Motor starting method comparison
Compare DOL, star-delta, soft start and drive starts.
Try a job 4
Each job fills in example values. Replace them with your values.
- Compare starters for a centrifugal pumpStart current and torque of six methods for a 15 kW pump.
- Check star-delta on a loaded conveyorWhich starts a 22 kW conveyor that needs high breakaway torque.
- Estimate the dip of a fan startThe voltage dip of each method for a 37 kW fan.
- Start a bore pump on a weak supplyThe dip of a 5.5 kW pump at the end of a rural line.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Shaft power
- 11 kW
- Line voltage
- 400 V
- Power factor
- 0.85
- Efficiency
- 0.9
- Locked-rotor current
- 6 x
- Locked-rotor torque
- 200 %
- Load breakaway torque
- 30 %
- Autotransformer tap
- 65 %
- Primary resistance, voltage at the motor
- 70 %
- Soft starter current limit
- 3.5 x
- Drive current limit
- 150 %
- Fault level at the motor
- 0 kA
Lowest start current that starts the load
Variable speed drive
- Full load current
- 20.75A
- Starting current, DOL
- 124.5A
- Starting current, chosen method
- 31.13A
- Starting torque, chosen method
- 150%
This result is a design aid. Verify it against the current standard.
Calculation steps
- Full load current20.75A
Induction motor at standstill: current with the voltage, torque with its square.
- Full load torque72.2Nm
- Locked-rotor current124.5A
- Locked-rotor torque200%
- Load breakaway torque30%
- Starting current, Direct on line124.5A
- Starting torque, Direct on line200%
- Starting current, Star-delta41.5A
- Starting torque, Star-delta66.7%
- Starting current, Autotransformer52.6A
- Starting torque, Autotransformer84.5%
- Starting current, Primary resistance87.15A
- Starting torque, Primary resistance98%
- Starting current, Soft starter72.63A
- Starting torque, Soft starter68.1%
- Starting current, Variable speed drive31.13A
- Starting torque, Variable speed drive150%
About this calculator
How this is calculated
You give the motor's shaft power, line voltage, power factor and efficiency. The tool works the full load current and full load torque the same way the motor tool does. Then you give the locked-rotor current and torque from the motor data, and the torque the load needs to break away from rest.
A cage induction motor at standstill behaves like a transformer with its secondary shorted. Its current follows the voltage at its terminals, and its torque follows the square of that voltage. Every starting method is a way of lowering the voltage at the motor, or of controlling the current directly. So the tool scales the locked-rotor current and torque for each method:
- direct on line takes the full locked-rotor current and torque;
- star-delta puts the line voltage divided by the square root of 3 across each winding, so the line current and the torque both fall to a third;
- an autotransformer at a tap gives the motor the tap voltage; the motor current falls with the tap, the line current and the torque with the tap squared;
- primary resistance leaves part of the voltage at the motor; the current falls with it, the torque with its square;
- a soft starter holds the current at its limit, and the torque falls with the square of that limit over the locked-rotor current;
- a drive starts at rated flux, so its torque follows its current limit.
A method starts the load where its starting torque is above the breakaway torque. With a fault level at the motor terminals, the tool estimates the dip as the starting current over the fault current.
| Step | Table or clause | What it decides |
|---|---|---|
| Full load current and torque | The motor tool's method | The base for every multiple |
| Direct on line | Locked-rotor data | The highest current and torque |
| Star-delta | One third of each | The current and torque in star |
| Autotransformer and primary resistance | Voltage ratio and its square | The current and torque at the tap |
| Soft starter and drive | Current limit | The current and torque each allows |
| Starts the load | Breakaway torque | Which methods start it |
Worked examples
- An 11 kW centrifugal pump. Every method starts it. Direct on line draws 124.5 A and star-delta 41.5 A. The lowest start that still turns the pump is the variable speed drive, at 31.13 A.
- A 22 kW loaded conveyor. Star-delta gives 73.3 % torque against a 120 % load, so it cannot start the belt. The lowest start that does is direct on line, at 290.57 A.
- A 37 kW fan on a board with 6 kA. The dip is 8.1 % direct on line and 2.7 % in star-delta. The lowest start is the variable speed drive, at 104.72 A and a 1.7 % dip.
- A 5.5 kW bore pump at the end of a rural line. Direct on line dips the supply 9.1 %, a soft starter at 3.5 times full load current 4.5 %. The lowest start is the variable speed drive, at 15.57 A.
Limits
The comparison is at standstill. It does not follow the torque through the run-up, so a star-delta start that breaks the load away can still stall before the change to delta. The drive figures assume rated flux; the supply side of a drive draws less than the motor side at low speed. The dip is an estimate for comparing methods, not a flicker study. Choose the circuit-breaker so its magnetic trip clears the starting current of the method used, and set the overload to the full load current.
See also: Motor starting methods: current and torque, Starting a loaded conveyor and Motor.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
How much does star-delta reduce the starting current?
To a third of the direct on line current, because each winding sees the line voltage divided by the square root of 3 at the start. The starting torque falls to a third as well.
Will star-delta start my load?
Only where a third of the locked-rotor torque is above the load breakaway torque. A fan or centrifugal pump usually starts; a loaded conveyor or crusher often does not. The tool marks each method that starts the load.
What does an autotransformer tap do?
At a 65 % tap the motor sees 65 % of the voltage. The motor current and the torque fall with the tap, and the line current falls with the tap squared, so a 65 % tap draws about 42 % of the direct on line current.
Why does a drive draw the least current at the start?
A drive starts the motor at low frequency with rated flux, so it gives full load torque at about full load current. The tool takes the drive current limit as the starting current and torque.
How is the voltage dip worked out?
As the starting current over the prospective fault current at the motor terminals. It is an estimate that takes the source and the start at the same power factor. Use it to compare methods, not as a flicker study.
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