Let's break down the surface grinder reversing application and address your questions about braking resistors, VFD suitability, and potential issues.

Braking Resistor - Rational or Overkill? Overkill, probably. You're right to question the need for a braking resistor, especially with the downsized motor. Here's why: Kinetic Energy

Smaller motor = lower inertia = less kinetic energy to dissipate during deceleration. A 3/4 or 1/2 HP motor will have significantly less rotating mass than a 1.5 HP motor.

Friction

Surface grinders have inherent friction from the spindle bearings, the grinding wheel contacting the workpiece (even lightly), and potentially the drive system. This friction will help with deceleration.

VFD's DC Bus Overvoltage

The primary reason for a braking resistor is to prevent the VFD's DC bus voltage from rising too high during regenerative braking. This happens when the motor acts as a generator while slowing down. With such a short ramp time and the inertia of the system, it's unlikely you'll generate enough energy to trip the VFD's overvoltage protection.

Cost & Complexity

Adding a braking resistor and the associated wiring increases the cost, complexity, and space requirements of the system.

VFD Limitations

As you've found, smaller, less expensive VFDs often omit the braking resistor circuit to keep costs down.

Alternatives to Braking Resistor

Consider these alternatives before jumping to a braking resistor:

Ramp Down/Coast

As you mentioned, a "coast to stop" deceleration is the simplest. While you want quick reversals, a brief coast won't necessarily significantly affect cycle time, and it will place the least amount of stress on the VFD and motor.

Controlled Deceleration

Try using a slightly longer deceleration ramp setting in the VFD. Experiment to find the shortest ramp down time that doesn't trigger overvoltage errors. The VFD may be able to bleed off the excess energy.

Dynamic Braking

Some VFDs have internal dynamic braking features that use internal components to absorb some of the regenerative energy. Check the VFD's documentation to see if your selected model has this capability.

VFD Suitability and Slam Reversing



VFD Capability

The biggest concern is the frequency of reversals. Slamming a motor from full forward to full reverse every second or faster is demanding, even with a small motor. This is what will likely shorten the lifespan of the VFD.

Considerations for Frequent Reversals



Motor Selection

Choose a motor specifically designed for inverter duty (VFD compatibility) and with good thermal protection.

VFD Sizing

Although you're reducing horsepower, consider oversizing the VFD slightly within the allowable range for the motor. This gives it extra headroom for handling the inrush current during reversals and helps dissipate heat.

Heat Dissipation

Ensure adequate ventilation for the VFD, especially in an enclosed cabinet.

VFD Parameters



Acceleration/Deceleration

Start with a conservative 0.5-second ramp time and gradually shorten it if the system operates well. If the system is going to be set to slam from FWD to REV, look for a VFD specifically rated for such duty.

Motor Parameters

Accurately program the motor's nameplate data (voltage, current, frequency, RPM, poles) into the VFD.

Carrier Frequency

Lowering the carrier frequency (switching frequency) can reduce switching losses in the VFD, potentially improving its lifespan. However, this can also increase motor noise. Experiment to find a suitable balance.

Current Limiting

Enable and properly configure current limiting in the VFD. This protects both the motor and the VFD from overcurrent during reversals.

Monitoring

Monitor the VFD's temperature and current draw during operation. If the VFD is running consistently hot or near its current limit, you may need to adjust parameters, further reduce the acceleration/deceleration rate, or even consider a more robust VFD.

Teco L510 Series

The Teco L510 is a good budget-friendly VFD. However, its limitations (no braking resistor support on 230V models, possibly less robust for frequent reversals) make it a less-than-ideal choice for this specific application. If you are able to find a coast to stop deceleration setting that the machine's owner deems to be acceptable, then the L510 could be suitable, provided it is adequately sized.

Recommendations


Experiment with the existing 1.5 HP motor before downsizing. Try the coast to stop/controlled deceleration methods with the existing motor and VFD (if you already have one). You might be surprised at how well it works, especially if the motor has inherently high friction.2. If downsizing, carefully consider the replacement motor's specifications. Inverter-duty is a must. Pay close attention to the motor's thermal characteristics.3. Thoroughly research VFD options. If the L510 does not work, look for a VFD that meets the following: Inverter duty rated and suitable for rapid forward/reverse cycling. Look for a VFD explicitly mentioning suitability for reversing applications. Some manufacturers offer models specifically designed for this type of duty. Slightly oversized for the motor (within the manufacturer's recommended range). Robust current limiting and overvoltage protection. Adequate heat sinking and cooling.* Adjustable carrier frequency.4. Document everything! Record all VFD parameters and operating conditions. This will help with troubleshooting and optimization.In conclusion, a braking resistor is likely unnecessary for your application. The bigger challenge is selecting a VFD and motor that can reliably handle the frequent reversals without overheating or premature failure. Careful experimentation and monitoring will be crucial for success. Flag for review