305C21 Datasheet, PDF (2/6 Pages) Vishay Siliconix – PTCR Motor Start Packages PSC Single Phase Motor Start Assist

305C Series
Vishay Cera-Mite
PTCR Motor Start Packages
START AND ACCELERATION TORQUES
SINGLE PHASE PSC HIGH EFFICIENCY COMPRESSORS
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 WK² (lb ft ²)
Accelerating Time (Seconds) = Avg. Torque (lb ft) x 308
(Avg. Torque = Curve B - Curve A)
Fig T-4
100
75
50
A1
A2
B1
Full Load
Operating Point
B2
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.
25
5th
7th
Slot
0
Cold Breakaway Torque
Possible Region of
Harmonic Torques
100
Percent Rated Torque
CURVE A1
Torque req’d
to accelerate
unloaded
compressor
CURVE A2
Torque req’d to
accelerate
compressor at
partial differential
pressure
CURVE B1
Motor speed
versus torque
with run cap
only
CONSIDERATIONS FOR CURRENT IN PTCR
APPROXIMATE EQUIVALENT CIRCUIT PSC MOTOR AT ZERO SPEED
Fig T-5
max.
Torque
200
CURVE B2
Motor speed
versus torque
with run cap
and PTCR
I_L (run) =
HP x 746
V_M ^{x pf x eff}
I_L (start) ≈ 6 x I_L run
For running conditions:
I_L
If V_aux = V_M, then I_M and I_aux =
√2
I_L
V_M
V_M
If V_aux ≠ V_M, then I_{aux =}
x
and Z_{aux =}
√2 ^Vaux
Iaux
For the greatest starting torque, PTCR should be chosen to make:
V_M x I_M = V_aux x I_aux. In many cases the auxiliary Volt-
Amperes are limited to about 50% of the main winding
Volt-Amperes to get 50% - 70% rated torque.
Then at start, with PTCR in series: Z'_aux = R_PTCR + Z_aux
I_{R start} through PTCR =
V_M
Z'_aux
I_{C start} through Run Cap =
I_{aux start} = I_{R start} + I_{C start}
V_M
Xc
; Xc =
1
ohms
2 πf C
If Z_aux is low impedance, less than 10% of R_PTCR
then it can be ignored and I_PTCR at start = V_M
R_PTCR
This closely approximates the condition for motors over 1/2 HP.
Line Voltage VM
L1
L2
I_L
I_aux
I_M
C ^Ic
R_{M *}
X_{M *}
PTCR
R_aux
X_aux
I_R
V_aux
*R and X are total of stator and rotor
Fig T-6
Simplified Voltage Diagram of the PSC Motor at Operating
Speed
*IA (auxiliary current) leads IM (main current)
by 80° to 90° when C (run capacitor) is
chosen for balanced operation at 3/4 to
full load. Line Power Factor = sine 2θ
V_M
Applied
θ
Voltage
= ^{I AC}
= ^VC
V ^PTCR
∗
θ
I_M
I_A
∗
V_A
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Document Number: 23086
Revision 14-May-02