Motor Thermal Protection; Electronic Thermal Relay - Danfoss VLT DriveMotor FCP 106 Design Manual

System Integration
•
The load drives the motor at constant output
frequency from the drive. That is, the load
generates energy.
•
During deceleration (ramp-down) when the
inertia moment is high, the friction is low, and
the ramp-down time is too short for the energy
to be dissipated as a loss in the drive, the motor,
and the installation.
•
Incorrect slip compensation setting can cause
higher DC-link voltage.
•
Back EMF from PM motor operation. When
coasted at high RPM, the PM motor back EMF can
potentially exceed the maximum voltage
tolerance of the drive and cause damage. To help
prevent this risk of damage, the value of
parameter 4-19 Max Output Frequency is automat-
ically limited. The limit is based on an internal
calculation, based on the values of:
-
Parameter 1-40 Back EMF at 1000 RPM.
- Parameter 1-25 Motor Nominal Speed.
- Parameter 1-39 Motor Poles.
The control unit can attempt to correct the ramp
(parameter 2-17 Over-voltage Control).
When a certain voltage level is reached, the inverter turns
off to protect the transistors and the DC-link capacitors.
Select the method used for controlling the DC-link voltage
level via:
•
Parameter 2-10 Brake Function.
•
Parameter 2-17 Over-voltage Control.
NOTICE
OVC cannot be activated when running a PM motor (that
is, when parameter 1-10 Motor Construction is set to [1]
PM non-salient SPM).
Mains drop-out
During a mains dropout, the drive keeps running until the
DC-link voltage drops below the minimum stop level. The
minimum stop level is typically 15% below the lowest
rated supply voltage of the drive. The mains voltage before
the dropout and the motor load determines how long it
takes for the drive to coast.
+
Static overload in VVC mode
When the drive is overloaded, the control reduces the
output frequency to reduce the load.
If the overload is excessive, a current can occur that makes
the drive cut out after approximately 5–10 s.
MG03M302
Design Guide

3.5.6 Motor Thermal Protection

Motor overload protection can be implemented using a
range of techniques:
•
•
•

3.5.6.1 Electronic Thermal Relay

ETR is functional for induction motors only. The ETR
protection comprises simulation of a bimetal relay based
on internal drive measurements of the actual current and
speed. The characteristic is shown in Illustration 3.9.
t [s]
2000
1000
600
500
400
300
200
100
60
50
40
30
20
10
Illustration 3.9 ETR Protection Characteristic
The X-axis shows the ratio between I
nominal. The Y-axis shows the time in seconds before the
ETR cuts off and trips the drive. The curves show the
characteristic nominal speed at twice the nominal speed,
and at 0.1 x the nominal speed.
It is clear that at lower speed, the ETR cuts off at lower
heat, due to less cooling of the motor. In that way, the
motor is protected from overheating, even at low speed.
Summary
ETR is functional for induction motors only. The ETR
protects the motor against overheating, and no further
motor overload protection is required. When the motor is
heated up, the ETR timer controls the duration of running
at high temperature, before stopping the motor to prevent
overheating.
When the motor is overloaded before reaching the
temperature where the ETR shuts off the motor, the
current limit protects the motor and application against
overload. In this case, ETR does not activate and therefore
a different method of thermal protection is required.
Activate ETR in parameter 1-90 Motor Thermal Protection.
ETR is controlled in parameter 4-18 Current Limit.
Danfoss A/S © 12/2018 All rights reserved.
Electronic thermal relay (ETR).
Thermistor sensor placed between motor
windings.
Mechanical thermal switch.
1.0 1.2 1.4 1.6 1.8 2.0
motor
3
f = 1 x f (par. 1-23)
OUT M,N
f = 2 x f
OUT M,N
f = 0.1 x f
OUT M,N
I
M
I (par. 1-24)
MN
and I
motor
33
3