "soft-start" packages for induction motors a few years back. Mostly they are intended for larger motors than the one described. Soft-start is often an option on larger variable start drives.
For induction motors with an RPM triggered start/run switch, there is a potential heat issue staying in the start mode for a longer than intended time.
Wiith air compressors, unloading at shut-off is very common and always a better choice. The down-side is that it demands both a series check valve and a pressure release valve.

 

Thanks, Albert, good video. While the motor runs after the soft start; (99.9% of time) the current passes trough the triac. That part am not confident with...
Perhaps a normally closed push-switch could be implemented in parallel to the circuit to manually operate the ramp period.

 

I would typically use an unloaded method, where there is either a manual valve or auto means of doing this,
What kind of reduction do you have between motor and compressor?
What is the nature of the compressor, Piston? Vane etc?
Post Photo?

 

Perhaps a normally closed push-switch could be implemented in parallel to the circuit to manually operate the ramp period.

That could be used to switch in a low resistance power resistor, similar to using a delay relay for that purpose.
You just have to remember to push the button before energizing the motor.

 

I would typically use an unloaded method, where there is either a manual valve or auto means of doing this,
What kind of reduction do you have between motor and compressor?
What is the nature of the compressor, Piston? Vane etc?
Post Photo?

Compressor is 4 piston radial 4000 rpm from a 3500 rpm motor for 4cfm/3600 psi..

 

So it would appear you definately would need an unloader of some kind at start up !?

 

So are you concerned about limiting starting current, or reducing mechanical startup stress?

Long live the deciBel!

 

Check this out:

https://es.aliexpress.com/item/1005009707092881.html

 

Those motors have a centrifugal switch which at low speed applies power to the start capacitor and winding. As the rotor spools up and reaches a certain speed, the capacitor and winding are disconnected.
Neither component is rated to be powered up by more than a couple of seconds, otherwise they will fail.
If you delay too much the opening of the centrifugal switch, you will overstress the capacitor. It perhaps won’t fail immediately, but its lifespan will be shortened.

 

Thank you all.
If 'unloader' means to start with no internal pressure builtup kept from the previous run; it has it automatic: purges pneumatically every time motor is turned off. The motor starts next run with less effort into a 'jolt' as there is no air kept compressed inside from a previous run
Am concerned in mechanical startup stresses of the compressor guts. The original motor was hydraulic; converted to electric now.

 

Thank you all.
If 'unloader' means to start with no internal pressure builtup kept from the previous run; it has it automatic: purges pneumatically every time motor is turned off. The motor starts next run with less effort into a 'jolt' as there is no air kept compressed inside from a previous run
Am concerned in mechanical startup stresses of the compressor guts. The original motor was hydraulic; converted to electric now.

I thought it strange that a 2 pole motor was used in this application, I suspect that the original hydraulic motor was much less RPM! ?

 

That could be used to switch in a low resistance power resistor, similar to using a delay relay for that purpose.
You just have to remember to push the button before energizing the motor.

NEVER count on a memorized sequence for any application that might need tobe done by somebody who has not been trained with the secret. THE REASON is that emergency situations DO HAPPEN, and that is not the time for a teaching session. Having to figure out a sequence to effect a life saving is a rather tense sequence, by the way.