Motors and Motor Control Systems

Motors and Motor Control Systems

Introduction

This chapter covers all the knowledge required for the Red Seal exam concerning electric motors and their control systems. You must master the operating principles, motor types, starting methods, protections, and control circuits in accordance with the Canadian Electrical Code, Part I (CE Code) . This chapter is structured to follow a logical progression: first the physical principles, then motor types, then starting and protection methods, and finally control circuits and troubleshooting.

Fundamental Principles of Motors

The Rotating Magnetic Field

Every alternating current (AC) motor operates on the principle of the rotating magnetic field. When a balanced three-phase system supplies three coils arranged 120° apart, the resulting magnetic field rotates at a speed called the synchronous speed (Ns).

The synchronous speed is calculated using the following formula:

Ns = 120 × f / P

Where:

Calculation example: For a 4-pole motor supplied at 60 Hz:

Ns = 120 × 60 / 4 = 1800 RPM

Slip

Slip (s) is the difference between the synchronous speed and the actual rotor speed, expressed as a percentage:

s = (Ns − Nr) / Ns × 100 %

Where Nr is the actual rotor speed.

For a typical induction motor, the slip at full load varies between 2% and 5%. Zero slip is impossible for an induction motor — it is the very principle of its operation. Torque is produced by the speed difference between the rotating field and the rotor.

Torque and Power

Torque (T) and power (P) are related by the following equation:

P (W) = T (N·m) × ω (rad/s)

Where ω = 2π × Nr / 60

For a three-phase motor, the electrical power absorbed is:

P = √3 × V × I × cos φ

Where:

Efficiency (η) is the ratio of mechanical output power to electrical input power:

η = P_output / P_input × 100 %

Types of Motors

Direct Current (DC) Motors

DC motors are classified according to the connection of the field winding relative to the armature:

The series motor is used for traction applications (overhead cranes, locomotives) due to its high starting torque. Caution: a series motor must never run at no load — the speed can become dangerously high (runaway).

Three-Phase Induction Motors

This is the most common type of motor in industry. The rotor can be:

The squirrel cage motor is the most common. Its main characteristics are:

Synchronous Motors

The synchronous motor runs at exactly the synchronous speed (zero slip). The rotor is supplied with direct current through slip rings and brushes. Typical applications:

Single-Phase Motors

Single-phase motors do not produce a rotating field by themselves — they require a starting method:

Nameplates and Rated Data

Reading the Nameplate

The motor nameplate provides essential information for installation and troubleshooting. You must know how to interpret each piece of data:

Service Factor

The service factor (SF) indicates the allowable continuous overload beyond the rated power. A motor with SF = 1.15 can operate continuously at 115% of its rated power without exceeding the maximum temperature of its insulation class.

Insulation Classes

Motor Protection

Overload Protection

Overload protection protects the motor against prolonged excessive currents that cause heating. It does not act on short circuits — that is the role of overcurrent protection devices (fuses or circuit breakers).

According to the Canadian Electrical Code, Part I, Rule 28-306, the overload protection device must be set at:

Example: 10 HP motor, 575 V, rated current 12.5 A, SF = 1.15.

Maximum setting = 12.5 A × 1.25 = 15.6 A → choose a relay set at 15 A.

Short-Circuit Protection

Short-circuit protection (fuses or circuit breakers) protects the conductors and the motor against fault currents. Rule 28-200 of the CE Code specifies the maximum values:

If these values do not allow the motor to start, the CE Code permits higher values under certain conditions (Rule 28-204).

Overload Relays — Trip Classes

Thermal overload relays are classified according to their trip time:

Ground Fault Protection

Rule 28-600 of the CE Code requires ground fault protection for motors over 750 V. For low-voltage motors, this protection is generally integrated into the circuit breaker or provided by a residual current relay.

Starting Methods for Three-Phase Motors

Direct-On-Line (DOL) Starting (Full Voltage)

The motor is connected directly to the supply. The starting current is 5 to 8 times the rated current. This method is acceptable for small motors or when the supply capacity is sufficient.

Star-Delta (Y-Δ) Starting

The motor starts in star (Y), which reduces the voltage across each winding to 58% of the line voltage (1/√3). After a time delay, the connection changes to delta (Δ).

Important: This method only applies to motors whose six winding terminals are accessible and which are designed to operate in Δ at the supply voltage.

Autotransformer Starting

An autotransformer supplies reduced voltage to the motor during starting. Typical taps are at 50%, 65%, and 80% of the rated voltage.

Example: With a 65% tap, the starting torque is 0.65² = 0.42 (42%) of the direct starting torque.

Stator Resistance Starting

Resistors are inserted in series with the stator windings during starting, then progressively short-circuited. This method is simple but results in significant energy losses.

Variable Frequency Drive (VFD) Starting

The variable frequency drive (VFD) adjusts both voltage and frequency to control speed and torque. Advantages:

The VFD modifies the V/f relationship to maintain a constant flux in the motor. Constant V/f control is the most common: voltage and frequency vary together to maintain the rated ratio.

Control Circuits

Basic Components

A control circuit typically includes:

Start-Stop Circuit

The classic start-stop circuit includes:

Operating sequence:

Circuit with Two Start Pushbuttons (Two-Location Control)

To control a motor from two locations, the start pushbuttons are connected in parallel and the stop pushbuttons in series.

Circuit with Timing

Timers can be:

Typical application: star-delta starting — the timer controls the transition from Y to Δ after a predetermined delay (generally 3 to 10 seconds).

Reversing Circuit

To reverse the direction of rotation of a three-phase motor, two phases are interchanged. The circuit includes two contactors:

Mandatory electrical interlocking: The two contactors must never be closed simultaneously — this would create a short circuit between phases. Interlocking is achieved by:

Canadian Electrical Code (CE Code) Rules — Section 28

Rule 28-106 — Supply Conductors

The supply conductors of a motor must have an ampacity of at least 125% of the motor's rated current.

Example: 12.5 A motor → conductors sized for 12.5 × 1.25 = 15.6 A → choose #14 AWG conductors (20 A at 75 °C) or #12 AWG depending on length and voltage drop.

Rule 28-110 — Overcurrent Protection

Each motor must be protected by an overcurrent protection device conforming to Rules 28-200 to 28-210.

Rule 28-500 — Disconnecting Means

A disconnect switch must be installed within sight of the motor or be lockable in the open position. It must be capable of being locked in the open position to permit safe maintenance.

Rule 28-600 — Ground Fault Protection

Mandatory for motors over 750 V. For low-voltage motors, the protection is generally integrated into the circuit breaker or provided by a residual current relay.

Rule 28-602 — Grounding

The motor frame must be grounded in accordance with Section 10 of the CE Code. The grounding conductor must be sized according to Table 16 of the CE Code.

Rule 26-256 — Control Circuit Protection

Control circuits must be protected against overcurrent. The protection must be installed at the origin of the control circuit.

Troubleshooting and Verification

Pre-Commissioning Checks

Current Measurements

The current of each phase must be measured during operation. A current imbalance between phases greater than 10% indicates a problem:

Common Problems and Causes

Pitfalls to Avoid

Summary

Review Questions

Normative References

This chapter covers the essential knowledge for the Red Seal exam in industrial electricity. Mastery of the formulas, CE Code rules, and operating principles is essential. Practice with varied calculations and control circuit diagrams to consolidate your understanding.