Control Techniques Unidrive User Manual

Unidrive SP Low Voltage DC Installation Guide 35
Issue Number: 1 www.controltechniques.com
Safety Information Introduction Product information System design
Component data
Index
5.3.2 Sizing an appropriate thermal overload relay to protect the brake resistor
The 3 main considerations when calculating the thermal overload relay are as follows:
1. The thermal overload relay with the minimum tripping time must not trip with the
brake current pulse.
2. The peak current through the resistor must not damage the overload relay, this can
be checked by the following equation.
3. The brake resistor capability must be greater than the thermal overload relay with
maximum tripping time.
Worked example
Select a thermal overload relay for an SP1401 operating in normal duty, which will be
braking at 150% of the normal duty rating with a deceleration time of 5 seconds.
Additional data
From Table 5-4 on page 33 the value of R_LVDC will be 6.9Ω.
From section 3.1.2 Low Voltage DC rating on page 9 the LVDC brake turn on voltage
will be:
1.325 x Pr 6.46 (48V) = 63.6V
Normal operation brake turn on voltage = 780V
1. To calculate the thermal overload relay setting (I
set
)
Power from motor at 48V =
Peak current over the braking period =
Nominal motor power Overload %
LVDC brake turn on voltage
Normal operation brake turn on voltage
----------------------------------------------------------------------------------------------------------------
××
1100W 150%
63.6V
780V
---------------
×× 135W==
I
rms
peak()
Peak power at beginning of brake period (W)
Brake resistor value (Ω)
-------------------------------------------------------------------------------------------------------------------------------=
135W
6.9Ω
----------------= 4.4A=

Control Techniques Unidrive Specifications

General

Brand	Control Techniques
Model	Unidrive
Category	Power Supply
Language	English