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5
305C Series
Vishay Cera-Mite
ceramite.support@vishay.com
Document Number: 23086
Revision 14-May-02
START CAPACITOR PTCR VALUE
50 microfarads 50 ohms
75 microfarads 37.5 ohms
100 microfarads 25 ohms
125 microfarads 20 ohms
200 microfarads 12.5 ohms
250 microfarads 10 ohms
EFFECT OF PTC RESISTANCE ON STARTING TORQUE OF PSC MOTORS
A. Rated torque is the torque at full speed rated load.
It is calculated as:
HP x 5250
Torque (lb - ft) =
RPM
The range shown includes both normal slip and high
efficiency low slip motors. Starting torque varies as:
(Line Voltage)2.
B. Figure T-4 shows effect of using PTCR to increase
starting torque. For reciprocating compressors, it is
advised to choose a resistance value that gives at least
50% rated torque at locked rotor. Scroll and rotary
compressors may require less torque.
Fig T-7
Table 2
Table 3
LOCKED ROTOR
STARTING TORQUE WITH RUN CAP
TORQUE WITH RUN
AND PTCR (% RATED TORQUE) (SEE B)
MOTOR HP
CAP ONLY RESISTANCE (Rdyn)
(TABLE 4)
% RATED TORQUE
(NOTE 7) (SEE A) 50 ohm 25 ohm 20 ohm 12.5 ohm 10 ohm
0.5 25% to 35% 70 - 100% 80 - 100% NA NA NA
1 25% to 35% 50 - 70% 70 - 100% NA NA NA
2 20% to 30% 40 - 60% 60 - 90% 70 - 100% 70 - 100% 80 - 100%
3.5 20% to 30% NA 40 - 60% 50 - 85% 60 - 90% 70 - 100%
5 15% to 25% NA NA 40 - 60% 50 - 75% 60 - 90%
6.5 15% to 25% NA NA NA 40 - 70% 50 - 80%
X
c
=
1
2
π
fC
PTCR Motor Start Packages
TYPICAL PTCR CURRENT VS. TIME SHOWING DEFINITION OF RDYN AND SWITCH TIME (T)
0.1
0.2
0.3
0.4
0.5
0.6
PTCR Switchout
0.7
0.8
Applied Voltage (E
RMS
)
I
PTCR
at .05 Sec
R
DYN
=
Peak
Current
2Current
I
P
(
Time to Switch (Seconds)
Motor
Closes
Switch Time (t)
OSCILLOSCOPE
SHUNT
I
PTCR
E
RMS
PTCR
Time (t)
≈
KM (130
°
C - T
0
)
M = PTCR mass
T
0
= PTCR temp at time 0
K = 0.75 J/g/
°
C
R
DYN
V
2
PTCR
START CAPACITOR REPLACEMENT
Capacitor Starting Comparison
Some PSC motors have historically been started with a
capacitor and relay. To deliver the same starting current as
a start capacitor, a PTCR resistance is available for approxi-
mately equal ohms. Table 3 can be used for conversion.
Even though the start current may be the same, the start
torques may differ depending on the motor design. The PTCR
has a fixed time built in. The start capacitor will
stay in the circuit until a relay switches it out. The
longer time provided by the capacitor and relay may
be needed on applications where equalization is
not present or adequate reset time is not available.
STARTING CURRENT APPROXIMATION
BASED ON