- Induction Motor Drives Definition: Induction motor drives are systems that control the performance of induction motors by adjusting frequency and voltage to manage speed, torque, and position.
- Starting Methods: Various methods like star-delta starters, auto-transformers, and reactor starters limit the high starting current in induction motors to prevent damage.
- Braking Types: Braking in induction motors includes regenerative braking, plugging, and dynamic braking, each providing different ways to reduce motor speed safely.
- Speed Control Techniques: Speed control of induction motors is achieved using methods such as pole changing, stator voltage control, and supply frequency control.
- Advantages of Induction Motors: Induction motors are increasingly used over DC motors due to their efficiency and ability to use advanced drives despite higher initial costs.
What is an Induction Motor Drive?
An induction motor drive combines a motor, power converter, controller, sensors and protection. It regulates speed or torque by controlling stator voltage, frequency and current. Scalar V/f control suits many steady loads, while vector control provides faster torque response and can support position control when the drive, feedback and motor are suitable. Energy savings depend on the load profile; a drive does not make every application more efficient.
Most industrial induction motors use a three-phase stator and start without a separate rotor supply. Their synchronous speed depends on supply frequency and pole count. Rotor speed is lower than synchronous speed during motoring, and the difference, called slip, increases as load torque rises. A line-fed motor is therefore a near-constant-speed machine, not an exactly constant-speed machine.
Before practical power-electronic converters, wide and efficient speed control was harder to obtain from squirrel-cage motors. Some variable-speed duties therefore used DC motors, wound-rotor schemes, pole-changing windings or mechanical control.
Power electronics developed from phase-controlled thyristors and GTOs to transistor inverters using devices such as IGBTs. These converters made variable-frequency induction motor drives practical across a wide power range.
System cost depends on power, control performance, braking duty, enclosure and installation. The sections below compare line-starting methods, electrical braking and established speed-control methods. Select equipment from motor, drive and load data rather than from a generic cost comparison.
Starting of Induction Motors
A three-phase induction motor develops starting torque when its stator is energised. At standstill, slip equals one and the motor presents its locked-rotor impedance, not zero resistance. Direct-on-line starting can therefore draw several times rated current while producing motor-specific starting torque. The permissible method depends on the supply, motor rating, load torque, acceleration time and allowed starts per hour:
- Star-delta starter
- Autotransformer starter
- Reactor starter
- Saturable reactor starter
- Part winding starter
- AC voltage controller starter
- Rotor-resistance starter for a wound-rotor motor
Star-delta starting applies only when the motor is designed to run in delta at the line voltage and all six winding leads are available. During star connection, each phase receives line voltage divided by √3. Motor line current and starting torque are approximately one-third of direct-on-line delta values. After acceleration, timed or current-based switching reconnects the induction motor drives winding in delta. The reduced torque must still exceed the load torque throughout acceleration.
An auto transformer starter applies a selected fraction of line voltage during acceleration. At standstill, motor current is approximately proportional to applied voltage, while starting torque is approximately proportional to voltage squared.
The transformer tap balances source-current reduction against the torque required by the load. Reduced voltage limits current, but it also lengthens acceleration if the available torque margin is small.
The selected transformer tap must keep supply current within the installation limit while allowing the induction motor to reach transfer speed. Contactors then bypass the autotransformer and connect the motor to full line voltage.
Series-reactor starting also reduces motor terminal voltage. Saturable-reactor controllers can vary reactance during acceleration, but they are a legacy solution and do not make starting torque nearly zero by design.
As reactor impedance falls, motor voltage and available torque rise. The acceleration profile still depends on the load and transition control, so a reactor does not guarantee a jerk-free start.
A specialised unbalanced scheme places impedance in one phase. This creates negative-sequence current and uneven winding heating. It should be used only with a motor and starter designed, rated and protected for that duty; unintended single-phasing is a fault.
After acceleration, the series impedance is bypassed and the motor follows its full-voltage torque-speed curve. Transfer can still cause a current or torque transient.
Part-winding starting requires a specially wound squirrel-cage induction motors design with parallel stator sections. One section is energised first, then the remaining section is connected after a short delay. The manufacturer’s connection and timing data determine the permitted duty.
A wound-rotor motor can start with external resistors in its rotor circuit. Added rotor resistance can increase starting torque while limiting rotor and stator current. It also dissipates slip energy as heat.
Sections of rotor resistance are removed as speed rises until the slip rings are shorted for normal running. This method can suit high-inertia loads and frequent starting only when the rotor resistors, switching equipment and motor thermal duty are rated for the resulting energy.
Braking of Induction Motors
An electric drivers system can develop torque opposite shaft motion to decelerate the motor and load. Braking of induction motors is grouped here by circuit action and energy path:
- Regenerative braking.
- Plugging or reverse-phase braking
- Dynamic braking which can be further classified as
- AC dynamic braking
- Self-excited braking using capacitors
- DC dynamic braking
- Zero-sequence braking
Using the sign convention in the diagram, regenerative operation gives negative stator real-power input:
Here, θs is the phase angle between stator voltage and current. Real power flows from the machine to the electrical system when the phase angle exceeds 90o, written θs>90o. Metering conventions may reverse the displayed sign, so the energy-flow direction should be stated.
On a fixed-frequency supply, a rotor driven above synchronous speed in the same direction has negative slip and operates as an induction generator.
The stator field does not reverse. Instead, the rotor overtakes the rotating field, reversing rotor-frequency phase relationships and electromagnetic torque. The resulting torque opposes the mechanical drive that is forcing the rotor above synchronous speed.
A variable-frequency drive can reduce commanded synchronous speed during deceleration, allowing the rotor to regenerate over a controlled shaft-speed range. The DC link and supply unit must accept the returned energy, feed it back to the mains or divert it to a rated braking path.
Plugging interchanges any two stator supply phases while the rotor is turning. The rotating field reverses immediately and produces counter-torque against the existing shaft motion.
If the original motoring slip is s, the plugging slip is (2-s). Plugging causes high current and thermal stress, and the supply must be disconnected near zero speed unless reversal is intended.
In a specialised AC dynamic-braking circuit, switching applies a single-phase supply to the stator after normal three-phase operation is interrupted. The resulting pulsating field contains positive- and negative-sequence components.
Leaving the disconnected lead open gives a two-lead connection. Reconnecting it to another phase gives a three-lead connection. Both are controlled braking arrangements, not permission to run a standard three-phase motor with an accidental open phase.
Positive- and negative-sequence fields produce opposing torques. Their net value depends on speed, voltage and rotor resistance. The motor, contactors and protection must be rated for the unbalanced current and heating.
Self-excited braking connects a capacitor bank across the stator terminals after the source is disconnected. The capacitors supply magnetising reactive current. Shaft motion, rather than the capacitor bank, supplies the mechanical energy being dissipated.
Residual magnetism and rotor motion can build terminal voltage if capacitance, speed and load conditions are suitable. Shaft kinetic energy then supplies generated real power, while the capacitors exchange reactive power with the machine.
Capacitance for an induction generator must be selected for the machine, speed and braking load. Generated energy is dissipated in machine losses and any connected resistor, and excitation may collapse as speed falls.
DC dynamic braking first removes the AC supply, then applies controlled DC current to the stator. The motor and drive limits determine the permitted current and duration.
DC stator current creates a stationary magnetic field. Rotor motion through that field induces rotor current and opposing torque. Kinetic energy becomes heat in the rotor and motor losses rather than returning to the supply. Braking torque falls towards zero with speed, and repeated DC injection can overheat the motor.
Speed Control of Induction Motors
Running speed follows synchronous speed and slip. For an ideal sinusoidal supply, synchronous speed is 120 times frequency divided by pole count, and rotor speed equals synchronous speed multiplied by (1 – slip).
Speed control of induction motors includes six established methods:
- Pole changing
- Stator voltage control
- Supply frequency control
- Eddy current coupling
- Rotor resistance control
- Slip power recovery
At fixed frequency, synchronous speed is inversely proportional to pole count. A pole-changing or multispeed motor uses a winding arrangement designed for two or more discrete pole counts. Reducing pole count raises synchronous speed; increasing it lowers synchronous speed. This method gives steps, not continuous adjustment.
Stator-voltage control keeps frequency and synchronous speed fixed but reduces developed torque. The operating point moves to higher slip, so the rotor slows under load. The useful range is limited and motor heating rises, making this method best suited to suitable fan-type loads or short-time duties.
At a fixed frequency and within the linear region, torque capability is approximately proportional to voltage squared. Motor current is not simply proportional to applied voltage under every load. Lower voltage increases the slip needed for a given load torque and can cause stall or overheating.
A variable-frequency drive changes the synchronous speed of an induction motor. Below base frequency, linear V/f control changes voltage in proportion to frequency to keep air-gap flux approximately constant. Keeping rated voltage while reducing frequency can cause magnetic saturation; speed adjustment changes frequency while the selected V/f characteristic remains appropriate. Above base frequency, voltage reaches its limit and the motor enters field weakening.
An eddy current clutch sits between a fixed-speed motor and a variable-speed load. Slip within the clutch provides speed adjustment but also creates heat.
The clutch has a rotating input member driven by the motor and a separate output member coupled to the load. A DC-excited field establishes magnetic coupling without mechanical contact between the active members.
Eddy currents induced in the drum interact with the excitation field to transmit torque to the load. Changing field current changes transmitted torque and clutch slip, which controls load speed below motor speed. The slip power becomes heat, so this is less efficient than direct variable-frequency control.
External rotor-resistance control applies only to wound-rotor machines. Increasing resistance moves the operating point to higher slip and lower speed for the same load torque, while wasting slip power as heat. It provides simple sub-synchronous control but poor efficiency and speed regulation. Slip-power-recovery systems instead route part of the rotor slip power through a converter, improving efficiency for suitable wound-rotor drives.
