现场有一台天车，其中大车机构用两个变频器各驱动两台电机，如果一台变频器出现故障，另一台变频器可以打单机运行。开始调试时两台变频器打单机时均报堵转故障，最近又试单机，发现能够打单机运行了。但是刚开始运行时速度不稳。如下图所示：

图1：1#单机运行

图2：2#单机运行

图3：双机运行
参数设置如下：电机控制方式：
p1300[0] Open-loop/closed-loop control operating mode [0] V/f control with linear characteristic
已完成的电机优化：
r3925[0] Identification final display 405H
r3925[0].0 Motor/control parameters calculated (p0340 = 1, p3900 > 0) Yes
r3925[0].2 Motor data identification carried out at standstill (p1910 = 1) Yes
r3925[0].3 Rotating measurement carried out (p1960 = 1, 2) No
r3925[0].4 Motor encoder adjustment carried out (p1960 = 1, p1990 = 1) No
r3925[0].10 Automatic parameterization only for V/f control (r0108.2 = 0) Yes
r3925[0].15 Motor equivalent circuit diagram parameters changed Not changed
电机额定数据：
p304[0] Rated motor voltage 380 V
p305[0] Rated motor current 169.60 A
p306[0] Number of motors connected in parallel 2
p307[0] Rated motor power 90.00 kW
p308[0] Rated motor power factor 0.840
p310[0] Rated motor frequency 50.00 Hz
p311[0] Rated motor speed 935.0 rpm

加速时间和减速时间
p1120[0] Ramp-function generator ramp-up time 9.000s
p1121[0] Ramp-function generator ramp-down time 9.000s
P1310=50%
请指教一下为什么双机运行时无问题而单机时速度开始时出现波动？