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Motor starting: voltage dip, current and starting time

See how the bus voltage drops and how long an induction motor takes to start, by source and method. Simplified nameplate study, not a dynamic one.

IEC 60034-12starting characteristics of squirrel-cage motorsIEEE 3002.7motor starting studiesNEMA MG 1locked-rotor code lettersIEC 60947-4-1thermal overload relay trip classesISO 8528-5diesel generator set in the transient state

Example cases
Motor starting: voltage dip, current and starting time

Only the basic data; the rest is taken with typical values.

ALearn

E1Motor
kW

From the nameplate.

V

Between phases; it is also the bus voltage.

From the nameplate, at full load.

%

From the nameplate, at full load.

Multiples of rated current at locked rotor.

NEMA MG 1; the upper end of the range is taken, the least favorable.

E2Network and transformer
MVA

Provided by the utility, at the transformer primary. If there is no transformer, at the motor bus.

kVA

0 = no transformer.

%
V
E3Diesel generator set
kVA

From the set nameplate, in continuous or prime duty.

%

From the generator datasheet. The higher transient reactance X′d is less favorable for a long start.

E4Starting method
%

Voltage applied to the motor.

%

Percentage of synchronous speed at which it switches to full voltage.

%
s

Voltage or frequency rise time.

% In
E5Load
kg·m²

It determines how long the start lasts.

%

Percentage of the motor's rated torque.

%

Percentage of the motor's rated torque.

E6Motor torque and slip
%
%

Percentage of rated torque.

%
%
E7Cable and protection
m

0 = motor on the bus.

mΩ/m
mΩ/m
%
% In

Use a decimal point. The calculation runs in your browser and updates as you type.

Minimum voltage at the bus

— %

Minimum voltage at the motor terminals

— %

Starting current, peak

— A

Starting time

— s

Thermal overload relay

—

Starting time

— s

Final speed

— %

Locked-rotor torque against load torque

—

Minimum surplus torque

—

Peak of the highlighted method

— A

Voltage dip at the bus

— %

Starting time

— s

Lowest network current

—

Minimum voltage at the bus

— %

Minimum generator for the limit

— kVA

Starting time

— s

Starting current, peak

— A

Voltage at starting
  • Busbar
  • Motor terminals
  • Chosen limit
Voltage when the motor starts and its recovery as it gains speed. The dashed line is the minimum your project allows.
Current during starting
  • Line
  • Motor
  • Rated
The peak holds while the rotor is slow and drops near rated speed. The motor current differs from the line current with star-delta, autotransformer and soft starter.
Torque against speed
  • Motor in this start
  • Motor at full voltage
  • Load
The motor accelerates where its curve is above the load curve; the crossing is the running speed. At reduced voltage the torque falls with the square of the voltage.
Current against speed
  • Motor in this start
  • Motor at full voltage
The current is highest at locked rotor and falls as the motor accelerates. At reduced voltage it is proportional to the voltage the motor sees.
Current peak by method
  • Highlighted method
  • Other methods
  • Rated current
Same data with the five methods. A bar marked "does not start" means the motor torque cannot overcome the load torque with that method.
Voltage dip and time by method
  • Highlighted method
  • Other methods
  • Chosen limit
The less current the method draws, the less the bus sags, but the longer the start lasts because the torque is lower.
Minimum voltage against generator rating
  • Minimum voltage
  • Chosen limit
  • Your generator
Same motor and same method with generators of different rating and the same reactance X″d. The minimum is the first size above the limit.
Voltage at starting with your generator
  • Busbar
  • Chosen limit
No voltage regulator or speed governor: the real voltage recovers faster than in this curve.

Step-by-step calculation

What it means for your installation

    Need it for a real project? We carry out the full study, with its report.

    Design and studiesSend via WhatsAppGo to the contact form

    What it is for

    Motor starting is a study in its own right. It answers whether the motor starts, how much the bus voltage sags, how long it takes and whether the thermal relay trips. The four tabs show voltage and current over time, torque against speed, the comparison of the five methods and the diesel generator case.

    The network is taken from the same data as the short-circuit tool. To carry it over from there, use the short-circuit power S″kQ of the bus; and to see the motor protection against the upstream protection, pass the relay setting to the coordination tool.

    How it is calculated

    IEC 60909-0network and transformer impedance

    Source (per unit on the motor base)

    ZQ = UnQ² / S″kQ · ZT = uk · Ur² / Sr · ZG = x″d · U2 / SG

    With c = 1: in a motor starting study the source voltage is the operating voltage, and the c factor of IEC 60909 applies to fault currents. The network impedance is referred to the motor side with the rated transformation ratio. The generator uses its own resistance, 0.15 · X″d at low voltage.

    Motor (current and torque against speed)

    I(ω) with I(0) = Ia · Ir² ∝ T · s · I(ωN) = In · T(ωN) = Tn

    The rotor current follows from the torque equation, Ir² ∝ T · s, normalized to Ia/In at locked rotor and to the rated current at the rated point. The torque curve passes through the locked-rotor torque, the pull-up torque, the breakdown torque and the rated point. It is a single-circuit approximation: if you have the manufacturer's curve, that one governs.

    NEMA MG 1code letterIEC 60947-4-1trip classes

    Acceleration and relay

    J · dω/dt = Tmotor · (m · V)² − Tload · ∫ (I / (7.2 · Ir))² dt

    Speed is integrated step by step with the voltage the network leaves at each instant. The thermal relay is evaluated with the integral of (I/(7.2·Ir))² over time and compared with the trip time at 7.2 times the setting, between the lower and upper limits of the class. The code letter gives kVA per horsepower at locked rotor, and the calculation takes the upper end of the range.

    Starting methods

    • Direct-on-line: full voltage, the highest current and the highest torque.
    • Star-delta: in star the motor sees 1/√3 of the voltage, a third of the line current and a third of the torque; it works if the load starts with little torque. The peak when switching to delta is not modeled with its transient.
    • Autotransformer: with a tap k, the line current is k² of the direct-on-line value and so is the torque.
    • Soft starter: raises the motor voltage along a ramp; the line current equals the motor current and torque grows with the square of the voltage. Starter harmonics are not modeled.
    • Drive: limits the current and delivers torque through frequency; the network sees a current close to rated.

    Limits of the calculation

    • An induction motor with a single torque and current curve, approximated from nameplate data and typical values for NEMA design B or IEC design N motors; it does not replace the manufacturer's curve or data.
    • Fixed-impedance source: the network, the transformer or the generator are represented by their impedance. In the diesel plant there is no voltage or speed regulator, so the calculated dip is more severe than the real one.
    • A single bus and a single motor: other running loads or simultaneous starts are not added, and the control voltage of contactors and sensitive equipment is not modeled.
    • The thermal relay is evaluated cold and without memory. Instantaneous and earth-fault protection are not checked: use the coordination tool.
    • The minimum voltage limit is yours to choose: no standard sets it. To verify a real start, the dynamic study of the installation is required.

    Frequently asked questions

    How much current does an induction motor draw when starting?

    With the rotor locked it draws five to eight times its rated current and keeps it while accelerating: the nameplate gives it as Ia/In or with a code letter in kVA per horsepower. The actual current depends on the voltage left at the terminals, and that depends on the network, the transformer and the cable.

    Why does the voltage drop when a large motor starts?

    The starting current flows through the network, transformer and cable impedance and produces a voltage drop across them. The weaker the source relative to the motor, the larger the dip. A diesel generator set has a much higher impedance than a distribution network, so it suffers more.

    When is a soft starter or a drive worthwhile?

    When direct-on-line starting pulls the voltage down more than the project allows, or when the load cannot tolerate a sudden torque. The soft starter limits the motor voltage along a ramp and keeps a lower current peak than direct-on-line. The drive delivers the current the torque demands and draws a value close to rated from the network.

    We carry out your motor starting study

    We model your system with the utility, transformer and motor data, verify the voltage dip and the starting time with each method and review the motor protection.

    See design and studiesSee the coordination study

    Related guides

    Large Motor Starting: Voltage Dip and StudyWhy starting a large motor drags down plant voltage, what effects it has and what a motor starting study (IEEE 3002.7) solves.Soft Starter vs VFD: Which One to ChooseDifferences between a soft starter and a variable frequency drive: how they start, torque, current, harmonics, energy savings and motor protection.27 Undervoltage Trip on Motor Starting: CausesWhy undervoltage 27 trips when a large motor starts: weak source, 27 time delay too short, simultaneous starts or an on-site generator.Motor 49 Overload Trip: Causes and DiagnosisWhy a motor thermal overload 49 trips: real overload, repeated starts, long acceleration, thermal model settings or RTD errors.

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