Relays or not? - 4QD - Electric Motor Control

Relays or not?

One question that often comes up is whether to choose a controller that uses relays or not? At 4QD we use both approaches and here we’ll take a quick look at the pros and cons.

To make an electric motor go in reverse we need to change the polarity of the voltage supplied to it, and there are two common ways to do this.

Method 1 – is to use a half bridge controller arrangement and use relays to swap over the polarity.

The basic diagram looks like this, which is the circuit we use for the lower power controllers in our range such as the DNO and Pro-150.

half bridge and relay circuit

Advantages of the half bridge and relay design

  • Fewer mosfets are needed, the PCB can be smaller, and the mosfet drive circuit can be simpler, all of which makes for a less costly controller.
  • The relays can be arranged to put a short circuit across the motor when stopped, providing a strong braking force.
  • The motor current only ever flows through a single mosfet bank which means less heat will be dissipated.

Disadvantages of the half bridge and relay design

  • The main issue with using relays is the longevity of the contacts. If the rating of the contacts is observed then the relays can last for a long time, for example our NCC series was designed back in 1993 and a lot of them are still in service 26 years later. However if the rating of the contacts is exceeded then problems can start to occur. Electric motors can draw stall currents sometimes three or more times greater than their rated value, and over time these momentary high currents can cause cumulative damage to the contacts which may eventually lead to failure.
  • If very short deceleration ramp times are used, the relay contacts can open before the motor is fully stopped, this causes arcing which will erode the contacts and can generate sufficient electrical interference to damage the controller.

Method 2 – Full H-bridge

A full H bridge uses twice as many mosfets arranged in an H configuration to do the switching, thus doing away with the need for relays.

The basic diagram for an H-bridge looks like this, which is the circuit we use in our higher power controllers like the Pro-160 and 4QD series.

Relays or not, a full H-bridge circuit

To go forward mosfets A and D are switched on, to go in reverse we switch on B and C.

Advantages of the H-bridge design

  • There are no relays in the circuit so long term reliability is enhanced, particularly in higher load situations.
  • Switching can be done very quickly which makes this design more suitable for say a robot which must change direction rapidly.
  • There is no short circuit across the motor when stopped which means that that it is easier to push a vehicle with the power off.

Disadvantages of the H-bridge design

  • Double the number of mosfets and bigger main capacitors are required for a given power level.
  • Because there are two mosfet banks in circuit the heat dissipation may be greater.
  • The circuit is more complicated. If software is used to control the switching it can get complex although modern chip design has mitigated this to a large extent.

In summary;

Half bridge controllers with relays are ok if;

  • The controller is conservatively rated with respect to the motor.
  • Gentle deceleration times are used.
  • A smaller, more cost effective design is required.

Full H-bridge controllers without relays are better for

  • Long term reliability.
  • Any application where rapid stops are called for.
  • The ability to push a vehicle is required.

Categories: Choosing a controller

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