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Braking
13 Braking
13.1 Types of Braking
The frequency converter utilizes 3 types of braking:
•
Mechanical holding brake
•
Dynamic brake
•
Mechanical brake control
Mechanical holding brake
A mechanical holding brake is an external piece of
equipment mounted directly on the motor shaft that
performs static braking. Static braking is when a brake is
used to clamp down on the motor after the load has been
stopped. A holding brake is controlled by either a PLC or
directly by a digital output from the frequency converter.
NOTICE
A frequency converter cannot provide a safe control of a
mechanical brake. A redundancy circuitry for the brake
control must be included in the installation.
Dynamic brake
Dynamic braking is accomplished internally within the
frequency converter and is used to slow down the motor
to an eventual stop. Dynamic braking is applied using of
the following methods:
•
Resistor brake: A brake IGBT keeps the
overvoltage under a certain threshold by
directing the brake energy from the motor to the
connected brake resistor.
•
AC brake: The brake energy is distributed in the
motor by changing the loss conditions in the
motor. The AC brake function cannot be used in
applications with high cycling frequency since
this function overheats the motor.
•
DC brake: An overmodulated DC current added to
the AC current works as an eddy current brake.
Mechanical brake control
For hoisting applications, controlling an electro-magnetic
brake is necessary. For controlling the brake, a relay output
(relay 1 or relay 2) or a programmed digital output
(terminal 27 or 29) is required. Normally, this output must
be closed for as long as the frequency converter is unable
to hold the motor.
If the frequency converter is brought into an alarm
condition, such as an overvoltage situation, the mechanical
brake immediately cuts in. The mechanical brake also cuts
in during Safe Torque Off.
MG37N102
Design Guide
NOTICE
For vertical lifting or hoisting applications, it is strongly
recommended to ensure that the load can be stopped if
there is an emergency or a malfunction. If the frequency
converter is in alarm mode or in an overvoltage
situation, the mechanical brake cuts in.
13.2 Brake Resistor
13.2.1 Selection of Brake Resistor
To handle higher demands by generatoric braking, a brake
resistor is necessary. Using a brake resistor ensures that the
energy is absorbed in the brake resistor and not in the
frequency converter. For more information, see the VLT
Brake Resistor MCE 101 Design Guide.
If the amount of kinetic energy transferred to the resistor
in each braking period is not known, the average power
can be calculated based on the cycle time and braking
time (intermittent duty cycle). The resistor intermittent
duty cycle is an indication of the duty cycle at which the
resistor is active. Illustration 13.1 shows a typical braking
cycle.
NOTICE
Motor suppliers often use S5 when stating the allowable
load, which is an expression of intermittent duty cycle.
The intermittent duty cycle for the resistor is calculated as
follows:
Duty cycle=t
T=cycle time in s
t
is the braking time in s (of the cycle time)
b
Load
Speed
Illustration 13.1 Typical Braking Cycle
Danfoss A/S © 6/2016 All rights reserved.
/T
b
ta
tc
tb
to
ta
tc
T
®
13
13
tb
to
ta
Time
123
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