Motor Starting Methods for EPC Electrical Engineers

Electrical EPC Design Course — Beginner to Professional Level

In Lesson 8, we learned the fundamentals of industrial motors, including motor rating, current, efficiency, power factor, and different starting methods.

Now we’ll go one step deeper.

This lesson focuses on an important question every EPC electrical engineer must eventually answer:

How do I decide the correct starting method for a motor?

Motor starting is important because a motor does not behave the same way when it is starting as it does when it is running.

1. Motor Running Current vs Starting Current

Imagine we have a:

100 kW motor

When the motor is already running, it may draw approximately:

160 A

However, when the motor starts, its current can temporarily become several times higher.

For example:

Running current ≈ 160 A

Starting current ≈ 6 × 160

Starting current ≈ 960 A

The basic sequence is:

START
  ↓
HIGH STARTING CURRENT
  ↓
Motor accelerates
  ↓
NORMAL RUNNING

This high starting current is one of the most important factors an electrical engineer must consider when designing the motor supply.

2. Why Does Motor Starting Current Matter?

Consider a plant where several motors are already operating.

              Transformer
                   │
                   ▼
                  PCC
                   │
        ┌──────────┼──────────┐
        │          │          │
        ▼          ▼          ▼
     Motor 1    Motor 2    Motor 3
     Running    Running    Running

                   │
                   ▼
                Motor 4
                STARTING

When Motor 4 starts, it may suddenly draw a large current.

This can cause a temporary:

Voltage drop or voltage dip

The voltage at other connected equipment may temporarily decrease.

Therefore:

Motor starting must be checked against the electrical power system.

3. Understanding Voltage Dip

Suppose the normal plant voltage is:

415 V

A large motor starts and draws a high starting current.

The voltage might temporarily fall to:

380 V

Conceptually:

Normal:
415 V

   ↓

Motor Starts

   ↓

During Starting:
380 V

This temporary reduction is called:

Voltage dip

If the voltage dip is excessive, it can cause problems such as:

• Other motors slowing down
• Contactors dropping out
• Sensitive equipment malfunctioning
• Equipment tripping
• Difficulty accelerating the starting motor

Therefore, the EPC electrical engineer needs to check whether the electrical system can support the motor during starting.

4. What Determines Motor Starting Current?

Motor starting current depends on several factors.

Important factors include:

• Motor design
• Motor size
• Motor voltage
• Motor characteristics
• Starting method
• Driven equipment
• Strength of the electrical supply system

For this lesson, we will focus mainly on how the starting method affects the electrical system.

5. Motor Starting Method 1 — DOL

What Is DOL?

DOL = Direct-On-Line

DOL is one of the simplest methods of starting a motor.

A simplified arrangement is:

       415 V Supply
            │
            ▼
         Breaker
            │
            ▼
        Contactor
            │
            ▼
          MOTOR

With DOL starting, the motor receives essentially the full supply voltage immediately.

Advantages of DOL

• Simple
• Low cost
• Reliable
• Easy to maintain

Disadvantage

• High starting current

So DOL is simple and economical, but the high starting current must be acceptable for the electrical system.

6. When Can DOL Be Used?

Suppose you have a relatively small motor.

The electrical system is strong, and starting the motor does not create an unacceptable voltage dip.

In that situation:

DOL may be perfectly suitable.

For example, a:

7.5 kW motor

might be suitable for DOL starting in an appropriate electrical system.

However, 7.5 kW should not be treated as a universal limit.

The actual permissible motor size depends on factors such as:

• Utility/source strength
• Transformer impedance
• Motor characteristics
• Starting requirements
• Client standards
• Permitted voltage-dip limits

Therefore:

Never select DOL based only on motor kW.

7. Motor Starting Method 2 — Star-Delta

Another traditional motor starting method is:

Star-Delta Starting

The motor starts in star and then transitions to delta for normal operation.

The basic sequence is:

START
  │
  ▼
 STAR
  │
  │ Motor accelerates
  ▼
TRANSITION
  │
  ▼
 DELTA
  │
  ▼
NORMAL RUNNING

The main purpose is to reduce starting current compared with DOL.

Advantages

• Lower starting current than DOL
• Relatively simple
• Less expensive than a VFD

Disadvantages

• Lower starting torque compared with DOL
• Transition can produce transients
• Not suitable for every motor/load
• Requires a suitable motor and application

Therefore, the motor and driven equipment must be checked before selecting star-delta starting.

8. Why Does Star-Delta Reduce Starting Current?

You don’t need the detailed mathematics yet.

The basic concept is:

During starting, the motor windings experience a lower effective voltage than with DOL starting.

This results in:

Lower starting current

But there is a trade-off:

Lower starting torque

This is why the characteristics of the driven equipment are important.

For example, a high-inertia load may require more starting torque than a particular starting method can provide.

9. Motor Starting Method 3 — Soft Starter

Now suppose we want the motor to accelerate more smoothly.

One option is a:

Soft Starter

A simplified arrangement is:

Supply
  │
  ▼
Soft Starter
  │
  ▼
Motor

A soft starter controls the applied voltage during acceleration.

Instead of applying full voltage abruptly:

0 V → FULL VOLTAGE

the voltage is controlled during the starting process.

Conceptually:

Voltage

100% |              ─────────
     |           /
     |        /
     |     /
     |   /
  0% |__/
     └────────────────────────
        Starting → Running

The result is a more controlled acceleration.

10. Benefits of a Soft Starter

A soft starter can provide:

• Reduced starting current
• Reduced mechanical shock
• Smoother acceleration
• Reduced stress on shafts and couplings
• Reduced pressure surges in some pump applications

For example, in a pump system, controlled acceleration can help reduce undesirable hydraulic effects.

However:

A soft starter is mainly a starting and stopping device.

It should not be confused with a VFD.

11. Motor Starting Method 4 — VFD

Now we reach one of the most important industrial motor-control devices:

VFD = Variable Frequency Drive

A VFD controls motor speed by controlling the frequency and voltage supplied to the motor.

A simplified arrangement is:

AC SUPPLY
    │
    ▼
   VFD
    │
    ▼
  MOTOR
    │
    ▼
   PUMP

Suppose a pump does not always require full flow.

Instead of operating at:

100% speed

all the time, the VFD can operate the motor at a lower speed when appropriate.

This makes the VFD much more than simply a starting device.

12. VFD Is More Than a Starting Device

This is one of the most important differences to remember.

Soft Starter

Mainly provides:

Controlled starting and stopping

VFD

Provides:

Starting + speed control + process control

For example:

50 Hz → Higher speed
40 Hz → Lower speed
30 Hz → Much lower speed

The actual motor speed depends on factors such as motor type, slip, control method, and load.

Therefore:

A VFD is selected when variable-speed operation is required, not simply because the motor is large.

13. Example — Pump Application

Consider a pump where the required flow varies.

Without a VFD

A simplified system could look like:

Motor
  ↓
Pump
  ↓
Valve
  ↓
Flow Control

The motor may continue operating near a fixed speed while the valve controls the flow.

With a VFD

The arrangement becomes:

Motor
  ↓
Pump
  ↓
Variable Speed
  ↓
Flow

Instead of controlling the process only by throttling a valve, the motor speed itself can be controlled.

For suitable variable-flow applications, this can provide useful process control and may provide energy savings.

14. How Does an EPC Engineer Select the Starting Method?

Now we reach the most important part of this lesson.

Suppose the Mechanical discipline gives Electrical the following information:

Motor = 250 kW

Voltage = 415 V

Application = Pump

Electrical engineering should not immediately say:

“Use a VFD.”

Instead, the engineer needs to ask several questions.

Question 1 — Does the Process Require Variable Speed?

If yes, a VFD becomes a strong candidate.

For example, if the pump flow needs to vary continuously, variable-speed operation may be required.

Question 2 — Can the Electrical System Handle Direct Starting?

If yes, DOL may be considered.

The engineer needs to check whether the resulting starting current and voltage dip are acceptable.

Question 3 — What Is the Starting Current?

The engineer needs to determine:

How much current will the motor draw during starting?

Then ask:

Will this cause excessive voltage dip?

Question 4 — What Starting Torque Is Required?

The driven equipment matters.

A high-inertia load may require substantial starting torque.

Therefore, a starting method that reduces starting current may also reduce available starting torque.

Question 5 — What Does the Client Specification Say?

The client may have a defined:

Motor Starting Philosophy

This could influence the permitted starting methods for different motor sizes or applications.

Question 6 — What Does the Process Require?

A compressor can have completely different requirements from a small fan.

Therefore:

The starting method must be selected according to the complete application, not motor size alone.

15. DOL vs Star-Delta vs Soft Starter vs VFD

The actual performance depends on:

• Motor
• Drive
• Electrical system
• Driven equipment
• Application

So this table is a starting point for understanding—not a substitute for project calculations.

16. Think Like an EPC Engineer

Let’s consider four different motors.

Motor A — 7.5 kW Pump

A small motor.

A possible starting method to consider first:

DOL

But the electrical system still needs to be checked.

Motor B — 75 kW Fan

Possible options could include:

• DOL
• Star-Delta
• Soft Starter

The final selection depends on the system and project requirements.

Motor C — 500 kW Compressor

Now we need a much more detailed analysis.

We need to consider:

• Starting current
• Starting torque
• Transformer capacity
• Source short-circuit strength
• Voltage dip
• Cable
• Protection
• Process requirements
• Client philosophy

This is no longer a simple “select a starter” decision.

Motor D — 500 kW Variable-Flow Pump

Here:

A VFD may be appropriate

because the process may require variable-speed operation.

This illustrates an important EPC principle:

Motor size alone does not determine the starting method.

17. Important EPC Calculation — Starting Current

Let’s take a motor with:

Motor rating = 250 kW

Running current = 400 A

Starting current = 6 × FLC

Then:

Istart = 6 × 400

Istart = 2400 A

Therefore:

Starting current = 2,400 A

So the motor may draw approximately 2,400 A during starting.

Now the EPC engineer asks the next important question:

“What happens to the plant voltage when 2,400 A flows?”

That question leads us to:

Motor Starting Studies / Calculations

This is where motor starting becomes a proper electrical system study rather than just a motor selection exercise.

18. Why Generator-Supplied Motors Can Be More Difficult

This is an important EPC situation.

Suppose a motor is supplied from the utility grid.

The grid may be relatively strong compared with a local generator source.

Now consider the same motor being supplied by a:

Diesel Generator (DG)

The arrangement may look like:

             DG
             │
             ▼
            PCC
             │
             ▼
         Large Motor

A generator has finite capacity and source impedance.

Starting a large motor can therefore produce a significant:

Generator voltage dip

A motor that starts successfully on utility power may create problems when started on emergency DG power.

This is one reason why:

Emergency-load studies and motor-starting studies are important in EPC projects.

19. The Complete EPC Motor Design Chain

Now connect everything we have learned:

Motor Data
    ↓
Motor Rating
    ↓
Full-Load Current
    ↓
Starting Current
    ↓
Starting Method
    ↓
Voltage Dip Study
    ↓
Transformer / DG Check
    ↓
Cable Check
    ↓
Breaker & Protection
    ↓
MCC Feeder
    ↓
Final Motor Supply

This is how:

Motor engineering becomes EPC electrical engineering.

The motor is not an isolated piece of equipment.

Its characteristics affect the entire electrical distribution system.

🧠 Key Takeaways from Lesson 9

Don’t try to memorize every detail yet.

Focus on these four starting methods.

DOL

Simple, but associated with high starting current.

Star-Delta

Reduces starting current but also reduces starting torque.

Soft Starter

Provides controlled acceleration and is mainly used for starting and stopping.

VFD

Provides starting plus variable-speed control.

And remember the most important EPC concept:

The starting method is not selected by motor size alone.

It is selected by considering:

Motor + Driven Equipment + Electrical System + Client Philosophy + Process Requirement

That is the real engineering decision. ⚡

🎯 Lesson 9 — Practical Exercise

Imagine a plant has the following motors:

For each motor, decide which starting method you would consider first:

A. DOL

B. Star-Delta

C. Soft Starter

D. VFD

Remember, there may not be one universally correct answer without knowing the complete project conditions.

The important part is:

Explain your reasoning.

Consider:

• Motor size
• Process requirement
• Starting current
• Starting torque
• Voltage dip
• Electrical system strength
• Client requirements

You don’t have to be 100% correct.

The goal is to start thinking like an EPC electrical engineer. 😊⚡

🚀 What Comes Next?

So far, we have learned how to understand motors and select a suitable starting approach.

Now we’re going to apply the electrical load information to one of the most important pieces of equipment in an industrial plant.

⚡ Lesson 10 — Transformer Sizing

In the next lesson, we’ll take what you’ve learned and work through:

kW → kVA → Transformer Capacity → Design Margin → Standard Transformer Selection

This will be your first proper EPC equipment-sizing exercise.

You’ll start seeing how the information from the:

Electrical Design Basis → Load List → Motor Data → Distribution System

ultimately becomes an actual equipment selection.