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305C Series

Vishay Cera-Mite

ceramite.support@vishay.com

Document Number: 23086

Revision 14-May-02

The use of a PTCR start assist insures sufficient acceleration

torque to overcome not only breakaway friction, but also

parasitic asynchronous torques associated with the 5th and

7th motor harmonics or lamination slot harmonics.

ACCELERATION TIME CONSIDERATIONS

The time to accelerate a rotating machine is:

RPM x WK

Accelerating Time (Seconds) =

Avg. Torque (lb ft) x 308

(Avg. Torque = Curve B - Curve A)

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 PTCR 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 torqe and time as long as several seconds.

PTCR starters not recommended.

Fig T-4

Fig T-6

Simplified Voltage Diagram of the PSC Motor at Operating

Speed

by 80° to 90° when C (run capacitor) is

chosen for balanced operation at 3/4 to

full load. Line Power Factor = sine 2

θ

Fig T-5

PTCR Motor Start Packages

HP x 746

For running conditions:

√2

I

L

x

√2

For the greatest starting torque, PTCR should be chosen to make:

Amperes are limited to about 50% of the main winding

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

1

; Xc =

ohms

Xc

2

πfC

This closely approximates the condition for motors over 1/2 HP.

START AND ACCELERATION TORQUES

SINGLE PHASE PSC HIGH EFFICIENCY COMPRESSORS

CONSIDERATIONS FOR CURRENT IN PTCR

APPROXIMATE EQUIVALENT CIRCUIT PSC MOTOR AT ZERO SPEED

100

75

50

25

0

5th

7th

Slot

100

200

C

PTCR

*R and X are total of stator and rotor

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

θ

Applied

Voltage

=

=

C

θ

∗

∗