PTC305C Series

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Vishay Cera-Mite

Revision: 11-Sep-15

3

Document Number: 23086

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

START AND ACCELERATION TORQUES SINGLE PHASE PSC HIGH EFFICIENCY COMPRESSORS

The use of a PTC start assist insures sufficient acceleration

torque to overcome not only breakaway friction, but also

Acceleration Time Considerations

The time to accelerate a rotating machine is:



1. If (curve B - curve A) is zero or less, the motor may stall.

2. In calculating torque available from curve B, allowance

should be made for cusps in the torque curve due to

harmonics. The time needed to accelerate from rest to

1/2 speed is critical, as the average torque available in

this region is limited. Select a PTC with sufficient

switching time (t) to accelerate the compressor.

3. Scroll and rotary compressors may have less breakaway

torque than shown.

4. A compressor with no equalization may require over

100 % starting torque and time as long as several

seconds. PTC starters not recommended.

CONSIDERATIONS FOR CURRENT IN PTC APPROXIMATE EQUIVALENT CIRCUIT PSC MOTOR



AT ZERO SPEED



For running conditions:







For the greatest starting torque, PTC should be chosen

to make:

.

In

many

cases

the

auxiliary

Volt-Amperes are limited to about 50 % of the main winding

Volt-Amperes to get 50 % to 70 % rated torque.

Then at start, with PTC in series:











This closely approximates the condition for motors over 1/2

HP

Fig. T-5

Fig T-6

Simplified voltage diagram of the PSC motor at operating

speed.

Note

(run capacitor) is chosen for balanced operation at 3/4 to full

load. Line Power Factor = sine 2



accelerating time (s) =

RPM x WK

2

lb ft

2



average torque lb ft

 x 308

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

average torque = curve B - curve A



100

75

50

25

0

5th

7th

Slot

100

200

CURVE B2

Motor speed

versus torque

with run cap

and PTCR

CURVE B1

Motor speed

versus torque

with run cap

only

CURVE A2

Torque req’d

to accelerate

compressor at

partial differential

pressure

CURVE A1

Torque req’d

to accelerate

unloaded

compressor

Cold Breakaway Torque

Percent Rated Torque

Full Load

Operating Point

Possible Region of

Harmonic Torques

max.

Torque

I

L

run

 = HP x 746

V

---------------------------------- x I

L

start



I

L



run

If V

I

L

2

-------

If V

aux

V

M



 then I

I

L

2

------- x

V

M

V

aux

------------ and Z

V

M

I

aux

----------

V

Z'

+ Z

aux

I

V

M

Z

aux

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

I

V

M

X

C

-------; X

1

2

fC

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



I

+ I

C start

V

M

R

PTCR

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

(1)

(1)

C

PTC

* R and X are total of stator and rotor

Applied

Voltage







(1)

(1)

=

=

x C