Hello Everyone,

I have a few fundamental doubts on the effects of back emf on a dc motor and was hoping to have them cleared.

Firstly , Let me explain the hardware setup. I have a DC-link capacitor voltage of about 300 V which connects to 3 parallel half bridges and the midpoint of each bridge connects to the motor.
What would be the effect of the back EMF at rated speed of motor on the DC Link voltage?, If the dc link voltage is not regulated, would the net resultant voltage across the dc bus be (dc bus voltage - Vback emf)?

Secondly:
I would like to include some kind of back emf protection in case the motor stops suddenly, the voltage across the DC link capacitor raises sharply,
My first thought is to use a brake chopper to waste the excess energy across it. But in order to do this, (calculate the maximum power that would be needed to be dissipated(hence choosing the appropriate brake resistor values)) , how do i calculate what will be the maximum voltage the dc-bus will raise to incase of a sudden motor stoppage? Can i assume the worst case to be 2x Dclink voltage?

Thanks!

 

Back EMF is a property of the motor and the speed of revolution. There is no practical way to reduce it except to use a different motor or run it at a slower speed.

 

Hello Papabravo,
Thanks for your reply, yes I am aware of that, i think my thread title was a misleading (don't know how to change it now),
What i meant was that during a sudden stop of the motor , there would be a steep raise in the dclink voltage due to a sharp raise in the back emf, how do we safely mitigate this ?

 

I guess I would need a schematic diagram of your setup. Since the motors are not directly connected to the DC-Link capacitor, I don't see why this would be a problem.

 

What is a DC link capacitor? Show us a schematic.

 

What is a DC link capacitor? Show us a schematic.

It’s just the capacitor across the DC supply to the three half-bridges.
The MOSFETs which form the bridges look like a three phase bridge rectifier when they are all switched off, so when the motor acts as a generator it charges the DC link capacitor.
Unless the motor is over speeding, I would not expect its generating voltage to be higher than the DC link voltage.

 

The DC link voltage will only increase if either the motor is overspeeding because of some external force, or because your 3f bridge is actively trying to brake the motor. Braking is IIRC done by running the PWM at lower duty cycle than what is appropriate for the motor at the current motor RPM. The half bridge then acts as a step up converter in the direction towards the DC link capacitor, and that is why phase converter IGBT bricks typically have three pairs of transistors for the bridge and one transistor to switch on a braking resistor whicht loads down the DC link and provides a sink for the energy coming from braking.

 

Hello Ian and Kubeek, Thanks for the reply, unfortunately I don't have a schematic yet, but Ian described it accurately!
@kubeek, but what happens if we remove the motor load suddenly?, Wont the motor temporarily overshoot?, thereby increasing the back emf?

 

Schematic:

 

If it’s a synchronous motor then why would it over speed when the load is removed? if it’s an induction motor, perhaps it could.

 

ah yes , I forgot about that!

 

A short overshoot could be due to self-inductance of the windings, but not due to generated EMF.