Motor Starters Part 4: Selecting and

Sizing Star-Delta Parts

December 02, 2022 by Simon Mugo

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This article makes the selection and sizing

of the star-delta motor starter parts very
easy. Here you will learn how to select and
size the fuse, circuit breaker, thermal
overload relay, and contactor.
Part 3 of the series discussed the basic theories of the star-delta motor stater,
noting the composition of contactors, fuses, circuit breakers, and overload
relays. These components come in different sizes and create challenges in
selection and sizing. This article will address how to select and size the fuse,
circuit breaker, thermal overload relay, and contactor.

A Sample Motor Challenge to Solve

In this article we will walk through the design of a star-delta motor starter to
meet the following requirements:

 415 V 3-phase motor

 10 HP
 Code A
 Non-inductive load
 Efficiency of 80%
 PF of 0.8
 Rotating at 600 RPM.
Motor Torque and Current

To work out the solutions, we must start with the

basic calculation of the motor’s torque and current.

The 5252 Conversion Factor

When calculating the motor-rated torque or the Full Load Torque (FLT) of the
motor, use the constant 5252. The number 5252 represents the point during
the revolution range of the motor where the torque and the horsepower cross
paths. It is derived from the relationship between the motor speed, power, and
torque as follows:

1 HP = 33000 ft-lb/min 1 HP=33000 ft-lb/min

Given that:

1 revolution = 360° = 2 π 1 revolution=360°=2π

The crossing path factor will be determined by:

330002 π = 5252.113 ≈ 5252 330002π=5252.113≈5252

Formula for the Motor Full Load Torque (FLT)

Let's now caluclate the value of the motor's full load torque:
 FLT = 5252 ⋅ HPRPM FLT=5252⋅HPRPM

Let's now input the values from our sample motor design challenge:

FLT = 5252 × 10600 = 88 lb-ft FLT=5252×10600=88 lb-ft

We can now converting FLT from lb-ft to Nm using

the folloowing:

1 lb-ft = 1.356 Nm 1 lb-ft=1.356 Nm

Therefore,

FLT = 88 × 1.356 = 119 Nm FLT=88×1.356=119 Nm

Calculating the Starting Torque of the Motor

For motors of capacity below 30 kW, then the induction motor starting torque
is 3 times the motor full load torque. We can plug in our previously calculated
value for the FLT to derive the starting torque:

Starting Torque of the Motor = 3 × Full Load Torque Starting

Torque of the Motor=3×Full Load Torque
Starting Torque of the Motor = 3 × 119 = 357 Nm Starting Torque
of the Motor=3×119=357 Nm

Calculating the Lock Rotor Current of the Motor

The Lock Rotor Current is in the form of a range that can be attained from the
standard table of Locked Rotor Current given the code of the motor as shown
below in Table 1. Note that the full table runs from code A to code V. For our
sample motor problem, the moder is code A.
 Table 1. Locked Rotor Currents

CODE MIN MAX

A 1 3.14

B 3.15 3.54

C 3.55 3.99

D 4 4.49

The formula for calculating the locked rotor current (LRC) of the motor is given
by:

LRC = 1000 × HP × Locked Rotor Current Code

Value1.732 × V LRC=1000× HP× Locked Rotor Current Code
Value1.732×V

Now, we can calculate the minimum and maximum motor LRC using the
horsepower and voltage of the motor:

$\text{LRC}_{min} =\frac{1000 \times 10 \times 1}{1.732 \times 415} = 13.91 \

text{ A}$
\$\text{LRC}_{max} =\frac{1000 \times 10 \times 3.14}{1.732 \times 415} =
43.69 \text{ A}$

Calculating the Full Load Current of the Motor

The formulas for the line and phase full load currents are given by equals:
 FL C LINE = 1000 × P1.732 × PF FLCLINE=1000×P1.732×PF

FL C p h ase = FL C Li ne 1.732 FLCphase=FLCLine1.732

Using those equations and plugging in the values for our sample motor:

FL C LINE = 1000 × 10 × 0.7461.732 × 415 × 0.8 = 12.97

A FLCLINE=1000×10×0.7461.732×415×0.8=12.97 A

FL C p h ase = 12.971.732 = 7.49 A FLCphase=12.971.732=7.49 A

Calculating the Star-delta Starter Motor Starting

Current
Finally, the formula for the star-delta starter motor starting current equals:

Motor Starting

Current = 3 × FL C = 3 × 12.97 = 38.91 A Motor Starting

Current=3×FLC=3×12.97=38.91 A

After calculating all the basic required values, it becomes easy for engineers
and technicians to size the components of the star-delta starter.
Selecting and Sizing the Fuse

TIME DELAY FUSE NON-TIME DELAY FUSE

MOTOR TYPE
(% OF FLC) (% OF FLC)

Single phase 300 175

3-phase 300 175

Synchronous 300 175

Wound rotor 150 150

Direct current 150 150

Table 2 contains data on standard fuse current ratings as per NEC 430-52,
which will make fuse selection and sizing easy.

Table 2. Sizes of fuses per NEC 430-52

The motor being powered is a 3-phase motor. The fuse time delay will be
300% of FLC and the non-time delay will be 175% of the FLC.
Time delay fuse maximum size = 300%×FLC = 300%×13 = 39A
Non - time delay fuse maximum size = 175%×FLC = 175%×13 = 22.75A≅23A
 Selecting and Sizing the Circuit Breaker
The table below represents NEC 430-52 circuit breaker sizes.

Table 3. Sizes of Circuit Breakers

CIRCUIT BREAKERS AS PER NEC 430-52

MOTOR TYPE INSTANTANEOUS TRIP IN % INVERSE TIME IN %

Single Phase 800 250

Three-Phase 800 250

Synchronous 800 250

Wound Rotor 800 150

Direct Current 200 150

Calculate the instantaneous trip circuit breaker using the formula below:
Instantaneous Circuit Breaker Maximum Size = 800%×FLC = 800%×13 =
104A
Now calculate the size of the inverse trip circuit breaker as shown below:
Inverse Trip Circuit Breaker Size = 250%×FLC = 250%×13 = 32.5A≅32A
Selecting and Sizing of Thermal Overload
Relay
As the sizing of the DOL starter demonstrated, the thermal overload relay has
an upper and lower value.

The Phase Thermal Overload Relay

Lower Limit of the Range = 70%×FLC phase = 70%×7 = 4.9A≅5A
Upper Limit of the Range = 120%×FLC phase = 120%×7 = 8.4A≅9A

The Line Thermal Overload Relay

For the start-delta starter, the thermal overload can be placed in the windings
or the line.
When placed in line
Supply > O/L > Main Contactor
Thermal Overload Relay = 100%×FLC LINE = 100%×13 = 13A
When placing O/L Relay in windings
Main Supply > Main - Contactor - Delta Contactor > Overload Relay
Thermal Overload Relay = 58×FLC LINE = 58%×13 = 7.54A≅8A

Selecting and Sizing the Contactor Type

Main and Delta Contactor
Here the contactors are smaller as compared to the one used in DOL Starter.
This is because the contractor here is only used to control the currents in the
winding only.
The winding current = 1.723×58%×Line current
They are rated at 58% of the motor current rating.

Star Contactor
This is the third contactor and it only carries the star current. The current here
is 58 % of the current in the delta contactor, which is 33% of the motor rating
currents.
As in the DOL calculation, the same types of contractors exist. From the chart
of standard types, the type of contactor is AC1.
Breaking Contactor Capacity = Value in the Chart×FLC LINE = 1.5×13 =
19.5A≅19A
Main Contactor Size = 58%FLC = 58%×13 = 7.54A≅8A
LINE

Star Contactor Size =3 3%× FLC LINE = 33%×13 = 4.29A≅4A

Delta Contactor Size = 58 % FLC LINE = 58%×13 = 7.54A≅8A

Key Takeaways of Selecting and Sizing

Star-Delta Motor Starter Parts
In brief:

 Torque conversion factor is 5252

 The size of the fuse is 39 A for maximum and 23 A for the minimum size
 Instantaneous circuit breaker size is 104 A and the inverse trip circuit
breaker size is 32 A
 The lower limit current of the thermal overload relay is 5 A and the upper
limit is 9 A
 The line thermal overload relay is 13 A while the windings thermal
overload relay is 8 A
 Main contactor is rated 8 A, star contactor 4 A while the delta contactor 8
A

Featured image used courtesy of Adobe Stock

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