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Types of Motors Used in an Elevator (Elevator Motors)
In this article, we will take a look at the different types of electric motors used in the elevators. We will also discuss the suitability of different motors in different types of elevators, power calculation, starting torque requirements, and speed control methods of elevator motors.
What is an Elevator Motor?
An elevator motor is an electric motor which is used for up and down movements of elevator car. The turning force produced by the elevator motor is transmitted to the rope winding drum to wind or unwind the rope, so that the elevator car moves up and down.
The elevator motors are specially designed electric motors to ensure the rapid acceleration and retardation, the inertia of moving parts of the elevator motor must be kept as low as possible. Consequently, the electric motors used for elevator service are designed with the armatures of comparatively small diameter. In the elevator service, slow speed motors are preferred, generally, the speed of the elevator motor should not exceed 900 RPM.
Types of Elevator Motors
Both AC and DC types of electric motors are being used in the elevator services. According to different service requirements, there are different types of DC and AC motors are employed.
DC Motors
There are different types of DC motors available (series motor, shunt motor and compound motor). The type of DC motor which is best suited to the given elevator service depends on the type of elevator service and the speed and the frequency of the slopes.
The elevator motor must not only lift up and down the load, but one of its principle duties is to accelerate and decelerate the elevator car rapidly. This task must be accomplished smoothly without jerks which might cause discomfort to the passengers.
The starting torque produced by the DC motor being used as the elevator motor should be at least 225% of the rated full-load torque. The DC motor having more than 15% speed variation from no-load to full-load is not suitable for elevator service.
Now, based on different elevator services, following types of DC motors are used −
For passenger service – Either compound wound DC motors or DC shunt motors give satisfactory results.
For freight service – In this service, compound wound DC motors are suitable. The series winding of the compound wound motor provides a high starting torque, which is an essential factor for heavy duty work.
Note – Most DC motors, whether compound wound or shunt, have suitable commutating pole winding so as to ensure the sparkles commutation in both the directions of rotation while using as elevator motor.
AC Motors
The AC motor used as the elevator motor are asynchronous motor (or induction motor). The following are the two types of AC motors suitable for the elevator service −
Squirrel Cage Motor − The squirrel cage motor is used extensively in the elevator services up to about 20 HP. It is used because of its simplicity and it also requires relatively simple form of controller. However, the power consumption of squirrel cage motor is slightly more than that of the slip ring motor, but due to the absence of slip rings and fewer controller parts, it is somewhat more reliable.
Slip Ring Motor − The slip ring induction motors are also being used in the different elevator services. Although, the slip ring motor of same rating is more expensive and has a lower power factor than the squirrel cage induction motor. Slip ring motors of low speed type are now available and are being used successfully on elevators whose car speeds are as high as 400 ft./minute.
Power Requirement of Elevator Motor
In order to determine the power required by an elevator motor for a given installation, following three factors are to be considered −
Speed of elevator car
Load (or net weight) to be hoisted
Efficiency
In the determination of the load to be hoisted, it is required to consider that the load of the car and the part of the load is counter balanced by the counter weight. Hence, only the unbalanced load must be taken into account.
The efficiency is the overall efficiency which considers various frictional and electrical losses.
Thus, the power (in H.P.) of the elevator motor required can be determined using the following formula −
$$\mathrm{Power\: required \: (in\:H.P.)\mathrm{\: =\: }\frac{Load \times Speed}{Efficinecy \times 33000}}$$
Starting Torque Requirements of Elevator Motor
An elevator requires higher starting torque than is required to keep the same elevator running at its rated speed and load. It is because of the static friction. Therefore, an electric motor to be suitable for elevator work, should be capable of developing a starting torque of 2.5 to 3 times of full-load torque.
Elevator Control System
The elevator control system is employed for the speed control of elevator motors and is broadly classified into two types −
Rheostatic control – In a rheostatic control system, a rheostat is included in the field and armature circuits of the motor and speed control is achieved by varying it.
Variable voltage control – In a variable voltage control system, the input voltage applied to the elevator motor is varied by some means and the speed control is achieved.
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