305C21 Datasheet, PDF (3/6 Pages) Vishay Siliconix – PTCR Motor Start Packages PSC Single Phase Motor Start Assist

PTCR Motor Start Packages
305C Series
Vishay Cera-Mite
EFFECT OF PTC RESISTANCE ON STARTING TORQUE OF PSC MOTORS
Table 2
LOCKED ROTOR
TORQUE WITH RUN
MOTOR HP CAP ONLY
(TABLE 4) % RATED TORQUE
(NOTE 7) (SEE A)
STARTING TORQUE WITH RUN CAP
AND PTCR (% RATED TORQUE) (SEE B)
RESISTANCE (Rdyn)
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%
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.
TYPICAL PTCR CURRENT VS. TIME SHOWING DEFINITION OF RDYN AND SWITCH TIME (T)
Fig T-7
Peak
Current
OSCILLOSCOPE
SHUNT
R_DYN
Time (t) ≈ KM (130°C - T₀) _V2PTCR
M = PTCR mass
T₀ = PTCR temp at time 0
K = 0.75 J/g/ °C
R _DYN
=
Applied Voltage
I _PTCR at .05
(E_RMS)
Sec
E_RMS
I_PTCR
PTCR
20% Peak
Current
Switch Time (t)
0
0.1
Motor
Contactor
Closes
0.2
0.3
0.4
0.5
0.6
0.7
0.8
PTCR Switchout
Time to Switch (Seconds)
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
1
X_c =
2πfC
Table 3
START CAPACITOR
50 microfarads
75 microfarads
100 microfarads
125 microfarads
200 microfarads
250 microfarads
PTCR VALUE
50 ohms
37.5 ohms
25 ohms
20 ohms
12.5 ohms
10 ohms
Document Number: 23086
Revision 14-May-02
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