A nontraditional wiring approach offers midspeed stepmotors high torque with low current.
Santa Clara, Calif.
Most engineers in the motion-control industry agree that a common frustration with stepmotors involves finding the right motor to solve their various application issues. Whether the application needs more torque, quieter resonance, cooler operation, or smoother motion, altering the electrical specification of a stepmotor helps modify performance to better suit specific application needs.
When speed and torque are crucial, stepmotors usually have traditional bipolar-series or bipolar-parallel connections. Typically, a series connection results in high inductance that lets the motor work well at low speeds. On the other hand, parallel connections work best for highspeedapplications. Altering input-voltage to the driver or output-current to the motor reduces resonance and noise. Likewise, motor-winding changes can shift the resonant spot away from the desired operating point for quieter motion at the crucial speed. Fewer windings lower resistance and, thus, reduce overall I2R power consumption for cooler operation.
Implementing these methods might help achieve a certain application requirement. However, altering one variable to solve a specific problem more than likely creates other unforeseeable problems. These modification side effects play a big role when selecting the right motor for a specific application.
There are times when the traditional series and parallel connections won't suffice. As previously mentioned, series connections are typically used to optimize performance for low-speed applications while parallel connections work for high-speed applications. Users who need optimized stepmotor performance at midrange speeds are stuck choosing between the trade-offs of the series and parallel connections.
To give users more options, stepmotor manufacturers continue to innovate and create new methods to help solve these problems. One such manufacturer, Lin Engineering, has created and patented the T-Connection to bridge the performance gap between series and parallel connections. The TConnection fits in-between the series and parallel connections to optimize torque, minimize resonance, and provide smooth motion at midrange speeds.
Typically, stepmotors are wired in series to keep current consumption low. While the combined resistance of each winding lowers overall current, it also combines the inductance of each winding. High inductance prevents operation at mid to high speeds.
The T-Connection also lowers current consumption by wiring two opposing windings in series. However, the mutual inductance of the two windings now becomes series-opposing. The effect helps lower the overall inductance and results in lower vibration and smoother operation than a normal series connection.
One specific application that benefited from a T-Connection step-motor was a pan and tilt surveillance-camera. Engineers were having-trouble with vibration and resonance at the desired operating speed. The problem was traced to resonance effects from the stepmotor.
Every time a stepmotor steps there is a period of oscillation, or back-andforth movement, of the rotor as the rotor settles into its new position. This oscillation creates stepmotor resonance. The faster a rotor can settle and stop at its next position, the smoother its motion will be. This smoother motion is a direct result of lower resonance and vibration.
Stepmotor resonance created a jittery motion as the camera panned from side to side. This often created viewing problems when the surveillance video was played back. Lin engineers determined the camera motor's operating speed fit the mid-range speed profile. Adding the TConnection resulted in a reduction of stepmotor resonance. This drop in resonance helped smooth motor response, creating a smoother pan.
Lin Engineering, (408) 919-0200, linengineering.com