TET4120 ELECTRIC DRIVES

PROJECT WORK

DESIGNING DRIVE SYSTEM FOR ESCALATOR

Mamta Maharjan, Stein Nornes, Linda Rekosuo

1 ii

iii

SUMMARY In this report the drive for specific escalator is designed. ABB’s motor DMR 112 SN-

473P with speed 2080 rpm and moment of inertia 0.05 kgm2 is selected to fullfil load

requirements.Motor is controlled by DC-DC fullbridge converter with bipolar pulse

width modulation. Control of current is done by using modulus optimum techique. For

speed symmetric optimum is used. The whole system is modeled with Simulink.

iv

Table of Content SUMMARY ..................................................................................................................... iii

1 Introduction ............................................................................................................... 1

2 Design of drive system .............................................................................................. 2

2.1 Modeling the load ............................................................................................. 3

2.1.1 Inertia of load ....................................................................................... 3

2.1.2 Load Torque ......................................................................................... 4

2.1.3 Simulink model .................................................................................... 4

2.2 Design of motor ................................................................................................ 5

2.2.1 Selecting motor .................................................................................... 5

2.2.2 Modeling motor ................................................................................... 6

2.3 Design of control system ................................................................................... 7

2.3.1 Design of power processing unit ......................................................... 7

2.3.2 Design of current regulator .................................................................. 9

2.3.3 Design of speed regulator .................................................................. 10

2.4 Model of whole escalator system .................................................................... 10

3 RESULTS ............................................................................................................... 12

4 Conclusion .............................................................................................................. 15

1 1

1 Introduction

In this project a drive for operating an escalator is designed. In order to do this, the esca-

lator in Simulink is first modeled. This model is used to calculate the minimum parame-

ters of the motor needed in order to meet the supplied requirement specification of the

escalator:

The escalator shall be able to take one person per step, each person weighing

100 kg

The escalator shall be able to operate in both the upwards and downwards

directions

The PPU shall be designed so that regenerative braking is possible

The escalator speed shall be 1.3 m/s and shall be able to accelerate from

standstill to rated speed in 6.5 s.

The escalator has a sensor that detects when no persons are riding the escalator.

The escalator speed is then reduced to 0.2 m/s. The escalator accelerates to 1.3

m/s when someone steps onto the escalator – this acceleration should take 5.5 s.

Based on the calculated minimum parameters a DC motor from the ABB datasheet is

chosen and Power Processing Unit and a control system for the drive are designed. Fi-

nally, entire system is simulated in Simulink and that the drive meets the requirement

specification is confirmed.

2

2 Design of drive system

In this chapter the drive system for the escalator is designed. Picture of the escalator is

shown in Figure 1. Escalator has to fulfill following requirements:

The escalator height difference is 5 m

The escalator angle is 25°

The height difference between steps is 25 cm

The escalator width is 1 m

Each step has a weight of 20 kg

The four horizontal sections of the escalator are each 3 steps long and the two

drums each take 6 steps around half of their circumference

The drums around which the escalator turns are made as a hollow cylinders with

an outer radius of 50 cm and an inner radius of 49 cm (spokes and hubs can be

neglected). The drum material is steel. Other drums and rollers than the two

large ones can be neglected.

The gear’s inertia is

Friction can be neglected

Figure 1 Structure of escalator

3

2.1 Modeling the load

2.1.1 Inertia of load

First inertia of load is defined. Inertia of drums can be calculated by using equations for

hollow cylinder. With outer radius r2, inner radius r1, height h and density ρ it can be

described as

(

) (1)

The density of steel varies depending on the alloy, and is usually between 7750 and

8050 kg/m3. For simplicity, we assume the density to be 8000kg/m

3. This gives the fol-

lowing inertia per drum:

(( ) ( ) ) (2)

Next inertia of steps is defined. Number of steps in each slanted region is

(3)

and total number of steps is

(4)

All of the steps are connected to the outside of the drum, and it gives the following iner-

tia

( ) (5)

Total inertia of load is then

(6)

Inertia seen in motor side is

(7)

4

2.1.2 Load Torque

The load torque is a result of the weight of the passengers. In accordance with the given

specification, we assume each person to weigh 100 kg. The force on the belt works in a

65 degree angle relative to the force of gravity, so the torque is

( )

(8)

Since there is only room for 20 persons on the slanted region, the maximum load torque

is

(9)

The maximum load torque seen from the motor side is

(10)

2.1.3 Simulink model

Simulink model for the load is shown in Figure 2. Its outputs are load torque and system

inertia seen in motor side. Inertia of whole system equals sum of inertia of load, gear

and motor.

Figure 2 Simulink model for the load

Figure 3 shows Simulink model for Subsystem. It models constant load inertia and load

torque which depends on number of passengers.

J_eq

75.21

gear ratio

T_L,motor side

No. of Persons

J_L

T_L

Subsystem|u|

2

Math

Function

0.05J_m

0.1

J_gears

J_L,motor side

5

Figure 3 Simulink model for the subsystem

2.2 Design of motor

2.2.1 Selecting motor

Motor is chosen by using ABB’s DC Motors, type DMR, motor catalogue. At Table 1

can be seen calculated motor requirements for every motor. Moment of inertia of motor

is placed on the first column. Optimum gear ratio is defined as

√

(11)

Because torque in this ratio would be very small, the optimum ratio reduced by 20 % is

used. Maximum torque, speed and power are defined as

( ) (12)

(13)

and

(14)

Motor has to be able to rotate both direction and this decreases the motor output a bit.

Reduction factors are given in motor catalogue and they have been taken account when

the actual motor requirements has been calculated.

2

T_L

1

J_L

0.5^2

r^2

0.5

r

20

kg/step

100

kg/pers

9.81

g

pi/180

deg2rad

cos

Trigonometric

Function

Saturation

Product

64

No. of steps

121.94

J_drum*2

65

Inclination deg

1

No. of Persons

6

Table 1 Calculated motor requirements for every motor

By comparing calculated values and technical data of motors the smallest and also the

cheapest motor DMR 112 SN-473P with speed 2080 rpm and moment of inertia 0.05

kgm2 has been selected. Data sheet for the motor can be seen in Appendix 1. Large gear

ratio (75.21) increases cost of gear but this has not been taken account in this case.

Nominal values of selected motor are seen on Table 2.

Armature

voltage

[v]

Speed

[rpm]

Power

[kW]

Armature

current

[A]

Torque

[Nm]

La

[mH]

Ra

[Ω]

Moment

of

inertia

[kgm^2]

Weight

[kg]

420 2080 14.7 41 67.4 11.70 0.75 0.05 100

Table 2 Nominal motor values of chosen motor

2.2.2 Modeling motor

Simulink model for the motor is seen in Figure 4. Calculated values for the model are

(15)

(16)

=

=9.5Ω (17)

(18)

(19)

(20)

(21)

7

(22)

(23)

(24)

(25)

Figure 4 Simulink model for the motor

2.3 Design of control system

2.3.1 Design of power processing unit

Fullbridge DC-DC converter seen in Figure 5 is chosen to control the motor. Four

eupec’s BSM 50 GB DLC IGBT-modules work as switches. Data sheet for modules is

seen in Appendix 2. Bipolar pulse width modulation (PWM) is used to control the

output. Modulation method consists of two signals: triangle waveform and control

voltage. Signals are compared together and switches are turned on and off in pairs. One

8

pair of switches, (T1, T3) or (T2, T4), is always on and output voltage gets either value

+VD or –VD. The average output voltage is defined as

(26)

Figure 5 Fullbridge DC-DC converter

Switching frequency, amplitude of triangle waveform and constant input voltage are

chosen to be fs=300 Hz, Vtri=5 V and VD=200 V. The transfer function of PPU is

( )

(27)

where

(28)

=

(29)

So the transfer function of PPU is

( )

(30)

VD

T1

T2

T4 T3

Vout

T1

9

2.3.2 Design of current regulator

Modulus optimum is used to design a current regulator. Transfer function ( )

( ) can be

defined by using Figure 4:

( )

( )

( )

(31)

Here Tm is very large so the speed changes very little during current regulation,

If2/sTm << ra(1+sTa). The transfer function can be simplified as

( )

( )

(32)

Open loop transfer function for current loop is then

( )

(1+

)(

)(

) (33)

Model of the PI controller is shown in Figure 6. Regulator’s zero is selected to cancel

large time constant Ta::

(34)

(34)

(35)

=

(36)

Figure 6 PI controller for current

10

2.3.3 Design of speed regulator

Speed regulator is designed by using symmetric optimum. Open loop transfer function

for speed is

( )

(1+

)(

)(

) (37)

Symmetric optimum gives values

(38)

(39)

(40)

(41)

Picture of speed controller is seen in Figure 7.

Figure 7 PI regulator for speed

2.4 Model of whole escalator system

The picture of whole escalator system is shown in Figure 8.

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Figure 8 Simulink model for whole system

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3 RESULTS

Escalator needs to operate in both directions. Escalator speed is 1.3 m/s and it is able to

accelerate from standstill to rated speed in 6.5 s. The escalator is equipped with sensor,

which detects if there is not any person riding the escalator. Without load escalator re-

duces its speed to 0.2 m/s. The escalator accelerates to 1.3m/s when someone steps onto

the escalator. Acceleration should take place within 5.5 s. This case is shown in Figure

9.

Figure 9 Reference speed Figure 10 Actual speed of escalator

It is assumed that the escalator is starting from the standstill and it accelerates to 1.3m/s

in 6.5s. Then it starts to move with the constant speed. On 15th

second the sensor senses

the escalator is empty and reduces the speed to 0.2 s. On 25th

second, after sensing that

person has stepped to the escalator, it accelerates to 1.3m/s again in 5.5s. The motor is

working on forward direction during acceleration and regenerative braking mode during

deceleration. The speed curve is seen in Figure 10. Reference speed and actual speed for

the reverse direction are seen in Figures 11 and 12.

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Figure 11 Speed reference in reverse Figure 12 Actual speed in reverse direc-

tion

In Figure 13 are shown the graph between speed, voltage and the current during forward

direction.

Figure 13 Speed, current and voltage during forward direction

Here during accelerating the current is positive and voltage is positive (forward motor-

ing) and during deceleration (reverse direction) the current is in negative direction but

voltage is still positive (regenerative braking). For the reverse direction shown in the

Figure 14, during accelerating the current is positive and voltage is negative and during

deceleration the current is positive but voltage is still negative. This means motor is

working regenerative braking mode. This is because weight of passengers is now accel-

erating speed of the escalator and it has to brake to be able to stay in desirable speed.

speed

Ea

Ia

14

Figure 14 Speed, current and voltage during reverse direction

Ia

speed

Ea

15

4 Conclusion

From the results, we can see that the chosen motor, DMR 112 SN-473P, is able to meet

the supplied requirement specification with the drive we designed. Whether this motor

is indeed the best (cheapest) choice, is not possible to determine when we do not know

the price of the motor or the gears. Generally, a smaller motor will be cheaper, but will

require a larger gear ratio, which will increase the cost of the gears.

Due to a large Ki relative to Kp in both the current and speed regulator, we can see

some oscillations in the step response. This is most prominent in the current curve, but

can also be seen in the speed curve with a high zoom. Since the maximum amplitude of

the oscillations in speed is 2 mm/s and the oscillations die out within a tenth of a se-

cond, this will not be noticeable for the passengers.

We see that the motor is able to work in regenerative braking mode. This is particularly

advantageous when the escalator is being run in reverse, where (due to our neglection of

friction) the motor is running in regenerative braking mode the entire time it is not idle,

and the escalator is actually generating electric power instead of consuming it. This is

because the force of gravity on the passengers is more than enough to run the escalator,

and the motor is needed to slow their descent.

If we had considered friction in our model of the load (this was not part of the assign-

ment), the results would have been somewhat different. With our model, the current is

zero when the motor is running in idle mode. In the real world, there would of course

need to be some current flowing in order to provide the necessary torque to cancel out

the effect of friction.

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Appendix 1

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Appendix 2

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19

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21

22

23

24

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