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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%

1

2

πfC

PTCR Motor Start Packages

TYPICAL PTCR CURRENT VS. TIME SHOWING DEFINITION OF RDYN AND SWITCH TIME (T)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

R

Peak

Current

20% Peak

Current

Time to Switch (Seconds)

Motor

Contactor

Closes

PTCR Switchout

Switch Time (t)

OSCILLOSCOPE

SHUNT

I

PTCR

E

RMS

PTCR

Time (t)

≈ KM (130°C - T

V

2

PTCR

M = PTCR mass

K = 0.75 J/g/ °C

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