Motor Technologies for Mid-Rise (Up to G+10) Elevators.

For a G+10 building, travel heights are typically between 30 to 35 meters. Speeds usually range from 1.0 m/s to 1.75 m/s. At this scale, the industry standard relies entirely on traction elevators, utilizing two primary motor categories:

1. Geared AC Induction Motors (The Legacy Standard)

Historically, the backbone of mid-rise elevators has been the 3-phase AC Induction motor paired with a reduction gearbox (usually a worm gear).

• How it works: The motor spins at high RPMs, and the gearbox steps down the speed to turn the traction sheave at the lower RPM required for a 1.0 m/s cabin speed.

• Control: Driven by Variable Voltage Variable Frequency (VVVF) inverters to ensure smooth acceleration and deceleration, easily interfacing with master controllers.

• Pros: Lower initial capital cost; highly durable for standard mid-rise traffic.

• Cons: The gearbox introduces mechanical losses (lower efficiency); requires regular oil maintenance; bulkier footprint requiring a dedicated machine room.

 2. Gearless Permanent Magnet Synchronous Motors (PMSM) (The Modern Standard)

For contemporary G+10 installations, especially Machine Room-Less (MRL) configurations, PMSM technology dominates. 

• How it works: These motors operate at very low RPMs with extremely high torque, allowing the traction sheave to be mounted directly to the motor shaft without a gearbox.

• Control: Requires advanced closed-loop VVVF drives with precise encoder feedback to maintain zero-speed holding torque and dynamic movement.

• Pros: Up to 50% more energy efficient than geared systems; no gearbox oil; ultra-compact, allowing the motor to be bolted directly to the top of the elevator shaft guide rails (MRL).

• Cons: Higher initial cost; complex drive tuning.

Part 2: Calculating Motor Capacity (The Mathematics)

(i) Q = Rated Passenger Payload (Kg) [ Industry Standard is 68Kg to 75Kg per passenger].

(ii) P = Weight of the empty elevator car (kg).

(iii) CW = Weight of the counterweight (kg)

(iv) V = Rated speed of the elevator (m/s)

(v) g = Acceleration due to gravity (9.81 m/s² )

(vi) µ  = Total system mechanical efficiency (Geared Systems = 065 to 0.75, Gearless Systems = 0.85 to 0.90)

Step 1: Determine the Counterweight

The counterweight is designed to perfectly balance the empty car plus 45% to 50% of the maximum payload. For this calculation, we use a 50% balancing factor.

                             CW = P + (0.5 X Q)

Step 2: Calculate the Maximum Unbalanced Mass (Mu )

 

The worst-case scenario for the motor is lifting a fully loaded car upwards.

The total mass on the car side is . The mass on the counterweight side is .

Mu = (P + Q) - CW

Substituting the counterweight formula:

Mu = (P + Q) – (P + 0.5 X Q)

Mu = 0.5 X Q

Note: The motor is only doing work against half of the payload weight, regardless of how heavy the steel cabin (P ) is.

 

Step 3: Calculate the Required Mechanical Force (F):

                             F = Mu X g

 

Step 4: Calculate the Required Motor Power (Pmotor):

 

Power is Force multiplied by Velocity. To convert the result to kilowatts (kW) and account for the frictional losses of the pulleys, ropes, and guides (µ), use the following equation:

                                     Pmotor (Kw) = F X V / 1000 X µ

 

Complete Combined Equation:

 

Pmotor (Kw) = (0.5 X Q) X 9.81 / 1000 X µ

 

 

Let’s see an worked example:

The Scenario: A G+10 residential building requires an 8-passenger elevator running at 1.0 m/s using a modern Gearless PMSM motor.

 

• Capacity (Q): 8 persons  68 kg = 544 kg

 

• Speed (V):

 

• Efficiency (µ):  (Typical for gearless MRL)

 

The Calculation:

 

Pmotor = (0.5 X 544) X 9.81 X 1.0 / 1000 X 0.85

 

Pmotor = 272 X 9.81 X 1.0 / 850

 

Pmotor = 2668.32 / 850

 

Pmotor = 3.13kW

Engineering Conclusion: The minimum required motor capacity is . In standard industrial practice, a designer would select the next commercially available size up, typically a 3.7 kW (5 HP) PMSM gearless machine.