Auto Transformer Starter Sizing for Motor Starting – Step-by-Step Guide
1. Understand the Purpose
An Auto Transformer Starter reduces the starting current and voltage of an induction motor
during startup. It provides a smooth start, reducing mechanical and electrical stress.
2. Collect Required Motor Data
You need the following motor information:
Rated Motor Power (kW or HP)
Rated Voltage (V)
Rated Current (A)
Starting Current (as a multiple of full load current, typically 5 to 7 times)
Motor Efficiency (usually between 0.85 and 0.95)
Power Factor (typically 0.8 lagging)
Starting Time (seconds)
Duty Cycle (how often it starts per hour)
3. Determine the Motor Full Load Current (FLC)
Use this formula:
FLC (A) = (Motor Power (kW) × 1000) / (√3 × Voltage × Efficiency × Power Factor)
Example:
Motor Power = 75 kW
Voltage = 400 V
Efficiency = 0.9
Power Factor = 0.85
FLC = (75 × 1000) / (1.732 × 400 × 0.9 × 0.85) = approx. 141.4 A
4. Choose Voltage Tapping Ratio for the Auto Transformer
Typical tapping ratios are: 50%, 65%, 80%
These percentages reduce the starting current proportionally.
Example:
Tapping = 65%
Starting current becomes (0.65)^2 = 0.4225 = 42.25% of direct-on-line starting current
So if DOL starting current is 6 × FLC, then:
Auto Transformer Starting Current = 6 × FLC × 0.4225
= 6 × 141.4 × 0.4225 = approx. 358 A
This is the line current; the motor still sees 6 × FLC at its terminals, but the supply sees much
less.
5. Select the Auto Transformer Rating
This should be based on starting kVA, not just running kVA.
Starting kVA = √3 × Line Voltage × Starting Current / 1000
Using our example:
= 1.732 × 400 × 358 / 1000 = approx. 248 kVA
You must select an auto transformer with a short-duty rating (typically 30 seconds, 60 seconds,
or 1 minute) equal to or greater than 248 kVA.
Note: You can oversize slightly to accommodate duty cycle or ambient conditions.
6. Select the Appropriate Tapping
Based on:
Load torque characteristics
Desired reduction in starting current
Supply network limitations
Low tapping (e.g., 50%) = less current but may not develop enough torque
High tapping (e.g., 80%) = more torque but higher current
Use 65% as a balanced choice unless you know the load requires more torque at start.
7. Select Contactor Ratings
Auto transformer starters use three contactors:
1. Main Contactor (M): Carries full load current
2. Star Contactor (S): Used during starting; rated for reduced current
3. Auto Transformer Contactor (T): Connects the transformer during starting; rated for
starting current
Use these guidelines:
Main Contactor: Rated for FLC (141.4 A in our example)
Star Contactor: Rated for motor current at the selected tap (65% of FLC = approx. 92
A)
Auto Transformer Contactor: Rated for line starting current (358 A in our case)
Use AC3 duty class contactors, as motors are inductive loads.
8. Thermal Overload Relay Sizing
Should be rated for 100% of FLC:
Set overload protection at 1.0 × FLC (or per motor datasheet)
In our example: Overload relay range should cover 141 A
9. Enclosure and Protection
Choose a suitable enclosure:
IP rating depending on installation site
Ventilation for transformer heat
Adequate short-circuit protection using MCCB or fuses
10. Wiring Considerations
Copper cable sized for FLC and short-circuit capacity
Proper earthing
Star-to-delta timer or programmable logic for switching
Summary Example (for 75 kW Motor)
Motor Power: 75 kW
Voltage: 400 V
FLC: approx. 141 A
Starting Tapping: 65%
Starting Current (line): 358 A
Starting kVA: 248 kVA
Auto Transformer Rating: ≥250 kVA for 30-60 seconds
Main Contactor: 150 A (AC3)
Star Contactor: 100 A (AC3)
Auto Transformer Contactor: 400 A (AC3)
Overload Relay: 140-160 A range
Cables: Rated for 150 A continuous
If both an Auto Transformer Starter and Star-Delta Starter are being considered (or
combined in some way), it's important to understand their differences, application contexts, and
why or when they might be used together.
However, Auto Transformer Starting and Star-Delta Starting are generally not combined
in practice — because they are two different reduced-voltage starting methods with different
wiring, timing, and performance characteristics.
But if you're asking from an analysis or comparison perspective (or designing for a scenario
where both methods are considered as options for the same motor), then here's how to handle
Auto Transformer Starter vs. Star-Delta Starter — including their sizing parameters,
selection criteria, and how you might document both in your Word report.
Comparison and Sizing Guide: Auto Transformer vs. Star-Delta Starter
1. Purpose of Each Starting Method
Auto Transformer Starter:
Provides adjustable starting voltage via tapping (50%, 65%, 80%)
Suitable for larger motors and long starting durations
Allows better control over starting current and torque
More expensive but more flexible
Star-Delta Starter:
Reduces starting current by connecting motor windings in Star (Y) first, then Delta (Δ)
Reduces starting voltage to 58% of line voltage
Simple and economical for motors >10 HP (~7.5 kW) with light starting load
Fixed torque reduction, cannot adjust
2. Current and Torque Characteristics
Starting Method Starting Voltage Starting Current Starting Torque
Direct-On-Line (DOL) 100% 600% 100%
Star-Delta 58% ~33% ~33%
Auto Transformer (65%) 65% ~42% ~42%
Auto Transformer (80%) 80% ~64% ~64%
Note: Percentages based on DOL full values.
3. Sizing Star-Delta Starter Components
Assuming the same 75 kW, 400 V, 141 A motor from before:
Motor Full Load Current (FLC):
Same as before: ~141 A
Star-Delta Starting Current (line):
~33% of DOL current
Starting current = 0.33 × 6 × 141 = ~279 A
Contactor Ratings for Star-Delta
Star-Delta uses three contactors:
1. Main Contactor (KM1): Carries full load current (141 A)
2. Star Contactor (KM2): Carries phase current during star connection
o Current = FLC / √3 ≈ 81.5 A
3. Delta Contactor (KM3): Carries full load current after changeover (141 A)
Sizing Table (Star-Delta)
Component Duty Type Required Current Rating
Main Contactor AC3 150 A
Star Contactor AC3 90 A
Delta Contactor AC3 150 A
Overload Relay - 140–160 A
4. When to Use Each
Criteria Auto Transformer Star-Delta
Motor Power > 75 kW ✅ Yes ❌ Not ideal
High Starting Torque Required ✅ Adjustable ❌ Low torque
Long Starting Time ✅ Suitable ❌ Not suitable
Cost Sensitive ❌ Expensive ✅ Economical
Adjustable Starting Conditions ✅ Yes ❌ Fixed
More Complex Control Logic ✅ Yes ✅ Yes
5. Final Recommendation Section (For Report)
Suggested Template for Your Word Document:
Motor Starting Mechanism Analysis – 75 kW Induction Motor
Options Evaluated:
Auto Transformer Starter (65% tapping)
Star-Delta Starter
Summary Table:
Criteria Auto Transformer Star-Delta
Starting Current 358 A (~42%) 279 A (~33%)
Starting Torque ~42% ~33%
Flexibility High (tapping) Low (fixed)
Cost Higher Lower
Suitability (>75 kW) Good Not preferred
Contactor Sizing Up to 400 A Up to 150 A
Application Suitability High torque loads Low inertia
Recommendation:
For high-torque or high-inertia loads (e.g., compressors, conveyors), Auto Transformer Starter
with 65% tapping is recommended. For simple, low-cost installations with low starting load,
Star-Delta may be considered for motors below 50–75 kW.
