How to change direction of three phase motor in Starters and VFD??

Phase Sequence Change in 3-Phase Motors

In electrical systems, there are certain applications where the phase sequence of a 3-phase motor needs to be changed. This usually happens in plants that rely on external power distribution companies instead of their own captive power supply, as utility-supplied phase sequence may vary at times.

The phase sequence determines the direction of rotation of a 3-phase induction motor. By interchanging any two supply phases, the motor’s direction can be reversed.

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1. Phase Sequence Change in Star-Delta Starter

• In a Star-Delta Starter, six motor leads are connected to the starter.

• Phase sequence can be changed by interchanging two phases in each group of three-phase connections.

• This ensures that the motor rotates in the desired direction when switching between star and delta modes.

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2. Phase Sequence Change in DOL / Soft Starters

• For Direct-On-Line (DOL) Starters and Soft Starters, phase sequence can be easily changed at the main incoming supply by swapping any two phases.

• This directly reverses the motor’s rotation without internal rewiring.

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3. Is Phase Sequence Change Required in VFDs?

• No, there is no need to change the phase sequence in Variable Frequency Drives (VFDs).

• Reason:

  • A VFD first rectifies the incoming 3-phase AC supply into DC.

  • Then, through its inverter section, it reconstructs a controlled 3-phase AC output to feed the motor.

  • Since the output is digitally synthesized, the input phase sequence has no effect on the motor rotation.

• The motor’s rotation direction can instead be set within the VFD’s programming/parameters, without physically altering the supply phases.

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✅ Key Takeaway:

• Star-Delta & DOL/Soft Starters → Phase sequence is changed by swapping two incoming phases.

• VFDs → Phase sequence does not matter; motor direction is controlled through drive settings.

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Three phase Motors Parameters calculations

📊 Electrical Parameters for Motor Installation & Testing

For every motor installation, an electrical engineer must calculate certain parameters to ensure correct sizing, protection, and performance monitoring. The following formulas apply to Alternating Current (AC) motors in both single-phase and three-phase systems.

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1. Current (Amperes) when Horsepower (HP) is known

• Single-phase:

$$I = \frac{HP \times 746}{E \times \eta \times pf}$$

• Three-phase:

$$I = \frac{HP \times 746}{1.73 \times E \times \eta \times pf}$$

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2. Current (Amperes) when Kilowatts (kW) are known

• Single-phase:

$$I = \frac{kW \times 1000}{E \times pf}$$

• Three-phase:

$$I = \frac{kW \times 1000}{1.73 \times E \times pf}$$

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3. Current (Amperes) when kVA is known

• Single-phase:

$$I = \frac{kVA \times 1000}{E}$$

• Three-phase:

$$I = \frac{kVA \times 1000}{1.73 \times E}$$

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4. Kilowatts (kW)

• Single-phase:

$$kW = \frac{I \times E \times pf}{1000}$$

• Three-phase:

$$kW = \frac{1.73 \times I \times E \times pf}{1000}$$

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5. kVA

• Single-phase:

$$kVA = \frac{I \times E}{1000}$$

• Three-phase:

$$kVA = \frac{1.73 \times I \times E}{1000}$$

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6. Horsepower (HP – Output)

• Single-phase:

$$HP = \frac{I \times E \times \eta \times pf}{746}$$

• Three-phase:

$$HP = \frac{1.73 \times I \times E \times \eta \times pf}{746}$$

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🔑 Key Symbols

• $I$ = Current (Amperes)

• $E$ = Voltage (Volts)

• $pf$ = Power factor

• $\eta$ = Efficiency (decimal)

• $HP$ = Horsepower

• $kW$ = Kilowatts

• $kVA$ = Kilovolt-Amperes

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📌 Practical Tip for Engineers:
Regular monitoring of motor current (ampere) is essential to:

• Detect overloads early

• Prevent overheating & insulation damage

• Improve energy efficiency

• Avoid penalties due to poor power factor

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Three phase motor formula

📘 Essential Induction Motor Formulas (Quick Reference for Electrical Engineers)

1. High Inertia Loads (Acceleration Time & Torque)

Acceleration Time (t):

$$t = \frac{WK^2 \times \text{rpm}}{308 \times T_{av}}$$

Required Torque (Tav):

$$T_{av} = \frac{WK^2 \times \text{rpm}}{308 \times t}$$

• $WK^2$ = Inertia in lb·ft²

• $t$ = Accelerating time in seconds

• $T_{av}$ = Average accelerating torque in lb·ft

👉 Usage:

• Use first formula if you know torque and want to find acceleration time.

• Use second formula if you know acceleration time and want required torque.

Inertia Reflected to Motor

$$J_{motor} = J_{load} \times \left(\frac{N_{load}}{N_{motor}}\right)^2$$

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2. Synchronous Speed, Frequency, and Poles

Synchronous Speed:

$$n_s = \frac{120 \times f}{P}$$

Frequency:

$$f = \frac{P \times n_s}{120}$$

Number of Poles:

$$P = \frac{120 \times f}{n_s}$$

• $n_s$ = synchronous speed (RPM)

• $f$ = frequency (Hz or CPS)

• $P$ = number of poles

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3. Horsepower, Torque, and Speed Relationship

Horsepower:

$$HP = \frac{T \times n}{5250}$$

Torque:

$$T = \frac{5250 \times HP}{n}$$

Speed:

$$n = \frac{5250 \times HP}{T}$$

• $T$ = torque (lb·ft)

• $n$ = speed (RPM)

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4. Motor Slip

$$\% Slip = \frac{n_s - n}{n_s} \times 100$$

• $n_s$ = synchronous speed (RPM)

• $n$ = actual motor speed (RPM)

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5. Symbols (Quick Legend)

• $I$ = Current (A)

• $HP$ = Output power (horsepower)

• $n$ = Motor speed (RPM)

• $n_s$ = Synchronous speed (RPM)

• $P$ = Number of poles

• $f$ = Frequency (Hz)

• $T$ = Torque (lb·ft)

• $EFF$ = Efficiency (decimal)

• $PF$ = Power factor (decimal)

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⚡ These formulas are widely used in motor calculations, programming of VFDs, and soft starters for acceleration, torque matching, and performance analysis.

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Single phase and Three phase useful formula's

Rules Of Thumb (Approximation)

At 1800 rpm, a motor develops a 3 lb.ft. per hp

At 1200 rpm, a motor develops a 4.5 lb.ft. per hp

At 575 volts, a 3-phase motor draws 1 amp per hp

At 460 volts, a 3-phase motor draws 1.25 amp per hp

At 230 volts a 3-phase motor draws 2.5 amp per hp

At 230 volts, a single-phase motor draws 5 amp per hp

At 115 volts, a single-phase motor draws 10 amp per hp

Mechanical Formulas

Torque in lb.ft. =

HP x 5250 rpm

HP =

Torque x rpm 5250

rpm =

120 x Frequency No. of Poles

Temperature Conversion

Deg C = (Deg F - 32) x 5/9
Deg F = (Deg C x 9/5) + 32 

Single phase and three phase faults effects on human body and human body perception level

Can ever imagine how dangerous are electrical faults???

Let me give you one fact about electrical faults and their levels.

Temperature produced during electrical arc-flash can reach 35,000 _F (19,500 _C).

 These extremely high temperatures will easily melt copper conductors.

Copper expands by a factor of 67,000 times at such high temperatures

Usually such faults occurred during Short circuit of copper bus bars.

When Human comes directly short circuit with two phases.

The dangers associated with these faults are:-

1. High pressures

2. High level of Sound

3. Very high currents

These high pressures can easily exceed 100-1000  pounds per square foot. Such faults produce high sounds which seems like a bomb get blasted and even causes damages to ladders, Humans etc.

The sounds associated with these pressures can exceed 160 dB.

The electrical current levels associated with electrical shock are measured in milli-amperes or one-thousandth of an ampere (0.001 Amps).

Electrical current flows through the body’s nervous system, muscles, and the blood system. Current flow

can damage organs , Can cause burning of body and have long-term effects on the body, which may not be readily visible at the time of the shock event.

HT Fault currents are such high that immediately all body get burnt in seconds and only ashes remains

Electrical shock current levels:-

Current range (mA) Physiological effect Condition description:-

1mA Threshold of perception Detect a slight tingling sensation in hands or fingertips

1–9 mA       Let-go threshold:- Unpleasant sensation but muscle control not impaired.

9–25 mA     Muscular contraction:- Painful and hard or impossible to release energized object in hand

26–59 mA   Muscular contraction Breathing difficult

60–100 mA Ventricular fibrillation Heart stoppage, respiration inhibition, death possible

Difference between XLPE and PVC Cables

Both Cables are widely used in Industries:

PVC - stands for polyvinyl chloride 
XLPE - is cross linked polyethylene cable. 

Main Thumb rule for differentiation is that XLPE cables can be used for both HT and LT lines.
But PVC Cables can be used only for LT lines.

XLPE can withstand a higher temperature than PVC without detriment. This means that more current can be conducted for the same cross sectional area of copper. This means a big saving in cables costs. 

Visit link below for more details:-
http://electrialstandards.blogspot.com/2014/04/66kv-to-132kv-tests-requirement.html

http://electricalsystembasics.com/2014/04/xlpe-cables-advantages-oil-filled-paper-insulated-cables.html

XLPE Cables useful properties are :-

XLPE Cables construction:-

1.Temperature resistance
2. Stress rupture resistance 
3. Environmental stress crack resistance 
4. Resistance to U.V light
5. Chemical resistance
6. Oxidation resistance

 XLPE Cables can be useful for following applications:- 

1) XLPE cables work for the working voltage of 240 V to 500 KV . 
2) Conductor Material can be either Copper or Aluminium. 
3) XLPE cables can be either Single Core cables or Multi core cables depending upon the number of 
cores. 
4) They can be Unarmoured or Strip Armoured or Wire Armoured or Tape Armoured type depending upon the presence or absence of Armour . 
5) HT / LT Aerial Bunched Cables 


Polyvinyl chloride Cables

They are commonly abbreviated as PVC, insulated cables are widely used in various fields. 

PVC's Cables are generally have following Properties:-

1. Low cost
2. Chemical resistance 
3. High tensile strength
4. Better flexibility 

For electric cables the PVC is mixed up with plasticizers.

Low voltage copper conductor PVC cables are extensively used for domestic home appliances wiring, house wiring and internal wiring for lighting circuits in factories, power supply for office 
automation, in control.

PVC CABLES 
1) Power Cables upto 1.1 KV 
2) Multicore Cables upto 61 Cores 
3) Shielded instrumentation Cables 
4) FRLS / FR / HR / Fire Survival Cable 

YWY, AYFY, AYY, AYCY, A2XCY Cables Meanings

📘 Cable Nomenclature Explained (PVC & XLPE Cables)

Cable designations like YWY, AYFY, A2XCY might look confusing at first glance. But once you understand the logic behind each letter, reading technical specs becomes straightforward.

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1. Basic Conductor Material

·         A → Aluminium conductor

·         (If “A” is absent, it means Copper conductor)

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2. Insulation Material

·         Y → TROPODUR = PVC insulation

·         2X → TROPOTHEN-X = XLPE insulation

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3. Armouring Types

·         W → Round Steel Wire Armouring

·         WW → Double Round Steel Wire Armouring

·         F → Flat (formed) Steel Strip Armouring

·         FF → Double Flat Steel Strip Armouring

·         Wa → Aluminium Wire/Strip Armouring

For Buying Best wires for your Home click here or image

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4. Screening

·         C → Copper metallic screen (overall)

·         CE → Copper screen on each core

·         Gb → Steel tape helix binder

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5. Sheath

·         Y (at end) → TROPODUR = PVC outer sheath

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6. Common Examples

Type	Full Form	Description
AYY	Aluminium, PVC insulated, PVC sheathed	Unarmoured heavy-duty cable
AYWY	Aluminium, PVC insulated, Steel wire armoured, PVC sheathed	Most common for power distribution
AYFY	Aluminium, PVC insulated, Flat strip armoured, PVC sheathed	For underground power networks
AYCY	Aluminium, PVC insulated, Copper screened, PVC sheathed	Used where EMC shielding is required
A2XCY	Aluminium, XLPE insulated, Copper screened, PVC sheathed	High-performance screened cable
AYCEFY	Aluminium, PVC insulated, Individually screened cores, Flat strip armoured, PVC sheathed	Special EMC + mechanical protection
A2XCEFY	Aluminium, XLPE insulated, Individually screened cores, Flat strip armoured, PVC sheathed	High-voltage EMC cable with XLPE

👉 If “A” is dropped → it’s the same type but with Copper conductor.
For example:

·         YY → Copper, PVC insulated & sheathed

·         YWY → Copper, PVC insulated, wire armoured, PVC sheathed

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7. Special Mining Cables

·         YWWGby → Copper, PVC insulated, double wire armoured, PVC sheathed, Mining grade

·         YFFGby → Copper, PVC insulated, double flat strip armoured, PVC sheathed, Mining grade

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🔑 Quick Decode Formula

[A or none] + [Insulation Code] + [Screen Code] + [Armour Code] + [Sheath Code]

Example:
AYFY = Aluminium + PVC insulation + Flat strip armour + PVC sheath

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