PTC305C Series

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Vishay Cera-Mite

Revision: 11-Sep-15

4

Document Number: 23086

THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT

EFFECT OF PTC RESISTANCE ON STARTING TORQUE OF PSC MOTORS

Table 2

A. Rated torque is the torque at full speed rated load.



It is calculated as:







The range shown includes both normal slip and high

efficiency low slip motors. Starting torque varies as:



B. Figure T-4 shows effect of using PTC 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 PTC CURRENT VS. TIME SHOWING DEFINITION OF RDYN AND SWITCH TIME (t)

Fig T-7

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 PTC resistance is available for

approximately 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 PTC

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



Table 3

MOTOR HP

(TABLE 4)

(NOTE 7)

LOCKED ROTOR TORQUE WITH

RUN CAP ONLY % RATED TORQUE

(SEE A)

STARTING TORQUE WITH RUN CAP AND PTC (% RATED TORQUE) (SEE B)

50



25



20



12.5



10



0.5

25 % to 35 %

70 % to 100 %

80 % to 100 %

NA

NA

NA

1

25 % to 35 %

50 % to 70 %

70 % to 100 %

NA

NA

NA

2

20 % to 30 %

40 % to 60 %

60 % to 90 %

70 % to 100 %

70 % to 100 %

80 % to 100 %

3.5

20 % to 30 %

NA

40 % to 60 %

50 % to 85 %

60 % to 90 %

70 % to 100 %

5

15 % to 25 %

NA

NA

40 % to 60 %

50 % to 75 %

60 % to 90 %

6.5

15 % to 25 %

NA

NA

NA

40 % to 70 %

50 % to 80 %

Torque lb - ft

 = HP x 5250

RPM

----------------------------

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

230 V

RMS

I

R

Peak

Current

20 % Peak

Current

Time to Switch (s)

Motor

Contactor

Closes

PTC Switchout

Switch Time (t)

OSCILLOSCOPE

SHUNT

I

PTC

E

RMS

PTC

M = PTC mass (g)

K = 0.75 J/g/°C

Time (t)

 KM (130 °C - T

V

2

PTC

START CAPACITOR

PTC VALUE

50 μF

50



75 μF

37.5



100 μF

25



125 μF

20



200 μF

12.5



250 μF

10



X

1

2

fC

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