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we are using 25HP AC induction motor for air compressor.

Due to frequent start/Stop, the contactor kit of starter burnt out frequently.
We want to change the starter to Solid state type.

VFD is the best solution for this but it is very expensive and we don't need so much accuracy in speed.

Can anyone suggest that idea/resources for ssr starter it?

Regards
PRakash

It sounds like you need a soft starter. ABB, among others, makes them.

Please define how frequently is the compressor motor cycling on and off.

Modern motor starters will take a fair amount of abuse. If it was me I would look for electrical problems before I start throwing money away on additional hardware.

I would look at loss of phase due to poor voltage regulation during starting. Poor voltage supply can also cause high starting currents. I would monitor/measure inrush current during starting assuming normal running amperage is OK.

Many air compressors have controls that start the motor unloaded. Once the motor is up to speed then the loading solenoid valve permits a load on the motor. This can make a big difference on starting currents and duration. Sometimes the motor is permitted to run continuously and it is loaded or unloaded as needed; this will permit the motor to cool during periods when it is unloaded.

I have never been a fan of definite purpose motor starters in heavy duty applications but they are cheaper for a manufacturer to use. Your air compressor manufacturer may have used one.

A variable frequency drive (VFD) is not needed here. A soft start motor starter works well with high inertia loads and is much more economical. I have used them quite satisfactorily to protect the belts in Reeves variable speed drives for mixer applications.

I hope this helps.

Responding to forstand's 20-Nov (19:42) comments:

1) Loss-of-phase would prevent the mmotor from starting.

2) Starting current is not dependent on load, but only motor parameters and supply voltage. I suggest you search the control list archives for a complete discussion on the subject!

3) During the starting phase a motor's impedance is constant, meaning that starting current is directly proportional to voltage!

Regards, Phil Corso (cepsicon@aol.com)

Hi,

Thanks for your suggestion.

Here power failure/fluctuation is the major problem. We recorded 120 time power failure per month. With every power failure we required 2 times on and off the machine. I did enquiry on soft starter. This is the best option for this problem.

I want to ask another question:
Can we use this soft starter for Speed control for small application (not required like VFD but slidly like celling fan dimmer)? Can we use Normal 3Phase SSR for motor control for small application up to 1HP, by controlling the gate voltage?

Prakash

I have had good luck with Allen Bradley SMC Starters http://www.ab.com/industrialcontrols/products/solid-state_motor_contro l/soft_starters/smc-flex.html