How to size components of a Direct on line (DOL) Motor Control

 


A DOL starter is the simplest and most common method for starting and stopping electric motors under 5 hp/4Kw. It typically comprises of some form of a circuit breaker, contactor and an overload protection.

It offers no way to limit the starting / inrush current consequently its components most me properly sized to accommodate he magnitude of voltage and current the system is expected to carry.

Components of a DOL Starter:

Fuse or circuit breaker: This device provides protection to the motor from overcurrent conditions caused by short circuits.

Contactor: This is the main switching device that that is energized to connect the motor to the power supply. It is it is energized and de-energized by a start/stop pushbutton or other control signal.

Overload relay: This device protects the motor from overcurrent conditions caused by motor overload. It trips and disconnects the motor from the power supply if the current exceeds a safe limit.


Typical Power Circuit of a DOL Starter

Sizing the Components:
Collect data for power rating (in kW), Nominal Voltage(V) Full Load Amps (FLA) and Locked Rotor Current (LRC) of the motor that you intend to start. This can often be found on the motor nameplate with the exception of the LRC or inrush current. However, we have an article that guides you on how to find the LRC.
Motor Nameplate


    Circuit Breaker:  National Electrical Code (NEC) Article 430 specifies rules and considerations for correctly sizing overload protection devices. It states that for continuous duty motors marked with a service factor of 1.15 or greater and a temperature rise of 40C, engineers should size the overload protection device at no more than 125% of the motor's full load amperage rating. NEC article 430 also states that all other motors should be sized at 115% of the motor's full load amperage rating.

Considering the NEC code if the nameplate FLA of the motor is 35A so the continuous current rating (Ir) of the breaker to be selected would be 35 *1.25 = 43.75A. However, there is no 43.75A breaker in the market so the next lower rating standard size of circuit breaker as per 240.6 would be selected 40A.
The breaker selected must be thermal-magnetic adjustable instantaneous trip setting (Im) and therefore be able to carry the inrush current without tripping when the motor is started.
Using data from name plate like HP, V and NEMA Design we have calculated the inrush current to be 133.3A, therefore the instantaneous magnetic trip dial would be adjusted to properly accommodate this current.
We used the actual Inrush current for close protection, but some engineers also set the magnetic trip setting to 800% of the FLA according to NEC 430-52  
 
    Contactor: NEC 430.22(A) requires that the contactor should be sized to handle the full load current (FLA) of the motor. This can be found on the motor nameplate. As a safety factor, it is recommended to select a contactor with a rating that is at least 10% higher than the FLC of the motor.
Contactors built using NEMA guidelines have a standardized rating system.  The current the contactor is expected to safely carry, and motor power is used to classify NEMA contactors. This classification starts at NEMA size 00 to NEMA size 9. 
From the Motor nameplate data and the NEMA size table, a NEMA 2 contactor is selected because can carry up to 45A is more than 10% greater than motor FLA. 

    Overload Relay: The overload relay is employed to trip the circuit when there is too much load on the drive ends of the motor, this condition or any other form of restriction cause the motor to pull more than its rated current. The range of the overload relay is selected would be within 70% to 120% of the motor FLA. As we know the max allowable overload is 125% for motors with SF of 1.15 and 115% for motors with less than 1.15 SF. Ideally the motor Overload relay should be set at FLA of the motor as a typical overload relay picks up at 1.25 of the dial setting in accordance with NEC.



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