Electrical Systems and Controls for Tower Cranes

Electrical Systems and Controls of Tower Cranes

Introduction to Electrical Systems

The tower crane is an essentially electrical machine. Unlike hydraulic mobile cranes, the majority of tower crane movements (hoisting, slewing, trolley travel, travel) are provided by electric motors. Mastering basic electrical principles, components, and safety devices is essential for the tower crane operator, both for safe operation and for pre-shift inspection.

The Canadian Electrical Code, Part I (CE Code) (CSA C22.1-21 standard) applies to the electrical installation of cranes. However, the operator is not responsible for the electrical installation itself, but must understand the systems to detect anomalies and react correctly.

Fundamental Principles of Electricity

Basic Electrical Quantities

Quantity	Symbol	Unit	Unit Symbol
Voltage	U (or V)	Volt	V
Current	I	Ampere	A
Resistance	R	Ohm	Ω
Active Power	P	Watt	W
Apparent Power	S	Volt-ampere	VA
Frequency	f	Hertz	Hz

Ohm's Law: U = R × I

Power in Three-Phase Alternating Current: P = √3 × U × I × cos φ

Where:

U = voltage between phases (volts)

I = line current (amperes)

cos φ = power factor (generally between 0.7 and 0.9 for motors)

Three-Phase Alternating Current

Tower cranes use almost exclusively three-phase alternating current (400 V or 600 V in Canada). Three-phase power allows:

More powerful motor starting

Higher efficiency

More economical distribution

The standard frequency in Canada is 60 Hz.

Transformers

The transformer steps down the utility voltage (often 600 V) to the control circuit operating voltage (24 V, 48 V, or 120 V). The isolation transformer also separates the control circuit from the power circuit, which improves safety.

Transformation Ratio: U₁ / U₂ = N₁ / N₂

Where N₁ and N₂ are the number of turns in the primary and secondary windings.

Electrical Components of the Tower Crane

Electric Motors

The motors used on tower cranes are primarily:

27.Three-phase squirrel-cage induction motors: robust, used for slewing and trolley travel.

28.Wound-rotor induction motors: allow progressive starting and speed control via rotor resistances, used for hoisting.

29.DC motors (rarer): offer excellent speed control, but require more maintenance.

Important characteristics for the operator:

The hoist motor is the most powerful motor on the crane

The slewing motor must withstand wind loads

Motors are equipped with electromagnetic brakes that engage automatically in the event of a power failure (safety braking)

Electromagnetic Brakes

The electromagnetic brake is a critical safety device. Its principle:

At rest: the brake is engaged by springs (safe position)

In operation: the electromagnet compresses the springs and releases the brake

This principle ensures that any loss of power results in the immediate stopping of the load.

Exam Point: The hoist brake must be capable of supporting 125% of the rated load (CSA Z248 standard requirement).

Contactors and Relays

Contactor: an electrically controlled switch, used to open or close power circuits

Relay: similar to a contactor but for control circuits (low power)

Contactors are equipped with:

Main contacts (power)

Auxiliary contacts (control and signalling)

Control coil (24 V, 48 V, 120 V)

Circuit Breakers and Fuses

Device	Function	Reset
Fuse	Overcurrent protection	Replacement
Thermal-magnetic circuit breaker	Protection against overloads and short circuits	Manual reset
Ground fault circuit interrupter	Protection against current leakage	Manual reset
Thermal overload relay	Motor protection against overloads	Manual or automatic reset

Important Rule: The thermal overload relay protects the motor against prolonged overloads, but not against short circuits. Fuses or circuit breakers provide this latter protection.

Control and Power Circuits

Distinction Between Power Circuit and Control Circuit

Power circuit: carries the current that supplies the motors (400 V, 600 V)

Control circuit: carries the control signals (24 V, 48 V, 120 V) that operate the contactors

This separation is essential for safety: the operator handles low-voltage circuits, while power is isolated in the electrical enclosures.

Schematic Diagram of a Hoisting Circuit

The hoisting circuit typically includes:

57.Main circuit breaker

58.Line contactor

59.Direction contactors (up/down)

60.Speed contactors (2 or 3 speeds)

61.Rotor resistances (for wound-rotor motors)

62.Hoist motor

63.Electromagnetic brake

64.Limit switch (up and down)

65.Load limiter

Hoisting Operating Sequence

67.The operator activates the hoist control lever

68.The control circuit (24 V) activates

69.The line contactor closes

70.The direction contactor (up) closes

71.The brake releases (with a slight delay)

72.The motor starts at first speed

73.The operator shifts to a higher speed if necessary

Trap to Avoid: The brake must never release before the motor is powered. A delay of 0.5 to 1 second is normal.

Limiters and Electrical Safety Devices

Limit Switches (Motion Limiters)

Limit switches are mechanical or electronic switches that cut the control circuit when the movement reaches a predetermined limit.

Type of Limit Switch	Function	Typical Location
Hoist limit switch (up)	Stops the hook from rising	Winch or jib
Lowering limit switch	Limits hook descent	Winch
Slewing limit switch	Limits rotation (if power cable)	Slewing ring
Trolley limit switch	Limits the trolley	Jib
Travel limit switch	Limits movement on rails	Travel bogies

Regulatory Requirement: The hoist limit switch (up) must be checked daily by the operator before the start of the shift (CSA Z248-17 standard, Article 8.2).

Load Limiter (LL)

The load limiter is an electronic device that measures the load moment (load × radius) and cuts dangerous movements when the limit is reached.

Operation:

Sensors (strain gauges) measure the deformation of the structure

The computer compares the actual load to the rated load for the given radius

If the limit is exceeded, the system cuts the hoisting (up) and trolley (outward) movements

Safety movements (lowering, trolley inward) remain available

Alarm Thresholds:

90% of rated load: audible and visual alarm

100% of rated load: cut-off of dangerous movements

110% of rated load: total cut-off (depending on model)

Moment Limiter (ML)

The moment limiter is an advanced function of the load limiter that takes into account the overturning moment. It uses the formula:

Overturning Moment = Load × Radius

This device is particularly important for luffing jib cranes where the radius varies with the jib angle.

Anemometer

The anemometer measures wind speed. It is connected to an alarm system:

Preventive alarm (generally 30 km/h): warns the operator

Stop alarm (generally 45 to 50 km/h depending on the manufacturer): cuts certain movements and requires work to stop

Requirement: The anemometer must be installed at the top of the crane, away from turbulence created by the structure itself.

Emergency Stop Buttons

Emergency stop button: located in the cab, immediately cuts all movements

Isolation switch: cuts all electrical power to the crane

Foot emergency stop button: on certain models, for situations where the operator cannot reach the main button

Procedure: After an emergency stop, the operator must identify the cause, correct it, then reset the system before resuming work.

Modern Control Systems

Variable Frequency Drives (VFDs)

Variable frequency drives (VFDs) are progressively replacing resistance systems. They allow:

Progressive starting and stopping (reduced shock loads)

Precise speed control

Energy savings

Reduced mechanical wear

Principle: The drive converts alternating current to direct current, then reconverts it to alternating current at a variable frequency. Motor speed is proportional to frequency:

Synchronous Speed: N = (120 × f) / p

Where:

N = rotational speed (rpm)

f = frequency (Hz)

p = number of motor poles

Programmable Logic Controllers (PLCs)

The programmable logic controller (PLC) is the electronic brain of modern cranes. It:

Receives signals from sensors and control levers

Executes logic sequences

Controls contactors and drives

Displays information on the cab screen

Records operating data (electronic logbook)

Safety Advantages:

Automatic fault detection

Precise diagnostics

Programmed safety interlocks

Telemetry and Display Systems

Modern cranes are equipped with screens that display in real time:

The suspended load

The current radius

The load moment

Wind speed

The jib angle (for luffing jib cranes)

Error codes and alarms

Requirement: The operator must understand all displayed information and know how to interpret alarms.

Electrical Cables and Connections

Types of Cables Used

Type of Cable	Use	Characteristics
Main power cable	Crane power supply	Multi-conductor, reinforced sheath
Power cable	Motor circuits	Cross-section matched to current
Control cable	Signalling circuits	Small cross-section, multi-conductor
Coaxial or fibre optic cable	Data transmission	For telemetry systems

Crane Power Supply Cable

The power supply cable is a critical component. It must:

Be mechanically protected (reinforced sheath)

Be suspended or guided to avoid any friction

Have sufficient length to allow all movements

Be visually checked daily by the operator

Daily Check Points:

Cuts, abrasions, deformations of the sheath

Connectors and plugs in good condition

No pinch points or friction points

Fasteners and supports in good condition

Grounding

Grounding is essential for the safety of people and equipment. It allows:

Discharging fault currents

Protection against lightning

Stabilizing voltages

Requirements:

The crane must be connected to ground by a grounding conductor

The ground electrode resistance must be less than 25 Ω (according to the Canadian Electrical Code)

The metallic masses of the crane must be interconnected

Canadian Electrical Code, Part I Rule: Rule 8-200 — Cranes must be grounded in accordance with the requirements of Section 10 of the Code.

Inspection and Verification Procedures

Pre-Shift Inspection

The operator must perform the following electrical checks before each shift:

165.Visual inspection of the power cable: sheath intact, no cuts

166.Test of limit switches: activate each limit switch and verify the movement stops

167.Test of the load limiter: perform a test lift with a known load

168.Test of the emergency stop button: verify immediate cut-off

169.Check of the anemometer: compare the reading with actual conditions

170.Check of indicator lights and alarms: ensure all indicators are working

171.Test of the brakes: perform a test lift and lowering

Periodic Inspection

Periodic inspections are performed by qualified personnel (electrician, technician) at a frequency defined by the manufacturer and regulations:

Frequency	Items to Check
Monthly	Contactors, relays, connections, torque checks
Quarterly	Resistors, motors, brakes, sensors
Annually	Cable insulation, grounding, electrical enclosures

Limit Switch Verification

Hoist Limit Switch (Up) Test Procedure:

177.Ensure the area is clear

178.Raise the hook at slow speed

179.Verify that the movement stops automatically before contact with the sheave

180.Verify that lowering remains possible

181.If the limit switch does not work, stop the crane immediately and report the fault

Trap to Avoid: Never use the limit switch as a normal means of stopping. It is a safety device, not an operating control.

Basic Electrical Troubleshooting

Common Symptoms and Possible Causes

Symptom	Possible Cause	Operator Action
No movement	Power failure, tripped circuit breaker	Check power supply, report
Only one movement not working	Limit switch activated, faulty contactor	Check limit switch, report
Slow hoisting	Overload, low voltage, motor problem	Check load, report
Brake does not release	Control voltage absent, faulty coil	Report immediately
Intermittent alarms	Loose connections, faulty sensor	Report, do not bypass
Display screen off	Screen power supply cut	Check circuit breaker, report

Golden Rules of Troubleshooting

187.Never bypass a safety device

188.Never open an electrical enclosure without authorization and training

189.Report any fault immediately

190.Never reset a circuit breaker without identifying the cause

191.Use lockout/tagout procedures before any intervention

Lockout/Tagout (LOTO) Procedure

The lockout/tagout procedure is mandatory before any intervention on electrical circuits:

194.Notify all persons concerned

195.Stop the crane (stop position)

196.Isolate the power supply (main circuit breaker)

197.Lock out the isolation device

198.Tag with the person's name and date

199.Verify the absence of voltage (by qualified personnel)

200.Perform the intervention

201.Remove the locks and tags after the intervention

202.Restore to service and verify operation

Applicable Standards and Regulations

Main Standards

Standard	Title	Application
CSA Z248-17	Code for Tower Cranes	Design, construction, installation, inspection, and use of tower cranes
CSA C22.1-21	Canadian Electrical Code, Part I	Electrical installation of cranes
CSA B149.1	Natural Gas and Propane Installation Code	Not directly applicable, but cited for auxiliary systems
CSA S16	Design of Steel Structures	Crane structure

Electrical Requirements of CSA Z248-17

The CSA Z248-17 standard contains several important electrical requirements:

Article 6.1: Cranes must comply with the Canadian Electrical Code

Article 6.2: Control circuits must be protected by fuses or circuit breakers

Article 6.3: Motors must be protected against overloads

Article 6.4: Electrical safety devices must be tested regularly

Article 8.2: Daily inspection must include electrical safety devices

Operator Responsibilities

The operator is responsible for:

Performing daily checks of electrical systems

Reporting any fault or anomaly

Never bypassing or neutralizing a safety device

Using the crane in accordance with the manufacturer's instructions

Understanding the information displayed by electronic systems

Traps to Avoid

221.Confusing the power circuit and the control circuit: The control circuit operates at low voltage (24 V, 48 V, 120 V), the power circuit at high voltage (400 V, 600 V). Never touch power circuit components.

222.Bypassing a faulty limit switch: This is a serious violation and a frequent cause of accidents. A faulty limit switch must be reported and repaired, never neutralized.

223.Using the limit switch as a normal control: The limit switch is a safety device, not an operating control. Repeated use can damage it.

224.Forgetting to test safety devices before the shift: Daily verification is mandatory. An undetected faulty device can lead to a serious accident.

225.Resetting a circuit breaker without identifying the cause: If a circuit breaker trips, there is a reason. Resetting without correcting the cause can cause a fire or major failure.

226.Confusing the load limiter and the limit switch: The load limiter protects against overload, the limit switch protects against excessive travel. These are two distinct devices.

227.Ignoring intermittent alarms: An alarm that triggers intermittently is often a sign of a growing problem. Do not ignore it.

228.Not knowing the meaning of error codes: The operator must know the common error codes for their crane and know what to do when they appear.

229.Forgetting lockout/tagout before an intervention: Even for a minor intervention, lockout/tagout is mandatory. Negligence can be fatal.

230.Not checking the power cable: A damaged cable can cause electric shock or fire. Daily visual inspection is essential.

Summary

Tower cranes use three-phase alternating current (400 V or 600 V, 60 Hz in Canada)

The power circuit (high voltage) is separated from the control circuit (low voltage)

Electromagnetic brakes are critical safety devices: they engage automatically in the event of a power failure

The load limiter measures the load moment (load × radius) and cuts dangerous movements

The hoist limit switch (up) must be tested daily

The anemometer triggers alarms at predetermined thresholds (approximately 30 km/h and 45-50 km/h)

Variable frequency drives and programmable logic controllers are modern control technologies

Grounding is mandatory and its resistance must be less than 25 Ω

The operator must perform daily checks and report any fault

Lockout/tagout (LOTO) is mandatory before any intervention on electrical circuits

CSA Z248-17 and the Canadian Electrical Code, Part I, govern electrical requirements

Golden Rule: When in doubt about the operation of an electrical system, stop the crane, report the problem, and do not resume work until it has been corrected by qualified personnel. Electrical safety allows no compromise.