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A three phase induction motor develops torque as soon as its stator receives a balanced three-phase supply. Its torque slip characteristic explains why the motor can start without a separate rotor drive.
At standstill, slip equals one and the motor develops positive starting torque. A starter is still needed to switch and protect the motor, and many applications also need it to limit current or control acceleration.
At standstill, an induction motor resembles an electrical transformer with a short-circuited secondary. The stationary rotor has no speed-induced counter-emf, so the motor can draw several times its rated current. Excessive starting current can cause a supply-voltage dip, while excessive starting torque can stress the driven equipment. The suitable starter therefore depends on the motor type of induction motor and the application. For a squirrel cage motor, consider three factors:
(a) Check how much starting current the supply network can support without an unacceptable voltage drop.
(b) Use the motor rating, winding connection and available terminals. Also check the permitted starting duty.
(c) Confirm that the available accelerating torque can start the load and overcome its inertia.
Squirrel-cage motor starting methods fall into two broad groups:
(i) Full voltage starting, which applies the line voltage directly, and
(ii) reduced-voltage starting, which limits current but also reduces starting torque.
The following sections compare the main methods.
Full Voltage Starting for a Squirrel-Cage Induction Motor
The standard full-voltage method is direct-on-line starting.
Direct-on-Line Starting Method
This arrangement is called direct-on-line (DOL) starting. A contactor connects the stator directly to the supply, so the motor receives full line voltage from the start. The resulting current is often several times the rated current; the exact value depends on the motor design and the supply impedance. DOL starting is simple and provides strong starting torque, but the motor, switching equipment, cables and supply must be suitable. The driven machine must also withstand the mechanical stress.
A high starting current can cause a sudden voltage drop in the supply. This may affect nearby equipment, so the network capacity and permitted voltage dip must be checked before choosing DOL starting. The following symbols are used to express starting torque in terms of full-load torque: Ts is starting torque
Tf is full-load torque
If is rotor current per phase at full load
Is is rotor current per phase at starting
sf is full-load slip
ss is starting slip
R2 is rotor resistance
Ws is synchronous speed
The general torque of induction motor expression is
The ratio of starting torque to full-load torque is
This simplified derivation treats rotor resistance as constant, although rotor frequency and skin effect can influence the actual starting behaviour.
Reduced-Voltage Starting for a Squirrel-Cage Induction Motor
Three traditional reduced-voltage methods are:
- Stator resistor starting method
- Autotransformer starting method
- Star-delta starting method
Each method trades lower line current for lower starting torque.
Stator Resistor Starting Method
The diagram shows a primary resistor starter:
A resistor or reactor is placed in series with each stator phase. The resulting voltage drop applies only a fraction x of the supply voltage to the induction motor, where x is less than one. As the motor accelerates, the impedance is bypassed and full voltage is applied. At a given slip, current falls approximately in proportion to voltage and torque falls approximately with the square of voltage. This method therefore suits a load that can accelerate with reduced torque. For the derivation, Ts is starting torque
Tf is full-load torque
If is rotor current per phase at full load
Is is rotor current per phase at starting
sf is full-load slip
ss is starting slip
R2 is rotor resistance
Ws is synchronous speed
The general torque of the induction motor expression is
The ratio of starting torque to full-load torque is
The simplified calculation assumes constant rotor resistance. If the starting phase voltage is xV1, the corresponding motor current is approximated as xIs. Substituting Is = xIs into the torque relationship gives
The result shows why starting torque varies approximately as x squared. A resistor dissipates real power as heat during starting. A reactor reduces that loss but lowers starting power factor. The choice depends on the required duty, cost and network conditions.
Auto Transformer Starting Method
An auto transformer is connected between the three-phase supply and the induction motor:
A selected tap reduces the motor phase voltage from V1 to xV1. The motor starting current then falls from Is to approximately xIs, while transformer action reduces the supply line current further. When the motor has accelerated sufficiently, the starter transfers it to full line voltage. Tap selection must provide enough torque for the load while keeping current within the network limit.
For the derivation, Ts is starting torque
Tf is full-load torque
If is rotor current per phase at full load
Is is rotor current per phase at starting
sf is full-load slip
ss is starting slip
R2 is rotor resistance
Ws is synchronous speed
The induction-motor torque expression is
The ratio of starting torque to full-load torque is
The simplified calculation assumes constant rotor resistance. With motor phase voltage reduced to xV1, motor current is approximated as xIs. Substituting Is = xIs in the torque relationship gives
Starting torque therefore varies approximately with the square of the selected voltage fraction.
Star-Delta Starting Method
The star-delta connection is shown below:
This method requires access to all six stator terminals, and the motor must be rated to run in delta at the supply voltage. The starter first connects the windings in star, so each winding receives the line voltage divided by the square root of three. It then changes to delta after the motor has accelerated.
The theoretical star-start line current and starting torque are about one-third of their DOL values. This method is therefore suitable only when the load can accelerate with the reduced torque. The transition must be timed correctly because changing from star to delta can produce current and torque transients. The symbols used below are
Tf for full-load torque
Ts for starting torque
If for rotor current per phase at full load
Is for rotor current per phase at starting
sf for full-load slip
ss for starting slip
R2 for rotor resistance
Ws for synchronous speed
The induction-motor torque expression is
The ratio of starting torque to full-load torque is
This simplified relationship assumes constant rotor resistance. If the line voltage is Vl, the phase current during a star start is Iss:
For a delta connection at the same line voltage, the corresponding current is
The comparison gives
Reduced-voltage starting limits current, but it also reduces torque. The selected method must still provide enough accelerating torque throughout the run-up period.
Starting Methods of Wound Rotor Motors
A wound-rotor motor provides access to its rotor windings through slip rings. This makes external rotor resistance the characteristic starting method, especially where a load needs high starting torque with limited current.
Addition of External Resistances in Rotor Circuit
External resistance can limit starting current and increase starting torque. It can also improve the starting power factor. The three slip rings connect to the rotor winding terminals. At start, the controller inserts the required externalresistance in each rotor phase. Contactors then short out the resistance in steps as speed rises, and the rotor circuit is normally short-circuited for continuous running. Correct step timing helps maintain useful accelerating torque without overheating the resistors or motor.
Induction and synchronous motors also differ in several operating characteristics:
(a) A standard induction motor draws magnetising current and normally operates at a lagging power factor. An overexcited synchronous motor can operate at a leading power factor.
(b) For a given frequency and motor parameters, induction-motor torque is approximately proportional to the square of applied voltage. Synchronous-motor torque has a different voltage and excitation relationship.
(c) An induction motor runs below synchronous speed and can be speed-controlled with a variable-frequency drive. A synchronous motor locks to synchronous speed when operating normally.
(d) A polyphase induction motor develops starting torque directly. A conventional synchronous motor needs a starting arrangement before its rotor can lock to the rotating field.
(e) An induction motor normally consumes reactive power. An overexcited synchronous motor can supply reactive power and support system power factor.
(f) A squirrel-cage induction motor needs only the stator supply. A wound-field synchronous motor also needs rotor-field excitation, although permanent-magnet designs do not.
(g) Induction-motor speed decreases slightly as load and slip increase. Synchronous-motor speed remains locked to supply frequency until the motor loses synchronism.
