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Electric Drives EEP 436 Course Contents

• Components and properties of electric drive systems
• Types of loads and four-quadrant operation
• Motor sizing
• Thermal considerations and duty types
• DC Drives
  • Fundamentals of DC machine speed control
  • Open-loop and closed-loop speed control
• Induction Motor (IM) Drives
  • Fundamentals of IM speed control
  • Three-phase voltage source inverters (VSIs)
  • Three-phase AC voltage controllers
  • Open-loop V/f speed control
  • Closed-loop V/f speed control
  • Braking techniques
  • Vector control

Text Books

• Fundamentals of Electrical Drives, Mohamed El-Sharkawy, Brooks/Cole, 2000
• Electric Drives: An Integrative Approach, Ned Mohan
• The Induction Machine Handbook, I. Boldea, S. Nassar

Assessment

• Projects: 20%
• Lab: 20%
• Midterm: 25%
• Final Exam: 50%

Motion Control and Servo Systems

• A servomechanism is an automatic device using error-sensing negative feedback to correct a mechanism's action. It has a built-in position/speed feedback mechanism to ensure the output matches desired position/speed.
• Drive systems employing electric motors as prime movers are called electric drive systems. The combination of electric motor, transmission, and control equipment is an electric drive system.

Reasons for Using Variable Speed Drives (VSDs)

• VSDs are used for applications like paper-making machines that can't operate without them, and for centrifugal pumps that benefit from energy savings.
• VSDs match motor speed/torque to the process requirement.
• VSDs save energy and improve efficiency.

Ward Leonard System for Variable Speed

• Introduced by Henry Ward Leonard in 1891.
• Disadvantages: very costly, low efficiency, large size, and weight.

Types of Electric Drives

• Based on motor type:
  • DC drive systems
    • Pros: simple control
    • Cons: regular maintenance, heavy, expensive, limited speed
    • Suitable for applications needing high performance (tractions, elevators, servos).
  • AC drive systems
    • Pros: less maintenance, lighter, less expensive, wide speed range.

Types of Electric Drives (Before power electronics development)

• DC drives were used in variable speed and High Performance Applications.
• AC drives were used in Fixed Speed Applications.
• Vector control of AC drives introduced in 1980, making AC drives the more preferred choice in most industrial applications.

Types of Classical Drive Systems

• Classical Drive Systems: Motors are directly fed from mains.
• Solid State Drives: Best suitable for VSDs applications.
• Trade-off between the two types depends on technology, application, performance capabilities, motor cost, and additional system component costs.

Components of Solid-State Drive Systems

• Power Source
• Power Converter
• Driving Motor
• Gating Signals
• Controller
• Measurements

Power Source

• Available supplies are commonly:
  • DC: Batteries, fuel cells, photovoltaic systems
  • AC: Single- or three-phase utility.
• Power sources may also be unregulated or fixed voltage/frequency.

Driving Motor

• Electric motors exhibit variations in torque-speed characteristics for varying mechanical loads.
• The basic selection criterion is meeting the required power level and performance during both steady-state and dynamic operations
• Motor types: DC motors (permanent magnet or wound field), AC motors (induction, synchronous, PMSM, brushless DC), and special motors (universal, switched reluctance, stepper).

Driving Motor (Considerations)

• Electrical Considerations: Running characteristics, Starting characteristics, Speed control characteristics, and Braking characteristics
• Mechanical Considerations: Type of enclosure, Type of bearings, Transmission, and Noise
• Sizing and Ratings: Load duty cycle, Heating and cooling time constants, and Overload capacity
• Cost Considerations: Initial cost and Running cost

Power Electronic Converter PEC (or Power Modules)

• Function: Regulates power supply depending on the motor type and available supply.
• May be a single converter or a combination.

Controller

• Controller complexity depends on performance requirements. Controller types: Analog (inflexible) and Digital (configurable)
• PLCs, DSPs, and microprocessors perform complex control functions. – Electric (or Galvanic) isolation between control and power circuits is mandatory for safety and avoiding control circuit malfunction or damage.

Mechanical Loads

• Active loads: Associated with gravitational or elastic deformation (change in potential energy), e.g., springs.
• Passive loads: Due to friction, shear, or deformation in non-elastic bodies (opposes motion). Examples: lathes, fans, and pumps.
• Load torque (TL) is empirically represented by various relationships (e.g., proportional to speed, proportional to square of speed, independent of speed).

Basic Types of Passive Loads

• Dry friction load (torque independent of speed)
• Viscous friction load (torque linearly dependent on speed)
• Torque proportional to the square of the speed
• Torque inversely proportional to the speed

Advantages of Solid-State Electric Drives

• Flexible control characteristics for shaping steady-state and dynamic load requirements.
• Wide range of torque and speed.
• Adaptable to various operating conditions (normal and hazardous).
• Automatic fault detection and ride-through capability.
• Four-quadrant operation.
• Energy saving and high efficiency.
• High power factor operation.
• Facility for remote control.
• Compactness and less floor space.
• Quiet operation.

Forms of Electric Drive Systems

• Individual Electric Drive (Single-motor, single-load drives):
  • Most common form
  • Dedicated to a single load
  • Higher initial cost
  • Applications include household equipment (washers, dryers), drills, fans, hard disks.
• Line Shaft or Group Drives:
  • Single motor drives multiple loads through a common line shaft and/or belts
  • Low initial cost
  • Motor rating sums less than individual loads
  • Disadvantages: motor failure shuts down whole system, inflexible speed control, high noise, and high mechanical losses.
• Multi-Motor Drives:
  • Several motors drive a single load
• Application examples: airplane actuation systems, cranes, robots

Driving Cycle Intervals

• Composed of acceleration, steady-state operation, and deceleration intervals.

Dynamic Equations of Electric Drives

• Governed by Newton's second law of motion.
• Simplest drive system comprises a rotating load directly coupled to the motor.
• Key variables: Motor torque (Te), Equivalent load torque (TL), Equivalent inertia (Jeq = Jm + JL), Equivalent friction (beq = bm + bL), and Equivalent stiffness (Keq).

Steady-State Stability

• Stability depends on friction and shaft stiffness.
• Operating point A is stable; point B is unstable.

Gear Input/Output Relations

• Assuming 100% gear efficiency, input and output speeds and torques are related.
• Gear ratios are important in relating input to output.

Gear Transmission

• Spur gear set
• Straight gear set
• Spiral bevel gear
• Worm gear set
• Helical gear set
• Planetary gear set
• Herringbone gear
• Hypoid gear

Equation of Motion

• Translational motion (Newton's law)
• Rotational motion
• Combination of rotational and translational motions

How To Refer System to Motor Side

• Load is coupled to the motor through a gearbox with ratios.
• Kinetic energy and power transfer are equivalent between motor and load sides despite the gearbox.

Translational to Rotating Motion

• For equivalent kinetic energy, relationships exist between translation and rotation with respect to gearing and radius.

Example (Equivalent Motor Torque, Inertia, and Friction)

• Includes cylindrical loads, winch loads, dry friction loads, viscous friction loads, and fan loads.

Motion Profiles

• Trapezoidal velocity profile (for analysis).
• S-curve velocity profile (practical). This profile leads to smoother motion.
• Jerk (jolt) of trapezoidal profile is infinite at four points resulting in discontinuities, while S-curve motion reduces discontinuous abrupt acceleration changes.

Example (Single-Axis Conveyor Vehicle)

• Defined parameters include mass, wheel diameter, speed, acceleration/deceleration, distance traveled, cycle time, gearbox ratio, and mechanical efficiency.

Load Side (Linear and Angular Motion)

• Defined parameters include time (t), velocity (v(t)), acceleration (a(t)), and displacement (s(t)) for linear motion.
• Parameters include time (t), angular velocity (ω(t)), angular acceleration (a(t)), and angular displacement (θ(t)) for angular motion.

Effect of Gearing-Equivalent System Inertia

• Motors designed for high speed, low speed applications use a gear to reduce the high speed.
• Gearing ratio (n = n₁/n₂) affects equivalent system inertia.

Load and Motor Steady-State Torque

• Pushing force, motion, friction, and relationships between torque and forces

Total Torque

• Rated torque (RMS) of the torque curve
• Trms (based on the torque curve)
• Tmax (based on the torque curve)

Four-Quadrant Operation

• During a driving cycle, the machine may act as a motor in either direction of rotation, or as a generator or brake depending on the terminal conditions.

Four-Quadrant Operation (Under Different Conditions)

• The machine operates in different quadrants based on the direction and magnitude of torque and speed (T and N).
• Specific diagrams show these relationships.

Four-Quadrant Operation Using Hoisting System

• The counterweight weight is adjusted to be greater than the unloaded cage and less than the loaded one (typically half the weight of the loaded cage).
• The machine runs as a motor (speed and torque have the same direction) in quadrants 1 and 3. –The machine acts as a generator or brake (Speed and torque are in opposite directions) in quadrants 2 and 4.

Motor Sizing

• Finding the best motor for a certain application.
• Motor selection depends on many factors: motion profile, inertia, speed, acceleration, torque, regenerative capacity, overload capacity, cost, resolution requirements, environment, power requirements, and physical size limitation.

Four Key Sizing Factors

• Inertia Ratio
• Speed
• Max torque at rated speed
• RMS torque at rated speed
• Motor inertia (Jm)
• Load inertia (Jl)

Inertia Ratio Specification

• 5:1 is typical
• 2:1 or less for high dynamic performance
• 10:1 or more for applications where performance is not critical
• Ease of control loop tunning and machine dynamic performance go up as inertia ratio goes down
• Low inertia ratio may entail an oversized motor, which can be avoided by using gear transmission.

Speed-Torque Curve

• Shows the relationship between speed and torque. A speed-torque curve shows possible torque values at different RPMs.
• Continuous versus Intermittent operating regions on the curve.

Motor Rating

• Rated torque is the maximum continuous torque at or below rated speed.
• Maximum speed and maximum torque are also determined.
• RMS torque should fall within the continuous region.

Max and RMS Torques

• Maximum torque at the application speed ideally falls in the intermittent region.
• RMS torque is equivalent to steady-state torque (for design purposes).

YASKAWA SigmaSelect

• Software package for selecting motor types.
• Allows load, motor, and gear designs parameters to be set and evaluated.

Selection of Motor Rating and Thermal Considerations

• Motor size and rating depend on heating effects, load conditions, duty type, and environmental conditions.
• A motor is selected such that the temperature rise (θ) is less than the permissible temperature rise (θp).

Motor Heating and Cooling

• General information.

Motor Thermal Model

• Heating/cooling equation—related to losses (Q, Watts), temperature rise (Δθ), and thermal time constants.

Heating and Cooling Time Constants

• Heating time constant (Th) and cooling time constant (Tc).
• Tc is generally two to three times larger than Th.

Insulation Classes for Electric Motors

• Insulation classes (A, B, F, H) specify allowable winding temperature rise with ambient temperature (40°C).
• Temperature rises in the motor windings as soon as the AC motor starts. Hot-spot temperatures are higher than ambient.
• Classes B and F are common. Specific temperature allowances exist for certain conditions and classes.

Insulation Classes for Electric Motors (Cont.)

• Operating at higher temperatures reduces service life. A 10°C increase in temperature can halve the expected life of the insulation.
• Hot-spot temperature allowances are provided to account for higher temperatures in hot-spot regions within windings. Insulation replacements should use equivalent or higher classes.

Example (Transformer)

• A 10KVA transformer has a normal load overload of 25KVA. Heating time constants are dependent on copper losses ignoring iron losses.

IEC 60034-1 Electric Motor Duty Cycles

• IEC standards for describing motor operating conditions. Duty cycles are important for selecting appropriate motors due to their impact on internal temperatures.

Duty Type S1 (Continuous running)

• Constant load, sufficient time for thermal equilibrium.

Duty Type S2 (Short-time duty)

• Constant load for a specific time period (less than time for reaching thermal equilibrium)
• Cycle includes pause for thermal equilibrium.

Duty Type S3 (Intermittent Periodic Duty)

• Series of identical cycles—operation followed by a pause
• Starting current is not significant for heating concerns.

Duty Type S4 (Intermittent Periodic Duty With Significant Starting Periods)

• Series of identical cycles with significant starting periods followed by constant load operation and pauses.

Motor Nameplate

• Contains information about the motor, including its specifications
• Examples shown on some of the nameplates.

Electric Motor Ingress Protection (IP) Ratings

• IP ratings indicate protection against water and solid foreign objects (dust).
• These ratings are provided on the nameplate.

IP Ratings (Solids and Liquids)

• IP ratings for solids and liquids, showing different levels of protection against these types of foreign matter.