Motors & Drives Information

Questions concerning various AC motors and ac drives often arise. Discussions with and answers from various users indicate that the operational behavior of ac drives and ac motors are still a mystery. This article will clarify some of the concerns raised by a typical user question.

Typical Question

I have been researching AC drives and regenerative braking on three-phase induction motors. Thus far, I have been able to find only basic information on how an AC drive and motor interact to provide regenerative braking.

It seems that AC drives run at a lower frequency than the free-run frequency of the motor causing a reverse current flow in the AC drive and back to the DC bus.

Is the ac drives just supplying the magnetizing current while capturing the torque current supplied by the motor during regeneration? From what I understand, the magnetizing current lags the voltage by 90° and the torque current is in phase with the voltage during normal drive conditions. Under regeneration, I’m not sure what the phase relationships are.

Is an external speed sensor required to keep the AC drive frequency below the motor frequency or can an AC drive subtract the drive signal from the motor current waveform to determine the motor speed?

Discussion Group Answers

Normally we think of an induction motor as supplying shaft power, but it can easily absorb shaft power. This commonly happens, for example, when a crane is lowering a load; the motor is turning one direction but it has to create torque in the opposite direction. For this condition, the motor rotor turns faster than the stator frequency and power is returned to the supply. This is called an over-running condition.

When an ac drive is decelerating an induction motor, the same condition applies, and the rotational energy in the motor rotor/load is returned to the power supply. This energy pumps up the power supply capacitors (DC bus), which is usually dissipated by electronically connecting a resistor across the DC bus as required to keep the DC bus value at some predetermined voltage limit.

There have been a lot of recent advances in regenerative AC drives. Regenerative AC drives work fairly simply. When you want to stop, the AC drive output frequency is driven to a very low level, say 10-15Hz. At that level the excitation is still present but virtually any continued rotation is exceeding the “synchronous” frequency applied to the motor.

Load inertia becomes the prime-mover. With excitation, the AC motor rotating over base speed is an induction generator. The faster it turns, the more negative torque it converts to power. That energy is converted by the reverse diode in the power modules of the AC drive to DC on the bus. Then a separate set of transistors on the front-end of the AC drives reconverts that DC into a fixed frequency voltage applied back to the supply source.

Once the motor speed goes below that threshold output frequency, the AC drive switches over to DC injection braking to finish the task. At low motor speeds, there is usually very little energy left in the load. You don’t need to know the exact rotor speed, but newer versions with Open Loop Vector control maintain a frequency difference for better control.

Some AC drives, in multiple drive applications, are set up to allow their DC link to be tied to the DC links of other AC drives so that the regenerative power from one motor can be used as motoring power for another.

There are companies that make aftermarket regen modules to apply to existing AC drives. They do not track anything, they just monitor the AC drive DC bus voltage, which means you must manually program the AC drive to go to a low frequency output (instead of Off) and DC inject separately from the regen module.

They still work fine, though, because once you establish the desired braking time, you simply set your decel rate a little faster than that. It almost always means the commanded frequency is lower than rotational speed.

If you look closely at the PWM bridge that drives the motor, there are inverse connected diodes that permit regenerated energy to flow back into the DC bus. These are usually part of the PWM power module. Anytime a PWM drive is powering a reactive load such as a motor, the drive needs these to handle reactive current in the motor.

When regenerating, the PWM transistors and the motor inductance act like a boost converter that pushes most of the power flow through the diodes. This is not altogether different than when a PWM drive for a DC motor is reconfigured to act as a boost converter to boost the voltage of the DC motor to match the DC traction power mains during regeneration.