AC Motor starting current calculation

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RohanDey

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Hello,

I have the following specification of an AC Motor:
- Single phase AC motor
- 1 HP
- Works at 230 Volts

I want to control this motor (ON/OFF) using a Solid State Relay. I am facing difficulty in finalizing the specifications of the SSR which I will buy.

Can anyone please give some insight on the things I should consider in selecting my relay properly? Like what will be the starting current of the above motor etc.

Thanks..
 

Tenfold rated current can be a rule of thumb. The duration of inrush current however depends on load inertia torque.

In any case, refer to the type plate current specification (or alternatively the specification of an industry standard motor) rather than calculating a current from the HP number.
 

Thanks for your reply.

Actually I am trying to generalize the relay spec for different single phase AC motors. 1 HP being the maximum rating among them.

I am getting 4 Amps at 90% efficiency from this calculator for the above specs.

So does it mean that I need to choose a SSR with 40 Amps output capacity?

I will try to get the nameplate of the motor and find the inrush current code from it.

Thanks..
 

You won't find inrush current on type plates.

Chosing a 40 A SSR would put you on the safe side and may be necessary if the SSR has no specification for short time (e.g. seconds range) overcurrent.
 

There are four basic types of single phase induction motors in that power range:
-Split phase
-permanent split capacitor
-Capacitor start
-Capacitor start/run

They all have different start current and torque characteristics. The higher the torque, usually the higher the start current.

Many times the start current is usually not listed. The exception to the rule are sealed compressors, which do have a LRA (locked rotor amps) rating.
So a 10X margin from your FLA (full load amps) is a safe bet.
 

the full load amps comes to about 4 amps
you can safely use a standard size 10 amp relay
 

According to crydom literature
**broken link removed**

1 HP 230V single phase motor's FLA is 7A.(page 4)
and suggested SSR rating is 30A.(page 5)
 

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