PTC305C Series

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Vishay Cera-Mite

Revision: 11-Sep-15

2

Document Number: 23086

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

ECONOMICAL SOLID STATE TORQUE ASSIST FOR HEAT PUMPS, ROOM AIR, COMMERCIAL AND

RESIDENTIAL AIR CONDITIONING AND REFRIGERATION SYSTEMS

Positive Temperature Coefficient Themistors (PTC) have

been used for many years in millions of HVAC applications

to provide starting torque assistance to Permanent Split

Capacitor (PSC) single phase compressor motors.

Sizes are available to cover the full range of 120 V/240 V

PSC compressor motors.

Safety Agency Recognition

Vishay

Cera-Mite

motor

start

PTC

thermistors

are

recognized by Underwriter Laboratories file E97640 in

accordance with standard for controllers and refrigeration

components UL 873 or UL 60730; and Canadian Standard

C22.2

No.

24.

All

packages

and

accessories

are

RoHS-compliant.

RELATIVE COMPARISON OF VARIOUS MOTOR STARTING METHODS

Three methods have historically been employed to generate

starting torque for PSC motors. All are well-proven

technologies and may be compared relative to one another

based upon categories shown below.

The importance of each category is dependent upon the

motor application and industry sector.

In general, if the PTC starter produces sufficient starting

torque, it is considered the simplest and most economical

choice.

Table 1

SIMPLIFIED PTC STARTING DIAGRAM

Start Sequence

When starting the compressor, contactor (M) closes; the

PTC, which is at low resistance, provides starting current to

the motor’s auxiliary winding. After time delay (t), the current

passing through the PTC causes it to heat and “switch” to a

very high resistance. At this point the motor is up to speed

auxiliary winding. The PTC remains hot and at high

resistance as long as voltage remains on the circuit. When

contactor (M) opens, shutting off voltage to the compressor,

the PTC cools to its initial low resistance and is again ready

to provide torque assist on the next startup.

Restart

It is important to provide time between motor starts to allow

the PTC to cool to near its initial temperature. This time is

usually 3 min to 5 min and is determined by the thermostat

(THERM) or separate time-delay relay (TR). Attempts to

restart in less time may be successful depending on

compressor equalization, line voltage, temperature, and

other conditions. If the motor were to stall in a locked-rotor

state, overload device (PD or TS) would open the line and a

further time delay would occur until the motor overload is

reset. Motor start PTCs are applied to compressors having

means to equalize pressure during shutdown.

TYPICAL PTC CHARACTERISTICS



AS A MOTOR START DEVICE

STARTING

METHOD

MECHANICAL

ELECTRICAL

FINANCIAL

EASE

OF

WIRING

PANEL

SPACE

REQUIRED

SENSITIVE

TO

MOUNTING

DIRECTION

ACCELERATION

TORQUE

PRODUCED

ACCELERATION

(SWITCH)

TIME

RESET

TIME

REQUIRED

EMI/RFI

GENERATED

TECHNOLOGY

INVENTORY

MIX

REQUIRED

RELIABILITY

PURCHASED

COST

PTC starter

Simple

2 wire

Lowest

No

Lowest

Fixed

3 min to

5 min

No

Solid State

Lowest

Highest

Lowest

Start cap

with PTC

acting as a

current relay

Moderate

2 wire or

3 wire

Medium

Yes

Medium

Fixed

2 min to

5 min

No

Solid State

Medium

Medium

Medium

Start cap

used with

potential or

current relay

Difficult

4 wire or

5 wire

Highest

Yes

Highest

Variable

based on

motor speed

None

Yes

Electro

Mechanical

Highest

Lowest

Highest

Fig T-2

M

Low Voltage Control Transformers Not Shown

Protective

Device

Optional OFF

Time Delay

Relay

Protective

Device

AC Line

120 or 240

VOLTS AC

Internal

Motor

Winding

Overtemp

Switch

MAIN

PSC

MOTOR

AUX.

PTC

TS

TR

TR

PD

M

M

M

L

1

L

2

C

R

Fig T-3

RESISTANCE

CURRENT

Switch Time (t)

0.01

50

100

150

1000

10 000

10

100

PTC Temperature °C

0.1

1.0

10.0