7/29/2019 Braking Calculation

1/3

YASKAWA

Frequently Asked Questions

Date: 15.Sept.01

Page 1 of 1Braking Calculation 01

Dynamic Braking Calculation

In some applications the motor works in regenerative mode during deceleration, i.e. the kinetic energy

of the machine is fed back into the inverter. This causes a rise of the DC bus voltage. If the fed backenergy is too much, the inverter may trip with an overvoltage fault (OV). In this case a DB resistor or acombination of a braking unit and a DB resistor must be used to dissipate the energy as heat.Typical applications where DB resistors must be used are hoist applications like cranes and elevatorsand applications where the machines inertia is very high like winder, washing machines and so on.

General way of DB resistor and braking unit dimensioning1. Calculation of the maximum braking torque MBmax.

The maximum braking torque depends on the speed at deceleration start n1, the speed atdeceleration end n2, the desired deceleration time tB and the total inertia of the system Jtot

(sum of all inertias regarded to the motor shaft).

( )load

B

21totmaxB M

t55.9

nnJM

=

2. Calculation of the maximum braking powerPBmax.( )55.9

nnMP 21maxBmaxB

=

3. Calculation of the maximum electrical braking powerPel.The braking power calculated in step 2 results of the energy that is fed back from themechanical system. Because of losses in the motor and the inverter not the whole energycomes back to the DC bus, i.e. the energy that has to be dissipated in the braking resistor asheat is lower than that which comes back from the mechanical system. If a gear is installedsome losses in the gear will occur additionally.

maxBGearrMotmaxBel P)1(PkPP =

The reduction factork depends on the rated motor powerPrMot. Typical reduction factors areshown the following table.

Motor Power Factor k

to 1.5 kW 0.25

2.2 kW to 4 kW 0.20

5.5 kW to 11 kW 0.15

15 kW to 45 kW 0.08

up from 45 kW 0.05

If the gear efficiency Gearis unknown, a value of 1 can be taken. In this case the resultingelectrical braking power value will be more on the safe side.

4. Calculation of the maximum allowable braking resistor value RB.The maximum braking resistor value can be calculated as follows:

el

2

BB

PUR

The selected resistance value should not be higher than this calculated value.UB is the DC bus voltage level at which the braking chopper is switched on. For the internalbraking chopper it depends on the power supply voltage and the input voltage setting(constant E1-01 at G5, F7 and G7; see table below). When an external braking chopper isused, it can be selected by jumper setting (for details refer the braking chopper manual).

7/29/2019 Braking Calculation

2/3

YASKAWA

Frequently Asked Questions

Date: 15.Sept.01

Page 2 of 2Braking Calculation 01

Dynamic Braking Calculation

Internal braking chopper turn ON level

Note:Make sure that the selected resistance value is not below the minimum connectableresistor value of the braking chopper. Otherwise the braking current will be too high whichmay result in braking chopper damage.

5. Selecting the braking chopperTo select the braking chopper the following data have to be known:- The duty cycle ED- The braking current IB

- The braking time tB- The overload curve for the braking chopper

The duty cycle ED is the ratio between the braking time tB and the cycle timeT.

n1

n2

T

tB

speed

time

T

tED B=

The braking current IB can be calculated using the braking start voltage UB and the selectedbraking resistor value RBsel.

Bsel

BB

R

UI =

Assuming that the braking torque is constant, the braking power is proportional to the speed.The speed is lowered linearly by the inverter, i.e. the braking power is also lowered linearly(see figure below).

n1

n2Tdecel

PBmax

PBave

Motor Speed

Load Power

E1-01 < 400V E1-01 >400V

G5 380V DC +-3% 660V DC +-3% 763V DC +-3%

F7, G7 387V DC +-3% 682V DC +-3% 788V DC +-3%

400V classInverter Type 200V class

7/29/2019 Braking Calculation

3/3

YASKAWA

Frequently Asked Questions

Date: 15.Sept.01

Page 3 of 3Braking Calculation 01

Dynamic Braking Calculation

Hence for the selection of the braking chopper the average braking power value PelAve can beused. It is calculated as follows:

2

PP elelAve =

Now the braking time tch, that is relevant for selecting the braking chopper is lower than thereal braking time tB. It can be calculated as follows:

2B

elAveBselB

elMax

elAveBch

V

PRt

P

Ptt

==

Using the values for the braking current IB, the time tch and the duty cycle ED together with thebraking chopper overload curves the proper braking chopper can be selected.