All electrical items carrying current get hot. The amount of heat is actually proportional to the square of the current, so doubling the current causes four times the heat. A high current motor and controller can carry a lot of current – so can generate a lot of heat. For these, cooling is important.
Motors – at least the ventilated ones – have a built in fan for cooling. Controllers usually do not. So a motor can generally be used at higher currents for longer than a controller since the heat will build up in the controller while the motor cools itself.
Fortunately most controller applications are land based and very ´peaky´: a machine on land generally only takes a lot of current to get up to speed or to climb a hill, both are usually only a few seconds.
This peaky heat generation is well suited to MOSFET technology and all controllers therefore tend to be rated for peaky use. To make a controller for continuous use, you either need to use a lot more power devices – i.e. use a bigger controller – or take steps to remove the heat quicker. Boats and other water craft need a continuous rating since the faster you go, the more the current: it takes power simply to move through water, not just to get up to speed.
The moral is: use a controller that will give a top current output perhaps 2 – 3 times the continuous motor current and do not expect a controller to run continuously at more than 30-50% of its short term rating. Also if you are running at high duty cycles then consider fitting additional cooling or heatsinking
4QD now have finned air and water cooling solutions for the 4QD series, and can also fit the larger DNO heatsink to the Porter range.