电机低速:龟兔赛跑式的应用情形

电机低速:龟兔赛跑式的应用情形

有时电机需要像兔子一样全速运行。

一般来讲,多数电机的最高效运行模式为全速运行。但有时电机需要运行在全速以下,甚至是龟速运行。

低速应用需要考虑多个因素,如电机支路供电、电机类型、控制器类型等。

应用项目的日常任务可能需要同时用到全速和低速,且应当同时提供这两种运行方式。低速的持续时间和程度决定了哪种方式最合适。

许多应用受益于在运行期间调节过程速度的能力。具体原因是需要定位、检查或调节对齐度、检验设备(如带锯)或者降低风机转速。

您有多种方法来实现相关应用中的低速控制,包括变频器和软启动器以及传统齿轮传动装置。

VFD 控制使用频率控制转速。电机同步转速随施加频率和电机极数的变化而变化。本例将帮助您理解频率调节的要点:

N = 120F/P

N = 每分钟转数 (RPM)

F = 施加频率

P = 电机极数

由于特定电机的极数不会改变,改变频率就会改变电机转速。根据控制方式,变频器在低速运行的同时还能提供满转矩。根据电机绝缘和类型,它能几乎无限地以低速运行。此时将始终精确执行定位功能。

软启动器 (SMCTM-50) 并非利用频率控制电机转速。多数软启动器利用跳周期方式实现低速运行。跳周期方式的转矩能力受到限制。该模式通常在正向和反向上以固定低速运行,而 SMC-50 的工作方式却不一样。

SMC-50 提供完全可调的低速运行能力,可从零速调至正向和反向 15%。这表示一台 1800 RPM 电机可实现反向 270 RPM 至正向 270 RPM 的低速运行。SMC-50 通过更频繁地触发晶闸管来实现低速运行,而非使用跳周期方式。

该方式的转矩能力比跳周期方式高得多,同时能够在低速状态下提供更多控制选项。该方式的低速运行持续时间为数分钟,而非数小时。因此,它是短时低速运行的理想选择。

无论处于低速还是全速状态,改变电机转动方向的传统方法是交换电机的 2 个相位,或者使用换向接触器。SMC-50 可轻松配置为正向或反向低速运行,无需借助换向接触器或人工反接引线。

使用 SMC-50 实现低速运行的实例。

  • 反向运转泵机疏通泵机进口,无需派专人清理进口,省时省钱。
  • 低压力测试泵机只需提供检查系统所需的流量,避免因泵机全速运转导致损坏。
  • 工厂应用利用低速调整并检查锯条对齐度。
  • 粮食让最终用户检验某个粮斗,而不必停止过程。
  • 基本定位转鼓上的门,用于装卸产品。您可利用低速到达正确位置停下来,或者以足够低的速度不间断地完成整个过程。
  • 维护设备以低速完成齿轮和机械部件的润滑。
  • 利用 Accu-StopTM 功能将电机从全速将为低速,然后再实施制动操纵以进行简单定位。

因此,应针对低速需求解决持续时间的问题。如果计划长时间低速运行 (比如说风扇),那么变频器将是不错的选择。如果应用项目不经常低速运行且持续时间只有数分钟,那么 SMC-50 将是最佳的选择。

并非每项应用都需要像兔子一样全速运行。有时龟速运行对应用项目更有帮助!总之,将两者相结合才能确保顺利完成工作!

阅读我们的白皮书使用软启动器实现低速应用,了解更多信息!

Whitepaper: Slow Speed Applications Using a Soft Starter

Because the number of poles of a particular motor does not change, changing the frequency changes the speed of the motor. The VFD has the ability to operate at slow speeds while simultaneously providing up to full torque, depending on the control. It has the ability to run at slow speed almost indefinitely, depending on the motor insulation and type. All this while accurately performing positioning function.

A soft starter, like the SMCTM-50, does not utilize frequency to control the speed of the motor.  Most soft starters utilize a method called cycle skipping to achieve slow speed.  Cycle skipping has limited torque capabilities.  While this method does provide fixed slow speed operation, usually in a positive and negative direction, the SMC-50 operates differently. 

The SMC-50 offers fully adjustable slow speed capability from zero speed to positive and negative 15%.  Meaning from negative 270 RPM to positive 270 RPM for an 1800 RPM motor.  Instead of cycle skipping, the SMC-50 accomplishes slow speed by firing the SCRs more frequently.

This method allows the development of much higher torque capabilities over the cycle skipping method while providing much more control while in slow speed.  This method provides minutes, not hours, of slow speed operation.  Perfect for lower time durations of slow speed.

Traditional ways of changing direction of a motor, whether it is slow speed or full speed, is to either swap 2 phases of a motor, or utilize a reversing contactor.  The SMC-50 can easily be programmed to perform slow speed in forward or reverse direction without the use of reversing contactors or physically reversing leads.

Examples of use of slow speed with the SMC-50.

  •  Reversing direction of a pump: unclogging the inlet of a pump without the need to send someone to clear the inlet saves time and money.
  • Low-pressure testing: getting the pump to flow just enough to check the system may be all that you need to do without the possibility of causing damage with the pump full on.
  • Mill application: adjusting and checking saw blade alignment by using slow speed.
  • Grain: when an end user would like to verify a grain bucket without having to stop the process.
  • Basic positioning: a door on a tumbler for adding or dumping product. You can use slow speed to get to the right position to stop or to slow enough to complete the process without stopping.
  • Maintenance of equipment: lubrication of gears and mechanical parts can be done at slow speed.
  • Utilizing the Accu-StopTM function to bring a motor from full speed to slow speed and then reapply a brake maneuver for simple positioning.

So the question that should be asked for slow speed needs is the duration. 

If meant to run in slow speed for a very long time, say for a fan, a drive would be a good selection. 

If the application calls for infrequent slow speed for minutes, the SMC-50 maybe the best option.

Not every application needs to run at full speed, like a rabbit.  Sometimes moving at a turtle’s pace will assist with the application! All and all, the two working together will get the job done!

Check out our whitepaper Slow Speed Applications using a Soft Starter for more information!

Bill Bernhardt
发布 September 4, 2017 作者 Bill Bernhardt, Senior Commercial Engineer, Rockwell Automation
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