This article describes the different types of electric elevator motors. We will also discuss the suitability of different types of electric motors for elevator applications, their speed-torque characteristics, power consumption, and speed control methodology.
What are Elevators?
An elevator is a car that moves in a vertical shaft to carry passengers or materials between the levels of a multistory building.
The elevators are propelled by electric motors using a counterweight through a cables and pulleys system.
What is an Elevator Motor?
An electric motor used for the up and down movement of the elevator car is known as an elevator motor. The motor produces torque and it is transmitted to the rope winding drum. The rope-winding drum wind and unwind the rope according to the forward and reverse rotation of the motor.
An elevator motor is used for the up and down movements of the elevator car. The turning force produced by the motor is transmitted to the rope-winding drum. The drum wind or unwind the rope, so that the elevator car moves up and down.
The mass of the motor is kept as minimum as possible to have a low moment of inertia. The low moment of inertia helps in the quick acceleration of the motor. Therefore, elevator motors are designed to keep the armature diameter small. The slow-speed motors below 1000 RPM are preferred for elevator applications.
Types of Elevator Motors
Both AC and DC types of elevator motors are used.
DC Motors
There are different types of DC motors-series motors, shunt motors, and compound motors- available. However, all types of DC motors are not suitable for elevator services. The selection of the elevator motor depends on the starting torque demanded by the elevator, the type of elevator service, speed and frequency of the slopes.
The principle requirement of the motor is that it should lift up and lift down the load with rapid acceleration and deacceleration without any jerks. The jerks during acceleration and deacceleration cause more wear and tear and moreover, also cause discomfort to the passengers.
The DC motor should be capable to produce at least 250 % of the rated full-load torque, and its speed regulation must be 15 % from no-load to full-load.
The elevators are used for passengers and materials. The different types of DC motors is used for the elevators used for freight and passenger.
- For passenger service – Both compound wound DC motors and DC shunt motors are suitable and give satisfactory results. It has less sparkless commutation and they are suitable for forward and reverse rotation.
- For freight service – The compound wound DC motors are most suitable for this application. The compound wound motor provides a high starting torque because of the series winding. The high starting torque is necessary for lifting and lowering heavy loads.
AC Motors
The following types of AC motors are suitable for elevator applications.
Squirrel Cage Motor
The squirrel cage motor is simple in construction and it is robust because it does not have slip rings and carbon brushes. Speed control can be achieved by the use of a variable frequency drive. It is possible to operate the motor in four quadrants i.e. forward and reverse motoring and braking zone through VFD. The squirrel cage motor of below 25 HP rating is suitable for elevator applications.
Slip Ring Motor
A slip Ring motor delivers more starting torque than a squirrel cage induction motor. It is used when a large rating of the motor is required for elevator application. These motors are suitable for elevators whose car speed is 400 ft./minute and above. A slip ring motors need more maintenance compared to a squirrel cage induction motor.
Power Requirement of Elevator Motor
In an elevator, the load of the car and the part of the load are counterbalanced by the counterweight. Therefore we need to take the unbalance load into consideration for the power requirement of a motor.
The power of the motor can be calculated by taking the following factors into consideration.
- Speed of elevator car
- Net weight to be hoisted
- The efficiency of the system
The power required in the horsepower of the elevator motor can be expressed by the following formula.
Starting Torque Requirements of Elevator Motor
The elevator demands higher starting torque in comparison to the torque it demands during the running conditions. In starting additional torque is required to overcome the static friction. Therefore, the motor should be capable to deliver 250-300 % starting torque of its full-load torque,.
Elevator Control System
The elevator control system is used for the speed control of elevator motors. There are two types of elevator control systems. They are-
Rheostatic control
A rheostat is added in the field and armature circuit of the DC motor to control the speed of the motor. By changing the value of resistance, the speed is controlled.
Variable voltage control
The input voltage to the elevator motor varies by some means in variable voltage control. The voltage can be varied by thyristor drive for the DC motor and VF drive for the squirrel cage induction motor.