Technical Article

Motor Starters Part 2: Selecting and Sizing DOL Parts

November 17, 2022 by Simon Mugo

This article instructs readers on how to size and select Direct On-Line motor starter parts like circuit breakers, contactors, and thermal overload relays.

Part 1 of this series took a broad look at the basics of Direct On-Line (DOL) motor starters. The DOL starter has many important parts–the circuit breaker, the contactor, and the thermal overload relay.

The three parts provide overvoltage protection. Due to their use as protective devices, the components should be selected and sized according to function. The process of sizing them requires an engineer or technician to be equipped with the necessary knowledge to calculate the parameters of these components. This article will examine how to size and select the circuit breaker, the magnetic contactor, and the thermal overload relay for the DOL motor starter.

 

DOL Motor Starter Circuit Breaker Selection and Sizing

From the main, the circuit breaker is the first component that current passes across before reaching the magnetic contactor. The circuit breaker is connected between the main supply and the magnetic contactors. For sizing and selection of the circuit breakers, the following steps are very significant:

  • Step 1: From the motor nameplate, read and record the size of the motor. This motor size is always in horsepower (HP).
  • Step 2: Determine the motor’s Full Load Current (FLC).
  • Step 3: Factor in safety by adding 50% to the FLC calculated in step 2 above.
  • Step 4: Select the best-suited circuit breaker as per available market ratings.

 

Step 1: Motor Size as per the nameplate

Motor Size = 6HP

1HP = 746W

Converting motor size to watts:

Motor Size = 6x746 = 4476W

 

Step 2: Determine the motor’s Full Load Current (FLC)

Power(P) = 4476W

The formula for calculating power in a three-phase is determined by:

\[P=\sqrt{3}VIcos\empty\]

Where

cos∅ = power factor (PF) = 0.8

V = 3 - Phase voltage = 415V

I = Full Load Current of the motor

Therefore, the motor’s Full Load Current can be determined by:

\[I=\frac{P}{\sqrt{3}V\times PF}=\frac{4476}{\sqrt{3}\times415\times0.8}=7.78A\]

The motor FLC is 7.78 A.

 

Step 3: Factor in safety by adding 50% to FLC calculated in step 2

Increase the calculated FLC by 50% or, in other words, multiply it by a factor of 1.5.

Circuit Breaker Rating = FLC×1.5 = 7.78×1.5 = 11.67A

Therefore, the calculated circuit breaker rating is 11.67 A.

 

Step 4: Select the best-suited circuit breaker by rating

The table below represents all standard circuit breakers available in the market. 

 

Table 1. Table of Standard Breaker Ratings

CIRCUIT BREAKER SELECTION TABLE(A)

6

10

16

20

25

32

40

50

63

80

100

125

160

180

200

250

300

350

400

630

800

1000

1500

1600

2000

2500

3000

3500

4000

4500

5000

 

 

CURRENT RANGE(A)

BREAKER TYPE

6 – 63

MCB

64 – 800

MCCB

Above 800 for LT Side

ACB

 

The circuit breaker’s calculated current rating is 11.63 A which falls between 10 A and 16 A in the table above. Therefore, the proposed breaker will have a 16 A current rating. 16 A lies between 6 and 63 A, so the circuit breaker will be MCB.

Hence, the proposed circuit breaker is 16 A TP MCB.

 

DOL Motor Starter Contactor Selection and Sizing

The contactor is connected between the circuit breaker and the thermal overload protection relay. The sizing and selection process for the contactor is similar to that of the circuit breaker in calculating the Full Load Current and the calculated current ratings. The difference is when it comes to the selection table for the components that are available on the market.

From the sizing and selection of the circuit breaker, we found that:

Motor Size = 6HP

Power(P) = 4476W

The motor FLC = 7.78A

Circuit Breaker Rating = FLC×1.5 = 7.78×1.5 = 11.67A

The circuit breaker rating is similar to the contactor rating. Select the contactor rating according to Table 2 below:

Table 2. Table of the Market Contactor Ratings

 

THE CONTACTOR RATINGS

9

12

18

22

25

32

38

40

45

50

65

70

80

95

115

150

185

200

225

265

300

330

400

460

500

580

630

780

800

1215

2250

 

 

The contactor size calculated current rating is 11.63 A, which falls between 9 A and 12 A in Table 2. Therefore, the proposed contactor will be a 12 A current rating. 

 

Selecting and Sizing Thermal Overload Relay for the DOL Starter

A thermal overload relay is connected between the motor and the contactor. It protects the motor against overvoltage. The following steps are used in the selection and sizing of the thermal overload relay.

  • Step 1: Take the motor size from the motor nameplate.
  • Step 2: Compute the Full Load Current of the motor.
  • Step 3: Calculate the lower limit of the thermal overload relay by subtracting 20% from the calculated FLC.
  • Step 4: Calculate the higher range limit of the thermal overload relay by adding 20% to the calculated FLC.
  • Step 5: Select the thermal overload relay from the sizes readily available in the market.

 

Step 1: Motor size from the nameplate

Motor size = 5HP

Motor size in watts = 746×6 = 4476W

This is the power needed to start the motor.

 

Step 2: Calculating the FLC

Power(P) = 4476W

The formula for three-phase power:

\[P=\sqrt{3}VIcos\empty\times Efficiency\]

Efficiency = 90%, V = 415V, COS∅ = PF = 0.8 and I = ?

Rearranging the formula above:

\[I=\frac{4476}{\sqrt{3}\times414\times0.8\times0.90}=8.65A\]

FLC = 8.65A

 

Tolerance

Tolerance will help solve for step 3 and step 4. The thermal overload relay is represented by a tolerance of 20% from the FLC.

Therefore,

Tolerance = 8.65×20% = 1.73A

 

Step 3: Calculate the lower limit of the thermal overload relay by subtracting 20% from the FLC

Lower limit = FLC - Tolerance = 8.65 - 1.73 = 6.92A

 

Step 4: Calculate the higher range limit of the thermal overload relay by adding 20% to the FLC

Higher Range = FLC + Tolerance = 8.65 + 1.73 = 10.38A

Therefore, the range is 6.92 A to 10.38 A.

 

Step 5: Select the thermal overload relay from the sizes readily available in the market

Table 3 represents all the thermal overload protection relays available in the market.

Table 3. Thermal Overload Relay Standard Sizes

 

THERMAL OVERLOAD RELAY SIZE RATINGS

0.1-0.16

0.16-0.25

0.4-0.63

0.63-1

1-1.6

1.25-2

1.6-2.5

2.5-4

4-6

5.5--9

7-10

9-13

12-18

48-65

23-32

28-36

30-40

37-50

55-70

63-80

80-93

80-125

100-160

125-200

160-250

200-315

250-400

315-500

400-630

0.25-0.4

 

From Table 3, the thermal overload relay current rating calculated range falls between 9-13, and this is the proposed thermal overload relay for the DOL starter.

Part 3 of this series will focus on Star-Delta motor starters.

 

Featured image used courtesy of Adobe Stock