Motors, Controllers, and Variable Frequency Drives
Red Seal Practice study guide with diagrams.
Motors, Controllers, and Variable Frequency Drives
Chapter Introduction
This chapter covers all the knowledge required for the Red Seal exam in construction electricity regarding motors, their controllers, and variable frequency drives. You must master the operating principles, installation methods, protection calculations, and regulatory requirements of the Canadian Electrical Code, Part I (CE Code) . This chapter is structured to follow the installation logic: from the motor itself, to its controller, then to its protection system, and finally to variable frequency drives.
Fundamental Principles of Electric Motors
Motor Classification
Motors are classified according to their power source and operating principle. For the exam, you must clearly distinguish:
Motor Type
Power Supply
Main Characteristic
Typical Use
Direct current (DC) motor
DC
Variable speed via armature voltage
Hoists, machine tools
Synchronous motor
Three-phase AC
Constant speed = frequency × 120 / number of poles
Large loads, power factor correction
Induction motor (asynchronous)
Three-phase AC
Slip required to produce torque
Pump, fan, conveyor
Single-phase motor
Single-phase AC
Requires a starting circuit
Small appliances, residential fans
Three-Phase Induction Motor — Operating Principle
The three-phase induction motor is the most common in industry. Its stator creates a rotating magnetic field at synchronous speed:
n_sync = 120 × f / p
Where:
n_sync = synchronous speed in revolutions per minute (rpm)
f = frequency in hertz (Hz)
p = number of poles per phase
Example: a 4-pole motor supplied at 60 Hz runs at a synchronous speed of 120 × 60 / 4 = 1800 rpm.
The rotor, lagging behind the rotating field, develops slip (s):
s = (n_sync − n_rotor) / n_sync × 100%
Typical slip at full load is 2 to 5%. Slip is essential: without it, no electromotive force is induced in the rotor and no torque is produced.
Starting Torque and Starting Current
At startup, an induction motor draws a current of 600% to 800% of its full-load current (rated current). This high current lasts 1 to 3 seconds for small motors, but can persist longer for large motors. Starting torque is typically 150% to 200% of rated torque.
Mechanical power formula:
P (kW) = √3 × U × I × cos φ × η / 1000
Where:
U = line-to-line voltage (V)
I = line current (A)
cos φ = power factor
η = efficiency (expressed as a decimal, e.g., 0.90)
Nameplate Data
The motor nameplate is a mandatory source of information for all calculations. You must know how to read and interpret each piece of data:
Rated voltage (e.g., 208 V, 240 V, 480 V, 600 V)
Full-load current (FLA — Full Load Amps)
Service factor (SF) — typically 1.15
Insulation class (A, B, F, H) — determines maximum allowable temperature
Locked-rotor code letter (e.g., Code J, K, L) — indicates starting current in kVA/HP
Rated speed (rpm)
Frequency (60 Hz in Canada)
Power (HP or kW)
Enclosure type (TEFC, ODP, etc.)
HP ↔ kW conversion: 1 HP = 0.746 kW; 1 kW = 1.34 HP
Motor Controllers
Definition and Functions
A motor controller is any device that controls the operation of a motor, whether for starting, stopping, speed, or reversing direction of rotation. The controller must be capable of interrupting the locked-rotor current of the motor.
Contactors and Relays
The contactor is the power element of the controller. It consists of:
Control coil (24 V, 120 V, 240 V, 480 V)
Main contacts (power)
Auxiliary contacts (for control logic)
The overload relay protects the motor against prolonged overcurrents. There are two main types:
Type
Principle
Advantage
Disadvantage
Thermal (bimetallic)
Differential expansion of two metals
Simple, economical
Less precise, slow response time
Electronic
Current measurement via current transformer
Precision, fine adjustment, phase-loss protection
More expensive
The overload relay must be set at no more than 125% of the motor's full-load current (Rule 28-308 of the CE Code), except for exceptions provided for motors with a service factor of 1.15 or greater, or with a 40 °C temperature rise, where the setting can be up to 140%.
Full-Voltage (Across-the-Line) Starters
The direct-on-line (DOL) starter applies full voltage to the motor as soon as the contactor closes. It is the simplest and most economical, but it imposes a high starting current on the supply. It is acceptable for low-power motors or when the supply can handle the inrush current.
Reduced-Voltage Starters
To reduce starting current, reduced-voltage starters are used:
1. Wye-Delta (Y-Δ) Starter:
The motor starts in wye (Y) configuration, where each winding receives U/√3.
After a time delay (typically 5 to 10 seconds), it switches to delta (Δ), where each winding receives full voltage.
Starting current is reduced to 33% of direct starting current.
Starting torque is also reduced to 33%.
Requires a motor with 6 accessible terminals and designed for Δ operation.
2. Autotransformer Starter:
Uses an autotransformer with taps at 50%, 65%, and 80% of the voltage.
Line current is reduced by the square of the voltage ratio (e.g., at 65%: 0.65² = 42% of direct current).
Torque is also reduced by the square of the voltage ratio.
3. Stator Resistance Starter:
Inserts resistors in series with the stator windings at startup.
The resistors are short-circuited after the motor reaches speed.
Current is reduced proportionally to the voltage drop across the resistors.
4. Electronic Starter (Soft Starter):
Uses thyristors (SCRs) to progressively control the applied voltage.
Allows continuous adjustment of starting voltage and ramp time.
Reduces mechanical shock and hydraulic surges.
Comparison of Starting Methods
Method
Starting Current (% of direct)
Starting Torque (% of direct)
Relative Cost
Complexity
Direct (DOL)
100%
100%
1×
Low
Wye-delta
33%
33%
1.5×
Medium
Autotransformer (65%)
42%
42%
2×
Medium
Stator resistance
50-80%
50-80%
1.5×
Medium
Electronic (soft starter)
50-80% (adjustable)
50-80% (adjustable)
2.5×
High
Exam trap: the wye-delta starter reduces current to 33%, but torque to 33% as well. If the load requires starting torque greater than 33% of rated torque, this method is unsuitable.
Motor Protection
Overcurrent Protection (Short-Circuit)
The conductors supplying a motor must be protected against short-circuits by fuses or circuit breakers. According to Rule 28-200 of the CE Code, the overcurrent protection device must be set at:
No more than 300% of full-load current for time-delay fuses (Class CC, J, RK1, RK5, L).
No more than 250% for fast-acting fuses.
No more than 250% for inverse-time circuit breakers.
If these values do not allow the motor to start (nuisance tripping), the CE Code permits increasing the value up to 400% for time-delay fuses and 300% for circuit breakers, provided the conductor is sized accordingly.
Overload Protection
Overload protection (thermal or electronic relay) protects the motor against excessive heating. Requirements of Rule 28-308:
Maximum setting of 125% of full-load current for standard motors.
Maximum setting of 140% for motors with a service factor of 1.15 or greater, or with a 40 °C temperature rise.
The overload relay must be located in the power circuit, between the contactor and the motor.
Rated Current and Conductor Calculations
Complete calculation example:
A three-phase motor of 25 HP, 600 V, power factor 0.85, efficiency 0.90.
95.Full-load current:
I = P / (√3 × U × cos φ × η)
I = (25 × 746) / (1.732 × 600 × 0.85 × 0.90)
I = 18650 / 795.3 = 23.4 A
99.Conductors (Rule 28-106): conductors must have an ampacity of at least 125% of full-load current.
I_min = 23.4 × 1.25 = 29.3 A
→ #10 AWG conductor (35 A at 75 °C) or #8 AWG depending on length and voltage drop.
102.Overload protection (Rule 28-308):
Maximum setting = 23.4 × 1.25 = 29.3 A → relay set at 29 A (or the nearest standard value).
104.Short-circuit protection (Rule 28-200):
Time-delay fuses: 23.4 × 3.00 = 70.2 A → standard 70 A fuse.
If nuisance tripping: up to 23.4 × 4.00 = 93.6 A → 90 A fuse.
Correction Factors
Conductors must be corrected for ambient temperature and bundling (Rules 4-004 and 4-006 of the CE Code). Remember to apply these factors before comparing conductor ampacity with the required current.
Variable Frequency Drives (VFDs)
Operating Principle
A variable frequency drive (VFD) controls the speed of an induction motor by simultaneously varying the frequency and voltage of the power supply. The fundamental principle is to maintain a constant V/f ratio to preserve motor torque.
Speed-frequency relationship:
n = 120 × f / p × (1 − s)
By varying f from 0 to 60 Hz, you obtain a speed variation from 0 to rated speed. Beyond 60 Hz, the voltage can no longer increase (supply limit), so torque decreases proportionally to 1/f (field weakening region).
Internal Architecture of a VFD
The VFD consists of three main stages:
1. Rectifier (AC/DC converter):
Diode or thyristor bridge
Converts alternating voltage to direct voltage
The DC voltage is approximately 1.35 × the RMS voltage (e.g., 480 V → 648 V DC)
Pulse width modulation (PWM) at high frequency (2 to 16 kHz)
Produces an alternating voltage of variable frequency and amplitude
Pulse Width Modulation (PWM)
PWM consists of switching the IGBTs at high frequency while varying the duty cycle to create an approximate sine wave. The switching frequency influences:
Switching Frequency
Advantage
Disadvantage
Low (2-4 kHz)
Fewer losses, less heat
Acoustic motor noise
High (8-16 kHz)
Quiet operation
More losses, more heat, more interference
VFD Wiring and Installation
Important requirements:
137.Shielded cables: cables between the VFD and the motor must be shielded to reduce electromagnetic interference (EMI). The shield must be grounded at both ends.
138.Circuit separation: VFD power cables must be separated from control and instrumentation cables (minimum distance of 300 mm or separation by a grounded metal partition).
139.Grounding: the VFD must be grounded in accordance with Rule 10-200 of the CE Code. High-frequency grounding may require flat braided conductors.
140.Upstream protection: the VFD must be protected by fuses or a circuit breaker sized according to the manufacturer's recommendations, typically between 125% and 150% of the VFD's rated current.
141.Line reactors: recommended when harmonic distortion is a concern or when the power source is weak.
Harmonics and Distortion
VFDs generate current and voltage harmonics due to electronic switching. The main effects:
Heating of transformers and capacitors
Nuisance tripping of protection devices
Disturbance of sensitive electronic equipment
Neutral overvoltage (3rd harmonic currents)
Solutions:
Line reactors (3% or 5% impedance)
Passive filters (tuned traps on dominant harmonics, typically 5th and 7th)
Active filters
Phase-shifting transformers (12 or 18 pulse)
Motor Braking with VFDs
The VFD can brake the motor in several ways:
1. DC injection braking:
The VFD applies a DC voltage to the stator
The motor stops quickly
Used for holding at standstill
2. Dynamic braking (braking resistor):
Deceleration energy is dissipated in a resistor
Required for high-inertia loads
The resistor is controlled by an internal chopper
3. Regenerative braking:
Energy is returned to the supply
Requires a four-quadrant rectifier
More expensive but energy-efficient
Essential VFD Parameter Settings
The following parameters must be configured during commissioning:
Parameter
Description
Typical Value
Motor rated voltage
Nameplate voltage
480 V
Motor rated current
Nameplate FLA
23.4 A
Maximum frequency
Base frequency
60 Hz
Minimum frequency
Low speed limit
0-10 Hz
Acceleration ramp
Time to reach speed
5-30 s
Deceleration ramp
Time to stop
5-30 s
Starting torque
Additional torque at startup
0-150%
Slip compensation
Speed correction under load
0-5%
Built-in VFD Protection
Modern VFDs incorporate many protection features:
Overcurrent protection (electronic, faster than a thermal relay)
DC bus overvoltage protection
Undervoltage protection
Drive overheat protection
Phase-loss protection
Ground fault protection
Output short-circuit protection
Important: the VFD does not necessarily replace the motor overload protection. According to Rule 28-308, the VFD can be considered an overload protection device if it is set in accordance with the requirements and if the manufacturer certifies it.
Specific Regulatory Requirements (CE Code)
Rule 28-106 — Conductors
Motor supply conductors must have an ampacity of at least 125% of the motor's full-load rated current. This rule applies to conductors between the last overcurrent protection device and the motor.
Rule 28-110 — Single-Phase Motors
For single-phase motors, the rated current is that indicated on the nameplate. Conductors must be sized at 125% of this current.
Rule 28-200 — Overcurrent Protection
As detailed previously, short-circuit protection must be set between 250% and 300% of rated current, with the possibility of increasing up to 400% (fuses) or 300% (circuit breakers) if needed.
Rule 28-308 — Overload Protection
The overload protection device must be set at no more than 125% of rated current (or 140% for special motors). It must be able to withstand the starting current without tripping.
Rule 28-500 — Disconnecting Means
Each motor must have a visible and accessible disconnecting means that allows cutting off power to the motor and its controller. The disconnecting means must:
Be capable of interrupting the rated current of the motor
Be lockable in the open position
Be located within 9 m of the motor, or be lockable and comply with the requirements of the rule
Rule 28-600 — Controllers
The controller must:
Be capable of interrupting the locked-rotor current of the motor
Have a voltage rating at least equal to that of the circuit
Be mounted in an accessible location
Interrupt all ungrounded conductors
Pitfalls to Avoid
204.Confusing full-load current and starting current: starting current is 6 to 8 times rated current. Overcurrent protection calculations use rated current, not starting current.
205.Forgetting the 125% factor for conductors: conductors supplying a motor must be sized at 125% of rated current, not 100%.
206.Neglecting correction factors: ambient temperature and conductor bundling modify the actual ampacity of conductors. Always apply correction factors before selecting conductor size.
207.Confusing wye-delta and electronic starters: wye-delta reduces current to 33% but imposes an abrupt switching; the electronic starter offers a progressive ramp.
208.Forgetting slip in speed calculations: the rated speed of an induction motor is always lower than synchronous speed. Do not confuse the two.
209.Ignoring harmonics with VFDs: VFDs generate harmonics that can cause problems on the supply. Plan for reactors or filters when necessary.
210.Using unshielded cables between VFD and motor: this causes electromagnetic interference and can damage sensitive equipment.
211.Misinterpreting the service factor: the service factor allows temporary overload, but does not change the rated current used for protection calculations.
212.Forgetting overload protection when using a VFD: the VFD must be programmed to protect the motor, or an external overload relay must be installed.
213.Confusing HP and kW units: 1 HP = 0.746 kW. A conversion error invalidates all current calculations.
Summary
The three-phase induction motor operates by a rotating magnetic field; its synchronous speed is n = 120 × f / p, and slip is essential to its operation.
The nameplate provides all necessary data: voltage, current, service factor, locked-rotor code, insulation class.
The controller must interrupt locked-rotor current; overload relays protect against prolonged heating.
Reduced-voltage starting methods (wye-delta, autotransformer, soft starter) reduce starting current but also torque.
Short-circuit protection is set between 250% and 300% of rated current (up to 400% for fuses if needed).
Overload protection is set at 125% (or 140% for special motors) of rated current.
Conductors are sized at 125% of rated current, with application of correction factors.
The VFD controls speed by varying frequency and voltage with a constant V/f ratio; it consists of a rectifier, DC bus, and IGBT inverter.
VFDs generate harmonics and require shielded cables, circuit separation, and adequate grounding.
The CE Code (Part I) imposes specific rules for conductors (28-106), overcurrent protection (28-200), overloads (28-308), disconnecting means (28-500), and controllers (28-600).
Practice reading a nameplate and performing the complete motor circuit calculation (conductors, protection, overload).
Know the CE Code limit values by heart: 125%, 250%, 300%, 400%.
For VFD questions, remember the constant V/f principle and the three stages of the drive.
Read each question twice: examiners often trap on units (HP vs kW), phases (single-phase vs three-phase), and voltages (208 vs 600 V).
Systematically use the CE Code tables for conductor ampacities and correction factors.
This chapter covers all the essential concepts to succeed on Red Seal questions about motors, controllers, and variable frequency drives. Practicing calculations and precise knowledge of CE Code rules are your best allies on exam day.
